Combustion Chamber Deposits in Spark-Ignition Engines

TRCP 3709

Combustion Chamber Deposits in Spark-Ignition Engines: A Literature review by Gautam T.Kalghatgi

Shell Research Ltd., Thornton Research Centre, P.O.Box 1, Chester CH1 3SH, U.K.

Paper proposed for presentation at the SAE Fuels and Lubricants Meeting, October 16-19, Toronto, Canada

April 1995

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Combustion Chamber Deposits in Spark-Ignition Engines: A Literature Review Gautam T. Kalghatgi Shell Global Solutions U.K.

viz carburetters, port fuel injectors and inlet valves has matured and the use of such additives is being increasingly mandated. There has been concern that some of this technology might cause an increase in CCD formation and associated engine performance problems. The spurt in interest in CCD is reflected in industry initiatives like the 1993 CRC Workshop on Combustion Chamber Deposits ; the Proceedings of this workshop (2) contain a lot of valuable information. An earlier source of information on CCD is (3). This paper is a review of the literature more focused on CCD than in (1). It contains material that was not considered in (1) and is meant to be read in conjunction with (1). The fuel of interest is unleaded gasoline unless otherwise mentioned.

ABSTRACT Deposits, derived primarily from the fuel but with some contribution from the oil are formed inside the combustion chamber of a spark-ignition engine with use. The growth of combustion chamber deposits (CCD) is a dynamic and, to an extent, reversible process which at any given time reflects the balance between the formation and removal processes. Engine surface temperature is the most important parameter that affects their formation and changes in engine operation which tend to increase surface temperature, reduce deposit growth. At a fixed temperature, less volatile fuels tend to form more deposits than more volatile fuels. Some detergent additive packages tend to increase CCD levels. CCD reduce the heat lost to the coolant and increase charge temperature thereby increasing flame propagation rates but reducing volumetric efficiency; they might also affect the final phase of combustion by as yet undefined chemical means. This is reflected as an increase in octane requirement and NOx and a reduction in maximum power but an improvement in fuel economy and a reduction in CO2 emissions. They might also lead to higher HC emissions but not necessarily always since there might be competing mechanisms which come into play. CCD effect on CO emissions is not clearly established. They can also cause other interference problems like carbon rap. It is not known to what extent engine performance is affected by small changes in CCD levels. There is a large variation in deposit growth and its response to changes in fuel, additives and engine operating conditions across the combustion chamber and between different engines. Similarly, the performance of different engines will be affected to different extents by the deposits. While assessing the effects of different fuels or additives on engine performance and emissions through their effects on CCD, the simultaneous effects on other aspects such as inlet valve deposits which might have their own effects on engine operation, should also be considered. The paper reviews the literature on these topics.

2. GROWTH OF COMBUSTION CHAMBER DEPOSITS 2.1 DEPOSIT FORMATION AND REMOVAL Combustion chamber deposits are derived primarily from the fuel and to some extent from the engine lubricating oil in typical modern engines with low oil consumption running on typical, full-boiling-range fuels (e.g. 4,8,9,10). The primary deposit formation mechanism appears to start with radical initiated addition/substitution reactions on fuel and lubricant components to produce coordinated oxidation products; these products then condense on the hot surfaces and undergo polymerisation (11,13). The polymerisation mechanism is not simple pyrolysis (12). For example deposits formed by heating oils in shallow dishes or from fuels burnt in diffusion flames contain less than 2% oxygen as opposed to 16-25% in CCD. However when films of oil were subjected to periodic ignition and extinguishment, deposits similar to CCD in terms of oxygen concentration and pyridine solubility were obtained (12). Lauer and Friel (12) suggest an "oxidative pyrolysis" of deposit precursors which condense on the surfaces. Price et al suggest that this process starts with the reactions caused by radicals that diffuse into the quench layer adjacent to the wall from the flame (13). Partial oxidation reactions occur ; further surface reactions prevent the evaporation of such material and the deposit formation process continues (13). The condensation of the deposit precursors on the surface is the critical step in this process and deposit formation is strongly dependent on the surface temperature (4,5,6,7,14,22) higher surface temperatures reduce deposit levels.

1. INTRODUCTION The literature on the deposits that form with use on the internal surfaces of a spark ignition engine was reviewed in (1). In the period following the publication of (1), new field problems like carbon rap (see below) attributed to combustion chamber deposits (CCD) have been recognised. Moreover, fuel and additives technology to ensure satisfactory control of deposits in other parts of the engine

As the deposits grow, the surface temperature increases because of the insulating properties of the deposits 2

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mechanisms like oxidation and gasification (29), physical mechanisms like desorption and evaporation of volatile and gaseous components and mechanical removal like abrasion, flaking brought about by thermal stresses and mechanical wash-off. In many of these mechanisms e.g., oxidation, as the surface temperature increases, the rate of deposit removal would increase (29). Price et al have studied the deposit removal process to an extent in a laboratory rig (13). The details of such removal mechanisms and the extent of their individual importance are difficult to assess in the engine context though some useful attempts have been made recently (29). Deposit weight and any other property measured at a given time reflect the balance of the deposit formation and removal processes upto that time.

(1,5,15,16). This can be inferred from the composition variation across the thickness of the deposit (12,44,51,60) with low-melting-point materials at the wall to high-melting-point materials at the deposit surface. For instance, Nakamura et al (44) demonstrated sharp concentration gradients from unleaded gasoline with high concentrations of carbon near the wall and high concentrations of inorganics such as Ca and Zn (presumably from the oil) at the deposit surface which Chapman and Williamson (60) have shown to be in the form of calcium sulphate and zinc phosphate. Lauer and Friel (12) speculate that acid and anhydride groups in the carbon-rich layers adjacent to the wall are responsible for their adhesion to the wall. The measurement and modelling of heat transfer through CCD (see section 5 below) also confirm the increase in surface temperature as deposits grow. Hence as CCD grow, only the less volatile deposit precursors will condense and contribute to deposit growth; the results of thermogravimetric analysis (TGA) of deposits collected at different stages of an engine test by Daly et.al (18) confirm this. Daly et al (18) also report that as an engine test progressed, the aliphatics to aromatics ratio as determined by solid state NMR decreased i.e, the newer deposits were relatively more aromatic. It has to be said that Daly et.al (18) interpret their results in terms of a more complicated model for deposit formation. However their observations are fully in line with the simpler physical picture of deposit growth described above and in (5). Ebert et al (42) have also shown the ability of the deposits to adsorb aromatics from the fuel. They suggest that deposit growth occurs through a chain-growth mechanism with adsorbed species being oxidatively bound to the existing deposit.

