ABSTRACT
Most of the current automotive engines, especially in North America, are of the spark ignition type, where a homogenous mixture of fuel vapor and air is spark ignited at high temperature and pressure to initiate turbulent combustion in a reciprocating-type piston engine� Through the resulting turbulent flame propagation, thermal energy is released and mainly through the action of piston expansion, work is produced� The typical influence of changes in the fuel to air mass ratio of the mixture on the power output and the work production efficiency is shown in Figure 12�1� It can be seen that maximum power output is associated with a slightly richer stoichiometric mixture, whereas maximum efficiency is associated with a slightly leaner one� Nowadays, for the effective control of the composition of the exhaust emissions out of these engines, a three-way catalytic converter is used� These devices require the employment of stoichiometric mixtures throughout, while power output is controlled by varying the total mass of the fuel-air mixture introduced�
The fuel used in this type of engine is usually liquid gasoline, which needs to have properties and combustion characteristics that would ensure its full vaporization and homogenous mixture with the air, well before the initiation of combustion by electric spark� Following ignition, the turbulent flame propagation must be sufficiently rapid to complete the combustion process before the rapid expansion by the piston action and before any part of the mixture undergoing autoignition ahead of the arrival of the consuming flame� Accordingly, the progress of the combustion process in these engines can be controlled through measures such as optimum spark timing (Figure 12�2)� The fuel employed should have the right physical and chemical characteristics to ensure clean, reliable, and efficient combustion while producing sufficiently high power output�
Commercial gasolines are complex mixtures of hydrocarbons and a very large number of other organic compounds� These can vary substantially in nature and concentration from one gasoline to another depending on the crude from which they were derived and the processes employed in refining� However, there are a number of specified standard requirements that gasolines must satisfy before they can be available for sale to the public� Among the principle properties controlling the performance of a gasoline in an engine are its volatility and the associated combustion characteristics, particularly resistance to the onset of autoignition and knock� These properties are adjusted to some extent according to local and seasonal conditions, the type of engine used and its mode of operation through refining, and the employment of suitable additives�
Gasolines must be sufficiently volatile to be readily vaporized to produce sufficiently rapidly a homogeneous fuel-air mixture before the commencement of ignition under all operating conditions� Gasolines must ensure engine easy starting, rapid warm-up and acceleration, and proper distribution of the fuel-air mixture among the different cylinders� Conversely, it must not be so volatile that some vapor will be lost directly from the fuel tank into the environment, or some fuel vapor can be formed prematurely in the fuel line to impede the flow of liquid fuel and produce a vapor lock� This may undermine the smooth fuel flow that can cause the engine to operate erratically or become inactive� On this basis, these conflicting requirements can be partly met by using, for example, a slightly more volatile gasoline in winter than in summer and through the extensive use of suitable modifying additives�
The volatility of liquid fuels, including gasolines, is assessed by standard empirical distillation tests, where the percentage of the liquid evaporated is related to the transient fuel temperature� Figure 12�3 shows a representation of the simple apparatus employed, and Figure 12�4 shows a typical volatility curve obtained� The low temperature portions of the volatility curve indicate the ease of vaporization that control the engine starting requirements and cold-weather operation� The higher temperature portions influence increasingly the warm-up and vehicle acceleration that require promptly high fuel flow rates� The highest temperature portion relates to the contents of more heavy fractions that may be considered to indicate the tendency for crankcase oil dilution and formation of deposits� The figure also shows the main consequences associated with the changes in the profile of the volatility
curve on the driving features of automobiles� Typically, the major boiling range of common gasolines is from 30°C to 210°C�
For pure immiscible liquid fuel mixtures under atmospheric pressure conditions, each component will have its own distinct constant boiling temperature� Figure 12�5 shows a schematic representation of a mixture of pure fuels where its four components are assumed to be ideally immiscible, with each component remaining unaffected by the behavior of the other components� The large number, wide variety, and concentration of the different components found in a gasoline will produce the continuous smooth curves shown with the changes in the gradient normally associated with commercial liquid fuels�
