Review of Problems on Combustion in Engines and Kinetics of Combustion Processes

COMBUSTION PROBLEMS IN INTERNAL COMBUSTION ENGINES

By W. G. LOVELL

Why the Problems are Important

The problems of combustion in engines are interesting principally for three reasons: first, they have great economic and social significance: second, the solution of them is limited by our lack of fundamental knowledge of the processes going on in the engines and especially of the chemical kinetics; and third, some of the problems can be solved with great rewards. They seem to indicate a good place to work.

The economic significance of combustion in engines is easy to understand. As indicated in Figure 1, of all the energy tha t we liberate or use in this country, a little less than 40 per cent comes from coal, a little less than 60 per cent from petroleum, and only 4 per cent from water power. This does not include the energy that is immediately derived from sunshine, but only our so-called mechanical or heat energy. Ahnost all of it comes from what we call combustion and this means that we have to have oxygen in addition to the fuel. Now the almost 60 per cent of the total that comes from petroleum out of holes in the ground is of special interest. For of this 60 per cent, nearly one-third is used for internal combustion engines, ahnost all of it in the form of gasoline.

There are many engines in this country--most of them in automobiles and trucks of which there are about 55 million. If we would think of their installed horsepower as being about 100 each, then the total amounts to about 50 times the entire installed central-station horsepower in this country. If we could increase the efficiency of these engines by even one per cent we would

save enormous amounts of fuel and in effect increase our total fuel supply.

We think these engines are going to be with us a long time yet; they are quite reliable, convenient and cheap in automobiles. There are more and more of them doing most of the hauling on rail- roads and on the farms as tractors and in many other places doing odd jobs. So the engines con- tinue to increase in numbers, and it is hard to see a replacement for them in the foreseeable future.

What is wrong with them, and can they be improved or made more efficient? As indicated before, they can be improved; but a principal obstacle is our ignorance about the chemical kinetics of combustion.

Chemical Kinetics in Spark-Ignltion Engines

There are two main avenues of approach to- ward improved engine performance. The first is by increasing the engine compression ratio; this results in more power because of increased ther- mal efficiency. The second way is by putting more fuel-air mixture into the cylinder by geared supercharging; this results in increased power output but the same thermal efficiency. Both of these methods encounter the important phe- nomenon called fuel knock. Knock limits both the permissible compression ratio and the per- missible amount of supercharge.

THE KNOCK

The knock occurs because as the charge of gasoline vapor and air is compressed by the pis- ton in the cylinder, chemical reactions may occur

2 REVIEW OF PROBLEMS

in the mixture. Then, after the mixture has been ignited by the spark plug, the expanding gases compress the unburned mixture still fur- ther, and during this further compression i t undergoes additional reactions. With many gaso- line hydrocarbons the reactions proceed to a critical state in the last portion of the unburned charge or the "end-gas" so that it autoignites, and this last portion burns extremely rapidly. I t knocks. The knock is thus a definite limitation, because it is not mechanically practical to run an engine with any significant amount of knock. If knock could be completely eliminated, engine efficiencies would be greatly increased. This is no small matter for even a 10 per cent relative increase in efficiency is like discovering whole new oil fields, or decreasing the total required amount of engine power by hundreds of millions

ER 4 ~

GAS 21"s

MOTOR FUEL (GASOLINE AND OP|

OTHER PETROLEUM FUELS 19%

FIO. 1. U. S. energy sources expressed in per cent equivalent Btu's. (Data from "Petroleum Facts and Figures," 10th Ed., 1952.)

of horsepower, or considerably extending our petroleum reserves.

I would like to digress for a moment, now that the importance of the phenomenon of knock is apparent, to consider it briefly. We think about it as autoignition of a fuel-air mixture ahead of the flame, and that sounds simple, even though the steps leading up to autoignition are quite complex. The autoignition is comple• also. If we photograph the autoigniting volume at high speed by means of the light it enlits, or by ob- serving its density gradients (as by schlieren photography) one comes up with some important observations. One is that it is an extremely non- homogeneous thing (and this is possibly a rational idea because we are dealing with very short time intervals) both as to temperature and extent of reaction. In fact it is convenient to think of a succession of autoignitions of little volumes. And if one thinks of the nonhomo- geneities, and rates of heat transfer, and reaction- rate temperature coefficients, there is a lot to

think about. Yet, one is encouraged to do some thinking about it because of the potential eco- nomic advantage in doing something about re- ducing the knock.