2.2 DYNAMIC NATURE OF DEPOSIT GROWTH Deposit growth is a dynamic and to some extent reversible process (e.g.30). For instance deposit levels in an engine can be reduced to a new equilibrium level if some deposit formation is switched off and deposit removal rate is increased say by increasing the surface temperature for a given fuel (4). Similarly for the same engine condition, replacing the fuel with a low boiling point fuel like isooctane (1) or even replacing the highest boiling fraction of the fuel with another of lower deposit forming tendency (30) will switch the deposit formation off temporarily because the surface temperature will be above the critical temperature for the new fuel and deposit weights and thicknesses will move to a new, lower equilibrium. 2.3 VARIATION IN DEPOSIT GROWTH - The average metal surface temperature varies widely across the combustion chamber, from around 120 C (31,32) in the cooler areas to about 320 C at the exhaust valve seats to over 800 C at the exhaust valves (33). During an engine cycle, surface temperature can increase by 400 C from its base line value (29) in the cooler areas. Piston surface temperatures vary between 200 C and 300 C depending on location and engine operating condition (32,33). Piston temperatures are also 10 C to 25 C higher than corresponding combustion chamber surface temperatures (32). The exposure of the surface to liquid fuel or additive droplets or to combustion products which might be important in deposit removal and formation mechanisms will also not be uniform across the cylinder. Hence there are very large differences in the weight, thickness and properties of deposits from different parts of the combustion chamber (1,21,22,34-36).

In general, at any given engine condition, it is the highest boiling fractions of the fuel that contribute most to CCD formation (20). For a given fuel, there will be a maximum surface temperature above which little deposit will form. This critical temperature was found to be 310 C for indolene by Cheng and Kim (4) and 320 C for another unleaded gasoline by Nakic et al.(14). For lubricating oil, which has a higher boiling point compared to the fuel, Cheng (5) reports that the critical surface temperature for deposit formation is about 60 C higher. For fuels of low boiling point like isooctane this critical temperature will be lower than any normally found in the combustion chamber and very little deposit from the fuel will be formed (e.g. 6,7,12,19). In such cases the deposits will primarily be derived from engine oil; Tsukasaki et al report that in cars running on methanol, which has a low boiling point, CCD consisted mainly of calcium originating from the oil (17). Engine oil appears to affect the formation of piston-top deposits more than the head deposits (see Section 5 below). For a given engine operating condition and hence a surface temperature regime, if higher boiling point materials, such as some fuel additives, are added to a given fuel, more deposits will be formed (e.g. 10,14,15, 18, 21-27) and this difference is likely to be more marked, the cleaner the original fuel is.

In general, surface temperatures increase with engine speed, throttle opening, coolant temperature and to a smaller extent with ignition advance and compression ratio except for exhaust valve temperatures which decrease slightly with an increase in compression ratio (32,33). They also increase as the mixture is made richer, because of the increase in the temperature of the burnt gas, peaking at an air fuel ratio about 15% fuel rich compared to stoichiometric conditions (33). Most of the experimental observations discussed below can be understood within the context discussed above. These studies range from single cylinder engine studies using

As the deposits are being formed, they are also being removed through various mechanisms as listed by Lepperhoff and Houben (28). These include chemical

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are high, are composed almost entirely of inorganic compounds (6,19,36,44). Deposits in the end-gas region i.e the region where the flame front arrives last, have lower oxygen content and higher carbon content (19,36), indicating a less oxidised state, compared to deposits from other parts. Gebhardt et al (19) also found lower quantities of volatile material compared to non-volatiles in deposits in the end-gas region which they interpret as indicating condensation of heavier aromatics in this area. There can also be substantial differences, e.g. in H/C values, in deposits from different engines (19,36) and the same engine operating at different conditions (36) .

removable probes (e.g. 4,8,9) to tests on production engines on test stands and road tests. 3. PROPERTIES OF COMBUSTION CHAMBER DEPOSITS In comparison with fuel and oil, on average, CCD from unleaded gasolines have much higher weight concentrations of oxygen (16%-25%) and nitrogen (1%-2.5%) and much lower H/C ratios (0.6 to 1 compared to 1.7 to 1.9 molar) (22,36,42,48,60); the carbon content is between 60% to 70% by weight. Thus a substantial change of the fuel and oil involving a large loss of alkyl components and a significant degree of oxidation takes place before their inclusion in CCD. The deposits also have a substantial volatile component - around 35 weight percent of the initial deposit is lost if it is heated to 500 C (19,42). The presence of lead increases deposit weights considerably - by a factor of up to 8 over unleaded gasoline ; the weight of the deposit is not related to lead concentration in the fuel above a limit of about 0.15 g/l (1). In deposits from leaded gasolines, lead constitutes a substantial (upto 70% wt) proportion of elemental composition (60). Other metallic elements, predominantly from the oil also show up in the deposits though, naturally, they are more prominent - between 2% and 6% - in deposits from unleaded gasolines (42,60). The concentrations of Zn, Ca and P, all coming from lubricant additives, are upto four times higher in piston-top deposits compared to head deposits (22,41,61); thus engine oil appears to affect the formation of piston deposits more compared to head deposits (22,61).

The most commonly measured deposit properties are thickness and weight. Thickness measurements can now be routinely made in different parts of the combustion chamber using commercial probes (e.g. 10,21,34,35,37-39) though other techniques like an optical microscope have also been used (15). Such measurements show the differences in deposit thickness in different parts of the combustion chamber, e.g lower thicknesses in hotter regions (5,15,37), and different ways of estimating an average thickness from such measurements have been recorded (e.g.34,35). Pistontop deposit weights are usually around 45% of the total CCD weight (21,40). In a given engine test with fixed fuel/additive combination, there is usually a reasonable correlation between any of the thicknesses measured and deposit weight although there is a lot of scatter (34,38,39). However, there can be significant differences in the response of different engines to the same additive - for instance, an additive which more than doubled deposit thickness compared to the base fuel in one engine test can produce a statistically insignificant effect in another engine test ( e.g. Engine C vs Engine F in 35).

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Densities of around 1530 kg/m have been reported for deposits removed from the combustion chamber for commercial unleaded fuels (41). In other studies where deposit volume was estimated through detailed thickness 3 measurements, average densities of between 1100 kg/m and 3 2000 kg/m were found with unleaded gasolines (43). Measured heat capacities of deposits removed from the combustion chamber vary from 0.84 kJ/kgK to 1.84 kJ/kgK (41,44); there appears to be a good positive correlation between the carbon content of the deposit and its heat capacity (44). Thermal conductivity is more difficult to measure and for deposits taken out of the engine depends very much on the way they are prepared into pellets prior to measurement (44). These values have ranged from 0.17 W/mK to 0.8 W/mK (see Table I in Ref.46). Thermal characteristics are likely to depend on the morphology of the deposit and need to be measured in-situ. Such methods have been developed and depend on making in-situ temperature measurements and inferring the thermal properties by modelling the heat transfer through the deposit (e.g.15,16). Thermal diffusivity of CCD from an unleaded gasoline has -7 2 been measured to be around 3.5 x 10 m /s (15). Hayes et al. report a wide variation in the thermal diffusivity from -7 -7 2 0.59 x 10 to 3.4 x 10 m /s in deposits from different locations in the engine (47).

