Reduction of Fuel Consumption and Exhaust Emissions
7.0 Introduction
Throughout the development of the internal combustion engine, there have been phases of concentration on particular aspects of the development process. In the first major era, from the beginning of the twentieth century until the 1950's, attention was focused on the production of ever greater specific power output from the engines, be they two- or four-stroke cycle power units. To accomplish this, better quality fuels with superior octane ratings were prepared by the oil companies so that engines could run at higher compression ratios without risk of detonation. Further enhancements were made to the fund of knowledge on materials for engine components, ranging from aluminum alloys for pistons to steels for needle roller bearings, so that high piston speeds could be sustained for longer periods of engine life. The text of this book thus far has concentrated on the vast expansion of the knowledge base on gas dynamics, thermodynamics and fluid mechanics which has permitted the design of engines to take advantage of the improvements in materials and tribology. Each of these developments has proceeded at an equable pace. For example, if a 1980's racing engine had been capable of being designed in 1920, it would have been a case of self-destruction within ten seconds of start-up due to the inadequacies of the fuel, lubricant, and materials from which it would have been assembled at that time. Should it have lasted for any length of time, at that period in the 1920's, the world would have cared little that its fuel consumption rate was excessively high, or that its emission of unbumed hydrocarbons or oxides of nitrogen was potentially harmful to the environment!
The current era is one where design, research and development is increasingly being focused on the fuel economy and exhaust emissions of the internal combus- tion engine. The reasons for this are many and varied, but all of them are significant and important.
The world has a limited supply of fossil fuel of the traditional kind, i.e., that which
emanates from prehistorical time and is available in the form of crude oil capable
of being refined into the familiar gasoline or petrol, kerosene or paraffin, diesel oil
and lubricants. These are the traditional fuels of the internal combustion engine and
it behooves the designer, and the industry which employs him, to develop more efficient engines to conserve that dwindling fossil fuel reserve. Apart from ethical considerations, many governments have enacted legislation setting limits on fuel consumption for various engine applications.
The population of the world has increased alarmingly, due in no small way to a more efficient agriculture which will feed these billions of humans. That agricul- tural system, and the transportation systems which back it up, are largely efficient due to the use of internal combustion engine driven machinery of every conceivable type. This widespread use of ic engines has drawn attention to the exhaust emissions from its employment, and in particular to those emissions which are harmful to the environment and the human species. For example, carbon monoxide is toxic to humans and animals. The combination of unburned hydrocarbons and nitrogen oxides, particularly in sunlight, produces a visible smog which is harmful to the lungs and the eyes. The nitrogen oxides are blamed for the increased proportion of the rainfall containing acids which have a debilitating effect on trees and plant growth in rivers and lakes. Unburned hydrocarbons from marine engines are thought to concentrate on the beds of deep lakes, affecting in a negative way the natural development of marine life. The nitrogen oxides are said to contribute to the depletion of the ozone layer in the upper atmosphere, which potentially alters the absorption characteristics of ultraviolet light in the stratosphere and increases the radiation hazard on the earth's surface. There are legitimate concerns that the accumulation of carbon dioxide and hydrocarbon gases in the atmosphere increases the threat of a "greenhouse effect" changing the climate of the Earth.
One is tempted to ask why it is the important topic of today and not yesterday.
The answer is that the engine population is increasing faster than people, and so too is the volume of their exhaust products. All power units are included in this critique, not just those employing reciprocating ic engines, and must also encompass gas turbine engines in aircraft and fossil fuel burning electricity generating stations.
Actually, the latter are the largest single source of exhaust gases into the atmosphere.
The discussion in this chapter will be in two main segments. The first concen trates on the reduction of fuel consumption and emissions from the simple, or conventional, two-stroke engine which is found in so many applications requiring an inexpensive but high specific output powerplant such as motorcycles, outboard motors and chainsaws. There will always be a need for such an engine and it behooves the designer to understand the methodology of acquiring the requisite performance without an excessive fuel consumption rate and pollutant exhaust emissions. The second part of this chapter will focus on the design of engines with fuel consumption and exhaust pollutant levels greatly improved over that available from the "simple" engine. Needless to add, this involves some further mechanical complexity or the use of expensive components, otherwise it would be employed on the "simple" engine. As remarked in Chapter 1, the two-stroke engine has an inherently low level of exhaust emission of nitrogen oxides, and this makes it an attractive proposition for future automobile engines, provided that the extra complexity and expense involved does not make the two-stroke powerplant non- competitive with its four-stroke engine competitor.
