Essay: Significance of viscosity in functioning of diesel engine

The diesel engine has been the engine of choice for heavy – duty applications in agriculture, construct ion, industrial, and on – highway transport for over 50 years. Its early popularity was because of its ability to use as a portion of the petroleum crude oil that was previously considered as a waste product of gasoline. Later, the other qualities of diesel like durability, better torque capacity and fuel efficiency assured its role as most demanding applications.1
The operating principles of diesel engines are significantly different from those of the spark- ignited engines. In a spark ignited engine, fuel and air that are close to the chemically correct, or stoichiometric, mixture are inducted into the engine cylinder and compressed which is ignited by a electric spark. The power of the engine is dependent on the quantity of fuel – air mixture that enters the cylinder using a flow – restricting valve commonly known as a throttle. In a diesel engine, also known as a compression- ignited engine, only air can enters through intake system of the cylinder. This air is compressed at high temperature and pressure and then finely atomized fuel is sprayed into the air at high velocity. When atomized fuels come in contact with high temperature air, the fuel vaporizes immediately and undergoes a series of spontaneous chemical reactions resulted into self-ignition or auto-ignition. Generally plug for spark is not required, although some diesel engines are equipped with electrically heated glow plugs to assist with starting the engine under lower temperature. The power of the engine is controlled by changing the volume of fuel provided into the cylinder and this does not require a throttle.
The timing of the combustion process must be precisely controlled to provide low emissions with optimum fuel efficiency. This timing is determined by ignition delay which is the fuel injection timing plus the short time period between the start of fuel injection and the auto ignition. When auto ignition occurs, the portion of the fuel which is ready for combustion burns very rapidly for a period of time which is called premixed combustion. When the fuel is prepared during the ignition delay is exhausted, the remaining fuel burns at a rate which is predetermined by the mixing of the fuel and air. This period is known as mixing-controlled combustion. The heterogeneous fuel – air mixture in the cylinder during the diesel combustion process contributes to the formation of soot particles, which is known to be one of the major challenges for diesel engine designers. These particles are formed because of higher air-fuel ratio. This mixture consists mostly of carbon with small amounts of hydrogen and inorganic compounds. Although the exact mechanism is still not yet clear. It was found that biodiesel produces less amount of soot, this appears to be associated with the bound oxygen in the fuel.2 The particulate level in the engine exhaust is composed of these soot particles along with high molecular weight hydrocarbons that adsorb to the particles as the gas temperature decreases during the expansion process in the exhaust pipe. This hydrocarbon material is known as soluble organic fraction. Generally these soluble organic fraction are increases when biodiesel is used, offsetting some of the decrease in soot.3 Biodiesel’s low volatility apparently causes a small portion of the fuel to survive the combustion process resulted into a thin coating on the cylinder walls, where it releases during the exhaust process.
A second difficult challenge for diesel engine designers is oxides of nitrogen (NOx) emissions. NOx emissions are associated with high gas temperatures and fuel lean conditions and, in contrast to most other pollutants, are usually observed to increase when biodiesel is used.3 NOx contributes to smog formation and is difficult to control in diesel engines because reductions in NOx tend to be accompanied by increases in particulate emissions and fuel consumption. While the bound oxygen on the biodiesel molecule may play a role in leaning the air-fuel ratio in NOx formation regions, the dominant mechanism seems to be the effect of changes in the physical properties of biodiesel, such as the sound speed and bulk modulus on the fuel injection timing.4 One of the most important properties of a diesel fuel is its readiness to auto ignite at the temperatures and pressures present in the cylinder upon injection of fuel. The laboratory test that is used to measure this tendency is Cetane Number Test (ASTM D613). This test gives comparison between the tendency to autoignite of the test fuel with a blend of two reference fuels, cetane (hexadecane) and heptamethylnonane. Fuels with a high cetane numberwill have short ignition delays and a small amount of premixed combustion since little time is available to prepare the fuel for combustion. Most biodiesel fuels have higher cetane numbers than petroleum – based diesel fuels. Biodiesel fuels from more saturated feed stocks have higher cetane numbers than from less saturated feedstocks.5 Biodiesel from soybean oil is usually reported to have a cetane number of 48 to 52, while biodiesel from yellow grease which contains higher saturated esters, is normally between 60 to 65.6
Effect of viscosity on engine life and its performance ‘ Previous studies
Fuel viscosity is specified in the standard for diesel fuel within a limited range. The boiling points of hydrocarbons in the diesel easily meet the viscosity requirement as a fuel. Most diesel fuel injection systems compress the fuel for injection using a simple piston and cylinder pump called the plunger and barrel. In recent development, to create high pressures the clearances between the plunger and barrel are set approximately one ten – thousandth of an inch. In spite of this small clearance, a small fraction of the compressed fuel leaks out from the plunger during compression. If fuel viscosity is low, the leakage will correspond to lesser power to the engine. If fuel viscosity is high, the injection pump will be not sufficiently supply the fuel to fill the pumping chamber which ultimately results into lower the engine energy. The viscosity range for typical biodiesel fuels overlaps the diesel fuel range with some biodiesels having viscosities above the limit.7 If fuel viscosity is extremely higher like with vegetable oils, there will be poor atomization in the cylinder because of lower degradation of hydrocarbons and resulted into production of black smoke.
The density and viscosity of the fuels affect the start of injection, the injection pressure, and the fuels spray characteristics which has influence on engine performance, combustion and exhaust emissions. Analysis of these results can easily be carried out when the key fuel properties of biodiesel-diesel fuel blends are known. 8
The weak van der Waal’s type forces between molecules provide cohesion to a body of fluid, and hence a resistance to internal displacement and flow. This resistance to flow is termed as viscosity. Liquid fuels expand when temperatures rise so that inter-molecular distances increase. Hence, the viscosity decreases.
The dynamic viscosity (??) of a sample may be defined as the tangential force on a unit area of either of two parallel plates at unit distance apart when the space between the planes is filled with the sample fluid, and one of the planes moves with unit velocity in its own plane relative to the other. Hence, it is the force per unit area to produce a unit velocity gradient. This is shown in Figure below:
In the metric system, the unit of dynamic viscosity is g/cm.s or poise, P. The poise is inconveniently large, so a centipoise, cP, is used where 1 cP = 0.01 P = 1 mNs/m2 = 1 mPas (in SI units).
Measuring the terminal velocity of a sphere falling through the sample gives us the dynamic viscosity of a liquid sample. In the case of opaque or slurried samples, measurement is made of the rotational swing of a cylinder suspended in the sample by means of a length of fine wire, or alternatively, by the resistance encountered by the motor-driver, rotation of one cylinder inside another when located in the sample. The latter method is used for fuels of high density and viscosity.
The kinematics viscosity (??) is defined as the quotient of the dynamic viscosity and the density of the sample. Hence
The units are given by
In the metric system, the unit of kinematic viscosity is cm2/s or stoke, St. Here also the smaller unit, the centistoke, cSt, is more convenient, where 1 cSt = 0.01 St = 1 mm2/s.
Kinematic viscosity is important in fuel technology in connection with the pumping and atomization of liquid fuels like diesel oil. Kinematic viscosity may be measured as a ‘conventional’ property, that is, a property that is dependent on the instrument, or alternatively as an ‘absolute’ property of the fuel. Both methods depend on the time taken by discharge of a sample of the fluid through a restriction under its own head. This follows because the force resisting the laminar (low speed) flow of a fluid through a restriction is approximately proportional to the dynamic viscosity, whereas the force promoting the flow is that due to gravity, and is proportional to the density of the fluid. Hence, the time taken for the gravity flow of a given 0volume of sample through a restriction is approximately proportional to the kinematic viscosity.