4. DEPOSIT STRUCTURE Different descriptions of the CCD structure have been given from different view-points. The carbon is amorphous (non-graphitic), extremely porous and characterised by a heterogeneous granular structure representative of a carbon produced by a pyrolitic mechanism (41). The exhaustive analytical studies of deposits from unleaded fuel by Ebert and co-workers (42) show that CCD are volatile carbonaceous materials more like bituminous coal than graphite. Between 85% and 90% of the carbon is aromatic this aromaticity is higher than that found for coals of comparable chemical composition. Their structure is a composite one consisting of a refractory "skeleton" which is rich in oxygen and poor in hydrogen compared to the volatile, smaller molecules adsorbed on this backbone (42). Similarly, Chapman and Williamson (60) report columnar and dendritic microstructures made up of inorganic material with amorphous carbon filling the gaps between these structures. The volatile material which is not bound so tightly to the deposit closely resembles the heavy ends of the fuel from which the deposit is probably formed. Unburned fuel components and oxidation products derived from fuel and oil can be detected in the deposits; these include carboxylic acids, metal carboxylates, esters, ketones, aldehydes and lactones (8).The primary molecular building

There are substantial differences in deposits from different parts of the combustion chamber. For instance, deposits on exhaust valves, where the surface temperatures

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blocks appear to be 2-4 ring PNAs originating in the fuel (11). These building blocks appeared to be independent of the fuel composition or the severity of the test cycle in a given production engine (11).

starts decreasing again if the mixture is made leaner beyond a limit ( also see 1.3 above). Compression Ratio - Changing the compression ratio from 7 to 8.75 had little effect on deposition rate (4). However Shore and Ockert (7) state that deposition rate went down as wall temperature was increased by raising the compression ratio though they do not show any direct evidence to support this claim. The change in surface temperature brought about by an increase in compression ratio is not uniform across the combustion chamber (33).

High-aromatic fuels have been shown to yield condensed, high-aromatic deposits and low-aromatic fuels produce "fluffier" deposits (11,49). The structures of the deposits on the head and on the piston were not found to be substantially different but there were cylinder to cylinder variations probably caused by temperature gradients (11). The molecular structure of deposits from two production engines running on two different cycles was similar (11,49) but differed significantly from the deposits from a Honda generator running on low load (11). This was attributed to differences in surface temperature and the consequent differences in the degree of condensation of deposit precursors (11). On increasing the load on this engine, deposit molecular structure changed, to resemble that in the production engines (11).

Ignition Advance -Increasing the spark advance increases metal temperatures (33) and Shore and Ockert (7) claim, again without showing direct evidence, that this reduces CCD formation. Surface Material - Cheng and Kim (4) found that deposition rate was independent of surface material as long as the thermal conductivity was high enough; they tested aluminium, brass, cast iron, sapphire, stainless steel and macor ceramic. Deposition rate was very low, however, on the ceramic coupon. Aluminium heads and pistons run roughly 40 C to 80 C cooler compared to cast iron ones (31) and consequently could be expected to be more prone to deposit formation.

5. FACTORS THAT INFLUENCE DEPOSIT GROWTH 5.1 EFFECT OF ENGINE PARAMETERS -In general, operating conditions that tend to increase surface temperatures lead to a reduction in CCD formation.

5.2 EFFECT OF FUEL PROPERTIES -The conclusions from the pioneering study of Shore and Ockert (7) have been confirmed in many other studies.

Engine Speed and Load - As engine speed or load is increased, deposit formation is reduced (7,40,50). In a fixed duration test, total fuel consumption increases as speed/load increases and it was found that there was a good negative correlation between total fuel consumption and CCD weight (40) as speed/load increased. In fact deposit levels can be substantially reduced in an engine already with CCD by running it at a high speed or load for a relatively short time (51-53).

The most important fuel property is the boiling point and the most critical fuel components in deposit formation are those with the highest boiling points (20). At a given boiling point, taking the major gasoline components, aromatics are the most prone, paraffins are the least prone with the olefins in between for CCD formation and boiling point is the only factor determining the deposit forming tendencies of aromatics (7). Price et al have confirmed these findings of Shore and Ockert (13). They also report that cyclic alkanes and olefins show greater deposit forming tendencies compared to their straight chain counterparts (13). Polynuclear aromatics (PNAs) are strong deposit formers depending on their structure; angular arrangements of aromatic rings produce more deposits than structures with linear configuration of rings (13). Also the deposit-forming tendencies of PNAs increases with the number and length of the alkyl side chains attached to the backbone of the molecule (13).

Coolant Temperature- Cheng and Kim found that as coolant temperature was increased from 40 C to 95 C, deposition rate decreased (4). At the higher coolant temperature, dark brown, smooth deposits which were hard to remove were formed as compared to black soot-like material at lower coolant temperature which could be easily removed with acetone. Nagao et al also report that increasing the coolant temperature from 50 C to 90 C reduced CCD levels in an engine test and deposits produced at the higher coolant temperature were less soluble in hexane and benzene (22). Inlet Temperature -When inlet temperature was changed from 26 C to 100 C in (4) there was no change in deposition rate. However, Myers et al. have reported that by reducing the liquid phase in the intake system by increasing the mixture temperature, CCD could be reduced (20); this might have been because they conducted their tests in carburetted engines.

Price et al have been able to unify the treatment of the CCD-forming tendencies of different generic forms of hydrocarbons by considering their molecular structure. They have defined a parameter derived from both the boiling point and the degree of unsaturation present in a precursor hydrocarbon - termed the nominal boiling point - which correlates well with deposit-formation rates in a laboratory rig spanning over four orders of magnitude with different hydrocarbon molecules (13).

Air/Fuel Ratio - Mixtures leaner than stoichiometric led to more deposits than rich mixtures (4) ; deposition rate

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Maxey (48) also found that even when some additives increased CCD growth, the additive chemical structure had little effect on the deposit chemical structure. They found no intact additive backbones or head groups in the deposits. Thus additives seem to undergo substantial transformation before being incorporated into the deposits. However Nagao et al were able to identify structures associated with the additives in the CCD (22).

There are now many published empirical studies which show that combustion chamber deposit formation tendency of a fuel increases as the aromatic content of a gasoline increases (8,11,24,30,49,55-57). Gibbs (57) states that CCD formation tendency increases, in terms of hydrocarbon groups, with saturates, C7+C8 aromatics, olefins and C9 and higher aromatics . However in all these studies the effects of changing composition are not decoupled from the effects of changes in the volatility characteristics. It is possible that, at the mechanistic level, the observed changes in CCD formation in many of these reported tests result primarily from changes in the boiling characteristics of the fuel .

Nagao et al (22) have been able to correlate the CCD forming tendencies of a polyether and four polyolefin detergents to the amount of residue left after three hours in a thermogravimetric analysis (TGA) rig at 300 C. Thus additives which are more thermally stable tended to cause more CCD in these tests. However, such detergents are also more likely to be effective for inlet valve deposit (IVD) control in engines which have high inlet-valve temperatures usually small engines runnning generally at higher speeds which are more common in Europe. It is also possible that some additives decompose at higher temperatures into components which promote CCD formation. Such additives might produce less residue in a TGA test such as described in (22) but might still lead to more CCD formation.

Jackson and Pocinki (27) have demonstrated a rough positive correlation between CCD weight and washed gum level in the fuel in two different engine tests. Takei et al (50) have shown reasonable positive correlations between the unwashed gum level in the fuel on the one hand and piston-top and cylinder-head deposit weights and thicknesses on the other. Nagao et al (22) have also shown a weak correlation between unwashed gum levels of a gasoline containing different additives and CCD levels. Gum levels will reflect the concentrations of the most involatile components in the fuel and hence the observed correlations are not surprising.

5.4 EFFECT OF ENGINE OIL - Deposits usually contain small quantities of Ca, Zn and Mg which come from lubricant additives. The concentration of these elements, usually in the form of sulphates or phosphates, is highest on the surface of the deposits (60). Compared to deposits in the head, deposits on the piston top have higher (by upto four times) concentrations of these elements (22,41,61) suggesting that oil plays a bigger role in forming piston deposits.