A less satisfactory means to assess the volatility is through determining variations in the vapor pressure of the fuel with temperature via the empirical approach of the Reid vapor pressure method, where a sample of the liquid fuel is placed in a standard closed cylinder immersed into a water bath at constant temperature and under controlled specified conditions (Figure 12�6)� The attained pressure at equilibrium is then taken to indicate an empirical measure of the volatility of liquid fuels� A volatile fuel at a
certain temperature will produce a high-pressure reading, whereas a low pressure will be associated with less-volatile liquid fuels when compared at the same liquid temperature�
Figure 12�7 shows schematically an example of the volatility characteristics of different common fuels� Curves A and B correspond to two pure liquid fuels, with A being more volatile than B� Curves C and D relate to gasolines, with C being more volatile than D, as in the case of winter grade versus summer grade� Curve E represents kerosene and jet fuels, curve F represents a diesel fuel, and curve G is for heavy and furnace fuel oil�
There are numerous properties, both physical and chemical, controlling the quality of a gasoline and its suitability as a spark ignition engine fuel� Some of these have been already introduced and others will be either discussed elsewhere in the book or included in the list of definitions of related terms at the
end� They include, for example, the heating value, octane number, flash point, autoignition temperature, viscosity, and sulfur as well as nitrogen contents�
All types of commercial gasolines sold to the public usually contain a wide range of additives to correct deficiencies in their properties and composition, improve performance, permit using a lower quality, cheaper fuel, reduce environmental pollution, enhance dependability, and protect the life of equipment and engine parts� These additives are added to the gasoline at the refinery in fuel supply lines or into finished gasoline tanks� Examples of the main additives employed are the following:
• Oxidation inhibiters inhibit the uncontrolled oxidation and gum formation within fuel systems, such as aromatic amines�
• Corrosion inhibitors prevent corrosion, mainly of iron such as carboxylic acids�
• Metal deactivators inhibit oxidation and gum formation that is catalyzed by certain metals, especially copper�
• Detergents prevent the formation of deposits and remove deposits, particularly for the fuel system and its injectors�
• Deposit controllers such as polybutene amines prevent and remove deposits from fuel and engine parts�
• Demulsifiers improve water separation from the fuel such as polyglycol derivatives�
• Antiknock compounds are most important to reduce the tendency to knock, like the largely disused organomanganese compounds and the banned lead alkyl additives�
• Anticing prevents the formation of icing in the fuel system, particularly with any water present in the fuel as impurity such as glycols�
• Dyes are usually added for identification purposes�
In general, there are mainly two ways to control emissions from fossil-fueled spark ignition engines� These operate either with a stoichiometric mixture, throttle control operation with a three-way catalyst, or through fuel-lean operation with spark ignition or diesel fuel pilot ignition, primarily without the need for a catalyst to oxidize the carbon monoxide and unburned fuel exhaust gas components� A suitable catalyst may be employed to reduce selectively the oxides of nitrogen when they are considered to be above the acceptable limit�
In general, the catalysts that may be used are made up of specially prepared mixtures of platinum, palladium, and rhodium applied to an inert washcoat of aluminum oxide supported on a ceramic substrate� The effectiveness of any catalyst depends on a number of operational factors that would include temperature, residence time or velocity of the gases, equivalence ratio, thermal cycling, and the extent of deterioration or poisoning that took place�
The products of combustion of the key low-molecular gases found in natural gas such as methane, ethane, and propane are usually less amenable to oxidize catalytically in comparison to those associated with gasoline or diesel fuels� The presence of sulfur in the exhaust gas does constitute a limitation to the action of the catalyst� The oxidation efficiency of catalysts when different common gaseous fuels are employed drops very rapidly with the reduction in the exhaust gas temperature� Hence, because the exhaust gas of lean mixtures is relatively at a low temperature, the effectiveness of the catalyst reduces very substantially�
The uncontrolled combustion phenomenon commonly encountered in spark ignition engines known as “knock” is associated with sudden exceedingly high rates of energy release, heat transfer to the walls, and rapid pressure