REDUCTION OF KNOCK

While there has been a good deal of thought given to using the knock, this approach has to date shown little promise. On the other hand, there are several possibilities for lessening the limitation of knock. Since the knock occurs in the end-gas, elimination of end-gas would re- move the knock. This may be accomplished by modifying the cycle somewhat and putting the fuel in at the same rate as it burns, so that the flame is stationary with respect to the engine cylinder but moving through a controlled swirl- ing gas. The ingenious mechanism for doing this is the result of straight-forward research, and there can be no doubt of its effectiveness for the complete elimination of knock.

This method for eliminating knock competes with another one which consists of changing the fuel. Which method may be better can only be determined by experience and by new knowl- edge of how best to go on both routes. For the present, modification of the cycle would be the best route only when fuel cost is high relative to engine cost. But for most engine-propelled auto- mobiles, depreciation of the value of engine and mechanism accounts for over three quarters of the total operating cost.

Other special at tention may be given the end- gas by mechanical means. Keeping it cool, diluting it, or shortening its time of existence in the engine cycle are methods that have been used successfully to reduce knock. These features of engine and combustion chamber design are sometimes termed "mechanical octane numbers." They move the barrier of knock ahead, but they do not eliminate it.

~UEL vxmABLES

So it is worth while to look at alternative methods for achieving the potentialities which can result from the elimination of knock. Knock can be eliminated by changing the molecular structure of the fuel to one which does not knock. Consider the two isomers, n-heptane and trip- tane (2,2,3-tr imethylbutane). Although they have identical composition and the same heat of combustion within a third of a per cent, their difference in combustion is so great that when they are used in un engine of varied knock-limited

COMBUSTION PROBLEMS IN INTERNAL COMBUSTION ENGINES 3

compression ratio, the thermal efficiency may vary almost twofold. The relationship between molecular structure and incidence of knock has been worked out on an empirical basis for some 300 pure hydrocarbons by an American Pe- troleum Institute Research project, so that firm fundamental data are available. The importance of fuel structure is evidenced by the fact that while the octane number scale of 0 to 100 covers only a part of the total range, it represents about a fivefold variation in the knock-limited power to be obtained by supercharging an engine.

Thus, the useful work to be obtained from a gasoline depends not only on the heat content (which cannot be changed very much) but also on the molecular structure of its hydrocarbon constituents. Hydrocarbons of some molecular structures can be used at higher compression ratios without knock because of the variations in the rate of chemical reactions in the end-gas ahead of the flame. Although it is probably not the original fuel that knocks or goes suddenly into autoignition, still it is the original fuel that determines to a large extent whether knock will occur or not. Consequently, the kinetics of some of these intermediate reactions are the deter- mining factor in the utility of the fuel.

EARLY C.OMBUSTION REACTIONS

Let us consider for a moment how important some of these intermediate reactions may he. These early reactions (or precombustion reac- tions, so-called because they do not necessarily result in the formation of the final combustion products), which are chemical changes occurring in the fuel-air mixture ahead of the flame front, have been shown to play an important role in engine operation. One of the most important aspects of these changes is that they are accom- panied by the release of appreciable amounts of heat- -perhaps exceeding 10 per cent of the total heat of combustion. Since the release of this heat alters the character of the entire combustion process, it is essential that we he able to deter- mine the progress of such precombustion reac- tions in order to evaluate their effect on engine operation.

A convenient way of doing this is by the use of pressure-time records, comparing different fuels, and with the spark ignition delayed in the cycle. One comes up with the ideas that the heat evolution may be a considerable amount, and that it depends on the fuel, but does not necessarily correlate with the tendency of the

fuel to knock. These are important concepts. They do not simplify the picture very much, but they enable one to do things to the com- bustion cycle, in the way of timing the heat release for better advantage.

