Design and Simulation of Two Stroke Engines

to be
cur
.3.20)
.3.21)
.3.22)
for
Chapter 4 - Combustion in Two-Stroke Engines
A4.1 and A4.2 present the fundamental theoretical base for the computation of exhaust emissions
within an engine simulation.
It is appropriate to point out here that the analysis of Eq. 4.3.3 shows quite clearly that
rich mixture combustion leads to an exhaust emission of carbon monoxide and unburned
hydrocarbons. As rich mixture combustion has insufficient air present to oxidize all of the
carbon and hydrogen present, the outcome is self-evident. The effects of dissociation have
been shown here to lead to both CO and HC emissions, even if the mixture strength is
stoichiometric.
The extent of the CO emission, either experimentally found or theoretically deduced, is
shown in Sec. 7.2.1, and Fig. 7.15 gives a classic picture of the profile of CO emission decaying
to a virtually non-existent level by the stoichiometric air-to-fuel ratio.
The extent of the hydrocarbon emissions is also shown in Sec. 7.2.1, but for the simple
spark-ignition two-stroke engine the fuel lost, by the inefficiency of all scavenging processes,
totally overshadows that emitted from the combustion process. While operating close to the
stoichiometric air-fuel ratio will minimize HC emissions, and the experimental data in Fig.
7.14 shows this quite clearly, the remainder is from scavenge losses and is very considerable.
The creation of nitrogen oxides occurs as a function of temperature, so it is maximized by
increasing load (i.e., bmep) levels or by combustion at the stoichiometric air-fuel ratio at any
given bmep value. This is illustrated most clearly in Fig. 7.55 at full load, and in Fig. 7.59 at
part load, for the same fuel-injected, two-stroke, spark-ignition engine.
For the two-stroke engine, much relief from combustion-related emissions is given by the
fact that the combustion chamber in the cylinder head is normally a much simpler, more
compact, and less mechanically cluttered design than that of the four-stroke engine. The "clutter"
referred to are the poppet valves, and the less compact shape which ensues from their
incorporation. This gives crevices into which the flame dies or is quenched, leading to incomplete
combustion in those regions.
Another point of relief is the scavenging process which leaves exhaust gas residual behind
in the cylinder, and this acts as a damper on the formation of nitrogen oxides. The twostroke
engine comes with this advantage "built-in," whereas the four-stroke engine requires
added mechanical complexity to recirculate exhaust gas, i.e., the EGR device which is commonplace
on an automobile engine to meet emissions legislation.
4.3.3 Heat availability during the closed cycle
From this discussion on dissociation it is clear that, at the high temperatures and pressures
which are the reality during a combustion process, carbon dioxide cannot be produced
directly at any instant during burning. Carbon monoxide is produced, but its heat of formation
is much less than that yielded by a complete oxidation of the carbon to carbon dioxide. Thus
the full heat potential of the fuel is not realized nor released during burning. There can be
further inhibitions in this regard. The mixture can be rich so that there is insufficient oxygen
to effect that process, even if it were to be ideal. There can be, and in a two-stroke engine
there certainly will be, considerable quantities of exhaust gas residual present within the
combustion chamber to inhibit the progress of the flame development and the efficiency of
combustion. Thus the complete combustion efficiency is composed of relative sub-sets related
to equivalence ratio and scavenging efficiency:
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