The conventional methods are generally simpler but less accurate. These are represented by the Redwood Instrument in England, the Saybolt in the US, and Engler in continental Europe. In each case, they comprise a sample cup filled with a standardized orifice tube at the bottom. The sample cup is surrounded by a water jacket containing a heating device (Figures above). When the temperature reaches the test level (this may be 25oC, 38oC, or 99oC for the Redwood test and 21oC, 38oC, 54oC, or 99oC in the Saybolt test), the orifice is unsealed, and the time taken to empty the cup is determined. The result is expressed as Redwood or Saybo1t Universal seconds, or as Engler degrees, which is the discharge time ratio for the sample and water. When the discharge time exceeds 2000 seconds, due to high viscosity, use is made of a Redwood II or a Saybolt ‘Furol’ (Fuel and Road Oil) viscometer which incorporates a larger diameter orifice.
The absolute kinematic viscosity is measured by employing a glass U-tube viscometer with a capillary tube built into one leg. See Fig. 8. The length-to-diameter ratio is such that end effects could be neglected, and higher precision could be achieved. The instrument is vertically dept in a thermostatically controlled water bath, and the time required for a given volume of sample to flow through the capillary is measured. This measured time t is used in the following equation to give the kinematic viscosity in centistoke
where C = instrument calibration constant and
B = instrument type constant, depending on the capillary diameter.
The operating range of the discharge time is from 120 to 1500 seconds and five sizes of instruments are available which cover the range of viscosity from 0.5 to 1500 cSt.
In most commonly used methods the discharge time is based on the movement of the trailing meniscus of the sample. With opaque liquids which tend to adhere to the glass walls, it is more convenient to reverse the flow direction and to time the movement of the leading meniscus.
Modifications of the design are available to increase the range from 0.2 to 300,000 cSt.9 For petroleum fuels; the variation of kinematic viscosity with temperature follows the rectangular parabolic law when plotted on uniform Cartesian coordinates. Straight line plotting is achieved by plotting the curve on a semi-log scale based on the relationship log log (?? + a) = n log T + b where a, b and n are constants. Viscosity of the fuel exerts a strong influence on the shape of the fuel spray. High viscosities cause low atomization (giving large sized droplets) and high penetration of the spray jet. In small combustion chambers, the effect of viscosity may be critical, hence maximum and minimum values may have to be specified. In a cold engine, with the resulting viscous oil, the fuel will be practically a solid stream discharging into the combustion chamber. This will make starting difficult and give a smoky exhaust.
If the viscosity of the fuel is low, leakage past the piston (plunger) in the fuel pump will be aggravated, especially after wear has occurred. The leakage may not be objectionable but accurate metering of the fuel may not be obtained. The lubricating properties of low-viscosity fuels are poor.9
10Fuels with high viscosity tend to form larger droplets on injection which can cause poor fuel atomization, higher engine deposits, require more energy to pump the fuel and wears fuel pump elements and injectors. High viscosity consequently also leads to poor combustion of fuel resulted into higher exhaust of smoke and emissions. 21 High viscosity also causes more problems in cold weather when the temperature is low, because viscosity has inverse relation with temperature as temperature decreases viscosity decreases.20, 22 Fuels with low viscosity may not provide sufficient lubrication for the precision fit of fuel injection pumps, resulting in leakage or increased wear.21 The higher heating value (HHV) is an important property defining the energy content and thereby efficiency of fuels i.e. vegetable oils, biodiesel. The HHV of biodiesel is around 10.0% less than that of petrodiesel (41 MJ/kg compared to ~ 46 MJ/kg). The higher heating value of a fuel increases with increasing carbon number in fuel molecules and also increases as the ratio of carbon and hydrogen to oxygen and nitrogen increases.23 Demirbas (2008)24 studied the correlation between viscosity and higher heating value (HHV) by performing a linear least square regression analysis and argued that there is high regression between the higher heating value and the viscosity values of vegetable oils and their methyl esters; the HHVs of vegetables oils and biodiesels increase with viscosity. The viscosity of trans-esterified oil, i.e., biodiesel, is about an order of magnitude lower than that of the parent oil.25 High viscosity is the major fuel property which explains why neat vegetable oils have been largely abandoned as alternative diesel fuel. 26 The difference in viscosity between the parent oil and thealkyl ester derivatives can be used to monitor biodiesel production.27, 28 Dynamic and kinematic viscosity data (which are related by density as a factor) of some individual fatty compounds are available in the literature. Kinematic viscosity (at 40 ??C), however, is the key parameter required by biodiesel and petrodiesel standards. However, the data in the literature varies, which are not limited to only by dynamic vs. kinematic viscosity but also the temperature. Kinematic viscosity at 40 ??C is limited between 3.5-5.0 mm??/s in the European biodiesel standard norms. The American specifications allow a quite broader range of 1.9-6.0 mm??/s. The corresponding limit for petrodiesel fuel is considerably lower (2.0 – 4.5).29 It is known that viscosity increases with chain length (number of carbon atoms). It is also true for alcohol moiety as the viscosity of ethyl esters is slightly higher than that of methyl esters.21 Geller and Goodrum (2000)30 reported that viscosity of pure and saturated TAGs of 6:0 to 18:0 correlated with the carbon number in a second order polynomial fashion. It was found that as the lengths of the acid and alcohol segments increased, the degree of random intermolecular interactions also increases resulted into higher viscosity. The effect becomes more evident at lower temperatures, because at lower temperature the molecular movements are very restricted.31 Viscosity also increases with increasing degree of saturation. Factors such as double-bond configuration may also influence viscosity i.e. cis double-bonds gives a lower viscosity than trans bonds. This is of significance for the use of ‘waste’ oils (such as frying oils) as biodiesel, as they usually are partially hydrogenated and frequently contain higher amounts of trans-fatty acid chains.21 Double-bond position affects viscosity less; terminal double bonds have a comparatively small viscosity-reducing effect. 25
The kinematic viscosity of several monounsaturated fatty acid methyl esters were also compared by them, among them cis and transisomers of methyl octadecenoate with the unsaturation in three different positions in the chain. For all neat transisomers of methyl octadecenoate, the kinematic viscosity exceeded the upper limit of 5 mm2/s as per European biodiesel standards but according to American biodiesel standard it is within the upper limit of 6 mm2/s. It has been reported by Rodrigues and co-workers (2006) that one double bond was shown to increase viscosity, whereas two or more double bonds caused a decrease in the viscosity. They have proved it by comparing the viscosities of methyl stearate and methyl oleate, (presence of one double bond) and methyl linoleate and methyl linolenate (two or three double bonds, respectively). They have suggested that presence of one carbon-carbon double bond in the structure of oleate gave rise to a stronger intermolecular interaction between the p electrons of the double bonds. This kind of interaction occurred because the spatial geometry of the cis configuration of the one double bond of the oleate still allowed a close packing between the molecules. Obviously, more strong orbital interactions could not be found between the steareate molecules, where only weaker vander Waals interactions were possible. On the other hand, the interactions between the p orbitals in linoleates and linolenates were reduced because of the spatial geometry of these molecules, where the alternate double bonds, all in cis-conformation, led to a coil configuration, hindering the approach of the sp2 atoms from the double bonds of neighboring molecules. These effects should be studied for the differences in viscosity. Branching in the ester moiety can alter viscosity, but it is not as much as unsaturation. Branching reduces the interactions between carbon chains by hindering neighboring molecules, as verified by Lee and co-workers (1995). 32
In a homologous series of compounds, viscosity increases directly with the increase in molecular weight. Unreacted triglycerides and polymers present in the frying oil enhances the viscosity of oil.33 The presence of hydroxyl groups in the molecule such as found in castor oil also increases the viscosity. This is very significance for the production of biodiesel from castor oil. These will produce a fuel which in its neat form exceeds all kinematic viscosity specifications in biodiesel standards due to its high content of ricinoleic acid.21 There are several reports on the kinematic or dynamic viscosity of fatty acid esters at various temperatures. The viscosity of oil is temperature dependent; rate of flow increases as the temperature increases.33 Wang and Briggs (2002)34 studied viscosity of soybean oils with modified fatty acid composition. The viscosity was expressed as:
??=Ae (Ea/RT )