5.3 EFFECT OF FUEL ADDITIVES -Modern detergent packages, many of which contain carrier oils and other co-additives in addition to the detergents, can lead to an increase in CCD formation (e.g. 10,21-28) in unleaded gasolines depending on their concentration and composition. Some detergent additives (10,23-25,30,35) and some carriers (27) are more likely to increase CCD than others in particular engine tests. In several engine and road tests, a polyether amine detergent has been shown to produce less CCD compared to polyolefin derived detergents (22,25,30). In other engine tests a PIB amine detergent package at the right dose rate was shown to be at least as good as a polyether amine for CCD (24,50,59). Of course, the detergent additives have to be used at sufficient concentrations to do their primary job, i.e. to keep the fuel and inlet systems adequately clean. Hence their propensity to form CCD has to be compared at concentrations that deliver comparable inlet system detergency performance. The ranking of additive packages, in terms of their ability to control inlet system deposits, can depend on the engine or vehicle test used for such ranking (1). This is quite likely to be the case also for CCD formation from additive packages.

CCD weights increase if oil consumption increases beyond a limit (9,36,63). However modern engines are designed to use little oil and if they are running normally, no significant correlation between CCD weight and total oil consumption is found (40). Thus engine oil contributes to CCD formation but the extent of this contribution is difficult to assess (41). When engines are run on fuels which themselves are unlikely to form deposits such as hydrogen (62), methanol (17) or alkylate (39), substantial amounts of deposits, predominantly derived from oil (17), can be formed. For example Moore (39) reported that in a car running on alkylate, 472 mg of deposits were formed in the squish area of a car in a road test compared to 2286 mg in the same car running on an unleaded gasoline. However, one would not be justified in concluding from these figures that in the latter case, 20% of the CCDs were derived from oil because the surface conditions which determine the dynamics of the CCD growth would have been quite different in the two cases. With normal full boiling range fuels running in modern engines which burn little oil, CCDs are likely to be predominantly derived from the fuel. If the route to deposit formation through the fuel is switched off e.g. by using a low-boiling-point fuel, the lubricant components come into play and CCD grow towards a different equilibrium.

As discussed in section 3, there can be significant differences in the response of different engines in terms of CCD formation to the same additive (e.g. 35). Also the effect of an additive in a given engine test would probably be more marked in a clean fuel than in a fuel which would produce more deposits by itself (24). Compared to CCDs from base fuel, deposits produced with fuels containing detergent additives tend to have lower aromaticity (18,48), higher hydrogen content (48), higher fraction of organic solubles (22) but similar quantity and kinds of organic oxygen functionalities (48). Keleman and

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ORI has been reviewed in (1) and only the salient points will be mentioned below.

The dominant oil component for CCD formation is the base oil - increasing the high-molecular-weight and low-volatility content of the oil increases CCD formation (9,64,65). Cheng (9) found that changes in lubricant additives had negligible effect on CCD growth. However, work by Takei et al (50) suggests that lubricant additive dosage levels might be important. They report that an SG grade oil, compared to an SE grade oil made from the same base oil, resulted in higher CCD weight but lower thickness (50).

In general, the larger the initial octane requirement (IOR), the lower the ORI (1). New cars, even of the same model, can have widely varying octane requirements but with use, the octane requirements are distributed in a narrower band. In very general terms, ORI is increased by factors that tend to increase combustion chamber deposit quantity such as fuel properties like high boiling points (77), aromatics (30,55,56) or the presence of some additives (64,77-79). However, some fuel additive packages have been shown to be either neutral or beneficial i.e. reduce ORI, compared to base fuel (74,80,81) in some engine and road tests. Similarly, factors that cause removal of CCDs such as hard driving modes (51,52,62,64,78) or running a dirty engine on isooctane (66), can reduce the octane requirement. In well controlled engine tests a reasonable overall correlation between CCD weight and ORI can be established (44,80). However this is the exception rather than the rule and usually, CCD weight does not correlate with ORI (44,79,82-85). Deposits in different parts of the combustion chamber have varying degrees of effect on ORI. For instance, deposits in the end-gas region, the region where combustion takes place last, appear to be the most significant (64) whereas piston-top deposits contribute little to ORI compared to head deposits (37,80). In some engines inlet valve deposits contribute significantly to ORI (63,74,87,89).

6. EFFECTS OF COMBUSTION CHAMBER DEPOSITS ON ENGINE OPERATION The effect of CCD on heat transfer at the combustion chamber walls has been demonstrated experimentally in many independent sudies (e.g.15,16,32,37,41,44,46,47,53, 54). They act as insulators and also as heat resesrvoirs, storing heat in one cycle and releasing it to the fresh charge in the next cycle. They also occupy volume, increasing the compression ratio. Finally they might also absorb and release unburnt fuel, pro-knock species and promote chemical reactions through catalytic effects. As deposits build up in the combustion chamber, the maximum (32) as well as the average (37) heat flux away from the combustion chamber decreases, heat lost to the coolant decreases (51,66,67), air consumption rate is reduced (51,66,68) volumetric efficency decreases (53,6870) and flame propagation rates increase (37,67,68). Thus the incoming charge is heated by the deposits; Harrow and Orman estimated that deposits increased the temperature of the trapped charge by about 11 C in their experiments (68). Modelling of the heat transfer through the deposit layer also shows that the deposit surface temperature, the temperature of the bulk gas and of the end-gas all rise as deposits build up. (69, 72-76). Studzinsky et al. estimate that the bulk gas temperature increases by about 10 C because of volumetric effects and by about 50 C from heat transfer changes owing to the deposits.

The primary reason why CCD cause ORI appears to be because they increase the end-gas temperature by storing up heat from one cycle and giving it up to the fresh charge in the succeeding cycle - the increase in surface temperature caused by CCD also plays a role in this. The volumetric effect of CCD on ORI accounts for about 10% of the total effect in unleaded gasolines (64). In addition, there might be a chemical mechanism - one possibility is that CCD absorb stable pro-knock species like hydrogen peroxide and release them in the end-gas to promote knock (75). The extent of such chemical mechanisms and the balance between chemical and physical mechanisms will almost certainly depend on the nature of the deposits and the fuel being used and at this stage is not understood. How inlet system deposits affect ORI, when they do, is also not understood. Thus ORI is a complex phenomenon which is not well understood even after decades of extensive study.

The observed effects of CCD on engine operation discussed below can be understood to an extent in the context of the discussion above. 6.1 OCTANE REQUIREMENT INCREASE ( ORI ) The tendency of a car to knock and hence its octane requirement (OR), increases with use as deposits build up. In bench engine tests or vehicle dynamometer tests where operating conditions are either constant or consist of repetitive cycles, OR increases rapidly in the early stages and reaches an equilibrium value as deposits stabilise. However in field tests, an equilibrium octane number as well as the distance when equilibrium is reached are some times difficult, if not impossible to define (61,63). It has been argued that concepts such as ORI can be applied to car populations only in statistical terms (63). Typical values of ORI ("stabilised" OR minus initial OR) for modern cars running on unleaded gasoline are between 4 and 5 numbers over 12000 miles; there can be large variations between individual cars - from 0 to 15 numbers. The literature on

Testing for ORI, like in other deposit related studies, is characterised by poor repeatability and reproducibility; it is also labour-intensive and expensive. The difficulties in ORI testing and in drawing sound and useful conclusions from such testing have been highlighted by many studies (e.g. 30,61,63,64,79,83,90). It is important to be aware of such difficulties while assessing the differences between say, the effect of two additives on ORI. 6.2 EMISSIONS - In general, as cars grow older, there is an increase, typically of the order of 25% though there could be a wide variation between individual cars, in engine-out hydrocarbon (HC),carbon monoxide (CO) and nitrogen

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into an active component of the deposits or oil rather than adsorption, a physical effect, of the fuel (100).

oxides (NOx) emissions (91-96). Some of the increase is caused by inlet valve deposits (1,93) and carburetter/port fuel injector deposits (1) and some presumably by changes in engine characteristics like valve seat and ignition system condition. The effects of CCD on emission have been separately assessed by measuring them before and after cleaning the combustion chambers of used engines.