Another way of looking at some of these early reactions is to consider not primarily their heat evolution but their effect on knock. One way to do this experimentally is to use two engines, one motored to compress and expand the fuel-air charge without ignition, the other firing this motored charge. What one observes is that as autoignition conditions are approached in the first engine, some reactions take place which do not evolve more than 1 per cent of the heat of combustion but which very significantly in- crease the tendency of the mixture to knock in the second engine. As the point of autoignition is approached closer, more extensive reactions take place, and the mixture has less tendency to knock in the second engine than even the original fuel-air mixture, probably because it is signifi- cantly decreased in heating value. One can do similar experiments in which the fuel and air are passed through a heated tube instead of an engine and come up with similar general ideas.

In summary, then, we can state the general concept that many reactions may occur before the flame occurs. The extent of these reactions are important, not only in determining the thermodynamics of the cycle, but also in deter- mining what happens ahead of the flame and whether knock occurs or not. These factors complicate the consideration of the combustion cycle, of course, but add intriguing possibilities for exploitation.

CONTROL OF KNOCK REACTIONS

More knowledge of the gross physical control of these reactions will help also. Knowledge of how to influence the pressure and also the tem- perature of the reacting gas and the temperature of the surrounding walls has already resulted in great progress in reducing knock. Such progress goes by the name of "mechanical octane num- bers", as mentioned previously. These gains are real, but they will not go far enough in per- mitting higher compression ratios to make us stop wanting better fuels, or more octane num- bers, or more extensive catalytic control of the chemical reactions of knock.

Catalytic control of the knock reactions is gained by the use of so-called antiknocks, metal- lic or nonmetallic, such as tetraethyllead or

4 REVIEW OF PROBLEMS

aromatic amines. They reduce knock when added to the fuel in small amounts. The chemical mechanism by which antiknocks or products of their decomposition or oxidation function can only be expressed in quite vague and general terms such as surface deactivation of chain carriers. In spite of our lack of knowledge of the mechanism by which lead catalysts operate, they are used in various amounts in over 98 per cent of the gasoline burned today in this country. If more were known about the chemical mechanism of antiknocks, they might be made more effective, since it is known that their effectiveness varies severalfold with changes in fuel composition, in contaminants such as sulfur compounds in the fuel, and with engine operating conditions. Certainly new knowledge on this subject would seem to offer great potentialities for practical improvement in the use of gasolines whether they contain commercially known antiknocks or new ones. Here again with antiknocks, as with fuels, we are concerned not with equilibrium states, but with the kinetics of combustion.

FLAME VELOCITY

Kinetics are also important in the control of flame velocity, and residence time of the charge is important in determining the incidence of knock. There would be less time for the knock- inducing reactions to take place ahead of the flame if the flame were to get there sooner. So far we have not been able to change flame velocity very much by reasonable amounts of additives or without gross fuel changes. We have been able to influence flame velocity importantly only by modifying physical factors such as turbulence. While turbulence reduces the knock, too much turbulence results in excessive heat loss to the walls. On the other hand an increase of flame velocity without increased turbulence would help reduce knock and would also probably make a small, but important, contribution to engine thermal efficiency because of less heat transfer a t the walls. What the fundamentals are which determine flame velocity are not entirely clear. Certain it is that we run engines at 600 rpm and also 10 times as fast at 6000 rpm. Over this range of speed there is only about a two-fold range in the degrees of crankshaft revolution required for the flame to consume the charge, and at the high-speed conditions, the flame travels about five times as fast in terms of feet per second. I t is this phenomenon that makes possible flexible engines, ones that will operate over a wide range

of speed and load. This increase in flame speed is, of course, due to turbulence; and that is some- thing, again, about which we have little useful knowledge.