where, R = gas constant, T = Kelvin temp and Ea = activation energy. The concept of effective carbon number was used to describe acyl chain length and degree of unsaturation and was correlated with viscosity and Ea. In the results linear relationships were established indicating that the more the saturation or the longer the fatty acyl chains, the more viscous the oil and the faster the viscosity changes with temperature. Further significant approaches for predicting biodiesel viscosity at various temperatures were carried out by Tangsathitkulchai and co-workers (2004)35 and Krisnangkura and co-workers (2006). Generally, fatty acid methyl esters have a Newtonian behavior above a temperature of about 5 ??C. Below this temperature, they present a pseudoplastic behavior. 36 Methyl esters also exhibit a thixotropic behaviour. In this behavior decrease in the viscosity of methyl esters with increase in temperature is at an exponential rate. The results obtained by Srivastava and Prasad (2001)36, with methyl esters from soybean and mustard seed oils, were also confirmed by Rodrigues and co-workers (2006)31 who also reported that this high viscosity at lower temperatures could be a result of micro-crystal formation and would cause serious problems in fuel lines and in engine filters. Tate and co-workers (2006)38 determined the viscosities of three biodiesel fuels (canola and soy methyl and ethyl esters of fish oil) using a Saybolt viscometer at temperatures from 20 ??C to 300 ??C. Using the measured densities over the same temperature range, the dynamic viscosities were obtained. An index termed the low-temperature viscosity ratio (LTVR) was proposed by Knothe and Steidley (2007)39 to determine individual compounds as well as mixtures by their low-temperature viscosity behavior. To determine this index, the kinematic viscosity value at 0 ??C of a compound is divided by the kinematic viscosity value at 40 ??C
LTVR = ??0 / ??40
The LTVR can be seen as a simplified index of change of kinematic viscosity with temperature compared to the viscosity index (VI) which is commonly determined by measuring the viscosity at 40 ??C and 100 ??C by means of an equation given in standards such as ASTM D2270. The low-temperature viscosity behavior of biodiesel, their components related to fatty materials and blends of fatty compounds, as well as blends with petro-diesel were also investigated. A variety of fatty esters was studied in pure form and in blends with petro-diesel. They also gavelow-temperature kinematic viscosity data of petro-diesel and biodiesel fuels and kinematic viscosity data for common fatty acid methyl esters, methyl decanoate (C10:0), methyl laurate (C12:0), methyl myristoleate (C14:1), methyl palmitoleate (C16:1), methyl oleate (C18:1), methyl linoleate (C18:2), methyl linolenate (C18:3), methyl ricinoleate (C18:1, 12-OH) as well as, for comparison purposes, two alcohols (1-decanol and oleyl alcohol), as well as a triacylglycerol (triolein). The blends showed a behavior closer to that of petrodiesel than of biodiesel or its neat components. Esters with shorter fatty acid chains with longer alcohol moieties display somewhat lower viscosities than esters with longer fatty acid chains and shorter alcohol moieties. Saturated esters with high melting points have only little influence on kinematic viscosity at lower temperatures and concentrations as observed in many common vegetable oils. The presented data can be used for determining fatty esters in terms of enriching them in biodiesel for the sake of improving fuel properties. Furthermore, the data presented can be used for predicting or verifying the viscosity of yet non-investigated compounds, as well as of mixtures. Other literature dealt with predicting the kinematic viscosity of biodiesel. Predicting the viscosities of biodiesel fuels from the knowledge of their fatty acid composition applying a logarithmic equation was investigated by Allen and co-workers (1999) and Rabelo and co-workers (2000) and verified by further studies (Noor Azian et al., 2001; Boyak et al., 2002).40 Beside the prediction of the viscosity in biodiesel, an attempt was also made for determining a blending rule for the viscosity of the blends of a general type of biodiesel with petroleum diesel was made by Yuan and co-workers (2005)41. However, a generalized equation for predicting the viscosities for blends was later given by Alptekin and Canakci (2008).42 The equation correlated the viscosity as a function of biodiesel fraction by empirical second-degree equation. Because, in a linear equation does not fit the data well in case of higher-degree equation. The general form of the equation as a function of biodiesel fraction is given by:
?? = Ax2 + Bx + C
where ?? is the kinematic viscosity (mm 2/s), A, B, C are coefficients which are different for different oils and x is the biodiesel fraction. This physical property can also be used to restrict the FA profile. For example, shorter-chain FA could be excluded by relatively high minimum value for kinematic viscosity as in EN 14214. Although the minimum value for kinematic viscosity prescribed in ASTM D 6751 overlaps petroleum based diesel fuels, the high minimum kinematic viscosity value for biodiesel prescribed in EN 14214 is higher than that of many petrodiesel fuels, underscoring the feedstock-restrictive nature of the EN 14214 limit. Also, biodiesel fuels derived from used frying oils has higher viscosity than fresh vegetable oils because of presence of higher content of trans FA and saturated, or, more generally speaking, less unsaturated FA. An upper limit of 5mm2/s for kinematic viscosity in biodiesel standards may exclude some frying oils as feedstock.29
Recent work by Sivebaek et al. [25] also considered the viscosity of DME, specifically with addition of lubricity and viscosity enhancing additives. They developed a volatile fuel viscometer (VFVM) that was designed to handle DME, neat or additized. They measured kinematic and dynamic viscosities of pure DME of 0.185 cSt and 0.122 cP at 25??C.Their measurements were performed at 5 bar pressure, roughly 75 psi. They have concluded that DME along with additive cannot reach the same viscosity and lubricity as diesel fuel. They have suggested that rather than using additives to allow fuel systems to tolerate DME, it is better to design fuel injection hardware to handle pure DME. McCandless and co-workers have developed models for the properties of DME and have developed fuel injection systems to accommodate neat DME (or more specifically, DME with 1% of a castor oil additive). Their approach has been to develop systems with less of the sliding wear that leads to pump and injector failures that were observed in earlier work on DME. 11
In contrast, in another work, accommodating DME within existing commercial fuel injection systems, there is a need to determine how to improve the viscosity of DME fuel blends, while keeping the DME content as high as possible to capture the emissions benefits of DME. To that end, the viscosity measurements of DME blended with various fuels and with various lubricity enhancing additives is intended to assess whether the viscosity of DME can be increased with only modest treat rates of other substances. The work presented here is the product of a joint effort by the Penn State Energy Institute and Multi – discipline Tribology Group, which have collaborated on studies of several alternative fuels including biodiesel fuels and oxygenated fuels. Significant improvements in friction and wear were demonstrated by vapor- phase oxidation of biodiesel fuels.11
Details of bioadditive and chemical additives, their production and consumption worldwide and at India level.
The modern history of lubricant additives began in the early 20th century with the use of fatty oils and sulfur in mineral oils to improve lubrication under high loads. World War II provided a major impetus to the development of lubricant additives as the military, engine builders, and machine manufacturers demanded more performance from their equipment. Reports say that consumption of lubricant additives in the U.S. has increased from 127 thousand metric tons to 710 thousand metric tons during in 1950 to 1978. Lubricants for internal combustion (IC) engines account for 72% of the market. Total world consumption of lubricant additives is estimated to be about three times that of the U.S12
The effects of fuel quality variations on diesel engine emissions is complicated by the wide variation of the engine response to the fuel quality changes and the extent of inter-correlation of the various fuel variables. Betroli et al. (1993) suggested that the particulate emission reduction could be attained using the ash less additive technology. They found that it is necessary to use a conditioning period prior to emission tests. Kouremenous et al. (1999) examined the effect of the fuel composition and physical properties on the mechanism of combustion and pollutant formation. A number of fuels having different viscosity, density, chemical composition, (especially aromatics type), are used in their investigation and found that the fuel properties namely density and viscosity are more important than fuel composition (aromatics) in respect of engine performance and emissions. The total aromatic content has more influence on engine performance and emissions rather than the individual aromatics.13
Hajdukovic et al. (2000) reported that the toxicity of diesel fuel is generally attributed to soluble aromatic compounds. Alkyl derivatives of benzene and polycyclic aromatic hydrocarbons are considered as most harmful. New oxygen and nitrogen derivatives of hydrocarbons are formed as a result of oxidative and pyrolytic processes during combustion.13
The diesel fuel being heavier and having higher carbon content has some problems when used in an engine. Due to its high freezing point, it can cause blockage of filters and nozzles especially under lower temperature. The routine use of fuel additive in diesel began in 1960’s in Europe as cold flow improvers. The additives added in parts per millions (ppm) levels achieve a specific objective of either improving the physical or chemical characteristics of the fuel or improving the combustion characteristics. There are many more functions of additives. Based on the function and additive concept, they are reported to be classified (Owen Kieth et al, 1990) as antioxidants and stabilizers, deactivators of metals, cetane improvers, combustion improvers, detergents, corrosion inhibitors, anti static additives, dehazers and demulsifiers, anti-icers, biocides, anti-foamants, odor masks and odorants, dyers and markers and drag reducers.13
Kidoguchi et al. (2000) in their investigations they have found that pyrolysis fuels with higher aromatics content could not be done. The aromatic compounds are very compact with very less surface to volume ratio compared to long chain normal polymers. They have higher C/H ratio and also cm ratio per unit volume. They are also more reactive because of lower C-C bond strength compared to C-H bonds.