Thus there are opposing mechanisms which determine the effect of CCD on HC emissions and this might explain why this effect is not clearly established. CO Emissions - There is one paper reporting significant effects of CCD on CO emissions (94). In this paper by Bitting et al, CO emissions were reduced, on average by around 30%, when CCD were removed . In another paper Bitting et al (101) show that in a road test, average CO levels either decreased or increased over the first 9000 miles depending on which of the two IVD control additives was used. In another road test where two IVD control additive packages were compared with the base fuel in 15-car fleets (45), average CO levels increased for the base-fuel fleet while they decreased for the cars running on the additised fuels during a 10,000 mile test. It might be that in this test the CCD effects on CO were counterbalanced by the effects of the detergents on IVD and the consequent effects on CO.

NOx Emissions -are reduced when CCD are completely removed (76,94,97,98,100). In fleet tests, NOx generally increases as the test progresses and deposits build up (45,101). Houser (95) found that in three engines, CCD weights were reduced by using a particular additive rather than another and this was accompanied by a reduction in NOx. Bitting et.al have demonstrated this effect in four vehicles of different models; their results were statistically significant at the 95% confidence level. The results of Studzinski et al. (76) suggest that piston-top deposits have a smaller effect compared to head deposits on NOx; their modelling study relates the increase in NOx to the increase in bulk gas temperature brought about by the CCD.

There is no convincing mechanistic explanation as to why CCD should increase CO. It is possible that because of the reduction in volumetric efficiency, as deposits build up, if there is no adequate closed-loop control, the mixture gets richer as air consumption is reduced. This might lead to an increase in CO and if this is the mechanism, the effect of CCD on CO emissions would be negligible if the mixture strength is maintained by proper closed-loop control.

Hydrocarbon (HC) Emissions- were reduced when CCD were removed in some studies (92,94,96,97,100) but Bower et al reported no change (99). Only Bitting et al. (94) have tried to assess the statistical significance of their results; their results were not significant at the 90% confidence level. In another paper, Bitting et al report that as deposits built up in another road test (on fuels containing IVD detergents and hence giving low levels of IVD) there was a trend that HC emissions actually decreased (101). Spink et al (45) also found that in three 15-car fleets each running on an unleaded base fuel and two different IVD control additives, average HC emissions decreased over a 10,000 mile test. Bitting et al speculate that some amount of CCD may be of benefit for HC emissions (101). Thus though there are indications that CCD may contribute to an increase in HC emissions, this effect has not been clearly established.

CO2 Emissions - Bitting et al report that CO2 emissions decreased during a road test running on fuels containing IVD detergents (101) i.e. as primarily CCD levels increased. Spink et al (45) also report that in three 15-car fleets, CO2 levels decreased on average by 3-4% over 10,000 miles in a road test. This is to be expected since CCD lead to improved fuel economy (see below). 6.3 FUEL ECONOMY - Nakamura et.al found, in eleven different cars, that as deposits grew and octane requirement increased, fuel economy improved, by 13% in the most extreme case; it was confirmed that most of this improvement was due to CCD by removing CCD and remeasuring fuel consumption (44,54). These cars were running on unleaded gasoline on the road and fuel economy was measured in a "10 mode" cycle. They also show (44) that in another car that ran on a chassis dynamometer at a constant speed of 60 kmph, fuel economy improved by about 9% in 10000 km, with most of the change occuring in the first 5000 km. In another experiment (44), they coated the cylinder head surface of a research engine with teflon and found that octane requirement increased and fuel economy improved depending on the thickness of the coating; 15.4% improvement in fuel economy at a fixed speed and load with a coating 0.08 mm thick was measured.

In fact, an increase in combustion chamber surface temperatures, usually brought about by increasing the coolant temperature, reduces HC emissions (100,103-105) and since CCD increase surface temperatures they might be expected to reduce HC emissions. Also, it is possible that deposits in the crevices could reduce crevice volumes which are major sources of HC emissions; CCD could reduce HC emissions through this mechanism (100). On the other hand CCD can absorb fuel hydrocarbons and release them, largely unchanged, during the exhaust stroke; this increase might be directly related to the relative solubility of the fuel in the oil which might be in the deposit (106). Tsukasaki et al have attributed the observed increase in formaldehyde emissions from a car running on methanol to the deposits absorbing the unvaporised methanol (17). Harpster et al (100) found that the observed increase in HC emissions with the growth of deposits was dependent on the fuel structure; of the fuels tested, MTBE and ethylbenzene showed the largest effects. They interpret this to mean that the primary mechanism responsible for the HC increase from CCD is fuel absorption

Kalghatgi et al (37) have measured a reduction in specific fuel consumption of about 7% at part throttle, with the reduction being most pronounced at the start of the test, as deposits built up in an engine test. By removing the deposits

8 Kalghatgi

Most modern engines have flat areas around the rim of the piston which cause radial gas motion known as "squish" as they approach similar flat areas on the head during the compression stroke. At top dead centre, the clearance between the head and the piston will be minimum in this squish area. There has been a design trend to reduce squish clearances, to as low a value as 0.7 mm by some manufacturers, since it helps to reduce emissions (107). The squish clearance will be inevitably less than the design value in some cars because of production tolerances. In addition, when the engine is cold, the piston is tilted slightly as it rises (107) particularly at low engine speeds (22). This rocking motion further reduces the minimum clearance between the piston and head. This minimum clearance can be eliminated by combustion chamber deposits and the piston top can actually hit the head surface under some circumstances. The clattering sound that results in the frequency range of 1kHz to 10kHz (22) has been variously described as "carbon rap", "carbon knock", "deposit induced noise" and "combustion chamber deposit interference". The problem usually occurs when a car which suffers from it is started at low ambient temperatures e.g. after it had been parked outside on a cold night, and disappears in around five minutes as the car warms up. However during this time the noise can be quite unpleasant and during 1992, several car manufacturers received hundreds of complaints about this problem (107109). No permanent engine damage was observed in cars which experienced CCDI (110) and the problem disappeared after prolonged shut-downs (110). Engines with small squish clearances did not suffer this problem before the summer of 1991 in the U.S. (107). It also appeared that the problem was more prevalent in the eastern part of the U.S.(108,109).