rise that must be avoided to ensure acceptable and safe engine operation� Spark ignition engine knock imposes very serious limits to the increase in power output, efficiency, type of fuel that can be used, and any further reductions in emissions in spark ignition engines� If an engine is allowed to continue running in a knocking mode for long, serious mechanical and thermal damage may ensue� Every effort is usually expended in the design, operation, and control of spark ignition engines to reduce the likelihood of the onset of knock to an absolute minimum� Often, this means that for any engine and fuel, the design and operating variables are selected conservatively to ensure knock-free operation, while sacrificing the potential for further improvements to the performance of the engine and fuel combination� If effective means can be found to predict the likelihood of the onset of knock for any set of operating conditions of the engine with a fuel, then protective measures can be devised more precisely, while ensuring optimum knockfree performance throughout�
The phenomenon of knock in spark ignition engines results from uncontrolled and rapid energy release well ahead of the turbulent propagating flame front arising from the autoignition of part of the unburned fuel-air mixture (Figure 12�8)� This autoignition is the outcome of a complex thermal and chemical interaction that takes place between the turbulent flame propagation processes and the preignition oxidation reactions of the “end gas” region of the mixture that is yet to be consumed by the flame�
Thus far, the reliable prediction of the onset of knock has not been easy� No relatively simple rapidly responsive approaches have been developed for quick and systematic application, including optimization approaches� This is largely because a detailed knowledge of the transient state of the unburned mixture and its increasingly accelerating reaction activity are needed� The extent of the preignition reactions going on within the reactants part of the mixture needs to be followed closely and its consequences for the combustion process and the corresponding engine behavior are to be established�
Some of the main features of spark ignition engine knock are the following:
• It causes exceedingly high rates of energy release, heat transfer to the cylinder surfaces, and pressure rise�
• It provides a serious barrier for improving engine performance� • It limits potential increases in engine power and efficiency� • It restricts the type of fuel that can be used and further reductions
in exhaust emissions� • It undermines engine life and lubrication�
If an engine is allowed to continue operating in a knocking mode even for a short time, then serious thermal and mechanical damage will result (Figure 12�9)� Every effort is usually expended in the design, operation, choice of fuel quality, and control of the engine to avoid encountering knock� Often design and operating parameters are selected conservatively to avoid knock, sacrificing achieving optimum performance� Automatic detection and control measures when fitted tend to be still of limited applicability and effectiveness�
In an engine, following external spark ignition, the propagation of the resulting turbulent flame front throughout the whole mixture needs to be completed before any part of the unburned cylinder charge can undergo autoignition� If the propagating flame does not manage to consume the entire mixture in time before autoignition takes place somewhere within the mixture yet to be burned by the flame, commonly known as the “end gas” region, then a sudden rapid and intense energy release takes place that can be detected as spark ignition engine knock� The intensity of this knocking condition can be readily detected through measures such as monitoring the rapid change in the rate of cylinder pressure rise (Figure 12�10) the distinctive noise emitted,
a rapid drop in power output and efficiency, excessive heat loss to the walls, and changes in the exhaust emissions and temperature� The intensity of the resulting knock can be related to the associated intensity of the resulting rate of pressure rise� This in turn will be a function of the net energy released by autoignition, which mainly controls the intensity of the temporal changes in cylinder pressure� These are dependent mainly on the mass of the end gas undergoing autoignition and the cylinder volume at the time�
The onset of autoignition and the associated extremely rapid energy release rates within a homogenous fuel-air mixture are normally the outcome of rapid acceleration of preignition chemical reaction activity due to a gradual increase in temperature and pressure and the buildup of concentrations of active species in the end gas that are mainly of the radicals type� Various indicators may be employed to define the end of this autoignition delay� These may include the observed rapid changes in temperature, pressure, or the concentration of some active species within the mixture that help in the rapid propagation of the overall reaction, such as the radical OH� Often, it is easier to associate the termination of the ignition delay with the consumption of a certain relatively small fraction of the fuel or oxidant or the release of a certain small fraction of the exothermic energy of the oxidation process� These different approaches for determining the effective delay in autoignition can be shown to yield essentially similar values when used�
