Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
200 -,
Q
E 150
sz
I
d 100
i
w
50
QUB270 CROSS SCAVENGED ENGINE
WOT, 3000 rpm
1000 2000 3000 4000 5000
ENGINE SPEED, rpm
AIRHEAD CLOSED
Fig. 7.47 Effect of airhead on hydrocarbon emissions.
AIRHEAD 50% OPEN
AIRHEAD 100% OPEN
While these are significant findings, the absolute levels of bsHC minimize at 60 g/kWh,
which is considerably above the 25 g/kWh theoretically achieved by the low bmep optimization
process described above for the simple two-stroke engine. It is not impossible to consider
a combination of the two approaches.
One of the advantages of the airhead approach is that lost charge from the exhaust port
tends to contain bypassed charge with a leaner air-to-fuel ratio than with homogeneous charging.
This provides excess oxygen in the exhaust system and would improve the conversion
efficiency of the pollutants by an oxidation catalyst placed in it.
7.4.2 Homogeneous charging with stratified combustion
As was pointed out earlier, direct in-cylinder fuel injection is one of the obvious methods
of reducing, or even eliminating, the loss of fuel to the exhaust port during the scavenge
process. Fig. 7.30 illustrates the positioning of such a fuel injector for the combustion of
gasoline in a spark-ignition engine. Figs. 4.1(b) and 4.9 show the positioning of injectors for
the injection of fuel into a compression-ignition engine, for it must not be forgotten that this
engine falls directly, and more convincingly in the view of some, into this category.
Gasoline injection into spark-ignition engines
The potential difficulties of evaporating a fuel spray in time for a homogeneous combustion
process to occur have been debated. The even more fundamental problem of attaining
good flammability characteristics at light load and speed in a homogeneous combustion process
has also been addressed. What, then, are the known experimental facts about in-cylinder
fuel injection? Is the combustion process homogeneous, or does the desirable possibility of
stratified burning at light load and speed exist? Does the fuel vaporize in time to burn in an
efficient manner? The answers to these questions are contained in the technical papers published
by several authors; answers to the question of cost and design approach for tradition-
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