in stages, they established that almost all this effect was attributable to head deposits and piston-top deposits had little effect. Spink et al (45) report that, in three test fleets, each of fifteen cars and each fleet running on a different unleaded fuel and additive combination, on-the-road fuel economy improved by around 17% over 10000 miles. Almost all of this improvement, part of which can be attributed to the reduction in friction losses during "running in", was in the first 3000 miles. However, in an ECE test cycle, for the same fleets, the fuel economy improvement was between 2.4% and 4.5% (45). The initial measurement in this case was 1000 miles after the start of the test and the final measurement, after 10000 miles. Fuel economy appears to change rapidly at the start of deposit build-up. If the "clean" engine measurement is not made in a truly clean engine, the first measurement of fuel consumption and the apparent change brought about by the deposits will be smaller. The fuel economy improvements reported by Spink et al (45) cannot be clearly attributed to CCD because they did not make any check-back measurements. Graiff measured fuel consumption at different speeds and road load conditions using two bench engines and a vehicle (87). He reports that removing combustion chamber deposits caused an increase of about 2% in fuel consumption at all conditions. A similar figure is quoted by Woodyard (38) for two bench engines. Tsutsumi et al (88) measured fuel consumption at wide open throttle at different speeds in an engine test as part of their investigations on the effects of surface finish of the combustion chamber. Their Figure 15 shows that when combustion chamber deposits built up, brake specific fuel consumption decreased by about 2% at all speeds upto 6000 RPM.

The problem was exclusively associated with engines with design squish clearances of 1 mm or less (39,107,108,110). There is a large variation in squish clearances associated with production tolerances even in engines of the same model; engines with smaller clearances ran into CCDI earlier- some as early as after 1500 km - compared to engines with larger squish clearances (39). CCDI could occur even with unadditised base fuel (39,85). Moore shows that there are significant differences between the effect of different deposit control additives on CCDI (39). However, in a fuel with a high aromatic content and high end-point, additives had little effect on CCDI (84) presumably because they do not affect CCD formation significantly (see 1.1 above) compared to such a base fuel. Even in cars susceptible to the problem, fuels like motor alkylate which are likely to produce low levels of CCD alleviate the CCDI problem (39,110).

Yonekawa et al have assessed the changes caused by CCD on the energy balance in a single cylinder engine by analysing the pressure curves and attribute the improvement in fuel economy at a fixed speed and load primarily to a reduction in heat lost to the coolant (67); faster flame development also plays a role. The heat lost to coolant constitutes a much larger fraction of fuel energy at low speeds and loads than at full throttle operation (Ref.112 Ch.12). Hence the fuel economy benefits of CCD are likely to be less marked at high loads. Thus CCD generally have a beneficial effect on fuel economy for some of the same reasons that they increase NOx and octane requirement, though the extent of this improvement might be different in different engines and operating conditions. The problem requires further study.

Moore has described a test protocol using a vibration signal from the engine to characterise CCDI intensity (39). He found that there was no correlation between CCDI intensity and CCD weight. As Megnin (110) suggests, the maximum deposit thickness is likely to be more relevant than the average deposit thickness or weight for CCDI. In fact Iwamoto et al (111) suggest that CCD thickness on the anti-thrust side in the squish region is important for CCDI.

6.4 COMBUSTION CHAMBER DEPOSIT INTERFERENCE (CCDI) OR CARBON RAP -The excellent paper by Moore (39) describes the phenomenon in detail and what follows borrows heavily from that work.

9 Kalghatgi

The problem could be eliminated quickly - in around 160 km - by using a very high dose, greater than 0.7% in (39), of a polyether amine detergent (39,110). However, this relief is temporary and the problem returns quickly. An effective and permanent cure was to increase the squish clearance (107).

These assessments need to be made on modern engines with unleaded fuels. However, during the process of establishing maximum power, maximum power will increase initially as CCD are removed because of the high temperatures reached in the engine (53).

The CCDI problem has drawn attention to combustion chamber deposits and helped generate new interest in controlling them. It is unlikely that it will continue to be a problem associated with CCD since there are simple design options available to engine manufacturers to eliminate it. This is in contrast to the other effects of CCD like ORI where there are no such simple engine design solutions.

6.6 OTHER EFFECTS -Pre-ignition and post-ignition are abnormal combustion phenomena in which the charge is ignited independent of the spark by hot spots in the combustion chamber and can cause serious engine damage (112,113). Build-up of CCD especially those containing lead or calcium and barium salts from the oil can promote these phenomena (113). It is unlikely that this problem is of much relevance in modern engines using unleaded gasolines. Spark plug fouling can be caused by deposits on the body of the plug. Greenshields published an authoritative study on plug fouling caused by deposits from leaded gasolines (114) on the ceramic core of the spark plug. An occasional modern problem is plug fouling caused by carbon deposits during the numerous very short trips taken by a car during its production and initial delivery (115-117). Moisture deposition on the carbon layer causes a conductive layer between the central electrode and the shell causing charge leakage and failure of the plug-gap to fire (116,117). In contrast, low electron work function deposits produced by spark-aider fuel additives on plug electrodes, particularly the cathode, can improve the energy transfer efficiency of the spark (118,119). This can lead to faster early flame development (120) and a dramatic improvement in the ignition ability of the spark near the ignition limits (118,119). This results in reduction in misfires,extension of the lean limit and improvements in driveability (121).

6.5 MAXIMUM ENGINE POWER -There does not appear to be any publication in this area after 1971 and many of the studies used leaded fuels. Cornetti et al.(53) report results from tests using 5 engines of different types of combustion chamber design after they had built up deposits using leaded fuels. The dirty engines were then rated at full throttle at 3000 RPM and the power increased smoothly by about 14% in 80 minutes ; they attribute this effect to the removal of some CCD. The final, stabilised power was 6% to 10% lower than for the clean engine. This is in line with earlier results (70,71). At a given engine speed, the torque is maximum at a particular ignition timing - Maximum Brake-Torque (MBT) timing. When deposits build up, at a given engine speed, MBT timing occurs earlier, reflecting the faster flame growth and the torque decreases (70); the change in the power/ignition timing curve has to be considered while assessing the extent of the power/torque loss. This loss in peak power is largely attributable to reductions in volumetric efficiency (53,70). The power loss depends on the combustion chamber design, being least (around 4%) for the most compact combustion chamber (70) ; the loss in maximum power reduced from 5.8% to 3.2% when the compression ratio was increased from 8 to 12 (71). The deposits in the area "most thoroughly scrubbed" by the incoming charge have the largest effects on the power loss; piston top deposits have a relatively smaller effect (70).

Deposits that form in the ring belt area could cause piston-ring sticking or ring plugging and ring breakage, all of which could lead to engine malfunction and short engine life (122). Such deposits are likely to be more associated with the lubrication system than the fuel/combustion system. Thus CCDs affect many aspects of engine performance and emissions. Some effects, like the improvement in fuel economy, are beneficial , for more or less the same reasons

TABLE I Major effects of CCD on engine performance and emissions Note: These effects are usually observed when CCD are completely removed. The effect of partial changes of say about 20% in CCD levels are not known. Parameter Possible Mechanism (s) Effect of CCD compared to a clean engine Octane Requirement Volumetric effect (small around 10% of total) Octane requirement increase (ORI) Thermal - heat reservoir/insulation (No clear correlation between CCD Chemical- absorption of knock precursors ? weights and ORI)

10 Kalghatgi

HC emissions (engine out)

Absorption of partly burned fuel by CCD Reduced crevice volume ( would reduce HC emissions) Increased surface temperature (would reduce HC emissions)

Increase in HC in some cases but not in others

CO emissions (engine out)

Not known. Possible that mixture is richened by reduction in volumetric efficiency. If so, the effect will not be seen in engines with good closed-loop control.

