Design and Simulation of Two Stroke Engines

Chapter 4
Combustion in Two-Stroke Engines
4.0 Introduction
This chapter will mainly deal with combustion in the spark-ignition engine, but a significant
segment of the discussion and the theory is about compression-ignition engines. The
subject will be dealt with on a practical design basis, rather than as a fundamental treatise on
the subject of combustion. It is not stretching the truth to say that combustion, and the heat
transfer behavior which accompanies it, are the phenomena least understood by the average
engine designer. The study of combustion has always been a specialized topic which has
often been treated at a mathematical and theoretical level beyond that of all but the dedicated
researcher. Very often the knowledge garnered by research has not been disseminated in a
manner suitable for the designer to use. This situation is changing, for the advent of CFD
design packages will ultimately allow the engine designer to predict combustion behavior
without being required to become a mathematics specialist at the same time. For those who
wish to study the subject at a fundamental level, and to be made aware of the current state of
the science, Refs. [1.3-1.4, 4.1-4.5 and 4.28-4.31] will help provide a starting point for that
learning process. There seems little point in repeating the fundamental theory of combustion
in this book, because such theory is well covered in the literature. However, this chapter will
cover certain aspects of combustion theory that are particularly applicable to the design and
development of two-stroke engines and which are rarely found in the standard reference textbooks.
The first objective of this chapter is to make you aware of the difference between the
combustion process in a real engine from either the theoretical ideal introduced in Sec. 1.5.8,
or from an explosion, which is the commonly held view of engine combustion by the laity.
The second aim is to introduce you to the analysis of cylinder pressure records for the
heat release characteristics which they reveal, and how such information is the most realistic
available for the theoretical modeler of engine behavior. In this section, other possible theoretical
models of combustion are presented and discussed.
The third objective is to make you aware of the effect of various design variables such as
squish action on detonation and combustion rates and the means at hand to design cylinder
heads for engines taking these factors into account.
Penultimately, the chapter refers to computer programs to assist with this design process
and you are introduced to their use.
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