Design and Simulation of Two Stroke Engines

Chapter 4 - Combustion in Two-Stroke Engines
The heat release characteristics of homogeneously charged, spark-ignition, two-stroke
engines have been found to be remarkably similar, which is not too surprising as virtually all
of the engines have rather similar combustion chambers. Such would not be the case for fourstroke
cycle engines, where the combustion chamber shape is dictated by the poppet valve
mechanisms involved. A uniflow two-stroke engine of the type illustrated in Fig. 1.5 would
conform more to a four-stroke combustion model than one for a two-stroke engine.
One of the simplifying assumptions in the use of a simple heat release model for combustion
is that the prediction of the heat loss to the cylinder walls and coolant is encapsulated in
the selection of the polytropic exponents for the compression and expansion processes. This
will be regarded by some as an unnecessary and potentially inaccurate simplification. However,
as all models of heat transfer are more or less based on empirical forms, the use of
experimentally determined polytropic exponents could actually be regarded as a more realistic
assumption. This is particularly true for the two-stroke engine, where so many engine
types are similar in construction. The key for success is to have a complete map of the polytropic
indices of compression and expansion to cover the entire speed and load range of any engine.
Unfortunately, such information does not exist, and much of that which is published is very
contradictory.
The modeler who wishes to use this simple approach, in the absence of better experimental
evidence from a particular engine, could have some confidence in using polytropic indices
of 1.25 and 1.35 for the compression and expansion phases, respectively, and 1.34 for the
numerical value of y in Eq. 4.2.12, together with a value between 0.8-0.9 for the overall
combustion efficiency, T|c.
4.4.2 A closed cycle model within engine simulations
A slightly more complex approach, but one which is much more complete and of greater
accuracy, is to employ all of the theory provided above on heat transfer, fuel vaporization,
heat release rates or mass fraction burned behavior, and solve the First Law of Thermodynamics
as expressed earlier in Eq. 4.2.8 at every step in a computation, but extended to include
vaporization of fuel.
8QR - 5QL - 5Qvap = P^-PiV, + Pl±£2 (v _ v )
Y - 1 2
All of the physical geometry of the engine will be known, as will physical parameters
such as all surface areas and their temperature. The open cycle model employed, such as
provided in Chapter 2, must give the initial masses, purities and state conditions of the cylinder
contents at the onset of the closed cycle. The air-fuel ratio will be known, so that either the
mass of fuel to be injected, or to be vaporized during compression, can be computed. A heat
release rate, or mass fraction burned, profile must be assumed so that at at any juncture during
combustion the heat to be released into the combustion chamber can be determined. The heat
transfer to or from the cylinder walls can be found at any juncture using the Annand model of
Sec. 4.3.4.
Using the theory of Sec. 2.1.6, at any point thereafter in the closed cycle, will yield the
properties of the cylinder gas at any instant, during compression and before combustion,
319