Hence, in the absence of air, they are prone to higher cracking, pyrolysis and agglomeration with other aromatic molecules nearby during the initial stages of combustion. Their adiabatic flame temperatures are also very high and as a result, soot formation increases (Hirao et al., 1988). Due to higher strength of O-H bonds compared to C-H and C-C bonds, O-H bonds break up in presence of high local temperatures and bring the local temperatures down. This decreases the possibility of formation of NOx. The O-H bonds are reformed as the temperatures decrease and the absorbed energy is given back.
Jensen et al (1983) observed that the concentrations of alkyl homologues of PAH and oxy-PAH in the particulates were found to decrease with increasing cylinder exhaust temperature. The degree of alkylation for the most abundant homologue of these compounds increased by one to two carbons as the cylinder exhaust temperature decreased. The inverse relationship between engine temperature and production of extractable organics suggests one possible emission control strategy. The post combustion reactor might achieve reduction of PM associated with organics. To evaluate the feasibility of such an engine modification, both particulate and vapour emissions need to be collected simultaneously. This is very important for proper correlation of particulate vapour with the engine conditions. Alkyl homologue analysis of diesel emissions provides information which may lead to selection of engine operating conditions that will reduce the environmental impact of diesel emissions.13
It is reported that
a) Iso-propyl nitrate can reduce aldehyde and CO level without much effect on NOx.
b) Iso and Iso-amyl nitrate and di-tertiary butyl peroxide reduce NO x by generating alkoxyl radicals.
Kulinowski et al. (1993) in his review suggested that diesel fuel additives such as cetane improvers, diesel detergents, combustion improvers, low aromatic and sulphur content and lubricity additives can give a desirable effect. They concluded that a properly formulated diesel additive with the above measures will result in desirable changes in the emissions and performance of the engine.13
Many scientists had tried vegetable oil as additive for diesel viscosity. Vegetable oil can be used as viscosity modifier because it has very high viscosity compare to diesel fuel these high viscosity of vegetable oil can be lowered by blending with pure ethanol. 25% of sunflower oil and 75% of diesel were blending as diesel fuel. The viscosity was 4.88 cSt at 40??C, while the maximum specified ASTM value is 4.0 cSt at 38??C. This mixture was not suitable for long-term use in a direct injection engine. A study was carried out by using the dilution technique on the same frying oil. Results with this technology have been mixed and engine problems similar to those found with neat vegetable oils as fuels were observed Basic problem with such vegetable oil as viscosity modifier is that it oxidizes while in contact with environment with respect to time which further lead to increase in fuel viscosity. On the other hand, the increased viscosity of biodiesel and the electronic control system may lead to some increase in the injection pressure and to some injection advance, both changes being associated in the literature to an increased number of small particles 43, 44 but it needs further investigation.
The investigation carried out by K. Pramanik in his research for finding Properties and use of Jatropha curcas oil and diesel fuel blends in compression ignition engine he declared that the high viscosity of the Jatropha Curcas oil which has been considered as a potential alternative fuel for the compression ignition (C.I.) engine was decreased by blending with diesel. The blends of different proportions of Jatropha curcas oil and diesel were prepared, analyzed and compared with base diesel fuel. The effect of temperature on the viscosity of biodiesel and jatropha oil was also studied. The performance of the engine using this blends of base fuel and Jatropha Curcas were studied in a single cylinder C.I. engine and compared. 14
With the performance obtained with base diesel. Significant improvement in engine performance was observed compared to vegetable oil alone. The specific fuel consumption and the exhaust gas temperature were reduced due to improve in viscosity of the blended fuel oil. Acceptable thermal efficiencies of the engine were obtained with blends containing up to 50% volume of Jatropha oil. From the properties and results of engine test, it was concluded that 40’50% of jatropha oil can be blended with diesel without any engine modification and preheating of the blends. The main aim of the present investigation was to adjust the viscosity of jatropha curcas oil and diesel blend close to that of conventional fuel to make it suitable for use in a C.I. engine and to determine the performance of the engine with the modified oils. Standard viscosity was achieved by preparing different dilution with diesel in varying proportions. Among the various blends, the blends containing up to 30% (v/v) jatropha oil have viscosity values close to that of diesel fuel. The blend containing 40% (v/v) vegetable oil has a viscosity slightly higher than that of diesel. The viscosity was further reduced by heating the blends. The viscosity of the blends containing 70.0% and 60.0% vegetable oil became close to that of diesel in the temperature ranges of 70’75 and 60’650C, respectively. The corresponding temperatures were found to be 55’60 and 450C for 50 and 40% blends, whereas only at 35’400C did the viscosity of the 30:70 Jatropha/Diesel blend become close to the specification range. Acceptable brake thermal efficiencies were achieved with the blends containing up to 50% jatropha oil. Blends with a lesser percentage of vegetable oils showed slightly higher exhaust gas temperatures when compared to an engine running with diesel but they were much lower than the jatropha curcas oil in all cases. Therefore, from the engine test results, it has been established that up to 50% jatropha curcas oil can be substituted for diesel for use in a C.I. engine without any major operational difficulties. However, the properties of the blends may be further improved to make use of higher percentage of jatropha oil in the blend using jatropha oil of purer grade which may be obtained by pretreatment of the oil. Moreover, the long term durability of the engine using this blend of diesel fuel requires further study.14
Methods to determine the viscosity
Elana Chapman and co ‘ workers did quantitative measurements of the viscosity of blends of DME in the federal low sulfur fuel were obtained using a high pressure viscometer, using capillary tubes that provided optimal measurement accuracy depending on the viscosity of the fuel mixture. The high pressure viscometer apparatus used for this work was designed and built at The Pennsylvania State University in 1962-63. This viscometer was modified accordingly for charging of pressurized liquid samples, as is necessary when dealing with highly compressed liquids. The viscometer instrumentation is made up of a pressure intensifying system, a pressure measurement system, a constant temperature bath and the viscometer pressure vessel.11
Kinematic viscosity may be measured as a ‘conventional’ property, that is, a property that is dependent on the instrument, or alternatively as an ‘absolute’ property of the fuel. Both methods depend on the time taken by discharge of a sample of the fluid through a restriction under its own head. This follows because the force resisting the laminar (low speed) flow of a fluid through a restriction is approximately proportional to the dynamic viscosity, whereas the force promoting the flow is that due to gravity, and is proportional to the density of the fluid. Hence, the time taken for the gravity flow of a given 0volume of sample through a restriction is approximately proportional to the kinematic viscosity.9
The conventional methods are generally simpler but less accurate. These are represented by the Redwood Instrument in England, the Saybolt in the US, and Engler in continental Europe. In each case, they comprise a sample cup filled with a standardized orifice tube at the bottom. The sample cup is surrounded by a water jacket containing a heating device (Fig 7 a & b). When the temperature reaches the test level (this may be 25oC, 38oC, or 99oC for the Redwood test and 21oC, 38oC, 54oC, or 99oC in the Saybolt test), the orifice is unsealed, and the time taken to empty the cup is determined. The result is expressed as Redwood or Saybo1t Universal seconds, or as Engler degrees, which is the discharge time ratio for the sample and water. When the discharge time exceeds 2000 seconds, due to high viscosity, use is made of a Redwood II or a Saybolt ‘Furol’ (Fuel and Road Oil) viscometer which incorporates a larger diameter orifice.9
The absolute kinematic viscosity is measured by employing a glass U-tube viscometer with a capillary tube built into one leg. The length-to-diameter ratio is such that end effects could be negligible, and the precision is therefore higher. The instrument is placed vertically in a thermostatically controlled water bath, and the time required for a given volume of sample to flow through the capillary is measured. This measured time t is used in the following equation to give the kinematic viscosity in centistoke
where C = instrument calibration constant and B = instrument type constant, depending on the capillary diameter.