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions Before embarking on the discussion regarding engine design, it is necessary to expand on the information presented in Chapter 4 on combustion, particularly relating to the fundamental effects of air-fuel ratio on pollutant levels and to the basic differences inherent in homogeneous and stratified charging, and homogene- ous and stratified combustion.
7.1.1 Some fundamentals regarding combustion and emissions
Although much of the fundamental material regarding combustion is covered in Chapter 4, there remains some discussion which is specific to this chapter and the topics therein.
The first is to explain the origins of exhaust emission of carbon monoxide, unburned hydrocarbons and nitrogen oxides from the combustion process. The reader will recall the simple chemical relationship posed in Eq. 1.5.16 for the stoichiometric combustion of air and gasoline. Also, the reader should remember the discussion in Chapter 4, wherein it is stressed that the combustion of fuel and air occurs with vaporized fuel and air, but not liquid fuel and air. Gasoline is defined as octane, the eighth member of the family of paraffins (alkanes) whose general formula is CxH,x+v The stoichiometric combustion equation is repeated here.
2*C8H|8 + 25*0, + (25*79/21)*N,
= 16*CO, + 18*H,0+ (25*79/21)*N, (7.1.1) The air-fuel ratio, AF, emanating from this balanced equation is calculated as:
AF=(25*32+25*79*28/21)/(16*12+36*l)=15.06 (7.1.2) However, should the combustion process not be conducted at the correct air-fuel
ratio then a different set of exhaust gas components would appear on the right-hand side of this equation. For example, consider the situation where the air-fuel ratio is 20% lean of the stoichiometric value, i.e., there is excess air present during the combustion process. The Eq. 7.1.1 is modified as follows:
2*C8H|g +1.2*{25*0, +(25*79/21)*N,}
= 16*CO, + 18*H,0+i.2*(25*79/21)*N, + 5*0, (7.1.3) It is observed that the exhaust gas now contains oxygen which is due to the excess
of air over that required to consume the fuel.
Consider the rich air-fuel ratio, for example at 20% rich of the stoichiometric value. The following "ideal" equation would ensue, based on the premise that the more active hydrogen consumes all of the oxygen before the carbon can so do.
2*C8H|8 + 0.8*{ 25*02 + (25*79/21)*N,)
= 10*CO + 6*CO, +18*H,0 + 0.8*(25*79/21)*N, (7.1.4)
The Basic Design of Two-Stroke Engines
It can be seen that the exhaust gas now contains a significant fraction of carbon monoxide in this theoretical combustion of a rich mixture.
In summary, from these simple examples of applied chemistry, one would expect to see a zero level of carbon monoxide and oxygen in the exhaust gas after a
"perfect" combustion process, but increasing quantities of carbon monoxide if the air-fuel mixture becomes richer, or oxygen if it is leaner. As with all real-life processes, no combustion process, even at the stoichiometric mixture, is ever quite
"perfect" for the molecule of octane. A more realistic combustion analysis would reveal that some of the hydrocarbon molecule never breaks down completely, leaving unburned hydrocarbons, and that some of the carbon monoxide never achieves complete oxidation to carbon dioxide, even in the presence of excess air.