The following is a list of some of the main negative effects on engine performance and the suitability of a fuel for spark ignition engine applications:
• Serious rapid drop in power and efficiency • Characteristic noise and roughness of the engine running
• Mechanical damage • Rapid and excessive increase in heat transfer to engine walls and
parts • Increased undesirable emissions • Increased carbon deposits • Lubrication deterioration and problems • Uncontrolled preignition ahead of the controlled passage of the
spark • Increased engine cyclic variation, combustion irregularities, and
rough running�
Knock can be established experimentally through a number of approaches that include observing the incidence of high-frequency combustion pressure oscillations combined with a distinctive noise due to knocking (Figure 12�10)� The knock-free operational limits are defined with reference to the prevailing equivalence ratio and/or the compression ratio at borderline knock or knock-free highest useful compression ratio under the specified set of operating conditions� Spark timing represents the time instant in the engine cycle at which the electric spark for ignition is passed� The value of the timing that brings about knocking is also sometimes used, (Figure 12�2)� For fuel comparative purposes, the knock limit usually needs to be determined in a standard special research engine, named Cooperative Fuel Research (CFR) engine, under some specified standard operating conditions while using a consistent criterion for establishing the knock intensity�
The onset of knock may be avoided by measures that include reducing sufficiently the equivalence ratio, compression ratios, spark timing advance, and intake mixture temperature� Reducing the boost pressure is needed for turbocharged engine applications� As shown in Figure 12�11, for a methaneoperated spark ignition engine, the knock-free operating mixture region narrows significantly with the increase in the engine compression ratio, leading to a reduction in the knock-free power and increasingly limited mixture control� When a sufficiently excessively high compression ratio is used, homogenous charge compression ignition (HCCI) requiring no spark ignition may be encountered� The knock-free operating range is severely limited by the increase in intake temperature�
The chemical nature of the fuel used is of paramount importance in avoiding the onset of knock� The blending of fuels may result in much-reduced knock resistance qualities that can be in some cases worse than those for the fuel components when employed on their own under the same operating conditions� For example, the presence of small amount of a liquid hydrocarbon
fuel vapor such as gasoline, n-hexane, or iso-octane with methane lowers its excellent knock resistance very significantly� Figure 12�12 shows the reduction in the knock-free equivalence ratio with increasing compression ratio for a range of fuels� The superior knock resistance of methane is clearly evident�
The octane number is an empirical indicator of the knock resistance quality of gasolines and other liquid fuels in spark ignition engines� It is determined under specified standard conditions by comparing the test fuel with a blend of iso-octane (2, 2, 4, tri-methyl pentane) and n-heptane that will produce exactly the same knocking tendency and intensity� The concentration of the iso-octane on a liquid volume basis in such an equivalent standard fuel blend in percent is the octane number of the test fuel�
There are mainly two types of octane numbers� These are described as research and motor octane numbers, which are related to two different test engine operating conditions, as shown in Table 12�1� They are meant to represent mild and somewhat relatively less mild operating conditions of actual engines, respectively� The octane number that is often quoted for commercial fuels is taken as the average of these two numbers�
There is a tendency to employ occasionally an empirical scale for rating mainly gaseous fuel using the methane number� This number is based on comparing the knock performance of the test fuel with that of a suitable mixture of methane and hydrogen� In this approach, methane is given a rating of a hundred and hydrogen a rating of zero�
The incidence of knock in an engine is encouraged by the following:
1� Increasing the value of the end gas mixture temperature that can result from a� Increasing the compression ratio b� Turbocharging and supercharging
c� Raising the inlet temperature d� Raising the engine coolant temperature e� Overheated working parts f� Increasing the load/power output g� Dirty cylinder and deposits on chamber walls reducing the heat
transfer h� Advancing spark timing, which increases the compression of the
end gas i� Increased hot residual gases from previous cycles 2� Increases in mixture density contribute to the increased tendency to
knock through a� Increasing the compression ratio b� Opening the throttle/increasing the mass of the charge and
engine load c� Supercharging/turbocharging d� Increasing intake pressure e� Advancing spark timing 3� Increases in reaction time of the end gas before flame arrival, which