The operating range of the discharge time is from 120 to 1500 seconds and five sizes of instruments are available which cover the range of viscosity from 0.5 to 1500 cSt..9
In most commonly used methods the discharge time is based on the movement of the trailing meniscus of the sample. With opaque liquids which tend to adhere to the glass walls, it is more convenient to reverse the flow direction and to time the movement of the leading meniscus.9
Identification of viscosity improver
As we have demonstrated, a fluid’s viscosity will decrease as it is heated. Viscosity Index Improvers are used to reduce the thinning effects caused by operation at elevated temperatures. They are the key component that allows for the production of multi-grade oils. These oils reduce the need for oil changes due to changes in ambient temperatures. Typical viscosity index improvers are polymers and copolymers of methacrylates, olefins, alkylated or dienes styrenes. Viscosity Index Improvers are long chain, with higher molecular weight that function by increasing the relative viscosity of oil more at high temperatures than at low temperatures. Viscosity index improvers can be thought of as springs. They ‘coil’ at cold temperatures and ‘uncoil’ as the temperature increases. Uncoiling makes the molecules bigger, which increases internal resistance within the thinning oil which reduces the overall viscosity loss of the fluid. The long molecules in viscosity index improvers (VII) can be subject to shearing in service to reduce their ability to minimize fluid viscosity loss. Permanent shear occurs when shear stress ruptures long molecules and converts them into shorter and lower weight molecules. The shortened molecules offer less resistance to flow and minimize their ability to maintain viscosity. Permanent shearing of viscosity index improvers can result in piston ring sticking (due to deposits), increased oil consumption and accelerated equipment wear. It should be noted that some VII’s are significantly more shear stable than others. Also, although the type of base stock used and the intended application determines the need for VII, many synthetic stocks may not require them at all. Loss of fluid viscosity can also occur due to a condition known as Temporary Shear. Temporary shear occurs when long viscosity index improver molecules direct themselves in the direction of the stress (flow). This alignment generates less resistance and allows for a reduction in fluid viscosity. When the stress is removed, the molecules return to their random arrangement and the temporary loss in viscosity is recovered.
Petroleum diesel are observed to have law value of viscosity then specified. Therefore, it is required to add certain additives called viscosity modifiers (VM; previously known as viscosity index improvers or VII). These are oil soluble polymers, which enable the oil to provide adequate hydrodynamic lubrication at high temperatures and good starting pumping performance at low temperatures. The mechanism of VII operations have been postulated by various researchers, i.e., Selby and Muller [1-6]. VM improves the viscosity index (VI). This function depends not only on particular polymer chemistry and constitution but also on shear rate and temperature [7]. ASTM D 2270-Procedure B describes the procedure for calculating the thickening efficiency and viscosity index of petroleum products. As the polymer chains break under the load, VII tolerates a permanent viscosity loss beneath the mechanical shear stresses at service conditions. The most common measure of lubricant degradation in service is kinematics viscosity. The reduction in kinematics viscosity, reported as percent loss of the initial kinematics viscosity, is a measure of shear stability of the polymer containing fluid as given in ASTM D 3945 [1]. Olefin copolymers (OCP) constitute one of the most important classes of polymers used as VII. They are oil soluble copolymers comprising ethylene and propylene and may contain a third monomer, a non conjugated diene. Although, ethylene/propylene copolymers represent the simplest case of vinyl copolymers, the structure of these copolymers can be extremely complex.
Despite related chemical structures, these copolymers exhibit different properties and they have commercial significance [2-7]. The chemical and associated physical characteristics, which make these copolymers suitable for using as VIIs, include molecular weight and its distribution, ethylene-to-propylene ratio, and comonomer sequence distribution. Therefore, the chemical nature of a given polymer needs to be understood for designing an effective multigrade formulation. Not only the appropriate range of weight average molecular weight (50,000 – 200,000 g/mol) but also the low polydispersity index (PDI) is the most important factors in control of the shear stability of OCPs. High molecular weight copolymers with broad PDI are able to increase permanent viscosity loss by concentrating the mechanical stress for molecular chain scission [6]. Further, the amount of branching can also be critical to the polymer behavior. No conjugated dienes are often used in the manufacture of ethylene-propylene copolymers to provide a site for cross-linking (in non-lubricant applications) or to reduce the tackiness of the rubber for ease of manufacturing and handling. Certain dienes promote long-chain branching. A disadvantage of long-chain branching is that it reduces lubrication of diesel. With a fixed arm length, an OCP having the higher degree of branching, increases the viscosity index rather than the one having a lower degree of branching. On the other hand, the viscosity index increases with increase in the arm length when the degree of branching is fixed. Adding OCP also causes a change in the pour point of the lubricant oil. The pour point decreases with increase in the degree of branching [8].
The specific grade of olefin copolymer is used as VII, which has low (40-60 mol%) propylene content [5]. This is a desirable characteristic for VII, because the propylene unit (with ??-hydrogen) is the point of attack by any oxidative degradation that occurs during its service in the engine. Consequently, oil undergoes more rapid and higher degree of permanent viscosity loss in engines. Hence, the thermo-oxidative degradation of the macromolecules is also related to the structural features. Due to the higher reactivity of ethylene to propylene, the formation of long sequences of ethylene is favoured rather than propylene sequences. This is substantiated by 13C NMR spectroscopy technique [6]. Also, higher content of ethylene or propylene blocks in polymer chain, which in turn increases crystallinity in the VII, leads to problems at low temperature. For optimum performance, amorphous polymers must contain less than 10% crystallinity. The longer sequence distribution of any monomer affects both the low and high temperature properties of the VII [7]. In addition, the arrangement of monomers must be random along the copolymer chain [5]. Modern diesel is formulated from a range of distillate and chemical additives. Many of the properties of the diesel are enhanced or created by the addition of special chemical additives to the base diesel. For example viscosity modifier (VM) or viscosity index improver (VII) , pour point depressant (PPD), antioxidant, corrosion inhibitor, extreme pressure agent etc. The base diesel also functions as the carrier for these additives in solution under all normal working conditions. Multifunctional additives satisfy more than one purpose and hence research throughout the world is increasing directed toward producing such type additives. In order to improve the viscosity and to meet the requirements of the technical specification viscosity improver are added. Viscosity improver functions by increasing molecular weight of the diesel. Viscosity index improvers(VII) also known as viscosity modifier(VM) are long chain, high molecular weight polymers used to resist the change of viscosity of the oil by increasing the relative viscosity of oil more at high temperatures than at low temperatures. The performance of viscosity modifier is very often expressed in terms of Viscosity Index (VI), which is an arbitrary number that indicates the resistance of a diesel to viscosity change with temperature. The higher the VI, the less the viscosity of diesel changes for a given temperature changes. The performance of the VII depends on the behavior of the polymer molecules in the oil, where the polymer solubility, molecular weight and resistant to shear degradation are determinant parameters. Although additives of many diverse types have been developed to meet special diesel needs, Acrylate based polymers as diesel oil additive are widely used. Several kinds of poly alkyl Acrylates are generally used as performance additive especially as PPD and viscosity modifier in diesel oil composition. In this paper the additive properties of different polymeric additive have been investigated. Physical characterization of the polymers was carried out. The behavior of the polymeric additives towards a specific solvent / base stock plays a significant role in their action as a performance additive in their end application. Viscosity provides very important data about the interaction of additive in base fluid and hence conformation of polymeric system in the base stock. Again, reports regarding such information are scanty and almost nil for polymers used as lube oil additives, present research also include viscometric study of this polymer. The higher the molecular weight, the more viscous the polymer solution will be. This is reasonable, when a polymer has a higher molecular weight, it has a bigger hydrodynamic volume; that is, the volume that the coiled up polymer takes up in solution. Being bigger, the polymer molecule can block more motion of the solvent molecules. It might be said that it can block off more lanes of the highway. Also, the bigger a polymer is, the stronger its secondary forces are. So the higher the molecular weight, the more strongly the solvent molecules will be bound to the polymer. This enhances the viscosity of the diesel.