Many of these effects are due to a phenomenon described as "dissociation"(4.1). A further experimental fact is the association of nitrogen with oxygen to form the nitrogen oxide pollutant, NO , and this undesirable result becomes more pro- nounced as the combustion temperature is increased. Thus, the stoichiometric equation, Eq. 7.1.1, is more realistically:
2*C
gH
18+ 25*0
2+ (25*79/21)*N,
=Z1*C0
2+ Z2*H
20 + Z3*N, + Z4*CO
+ Z5*C H + Z6*NO + Z7*0, (7.1.5) It should be remembered that the molecular quantities of the pollutants. Z4-Z7,
are quite small for a stoichiometric combustion situation. Exhaust gas analysis is usually conducted with a dry exhaust gas sample, i.e., the steam is normally removed. To put an approximate number on the values of Z4-Z7 for a "dried"
exhaust gas sample of exhaust emanating from the combustion process alone, i.e., not confusing the hydrocarbon analysis with poorly scavenged fresh charge in the two-stroke engine, one would record values such as: 0.15% CO by volume, 1% 0
2by volume, unburned hydrocarbons as 600 ppm CH
4(methane) equivalent, and 500 ppm NO equivalent. As a first approximation, ignoring the values of Z4-Z7 as being non-significant arithmetically, the total moles in the dried exhaust gas sample are principally derived from the carbon dioxide and the nitrogen, i.e., (16+25*79/21), or 110.05. The volumetric proportions of the pollutants are also (from Avogadro) molecular proportions. Therefore:
for CO Z4=0.15*l 10.05/100=0.165 forO, Z5=l*l 10.05/100=1.10 forCH, Z6=600* 110.05/1 E6=0.066 for NO Z7=500*l 10.05/1 E6=0.055
It will be observed that the original coefficients for carbon dioxide, steam and nitrogen would be barely affected by this modified exhaust gas analysis. The proportion by volume of carbon dioxide in this dried exhaust gas sample would be given by:
%vol CO
2=100*16/l 10.05=14.5
It is reasonably clear from the foregoing that, should the air-fuel ratio be set correctly for the combustion process to the stoichiometric value, even an efficient combustion system will still have unburned hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas from the engine. Should the air-fuel ratio be set incorrectly, either rich or lean of the stoichiometric value, then the exhaust pollutant levels will increase. If the air-fuel mixture is very lean so that the flammability limit is reached and misfire takes place, then the unburned hydrocarbon and the carbon monoxide levels will be considerably raised. It is also clear that the worst case, in general, is at a richer air-fuel setting, because both the carbon monoxide and the unburned hydrocarbons are inherently present on theoretical grounds.
It is also known, and the literature is full of technical publications on the subject, that the recirculation of exhaust gas into the combustion process will lower the peak cycle temperature and act as a damper on the production of nitrogen oxides. This is a standard technique at this time for production four-stroke automobile engines to allow them to meet legislative requirements for nitrogen oxide emissions. In this regard, the two-stroke engine is ideally suited for this application, for the retention of exhaust gas is inherent from the scavenging process. This natural scavenging effect, together with the lower peak cycle temperature due to a firing stroke on each cycle, allows the two-stroke engine to produce very reduced nitrogen oxide exhaust emissions at equal specific power output levels.
Any discussion on exhaust emissions usually includes a technical debate on catalytic after-treatment of the exhaust gases for their added purification. In this chapter, there is a greater concentration on the design methods to attain the lowest exhaust emission characteristics before any form of exhaust after-treatment takes place.
Asa postscript to this section, there may be readers who will look at the relatively tiny proportions of the exhaust pollutants in Eq. 7.1.5 and wonder what all the environmental, ecological or legislative fuss is about in the automotive world at large. Let such readers work that equation into yearly mass emission terms for each of the pollutants in question for the annual consumption of many millions of tons of fuel per annum. The environmental problem then becomes quite self-evident!
7.1.2 Homogeneous and stratified combustion and charging
The combustion process can be conducted in either a homogeneous or stratified manner, and an introduction to this subject is given in Sect. 4.1. The words
"homogeneous" and "stratified" in this context define the nature of the mixing of the air and fuel in the combustion chamber at the period of the flame propagation through the chamber. A compression ignition or diesel engine is a classic example of a stratified combustion process, for the flame commences to burn in the rich environment of the vaporizing fuel surrounding the droplets of liquid fuel sprayed into the combustion chamber. A carburetted four-stroke cycle si engine is the classic example of a homogeneous combustion process, as the air and fuel at the onset of ignition are thoroughly mixed together, with the gasoline in a gaseous form.
Both of the above examples give rise to discussion regarding the charging of the
cylinder. In the diesel case, the charging of the cylinder is conducted in a stratified
manner, i.e., the air and the fuel enter the combustion chamber separately and any mixing of the fuel and air takes place in the combustion space. As the liquid fuel is sprayed in 35s before tdc it cannot achieve homogeneity before the onset of combustion. In the carburetted four-stroke cycle si engine example, the charging of the engine is conducted in a homogeneous fashion, i.e., all of the required air and fuel enter together through the same inlet valve and are considered to be homoge- neous, even though much of the fuel is still in the liquid phase at that stage of the charging process
It would be possible in the case of the carburetted four-stroke cycle si engine to have the fuel and air enter the cylinder of the engine in two separate streams, one rich and the other a lean air-fuel mixture, yet, by the onset of combustion, be thoroughly mixed together and burn as a homogeneous combustion process. In short, the charging process could be considered as stratified and the combustion process as homogeneous. On the other hand, that same engine could be designed, viz the Honda CVCC type, so that the rich and lean air-fuel streams are retained as separate entities up to the point of ignition and the combustion process is also carried out in a stratified manner. The main point behind this discussion is to emphasize the following points:
(a) If a spark-ignition engine is charged with air and fuel in a homogeneous manner, the ensuing combustion process is almost inevitably a homogeneous combustion process.