lead to the lengthening of the autoignition delay can come about from
a� Increasing flame travel distance due to combustion chamber shape
b� Location of the sparking plug off the center within the chamber c� Increased engine cylinder size d� Increasing the number of spark plugs employed e� Decreasing mixture turbulence f� Decreasing the flame speed by having the fuel-air ratio employed,
not stoichiometric g� Decreasing engine speed 4� Increased reaction activity of the end gas resulting from a� Reducing the fuel octane number b� Using a stoichiometric fuel/air ratio c� Changes in humidity of the air d� Effects of deposits and lubricants
It is helpful to consider that there is a race between the propagating flame starting from the sparking plug and the autoignition reaction activity in regions away from the propagating flame to consume the entire fuel-air contents of the cylinder� If the flame fails to finish consuming the mixture in
time and to win this race, then knock will be encountered� The intensity of the resulting knock will depend on the mass of the mixture that autoignites�
Some control measures to reduce the tendency to knock are obtained by increasing the time to autoignition relative to that required by the turbulent flame to consume the entire mixture� Some of the measures that can be taken optimally are the following:
• Reduce the compression ratio • Use a higher octane number fuel • Lower the intake mixture temperature • Retard the spark timing • Increase the engine speed • Use a cooler cylinder jacket water • Make use of leaner mixture operation • Reduce the intake pressure • Increase the turbulence • Relocate optimally the spark plug • Use a smaller size cylinder • Increase the cooled exhaust gas recirculation • Ensure clean chamber surfaces
Of course, implementing such measures will often lead to the undermining of the efficiency, power output, and exhaust emissions of the engine�
Figure 12�11 shows the corresponding operational knock limits when the intake mixture is heated, showing not only the more intense earlier and wider knocking mixture region but also the appearance at high compression ratio of autoignition engulfing the whole mixture in the absence of a spark� This is an example of the HCCI engine�
As has been described earlier in this chapter, the three-way catalytic converter has been developed for spark ignition gasoline-fueled automotive engines to remove simultaneously the three major pollutants of the products of combustion� Those are the unburned hydrocarbons, carbon monoxide,
and oxides of nitrogen� These catalytic arrangements can reduce the oxides of nitrogen to harmless molecular nitrogen and at the same time oxidize the hydrocarbons and carbon monoxide to carbon dioxide and water vapor� The active catalysts in this arrangement are those of platinum and rhodium and sometimes palladium�
It is to be noted that if there is too much oxygen in the exhaust gas, then the reduction of the oxides of nitrogen will be incomplete, whereas if the exhaust gas does not contain sufficient oxygen, then the carbon monoxide and the unburned hydrocarbons will not be fully oxidized� On this basis, a compromise will see the required operating mixture to be essentially of stoichiometric value� However, saturated hydrocarbons, especially methane, react in this setup most slowly�
1� A primary reference fuel of 80 octane number (i�e�, 80% C8H18 and 20% C7H16 on a liquid component volume basis) is tested by using 10% excess air� Determine the volume of air needed per kilogram of fuel at 303 K and 87 kPa as well as the mass of fuel to air ratio� Take the density of the liquid fuels to be 0�75 kg/L� (Answer: 16�61 m3/kg, 0�0619, 0�619)
2� The schematic diagram given below shows curves of the volatility characteristics of different commercial fuels (not to scale)� Match these curves to the following list of fuels: (a) kerosene, (b) diesel fuel, (c) summer-grade gasoline, (d) winter-grade gasoline, (e) iso-octane, (f) n-pentane, and (g) bitumen� Very briefly outline the basis for your choice�
3� Indicate whether an increase in the value of the following operating and design variables is likely to reduce the minimum value of
the fuel octane number required by a spark ignition engine� Explain very briefly the reason behind your answer� (a) compression ratio, (b) leaner equivalence ratio, (c) spark timing
advance, (d) water jacket temperature, (e) cylinder diameter, (f) intake temperature�
(Answer: a: no, b: yes, c: no, d: yes, e: no, f: no) 4� Arrange the following potential spark ignition fuels in a descending
order of their octane number and their suitability for resisting the incidence of knock� Very briefly outline the basis for your order� (a) benzene, (b) cyclohexane, (c) n-decane, (d) hydrogen, (e) methane,
and (f) n-hexane� (Answer: e, a, b, d, f, c) 5� Distinguish clearly between the following properties of a fuel:
(a) flash point and (b) autoignition temperature� Briefly explain why the flash point of propane is usually listed as 169 K, yet its ignition temperature is 745 K� Also, why the flash points of n-octane and isooctane are 263 K and 285 K, yet their corresponding ignition temperatures are 479 K and 691 K, respectively?