One additive that is becoming more important to the formulator in meeting this goal is the Viscosity Index (VI) improver. This additive class helps the refiners to keep their diesel product at high performance levels over a wide temperature range. VI improvers are polymeric materials taken from the following technologies: olefi n copolymers (OCPs), polyalkyl methacrylates (PAMAs), poly – iso butylenes (PIBs), styrene block polymers (such as styrene isoprene, styrene butadiene) and ethylene alpha olefi n copolymers. They are prepared by the polymerization of the appropriate monomers.
Key Functions of VI Improvers:
‘VI improvers change the viscosity temperature relationship of a fluid to temper the natural tendency of fluids to thin with increasing temperature and to thicken at lower temperatures. In essence, VI improvers optimize the rheological properties of the lubricant and enable lubricant formulators to expand the temperature operating window of their products.’ ‘VI improvers provide a ‘boost’ to the high-temperature viscosity while having minimal effect on the lube oil viscosity at low temperature. They also reduce the viscosity of oils in response to shear.’ The relationship between the viscosity of polymers in oil and temperature is expressed as the numerical VI scale. The VI is calculated from the viscosity of the polymer in oil solution at 40 C and at 100 C. The smaller the difference in viscosity at low (40 C) and at high (100 C) temperatures, the higher the VI number or index obtained. ‘Most straight paraffinc oils without a VI improver have a viscosity index in the 95-105 range,’ Vargo says. ‘Multi grade oils are formulated within a specific viscosity range by adding polymer. The resulting oil can have a viscosity index in the 105-300 range depending upon the polymer chosen.’ ‘VI improvers provide the thickening normally obtained through use of a high-viscosity base stock. This allows the formulation of the proper viscosity lubricant that has improved low temperature fluidity and retains viscosity better at higher temperatures.’ VI improvers raise the lubricant’s viscosity index, which means that a higher-VI lubricant will change viscosity less as the temperature changes so it retains proper viscosity over a wider temperature range. The third function for VI improvers involves formulation of multi grade lubricants. Dimitrakis explains, ‘VI improvers allow the formulation of ‘multi viscosity’ lubricants, which meet the low-temperature viscosity requirements of lighter grade oil and the high-temperature viscosity of heavier grade oil.’ VI improvers provide important non viscometric performance such as improved piston cleanliness and deposit control, reduced viscosity increase and control of soot-mediated viscosity increase or wear, along with durability of seals and friction materials.’ ‘Besides reducing the viscosity dependence of lubricants on temperature, recent market needs require VI improvers to maintain the viscosity of the lubricant for a longer operating interval than before.’ ‘Besides reducing the viscosity dependence of lubricants on temperature, recent market needs require VI improvers to maintain the viscosity of the lubricant for a longer operating interval than before.’ Dewey Szemenyei, director of customer technical services-engine oil for Afton Chemical Corp. in Richmond, Va., says, ‘VI improvers need to have a greater relative thickening effect at high temperatures than low temperatures, not adversely impact low temperature, display low shear viscosity and have an appropriate level of permanent and temporary shear loss for the application
Working of VI Improvers
VI improvers act through swelling of the polymer chain as the temperature rises to offset the decrease in base oil viscosity. Vargo says, ‘The addition of polymer to base oil in the lubricant results in the interaction (diffusion) of the oil into the space around the polymer molecules. VI improvers act because the hydrodynamic polymer coil size increases as the temperature rises to offset the decrease in base oil viscosity. When the VI improver polymer dissolves in oil, long molecular chains form polymer coils in the oil. ‘As the temperature raises, the polymer chains become more relaxed and tend to be fully extended,’ Vargo adds. ‘This results in an increase in the hydrodynamic polymer coil size, which increases the fluid flow resistance. The net result is a relatively stable viscosity balance over a wide temperature range.’ Briggs notes how VI improvers eliminate the double-exponential dependency between viscosity and temperature: ‘The viscosity of base oils exhibits double-exponential dependence on temperature, meaning the viscosity drops very rapidly with increasing temperature and conversely increases as temperature drops. VI improvers work by adding practically the same percentage of viscosity at any temperature, thus eliminating the double-exponential dependence. This phenomenon is due to the fact that the volume that VI molecules occupy in the base oil is almost independent of temperature, and the viscosity boost is proportional to this volume.’ The uncoiling effect of the polymer is shown in Figure 2 in comparing viscosity- temperature curves of base oil and base oil blended with VI improver.
A fairly condensed polymer chain contributes less to fluid viscosity at low temperatures. But at higher temperatures, the polymer is more solvated by the base oil and uncoils to impart viscosity to the base oil. Iyer adds, ‘The viscosity response to thermal changes of VI improver added base oil is both nonlinear and reversible. A fluid containing a VI improver will be more viscous than one without at any temperature but will be relatively much thicker at higher temperatures when compared to a fluid without a VI improver.’ Abe says oxidative stability is also a factor in ensuring that the viscosity remains relatively stable. He says, ‘Oxidative stability is an important performance property in a lubricant operation that is needed for stability at higher temperatures. VI improvers prepared from fully saturated hydrocarbon polymers will show only minimal oxidation during use.’ Dimitrakis points out that use of VI improvers also can lead to improved lubricant properties at low temperatures. He says, ‘By reducing or eliminating the need for heavier lubricant basestocks, the effect of wax in those oils crystallizing at very cold temperatures is also reduced.’ Nass terms VI improvers as being composed of long and fl exible polymer molecules that interact with the base oil and themselves. He says, ‘This interaction leads to increased resistance to flow, particularly at higher temperatures where VI improvers havea greater impact on lubricant viscosity.’
Polymer Properties Affecting VI Improvers
Szemenyei considers the molecular weight of the polymer to be an important factor: ‘The higher the molecular weight, the greater the thickening for a given type of VI improver.’ Szemenyei also indicates that the concentration of the individual monomers used as building blocks to prepare a VI improver is important. ‘In the case of OCPs, these polymers are based primarily on ethylene and propylene,’ he says. ‘As the ethylene content increases, the thickening effect of the polymer also rises. But there is a tradeoff because higher ethylene content leads to worse performance in the mini-rotary viscometer test (MRV – ASTM D4684).’
The MRV procedure measures the yield stress and viscosity of a lubricant as it is cooled at a controlled rate over a time frame exceeding 45 hours to a temperature between -10 C and -40 C.
Szemenyei says, ‘Another issue is that all mechanical engines shear these polymers differently, meaning that new polymers will have different structures than sheared polymers. Polymers with high ethylene contents may have some of the long ethylene linkages hidden within the molecule.