Not clear

NOx emissions (engine out)

Increase in charge temperature because of heating of charge (see ORI above), reduction in heat lost to coolant.

Increase in NOx

Fuel economy and CO2 emissions

Reduction in heat lost to coolant, faster flame development because of increased charge temperature leading to higher combustion efficiency. (same as for NOx).

Improvement in fuel economy. Reduction in CO2 emissions

Maximum Power

Reduction in volumetric efficiency because of heating of charge.

Reduction in maximum power (No publications since 1971)

Carbon Rap

Mechanical interference between piston and head surfaces in the squish area

deposit growth vary across the combustion chamber. Hence there is a large variation in thickness, properties, structure and the effect on engine operation of deposits across the cylinder.

that CCD might increase octane requirement or NOx emissions. Table I summarises these effects. These effects have generally been observed by completely removing CCD. The effect of smaller changes in CCD levels of say about 20% are not known. While assessing the effect of fuels and IVD control additives, some of which might lead to higher levels of CCD, on engine performance and emissions, it is important not to concentrate exclusively on CCD. There are simultaneous effects of fuels and additives on other factors such as IVD levels which have effects on engine performance and emissions which might counteract some of the effects of CCD. What should matter is the overall effect of a fuel or additive. Fleet tests often demonstrate an overall benefit in emissions and fuel economy brought about by good IVD control additives compared to the base fuel alone (e.g. 45).

Changes in engine operating parameters that tend to increase surface temperatures reduce deposit formation. Thus increasing the speed,load and coolant temperature or making the mixture strength slightly richer than stoichiometric all reduce deposit growth. Materials of low conductivity like ceramics can attain high surface temperatures and on such surfaces, deposit growth is reduced. Boiling point is the most important fuel or lubricant property in CCD formation - the higher the boiling point, the greater the deposit forming tendency. Of the common fuel components aromatics have been reported to be more prone to form deposits than olefins and saturates but this might be primarily because they have higher boiling points. Many fuel detergent additive packages cause an increase in CCD formation but there can be a large variation between different additives and even for the same additive, between different engines and in different fuels in this regard. Some additive packages have been shown to be neutral or beneficial (i.e lower CCD) compared to the base fuel in some engine tests. Changes in engine oil additives have little effect on CCD formation.

7. CONCLUSION In a modern engine with low oil consumption running on a typical full boiling range fuel, combustion chamber deposits are primarily formed by the fuel though the lubricant also contributes to their formation. Deposit growth is a dynamic and reversible process which at a given time, reflects the balance between the formation and removal processes. The single most important parameter that controls deposit formation is the surface temperature which itself changes as deposits grow because of their insulating properties. This and other conditions which might affect

Combustion chamber deposits increase the charge temperature and flame propagation rate, and reduce volumetric effeciency and the heat lost to the coolant and 11

Kalghatgi

Increased likelihood. Can be designed out by increasing squish clearances

5. Cheng, S.S., " A Physical Mechanism for Deposit Formation in a Combustion Chamber", SAE Paper No. 941892, also in SAE SP-1054, 1994.

may affect the final phases of combustion through, as yet undefined, chemical means. This is reflected as an increase in octane requirement and NOx and a reduction in maximum power but an improvement in fuel economy and so a reduction in CO2 emissions. Their effect on HC and CO emissions is not clear though there is evidence that they might cause HC emissions to increase in some cases. There are different opposing mechanisms which determine the CCD effect on HC emissions and this might explain why this effect is not unequivocally established. How CCD effect CO emissions has not been established but if it is through their effect on mixture strength, engines with effective closedloop control should not suffer any increase in CO because of CCD. They can lead to carbon rap or combustion chamber deposit interference which might be avoided by simple design changes to the combustion chamber and hence is unlikely to remain a problem in the future. Deposits in different parts of the combustion chamber affect engine performance to varying degrees e.g. piston-top deposits have relatively small effects on ORI, NOx emissions, fuel economy and power. It is very likely that different engines will respond to different degrees to the build up of CCD. The mechanisms by which CCD affect engine performance are not fully understood. Further work is needed in establishing and understanding the effects of CCD on emissions and engine performance.

6. Shore, L.B. and Ockert, K.F. ," Combustion Chamber Deposits - A Radio Tracer Study", SAE Transactions, vol.66, pp 285-294, 1958. 7. Newby, W., " Emphasises the effect of boiling point on deposit formation", SAE Transactions, vol 66, p 294, 1958. 8. Kim,C., Cheng, S.S. and Majorski, S.A. " Engine Combustion Chamber Deposits : Fuel Effects and Mechanisms of Formation", SAE Paper No. 912379, 1991. 9. Cheng, S.S., " The Effects of Engine Oils on Intake Valve Deposits and Combustion Chamber Deposits", SAE Paper No.932810, 1993. 10. Shipinsky, J.H., p 4-19 in (2) above. 11. Edwards,J.C. and Choate,P.J.," Average Molecular Structure of Gasoline Engine Combustion Chamber Deposits Obtained by 13C,31P and 1H Nuclear Magnetic Resonance Spectroscopy" , SAE paper No.932811, 1993 12. Lauer, J.L. and Friel, P.J., " Some properties of carbonaceous deposits accumulated in internal combustion engines", Combust. Flame, vol 4, p 107, 1960

Engine and vehicle tests used in CCD studies are difficult, long, expensive and are characterised by poor repeatability and reproducibilty reflecting the complexity of the problem. This makes it difficult to arrive at definitive conclusions or to generalise such conclusions to embrace other fuels and engines and test procedures unless such experiments are conducted with care. The effects of CCD on engine performance and emissions have been assessed by removing the deposits completely. It is not clear to what degree performance and emissions are affected by smaller changes, say of 20% , in CCD levels. While assessing the effects of different fuels or additives on engine performance through their effect on CCD, it is important to remember that there would be simultaneous effects on other aspects such as inlet valve deposits which might have their own, counterbalancing effects on engine performance.

13. Price, R.J., Wilkinson,J.P.T., Jones,D.A.J. and Morley,C.," A laboratory simulation and mechanism for the fuel dependence of SI combustion chamber deposit formation", SAE Paper No.95 , 1995 14. Nakic,D.J., Assanis,D.N. and White,R.A.," Effect of elevated piston temperature on Combustion Chamber Deposit Growth", SAE Paper No.940948, 1994. . 15. Hayes,T.K., White,R.A. and Peters,J.E.," The In-situ Measurement of Thermal diffusivity of Combustion Chamber Deposits in a Spark Ignition Engine", SAE paper No.920513, 1992

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12 Kalghatgi

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76. Studzinsky, W.M., Liiva, P.M., Choate, P.J., Acker, W.P., Smooke, M., Brezinsky, K., Litzinger, T. and Bower, S., " A Computational and Experimental Study of Combustion Chamber Deposit Effects on NOx Emissions", SAE Paper No. 932815, 1993.

63. Alquist, H.E., Holman, G.E. and Wimmer,D.B., " Some observations of factors affecting ORI", SAE Paper No. 750932, 1975.