ENGINE DEPOSITS

So far, in thinking about engine combustion, little direct attention has been paid to the walls of the combustion chamber. They influence combustion and engine operation, and they are especially important because they accumulate deposits. These deposits consist of carbon- aceous residues from the fuel and the oil and lead salts from the combustion catalyst, tetra- ethyllead. These deposits are bad, for two prin- cipal reasons.

The first reason is that they have a pronounced effect on heat transfer; they get hot from one cycle, and transfer heat to the incoming charge on the next cycle. This is the most important reason that engines with deposits knock more than clean engines.

A second important bad effect of deposits is that they induce ignition of the charge. This comes about because many lead salts catalyze the combustion of carbon and carbonaceous materials, and under some conditions the de- posits will glow and ignite the charge. This uncontrolled ignition of the charge may cause knock when it is early enough in the cycle, for example before the spark. This random ignition of the charge by the deposits takes the con- trol of ignition time away from the designed spark timing and location, and, of course greatly disturbs the order of the engine cycle.

Various materials may be added to the fuel to correct this. Many of them contain phosphorus, which acts as a "poison" for the catalytic action of some lead salts, and such additives are cur- rently quite widely used.

C o m p r e s s i o n - I g n i t i o n Engines

When we turn to the diesel cycle, another set of problems faces us in engine-fuel relation- ships; and here, again, is a situation where more knowledge of chemical kinetics may lead to improvements in fuel utilization. In the diesel cycle, a charge of air is compressed in the engine cylinder and then the fuel is injected. The air is so hot, because of the approximately adiabatic compression, that the injected fuel ignites and burns. In the spark ignition engine, it is desirable to run at high compression ratios, but this can- not be done because of a kind of ignition ahead

COMBUSTION PROBLEMS IN INTERNAL COMBUSTION ENGINES 5

of the flame. But in the compression-ignition engine, high compression ratios must be used in order to get the air hot enough to ignite the fuel in a reasonable time and before too much fuel is injected, so that too much will not burn at once. In fact, the required compression ratios are pos- sibly a little higher than the ideal, not from the standpoint of thermodynamic efficiency, but from the standpoint of mechanical and metal- lurgical design required to withstand the high pressures. Thus, in the spark ignition engine a form of spontaneous ignition (knock) is un- desirable, but in the compression-ignition en- gine it is essential.

FUEL CH~kRACTERISTICS

In general, the autoignition characteristics of fuels for spark-ignition engines (characteristics which are roughly measured by knock ratings or octane numbers) are about the opposite of combustion characteristics for fuels used in compression-ignition engines, characteristics measured in terms of cetane numbers. Usually, high cetane number in a fuel goes with low octane number, and vice versa. Generally, compact structural formulas for hydrocarbons go with low cetane numbers (or high octane numbers), and long-chain formulas are typical of high cetane numbers (or low octane numbers). If the in- dividual hydrocarbons of petroleum and its products could be separated into two classes, high and low cetane numbers, compression- ignition engines could be operated on one class, and the spark-ignition engines on the other class. The two types of engines would then run at about the same compression ratios and consequently at about the same thermodynamic efficiencies. The choice of which of the two to use would then depend upon the type of service and the costs of manufacture of the two types of engines. But we do not know how to make such a separation of hydrocarbons, based not on physical properties but on combustion or autoignition characteristics. A separation on the basis of combustion inevi- tably destroys the material, and a suitable method of separation based on structure has not yet been devised. Perhaps in the future the equivalent of such a separation may be accomplished, even though it may entail the development of processes for manufacturing specific and fairly pure hydro- carbons.

Present fuels for compression-ignition or diesel engines are imperfect primarily because of de- ficiencies in their ignition. Better and faster

ignition would be an improvement for many applications. The gains, however, from better ignition or higher cetane numbers in diesel fuels are not comparable to those from higher octane numbers in gasolines. Higher octane numbers offer the potentiality of higher com- pression ratios, with better efficiency and more power. Higher cetane numbers fundamentally enable lower compression ratios to be used with a consequent loss in thermodynamic efficiency. And so, the gains resulting from superior diesel fuels are gains in the operations of engines and not those of fundamental thermodynamics.