These long ethylene links may more readily co-crystallize with other components in the oil, leading to solidification at a relatively high temperature. This problem has become more acute with the higher paraffin content Group II+ and Group III base oils now used in the low-viscosity grades that require such good low temperature qualities.’ Nass gives a more general view of which properties affect VI improver performance. ‘Most VI improvers are copolymers made from polymerization of two or more monomers,’ he says. ‘The chemical types of the monomers, their relative proportions, their sequence distribution, the overall molecular weight and the molecular weight distribution are the typical polymer properties that affect VI improver performance. Adjusting any of these properties can change the performance of VI improvers.’
The direct relationship between backbone molecular weight and thickening is readily seen in the curve in Figure 3 for PAMA, polyisobutylene and OCP. A greater percentage of mass of the polymer in the backbone means a larger thickening effect. STLE-member Joan Souchik, technical service manager of Evonik Oil Additives USA Inc., says, ‘The side chains, chemistry and geometry of the polymer play a major role in determining additional performance benefits like low-temperature fluidity or VI. Using a variety of monomer combinations and processing techniques, one can produce numerous different types of polymer architectures such as linear, branched, hyper branched, star and comb polymers, each of which can have homopolymers, block copolymers or random copolymers.’ Souchik continues by stating that this versatility of PAMA polymers can be used to meet specific lubricant application needs. She says, ‘PAMAs can be made to impart specific properties to a variety of fluids with different performance requirements. As examples, they can be made specifically to function as pour point depressants that boost low-temperature fluidity without providing thickening or as VI improvers which provide efficient thickening with excellent shear stability.’ But higher molecular weight polymers are more susceptible to shearing which will reduce their durability. Abe says, ‘In practical operation with polymer- type VI improvers, viscosity drop after a long interval operation causes metal-to-metal contact of gears or bearings. The market trend is moving toward more shear-stable lubricants with lower molecular weight VI improvers.’ These species are based on liquid ethylene alpha olefin copolymers or lower molecular weight PAMAs. Dr. Shanshan Wang, a consultant to Functional Products, discusses how the structure of a VI improver can be modified to optimize lubricant performance. She says, ‘High molecular weight linear polymers give good thickening efficiency and VI performance. Polymers with long branching structure, multi-arm branching or star structure can give better shear stability. By optimizing the molecular weight, the branching lengths, the crystallization behavior of the polymer, a viscosityviscosity modifier with good VI performance, shear stability and low temperature performance can be achieved.’ Meldrum indicates where the current VI improver polymers can best be used in lubricant applications. He says, ‘Elastomeric vinyl monomer based polymers such as OCPs or styrene block polymers can be more cost effective in multi grade engine oils. PAMAs, styrene ester copolymers and other types can be more readily tailored for the specific application, whether a transmission fluid, a hydraulic oil, a gear oil or another fluid.’ Briggs also examines how the main polymer types can be used in lubricant applications. ‘The key is to achieve the right balance in properties,’ he says. ‘For example, the optimum balance of shear stability index (SSI) and thickening efficiency (TE) allows reduced polymer treat rates for engine cleanliness and stay-in-grade performance while maintaining adequate wear protection. PAMAs can deliver asignificant VI boost but have poor TE. High ethylene content OCPs provide good thickening efficiency but because of inherent crystallinity on the molecule can jeopardize low-temperature pumpability.’ 15
Characterization of viscosity improver
Evaluating VI Improvers
All of the respondents indicate that two of the most important tests are to measure TE and SSI. Dr. Isabella Goldmints, viscosity modifier technologist for Infineum USA LP, says, ‘TE measures added viscosity per unit mass of VI improver, and SSI measures the percent of polymer-added viscosity loss after a 30-cycle Kurt Orbahn test.’ On the matter of shear stability,
Vargo says, ‘Increasingly, oil manufacturers are requiring more shear-stable polymers with Permanent Shear Stability ratings in the range of 25.0%. This means the oil should retain 75.0% of its viscosity and loses 25.0% of the viscosity imparted in the oil after the oil-polymer blend has been mechanically worked upon and sheared.’ The Permanent Shear Stability Index (PSSI) test measures the viscosity decrease under actual operating conditions as a hydraulic fluid, compressor oil or motor oil. While simple test methods readily measure VI and thickening efficiency, the Polymer Shear Stability Test is more precise and is measured by the Sonic Shear Test (ASTM D2603-01). More real world test procedures used include the Mechanical Share Test (ASTM D6278) and the Kurt Orbhan test.’ Other parameters that can be very important in assessing the performance of a VI improver are listed by Dimitrakis. He has mentioned that, ‘Depending upon the use, the VI improver should be evaluated for its efficiency to provide dispersancy, rate of oxidation of related deposits and viscosity increase, effect of soot-related viscosity increase and soot-related wear, effect of temperature, shearing, traction or internal fluid friction properties and film thickness
USING VI IMPROVERS OPTIMALLY
With wide choices of VI, the lubricant formulator must determine the best practice to use VI improvers to maximize product performance. ‘There are number of ways for VI improvers to enhance their ability of a lubricant in a specific application,’ Nass says. ‘VI improvers having a high TE could be used at low treat rates which minimizes deposit and sludge formation. It was found that some VI improvers have no significant impact on cold cranking simulator viscosity which allows the oils to be formulated with heavier, lower volatility basestocks. ‘Some VI improvers works very efficiently in maintaining high-temperature, highshear viscosity, even after mechanical shear and helps to protect against wear,’ Nass also added that fuel economy is another area improved by using VI improvers that impart specific viscometric properties to the oil. The benefit of a VI improver could be focused on improving the durability and efficiency of the lubricant,’ says STLE-member Doug Placek, presidentof Evonik Oil Additives USA Inc. He has also added that finished lubricants are typically constrained by various performance specifications. A formulation which incorporates an appropriate VI improver will facilitate an economic solution between both the formulation cost and performance. This can meet and often can exceed the required specifications.’ Szemenyei has mentioned that formulating the proper VI improver leads to a lubricant which works well at the optimum operating viscosity. He also said that VI improvers enable the lubricant for providing minimum energy consumption alongwith equipment protection.’
Szemenyei also warned that formulators should think beyond the treat cost of the VI improver. And said one should not make a mistake in judging the cost of using a VI improver by the VI improver cost alone. He also suggested that the lubricant producer should look at the overall impact of the VI improver on the fully formulated oil. Dimitrakis pointed out that the choice of a VI improver is application dependent. The formulator should select the high- and low-temperature viscometrics as per requirement. Dimitrakis further said that formulators should follow several steps in using VI improvers. He said that people feel that formulators should follow industry-recognized interchange protocols for VI improvers and produce the necessary data for a specific formulation using a control VI improver. The OEM must approve any interchange, by comparing with OEM-approved lubricants. Non-viscometric tests should also be done because the VI improvers possible have a substantial effect on performance areas outside of viscosity. It was also found that efficient compromise between thickening efficiency and shear stability could be achieved by treating the VI improver with a narrow molecular weight distribution. Abe said that formulators should mention the target performance of the lubricants they are synthesizing. Usually there is a substitution between thickening efficiency and shear stability for most polymers, therefore formulators should decide on the molecular weight of the VI improvers. But it was noted that narrowing the molecular weight distribution of the polymer can effectively compromise since polymers with narrower molecular weight distributions have lesser higher molecular weight chains. Abe also added that the formulator can work with a polymer which has both good thickening efficiency and shear stability. Besides looking at TE and SSI, Goldmints has given importance to formulation cost and flexibility. She said that some VI improvers require the addition of higher quality (and thus higher priced) base oil to meet target fresh oil viscometrics and those should be considered in formulation cost. Other important characteristics of VI improvers like compatibility with a number of basestocks and additive components those are added in formulation flexibility should also be considered. The type of VI improver chosen should have a significant impact on total formulation cost and one of the key criterions in formulation development.’ 15
Optimization of viscosity improver
Capabilities of Current VI Improvers
Currently it is believed that all the VI improvers available now a day can provide adequate performance. Although there is many more improvement need to be done. Till date hardware continues to change for reduced emissions, increased fuel economy and better durability, and along with this the lubricant performance will also have to change as well. This means it has created new requirements for both the performance additive and VI improver.’ Meldrum adds, ‘Significant strides have been made in the past few years to increase VI improver performance, permitting lubricants to be blended to a higher viscosity index, provide better fuel economy and lead to fewer engine deposits. Additional VI-improver improvements to provide increased engine and after treatment system durability, while maximizing fuel economy, can be expected in the future.’ Goldmints comments on several other challenges for VI improvers. ‘Market trends such as longer drains, increasing use of Group III base oils and the growth of biodiesel require careful consideration in VI improver selection,’ she says. ‘The lubricant industry is driven to make every possible improvement in fuel economy, which means that new VI improver technologies are under development to reduce fuel consumption while providing adequate engine wear protection.’