(b) If an engine is charged with air and fuel in a stratified manner, the ensuing combustion process is possibly, but not necessarily, a stratified combustion process.
In the analysis conducted above for the combustion of gasoline (see Eqs. 7.1.1- 7.1.5), the air-fuel ratio is noted as the marker of the relationship of that combustion process to the stoichiometric, or ideal. The reader will interpret that as being the ratio of the air and fuel supply rates to the engine. This will be perfectly accurate for a homogeneous combustion process, but can be quite misleading for a design where stratified charging is taking place.
Much of the above discussion is best explained by the use of a simple example illustrated in Fig. 7.1. The "engine" in the example is one where the combustion space can contain, or be charged with, 15 kg of air. Consider the "engine" to be a spark-ignition type and the discussion is pertinent for both two-stroke and four- stroke cycle engines.
At a stoichiometric air-fuel ratio for gasoline, this means that a homogeneously charged engine, followed by a homogeneous combustion process, would ingest 1 kg of octane with the air. This situation is illustrated in Fig. 7.1(al) and (bl). The supplied air-fuel ratio and that in the combustion space are identical at 15.
If the engine had stratified charging, but the ensuing mixing process is complete followed by homogeneous combustion, then the situation is as illustrated in Fig.
7.1(a2) and (b2). Although one of the entering air-fuel streams has a rich air-fuel ratio of 10 and the other is lean at 30, the overall air-fuel ratio is 15, as is the air-fuel ratio in the combustion space during burning. The supplied air-fuel ratio and that in the combustion space are identical at 15. In effect, the overall behavior is the same as for homogeneous charging and combustion.
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
AF=15-
AF=20.
AF=15 INLET 1 kg OCTANE 15 kg AIR
COMBUSTION SPACE
v j INLET
COMBUSTION SPACE
• s
1 kg OCTANE 15 kg AIR AF=15
^ J ( A1) HOMOGENEOUS CH ARC ING (B1) HOMOGENEOUS COMBUST ION
AF=10 AF=30
COMBUSTION SPACE
INLET NO. 1 , ^ 0.75 kg OCTANE
7.5 kg AIR 0.25 kg OCTANE 7.5 kg AIR
INLET N0.2 ^
COMBUSTION SPACE INLET NO 1
INLET NO.2
1 kg OCTANE 15 kg AIR AF=15
C A2) STR AT IF IED CH ARG ING (B2) HOMOGENEOUS COMBUST ION
~AF=10 AF=oo
COMBUSTION SPACE
INLET NO. 1 f -, 0.75 kg OCTANE
7 5 ka AIR 7.5 kg AIR -
INLET NO .2 J
COMBUSTION SPACE INLET NO 1
INLET NO .2
"BURN EONE" / 0.75 kg O C T A N E /
11.25 kg A I R / AF=15 /UNBURNED
/ 3.75 kg AIR J (A3) STRATIFIED CHARGING
Fig. 7.1 Homogeneous and stratified
(B3) STRATIFIED COMBUSTION
charging and combustion.
If the engine has both stratified charging and combustion, then the situation portrayed in Fig. 7.1 (a3) and (b3) becomes a real possibility. At an equal "delivery ratio" to the previous examples, the combustion space will hold 15 kg of air. This enters in a stratified form with one stream rich at an air-fuel ratio of 10 and the second containing no fuel at all. Upon entering the combustion space, not all of the entering air in the second stream mixes with the rich air-fuel stream, but a sufficient amount does to create a "burn zone" with a stoichiometric mixture at an air-fuel ratio of 15.
This leaves 3.75 kg of air unburned which exits with the exhaust gas. The implications of this are:
(a) The overall or supplied air-fuel ratio is 20, i.e., it gives no indication of the air-fuel ratio during the actual combustion process and is no longer an experimental measurement which can be used to optimize the combustion process. For example, many current production automobile engines have "engine management systems"
which rely on the measurement of exhaust oxygen as a means of electronically controlling the overall air-fuel ratio to the stoichiometric value.
(b) The combustion process would release 75% of the heat available in the homogeneous combustion example, and it could be expected that the BMEP and power output would be similarly reduced. In the technical phrase used to describe this behavior, the "air-utilization" characteristics of stratified combustion are not as