Design and Simulation of Two Stroke Engines

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Chapter 4 - Combustion in Two-Stroke Engines Plate 4.1 Compact combustion chamber for a loop-scavenged two-stroke engine. Plate 4.2 The complex combustion chamber of the conventional cross-scavenged engine. 287
Design and Simulation of Two-Stroke Engines zones in the combustion space which are at differing air-fuel ratios. This is known as a stratified combustion process. This stratification may be deliberately induced, for example, to permit the local efficient burning of a small mass of air and fuel of the correct proportions to overcome the problems of "four-stroking." It is also possible to utilize charge stratification to help alleviate detonation behavior. If the extremities of the combustion chamber contain air only, or a very lean mixture, then there exists the possibility of raising engine thermal efficiency with a higher compression ratio, while lowering the potential for detonation to occur. Further detailed discussion on this topic can be found in Chapter 7, which covers the design of engines for good fuel economy and exhaust emissions. 4.1.6 Compression ignition This subject will be dealt with more fully in Sec. 4.3.7, but the fundamentals of the process will be discussed here to distinguish it clearly from the spark-ignition process described above. A sketch of a typical combustion chamber and fuel injector location for a direct injection (DI) diesel engine is shown in Fig. 4.1(b). In the compression-ignition process the compression ratio employed for the engine is much higher so that the air temperature by the end of compression is significantly above the auto-ignition temperature of the fuel employed. When fuel is sprayed directly into this air at this state condition and at that juncture, the fuel commences to vaporize around the droplets and, when the vapor temperature has risen to the auto-ignition temperature, the combustion commences with that vapor and the air in its immediate vicinity. Thus, in diesel combustion by complete contrast with homogeneous spark ignition, it is possible to burn one microscopic droplet of fuel in a whole "sea" of air. In such lean combustion the great advantage is that it is possible to have it occur at air-fuel ratios four or five times leaner than the stoichiometric mixture. The penalty is that efficient combustion cannot occur at air-fuel ratios which approach the stochiometric value, for now the fuel is, as it were, chasing through the combustion chamber searching for the air. In the limited time available for combustion, it is not possible to have that take place for all of the carbon component of the fuel, and thus there is a "rich limit" for diesel combustion. This rich limit is typically some 40% leaner than stoichiometric, or at an air-fuel ratio of about 20. If the air-fuel ratio is richer than this value, then the exhaust gas exhibits considerable quantities of unburned carbon particulates, or black smoke. Today, exhaust emissions legislation specifies and limits the amount of carbon particulates which may be emitted. This effect is compounded by the heavier hydrocarbon fuel employed for diesel engines. Gasoline is typically and ideally octane, CgHig, i.e., the eighth member of the family of paraffins whose general family formula is CnH2n+2- This is much too volatile a fuel to be used in compression ignition as the ensuing pressure and temperature rates of rise would be so rapid as to cause mechanical damage to the cylinder components. The fuel employed for most automotive diesel engines is typically and ideally dodecane, Q2H26, i.e., the twelfth member of the family of paraffins. As it is heavier with a more complex structure, this molecule requires more energy to separate the carbon and hydrogen atoms from each other and thus it burns more slowly, giving a less rapid rate of pressure rise during a compression-ignition process than would octane. Nevertheless, this more complex molecule does not burn to completion so readily and hence the compounding effect on black smoke production alluded to at the end of the last paragraph. 288
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Design and Simulation of Two-Stroke
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A Second Mulled Toast When as a stu
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Acknowledgments As explained in the
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Table of Contents Nomenclature xv C
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Table of Contents 2.10.1 Flow at pi
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Table of Contents 3.5.3.1 The use o
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Table of Contents 6.1.3 The effect
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Nomenclature NAME SYMBOL UNIT (SI)
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speed of rotation speed of rotation
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Design and Simulation ofTwo-Stroke
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Page 258 and 259: Design and Simulation of Two-Stroke
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Page 302 and 303: Chapter 4 Combustion in Two-Stroke
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CURRENT INPUT DATA FOR 2-STROKE 'SP
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Chapter 4 - Combustion in Two-Strok
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Cylin- PP £ Pa- tics of Paper Chap
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vfitro- ;titi >tants 1970. i(In- Ch
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£2 = Pi P2 I Pi J Chapter 4 - Comb
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CHAINSAW AT 9600 rpm. Chapter 4- Co
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Chapter 7 Reduction of Fuel Consump
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Chapter 7 • Reduction of Fuel Con
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Chapter 7 - Reduction of Fuel Consu
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§ 2000
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y, ,6 Chapter 7 - Reduction of Fuel
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Chapter 7- Reduction of Fuel Consum
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Chapter 8 Reduction of Noise Emissi
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Chapter 8 • Reduction of Noise Em
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Chapter 8 - Reduction of Noise Emis
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• • / . • Appendix Listing of
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Active radical (AR) combustion and
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initial conditions, assumptions reg
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in two-stroke engine (schematic dia
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exhaust timing control valve, effec
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I Exhaust emissions/exhaust gas ana
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Exhaust systems (continued) untuned
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gas constant/universal gas constant
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in crankcase heat transfer analysis
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Mercury Marine air-assisted fuel in
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I PISTON POSITION (computer program
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Pressure wave reflection (in pipes)
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local acoustic velocity, 60 local M
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Scavenging (continued) scavenging e
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Simulation, engine See Computer mod
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temperature vs. crankshaft angle (c
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work per thermodynamic (Otto) cycle
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Design and Simulation of Two Stroke Engines

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