Design and Simulation of Two Stroke Engines

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Chapter 4 - Combustion in Two-Stroke Engines
Prior to the combustion process, the fuel is injected into the air in the cylinder, which by
dint of the high compression ratio is above the auto-ignition temperature of the fuel. It requires
trapped compression ratios, CRt, of about 18 to accomplish this latter effect, by comparison
with the 7 or 8 normally employed for spark ignition. The fuel droplets must first be
heated by the compressed air to vaporize the fuel, and then to raise that vapor temperature to
the auto-ignition temperature. This, like all heat transfer processes, takes time. Naturally,
there will be an upper limit of engine speed where the heating effect has not been fully accomplished
and combustion will not take place efficiently, or even not at all. Thus, a diesel engine
tends to have an upper speed limit for its operation in a fashion not seen for a spark-ignition
power unit. The limit is imposed by the effectiveness of the heat transfer and mixing process
for the particular type of diesel engine. For diesel engines in automobiles using the indirect
injection (IDI) method that limit is about 4500 rpm, and for larger direct injection (DI) diesel
engines for trucks or buses it is about 3000 rpm. There is a more extensive discussion of this
topic in Sec. 4.3.7, in terms of the considerable differences in combustion chamber geometry
for these two dissimilar approaches to the generation of combustion by compresson ignition.
4.2 Heat released by combustion
4.2.1 The combustion chamber
The combustion process is one described thermodynamically as a heat addition process
in a closed system. It occurs in a chamber of varying volume proportions whose minimum
value is the clearance volume, Vcv. This was discussed earlier in Sec. 1.4.2, and Eqs. 1.4.4
and 1.4.5 detail the values of Vcv in terms of the swept volume, Vsv, and the trapped swept
volume, Vtsv, to attain the requisite parameters of geometric compression ratio, CRg, and
trapped compression ratio, CRt. In Fig. 4.2, the piston is shown positioned at top dead center,
tdc, so the clearance volume, Vcv, is seen to be composed of a bowl volume, Vb, and a squish
volume, Vs. The piston has a minimum clearance distance from the cylinder head which is
known as the squish clearance, xs. The areas of the piston which are covered by the squish
band and the bowl are As and Ab, respectively, and this squish action gives rise to the concept
of a squish area ratio, Csq, where:
C =
area squished
bore area
*s
"a 2
bo
Ac + Av (4.2.1)
The definitions above are equally applicable to a compression-ignition engine, including
the IDI engine in Fig. 4.9.
4.2.2 Heat release prediction from cylinder pressure diagram
In Sec. 1.5.8 there is a discussion of the theoretically ideal thermodynamic engine cycle,
the Otto Cycle, and the combustion process is detailed as occurring at constant volume, i.e.,
the imaginary explosion. The reality of the situation, in Figs. 1.14 and 1.15, is that the engine
pressure diagram as measured on the QUB LS400 research engine shows a time-dependent
combustion process. This is pointed out in Sec. 4.1.1, where the flame speed is detailed as
having been measured at 24.5 m/s at the same test conditions.
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