IGNITION PHENOMEN2k

The word ignition, as used here in relation to diesel fuels, covers a multitude of ignorances about how soon and how much vaporization occurs and how fast some undefined chemical reactions of oxidation take place after the in- jection of the fuel into the hot air. We do know that ignition is greatly influenced by fuel com- position and by the addition of small amounts of materials such as some organic nitrates. An early and definite ignition would insure con- trol of the diesel cycle, since too much fuel could not then be injected before burning.

Our ignorances about ignition in the diesel cycle also extend to what is included in the word burning. The mixture in the diesel combustion chamber is far from homogeneous since it con- sists of air, and some exhaust-gas from the previous cycle, and into this mixture liquid fuel is injected. I t is easy to imagine droplets of fuel surrounded by vapor undergoing oxidation re- actions leading to ignition and more complete combustion with a range of fuel-air ratios giving various products of combustion. On a larger scale of volumes, we can imagine fairly large places where there is no fuel and other places where there is too much. As a practical matter, one cannot burn all the air in all parts of the combustion chamber because if the average fuel- air ratio is made sufficiently rich to accomplish this, there will be some volumes where there is so much excess fuel that smoke results. The practical power rating of a diesel combustion chamber depends not upon how much air there is available for combustion but rather upon how much fuel can be put in without there being some volumes or pockets which are so rich that smoke results, even though there is an over-all average excess of air. For these reasons, it is obvious that any progress that can be made in

6 REVIEW OF PROBLEMS

sorting out the simultaneous and consecutive phenomena of fuel injection, dispersion, vapori- zation, ignition, and burning will lead to im- portant advances in fuel utilization. A practical end, of course, is a more powerful, smaller and smoother operating engine. This requires a fairly fundamental approach.

W h a t K i n d o f l l e sea reh

The kind of information we might wish for on this kind of problem of chemical kinetics of combustion is quite fundamental. We would like to know the chemical equations and rates of all the oxidation and other reactions involved in

IOCp /

8c :_ T/J e"

S . ~- 6C �9

a.

/ - [ , / ,~

20 / // , 0 20 40 60 80 I00

CAI,.CULATED

FIG. 2. Correlation of calculated and experi- mental values of TEL decomposition expressed as per cent undecomposed. Different symbols repre- sent different fuel-additive combinations.

the burning of many different hydrocarbons, expressed as a function of temperature and pressure. This is too much information to obtain in an)- reasonable time, inasmuch as the com- bustion of a relatively simple substance such as hydrogen is far from being completely under- stood. Consequently, the attack must be on several fronts: the slow front-d t~ttack on the fundamentals by relatively indirect methods, and the logical, or intuitive, or empirical utiliza- tion of an individual piece of fund'nnental or basic information.

However, it is possible to make quite specific and practical use of combustion kinetics in engine combustion. I have mentioned a specific application to engine deposits. A later palter in this symt~osium discusses the application of rapid-compression machine data on ignition

delay to problems of engine knock. Other papers discuss the analyses of intermediate products from an engine, and the correlation of cool flame reactions. There will be photographs of cool flames and their spectra in engines.

As an interesting specific example of how it is possible to apply kinetics to the engine cycle, I would like to mention some work, which will be in a forthcoming paper, on the decomposition of tetraethyllead in the engine. This decomposi- tion is interesting because the products of the decomposition are the effective antiknock cata- lysts and they must be present at the right time in the engine cycle to be most effective.

An engine, with certain modifications, can be a useful rese'~rch tool, because with modern instrumentation one can measure the basic parameters of time, temperature, and pressure. Therefore, an engine is a reactor, more complex, it is true, than a simple heated vessel or a flow tube, but particularly useful. Furthermore, an engine has certain advantages for the investi- gation of rapid high-temperature reactions, in that cycles are repetitive and permit the use of devices like stroboscopes or sampling valves. Also, an engine permits the measurement of very high-temperature reactions which cannot be measured in conventional ways, because flow systems take longer to heat up the material than desired. Finally, if an engine is used as an experi- mental tool, there is less trouble in correlating the data with engine conditions.