77. Niles, H.T., McConnell, R.J., Roberts, M.A. and Saillant, R., " Establishment of ORI characteristics as a function of selected fuels and engine families", SAE Paper No. 750451, 1975.

64. Benson, J.D.," Some factors which affect octane requirement increase", SAE Paper No. 750933, 1975. 65. McNab, J.B., Moody, L.E. and Hakala, N.V., " Effect of lubricant composition on combustion chamber deposits", SAE Transactions, vol. 62, pp 228-242, 1954.

78. Saillant, R.B., Pedrys, F.J. and Kidder, H.E., " More data on ORI variables", SAE Paper No. 760196, 1976. 79. Fuentes-Afflick, P., p 9-29 in (2) above.

66. Warren, J.," Combustion Chamber Deposits and Octane Number Requirement" SAE Transactions, vol 62, pp583-594, 1954.

80. Peyla, R.J., p 7-109 in (2) above.

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81. Schreyer, P., Starke, K., Thomas, J. and Crema, S., " Effect of multifunctional fuel additives on octane number requirement of internal combustion engines", SAE Paper No.932813, 1993.

98. Huls, T.A. and Nickol, H.A., " Influence of engine variables on exhaust oxides of nitrogen concentrations from a multicylinder engine", SAE Paper No. 670482, 1967. 99. Bower, S.L., Litzinger, T.A. and Frottier, V., " The effects of fuel composition and engine deposits on emissions from a spark ignition engine", SAE Paper No.932707, 1993.

82. Mitchell, K., p 6-77 in (2) above 83. Wittenbrock, D.G., p 6-89 in (2) above.

100. Harpster, M.O., Matas, S.E., Fry, J.H. and Litzinger,T.A., " An experimental study of fuel composition and combustion chamber deposit effects on emissions from a spark ignition engine", SAE Paper No. 950740, Also in SAE SP-1095, 1995

84. Cunningham, L., p 7-15 in (2) above. 85. Avery, T., Axelrod,J and Carey, J., p 8-43 in (2) above. 86. Haury, E.J., p 9-41 in (2) above.

101. Bitting,W.H., Firmstone,G.P. and Keller,C.T., " A fleet test of two additive technologies comparing their effects on tailpipe emissions", SAE Paper No.950745, Also in SAE SP-1095, 1995.

87. Graiff, L.B., " Some new aspects of deposit effects on engine octane requirement increase and fuel economy", SAE Paper No. 790938, 1979. 88. Tsutsumi, Y., Nomura, K. and Nakamura, N., " Effect of mirror-finished combustion chamber on heat loss", SAE Paper No. 902141, 1990.

102. Wentworth, J.T., " More on origins of exhaust hydrocarbons - effect of zero oil consumption, deposit location and surface roughness", SAE Paper No.720939, 1972.

89. Eng, K.D., Carlson, C.A., Haydn, T.E. and Sung, R.L., " Engine test procedures to evaluate octane requirement increase and intake system cleanliness", SAE Paper No. 892122, also in SP-794, Fuel and Induction System Deposits, SAE, 1989.

103. Wentworth, J.T., " Effect of combustion chamber surface temperature on exhaust hydrocarbon concentration", SAE Paper No.710587, 1971 104. Myers, J.P. and Alkidas, A.C., " Effects of combustion-chamber surface temperature on the exhaust emissions of a single -cylinder spark-ignition engine", SAE Paper No. 780642, 1978.

90. Sengers, H.P.M. and ter Rele, R.R.J., " Improved precision in ORI field and engine tests", Paper No. CEC/93/EF18, Fourth international symposium on the performance evaluation of automotive fuels and lubricants, Birmingham, 1993.

105. Russ,S.G., Kaiser,E.W., Siegl,W.O., Podsiadlik,D.H. and Barrett, K.M., " Compression ratio and coolant temperature effects on HC emissions from a spark-ignition engine", SAE Paper No.950163, also in SAE SP-1094, 1995.

91. Gagliardi, J.C. and Ghanam, F.E., " Effect of tetraethyl lead concentration on exhaust emissions in customer type vehicle operation", SAE Paper No. 690015,1969. 92. Leikkanen, H.E. and Beckman, E.W.," The effect of leaded and unleaded gasolines on exhaust emissions as influenced by combustion chamber deposits", SAE Paper No.710843, 1971.

106 Adamczyck, A.A. and Kach, R.A., " The effect of engine deposit layers on hydrocarbon emissions from closed vessel combustion", Combust.Sci.Tech., vol 47, pp 193-212, 1986.

93. Houser, K.R. and Crosby, T.A., " The impact of intake valve deposits on exhaust emissions", SAE Paper No. 92259, 1992.

107. Russ, M.J., p 4-31 in (2) above. 108. Bannon, S.A., p 4-1 in (2) above.

94. Bitting, W.H., Firmstone, G.P. and Keller, C.T., " Effects of combustion chamber deposits on tailipipe emissions", SAE Paper No. 940345, 1994.

109. Carlson, C.A., p 4-7 in (2) above. 110. Megnin, M.K., p 8-57 in (2) above.

95. Houser, K., p 9-85 in (2) above. 111. Iwamoto, S. et al, " Carbon knock analysis concerning combustion chamber deposit", JSAE 9433290, 1994.

96. Pahnke, A.J. and Conte, J.F., " Effect of combustion chamber deposits and driving conditions on vehicle exhaust emissions", SAE Paper No.690017, 1969.

112. Heywood, J.B., " Internal combustion engine fundamentals", McGraw Hill Book Co., 1988.

97. Gagliardi, J.C., " The effects of fuel anti-knock compounds and deposits on exhaust emissions", SAE Paper No.670128, 1967.

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spark ignition engines", (Ed.s, Hilliard, J.C. and Springer, G.S.), Ch.6, Plenum Press, New York, 1984.

119. Kalghatgi, G.T., " Flame initiation and development from glow discharges - Effect of electrode deposits", Combustion and Flame, vol 77, p 321, 1989.

114. Greenshields, R.J., "Spark plug fouling tendencies", SAE Trans., vol 61, p 3, 1953.

120. Kalghatgi, G.T., " Improvement in early flame development in a spark ignition engine brought about by a spark-aider fuel additive", Combust. Sci. Tech., vol 52, p 427, 1987.

115. Kimbara, Y., Noguchi, Y. and Ishiguro, T., " Study on spark plug carbon fouling", SAE Paper No. 800832, 1980. 116. Quader, A.," Spark plug fouling: A quick engine test", SAE Paper No. 920006, 1992.

121. Kalghatgi, G.T., " Effect of a spark-aider fuel additive on the misfire characteristics of a spark ignition engine", Comust. Sci. Tech., vol 62, p 1, 1989.

117. Collings, N., Dinsdale, S. and Hands, T., " Plug fouling investigations on a running engine - an application of a novel multi-purpose diagnostic system based on the spark plug", SAE Paper No.912318, 1991.

122. Kipp, K.L., Ingamells, J.C., Richardson, W.L. and Davis, C.E., " Ability of gasoline additives to clean engines and reduce exhaust emissions", SAE Paper No. 700456, 1970.

118. Kalghatgi, G.T., " Improvements in the ignition ability of glow discharges brought about by deposits of potassium sulphate on the electrodes", Paper No. C50/80, International Conference on combustion in engines- Technology and applications, I. Mech.E., London, 1988.

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