Nass believes that for the lubricant industry to develop the ideal VI improver, improvements must be made in a number of areas. He says, ‘The ideal VI improver would be an inexpensive polymer that thickens the oil at low treat rate (high TE), maintains high-temperature, high-shear viscosity above the minimum required by SAE J300 and OEMs, provides optimal viscometrics for fuel economy and has minimal impact on the low-temperature properties as evaluated by the Cold Cranking Simulator and Mini- Rotary Viscometer tests. For all VI improver technology types, there are opportunities for improvement in each of these areas.’
Iyer notes an ongoing industry trend toward a smaller and leaner equipment footprint. ‘The reduction in equipment size is placing more pressure on the lubricant to provide excellent performance,’ he says. ‘In our experience, VI improvers with good thickening efficiency, shear stability and low-temperature performance such as PAMAs have proven to be very effective in applications testing done over the past decade.’ Iyer adds, ‘In hydraulic fluids systems, the trend is moving toward smaller sumps. In the past, the ratio of the size of a sump to the flow rate of a pump was 3:1. Now manufacturers are pushing the ratio to be equal or even less than 1:1, which means that hydraulic fl uids cycle through the system more frequently and, as a result, pick up more heat.’ He concludes, ‘The challenge is to improve fuel efficiency and maintain fluid life by either cooling the oil, which expends more energy, or improving the hydraulic fluid so it operates effectively and efficiently at higher temperatures. Newer, high-VI hydraulic fluids with better additive packages are making inroads into meeting this challenge.15
Determination of blending ratio with diesel or fuel
Incorrectly blended fuel can result in engine problems and warranty issues and also may affect the tax credits associated with blending biodiesel. The myriad of blending methods, biodiesel feed stocks, and analysis techniques can all add to the uncertainty of the fuel blend. Understanding the issues involved with blending and determining blend ratios will help to reduce inaccuracies and in turn will reduce the frequency of problems associated with incorrect blends. Biodiesel has been blended with diesel for many years; how-ever, recent consumer demand coupled with government tax incentives and environmental pressures has resulted in a dramatic increase in production volume.
The method used to blend the fuel is the most important factor contributing to blend accuracy. The two major blending techniques used are splash blending and in line (injection) blending. Currently, the most widely implemented technique is splash blending. This blending process involves adding biodiesel to a fuel truck that is partially filled with diesel fuel. The blending occurs as the truck drives and the fuel splashes around in the tank.
Unfortunately, in many cases, the truck does not drive far enough for the two fuels to blend uniformly. In addition, environmental factors such as temperature and humidity can affect the speed at which the fuels blend.
A recent analysis, carried out jointly by Wilks Enterprise, Inc. (South Norwalk, Connecticut, USA) and Pacific Biodiesel (Santa Cruz, California, USA), on a fuel truck using the splash-blending technique that supposedly had a B20 blend, showed approximately 12% biodiesel at the top of the tank, and 24% at the bottom. Clearly this is not an ideal biodiesel blending method; however, with little biodiesel infrastructure in place, this meth-od is the cheapest and easiest blend method for a fuel distributor to use.
A second, more accurate blending method is in-line blending. This type of blending occurs at a fuel rack, where dedicated blending equipment delivers a metered amount of fuel into a waiting truck. Ethanol and other fuel additives are commonly blended using this method. With in-line blending, the correct ratio of biodiesel is metered with automated control valves into the diesel fuel before it is dispensed into a truck. Since the resulting fuel is blended prior to entering the truck, the mixing problem associated with splash blending is eliminated. Although in-line blending offers a more accurate blending method than splash blending, any mechanical system is subject to wear and/or failures. The need to test the biodiesel blend ratio after final mixing is necessary regardless of the blending method. An accurate method to determine the biodiesel blend is just as important as an accurate blending method. Infrared (IR) spectroscopic analysis is currently the most popular method for measuring biodiesel blend composition ow-ing to its short analysis time and accuracy. Both the EN 14078 method and the recently passed ASTM D 7371 method specify mid-IR for the measurement of the biodiesel blend ratio. Both methods call for an FTIR (Fourier transform IR) spectrometer for the analysis. Whereas FTIR spectrometers are excellent laboratory analyzers, their cost, size, and delicate parts make them difficult to bring to the field. 16
To test the accuracy of biodiesel blending, they developed and validated a radiocarbon-based method and then analyzed a variety of retail biodiesel blends. Error propagation analysis demonstrated that this method calculates absolute blend content with ?? 1% accuracy, even when real-world variability in the component biodiesel and petrodiesel sources is taken into account. They independently confirmed this accuracy using known endmembers and prepared mixtures. This is the only published method that directly quantifies the carbon of recent biological origin in biodiesel blends. Consequently, it robustly handles realistic chemical variability in biological source materials and provides unequivocal apportionment of renewable versus nonrenewable carbon in a sample fuel blend.17
Sasaki Shigehiko et. al has developed a blending operating system which calculates optimum blend ratios, based on values measured in a continuous online analysis of product properties and corrects control set points automatically. 18
Effect of environmental parameters on viscosity with the data of previously done studies and their recommendations
Lubricant additive packages are multi-component blends sold to industrial customers. The risk of exposure of these products to man and the environment is already tightly controlled by existing EU legislation concerning protection of workers, emissions to air, water and soil, and disposal of waste.
In addition to legislative drivers, the lubricant additives industry, represented in Europe by the ATC, has an ongoing commitment to provide accurate and up-to-date health, safety and environmental advice to all downstream users. In particular, the ATC has taken an active and informed role in communications on Health, Safety and Environmental matters, such as providing nomenclature on additives, disclosure of appropriate detail on composition and hazard information and producing best practice guidelines for specific substances.19
Historically, additive manufacturers have collaborated in the testing of major additive classes. A recent example of this was the voluntary participation in national and international chemical initiatives (e.g. US EPA High Product Volume; ICCA SIDS) to generate data on a significant number of chemical classes, covering the main additive families included in crankcase lubricants.
The majority of lubricant additives are of low mammalian toxicity2 and are typically less harmful when ingested than familiar household products. Some lubricant additives are suspected of being harmful to aquatic organisms and most show a degree of persistence but these additives are typically of low water solubility and when handled and disposed of according to manufacturers’ recommendations are considered not to present a significant environmental risk.20 Additive suppliers provide this information to downstream users through hazard communication documents such as the Safety Data Sheet (SDS) and product label, which contain relevant instructions for safe storage, use and disposal.
The composition of the crankcase lubricant changes during use. Some additives are chemically changed or even destroyed as part of their functionality, and any contaminants of combustion generated during the course of engine operation which are not swept into the exhaust stream are neutralised and dispersed within the lubricant. It is widely accepted that used oil drained from the engine sump will contain polyaromatic hydrocarbons generated by the combustion process and this waste is suspected to pose a carcinogen risk through accidental skin contact. A significant number of motorists routinely perform engine oil changes themselves and so consumer product labelling and consumer education against inappropriate contact or disposal of used lubricants is part of an active safeguards programme within the oil marketing industry. This, together with a legal requirement to dispose of used oil at dedicated collection facilities serve to minimise the environmental and human risks from used oil.22
Most widely used viscosity improver and their chemical characteristics (in form of table)
References
1. Broge, J.L., ‘Revving Up For Diesel,’ Automotive Engineering International, V. 110, No. 2, February 2002, pp. 40- 49
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