For the study of the decomposition of tetra- ethyllead we may use a motored engine and avoid some of the difficulties with the nonhomogeneity of the charge when "t flame goes through it; and we may determine the amount of tetraethyllead decomposed by s'mlpling the exhaust. We may also compute the extent of decomposition by assuming a first-order homogeneous reaction, as has been reported in the literature, and using an Arrhenius equation:

t 2 t 2

In ~'~t= - - f k d t = -Af e - B I T d t 77"2 1 J ~ [

where n~ and n2 represent the concentration of TEL at the start and end of the cycle, respec- tively; k is the reaction-rate constant expressed in terms of the two constants A and B and the variable temperature T; and t is the time. We can use temperature values obtained from ac- curate pressure records and gas laws. By trial we may find consistent values of the constants.

COMBUSTION PROBLEMS IN DIESEL ENGINES 7

Figure 2 shows the results of 33 engine tests in which the amount of tetraethyllead decom- posed was calculated from accurate pressure- time data, and compared with the experimental values for TEL decomposition. These tests covered a wide range of fuel types and engine temperatures, and the calculated values differ from the experimental results by an average of less than l0 per cent, which is considered satis- factory for this type of investigation: I t would be quite difficult to carry out this work in a reactor other than an engine. The half life of tetraethyllead at the temperatures of interest is of the order of nfilliseconds, and conventional apparatus for kinetic studies under these con- ditions is not completely satisfactory.

The important thing about this may be that there is nothing unusually mysterious about an engine. Chemical reactions go on in it about the same as they do anywhere else. But it is particu- larly important that we consider the rapidly changing temperatures and pressures.

We can, consequently, see a good deal of light ahead, and although we have largely used up, in the field of combustion in engines, our avail- able stock of fundamental information we see several ways to apply new fundanmntal concepts. And finally, any new pieces of information are almost certain to have practical significance in this field in which, as has been indicated, the economic rewards are so great.

II

C O M B U S T I O N PROBLEMS IN DIESEL ENGINES

P. H. SCHWEITZER

The topic assigned is Combustion Problems in Diesel Engines. The following presentation is intended to highlight briefly some of these problems as they appear to this observer.

I t should first of all be pointed out that when we discuss problems of combustion, it is in order to ask whose problems we are talking about.

Various groups are affected by engine com- bustion. The principal interest of the scientists is in such things as flame and preflame kinetics, of the engineers in thermal efficiency, of the owners of the machine in operating economy and reliability; but a party intensely interested in combustion is seldom mentioned and usually completely forgotten: the general public.

A little reflection on the situation as it affects the public leads us to pinpoint certain diesel combustion problems. Noise, smoke and odor are the principal complaints, and the diesel excells in all three of them.

Even enlightened self-interest should induce the industry to take this matter seriously, more seriously than it has in the past. I t is easy to predict that government--s tate or municipal-- will soon act if we do nothing about it. An in-

censed public may force legislators to enact un- wise laws to the detriment of all of us.

Noise, smoke, and odor are the problems con- cerning which the public looks to us for solution. Just where are we in these items?

A good deal of the engine noise is combustion knock. Combustion knock is some function of the rate of combustion or the rate of cylinder pres- sure rise. While it is not a rule, it is a fairly vMid observation that if the rate of pressure rise per degree crank angle exceeds a certain value, the engine is sm'e to be rough. Why the pressure rise per degree should be the controlling factor in- stead of pressure rise per see is a puzzle.

Another puzzle that may be mentioned in passing pertains to ignition temperature. I t is a common observation that a liquid spray ignites more easily than gas or dry vapor. There is still a school in Germany which maintains that ignition takes place from the liquid phase. The self-igni- tion temperature is lower in the liquid phase than in the vapor phase. In fact, it is often below the vaporization temperature. Chemists correctly object that no ignition can take place in the li:luid phase. Yet you will find that, although a