Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
potential for the application of a catalyst to oxidize the bypassed fuel and carbon monoxide,
without incurring an excessive rise in exhaust gas temperature, becomes a possibility for the
optimized design but is a most unlikely prospect for the orginal concept.
The engine durability should also be improved by this methodology as the thermal loading
on the piston will be reduced. There is a limit to the extent to which this design approach
may be conventionally taken, as the engine will be operating ever closer to the misfire limit
from a scavenging efficiency standpoint.
As a historical note, in the 1930s a motorcycle with a two-stroke engine was produced by
the English company of Velocette [7.27]. The 250 cm 3 single-cylinder engine had a bore and
stroke of 63 and 80 mm, respectively, with seperate oil pump lubrication and was cross scavenged
by a deflector piston design very similar in shape to Fig. 3.32(a). The "part-spherical"
combustion chamber was situated over the exhaust side of the piston and the port timings
were not dissimilar to those discussed above for the optimized low bmep engine. It produced
some 9 hp at 5000 rpm, i.e., a bmep of about 3 bar. It sold for the princely sum of £38 (about
$52)! The road tester [7.27] noted that "slow running was excellent. The engine would idle at
very low rpm without four-stroking—one of the bugbears of the two-stroke motor." Perhaps
an optimized two-stroke engine design to meet fuel consumption and emissions requirements,
not to speak of the incorporation of active radical combustion, is nothing new!
7.3.4 The optimization of combustion
The topic of homogeneous combustion is covered in Chapter 4. Since Chapter 1, and the
presentation of Eq. 1.5.22, where it is shown that the maximum power and minimum fuel
consumption will be attained at the highest compression ratio, you have doubtless been waiting
for design guidance on the selection of the compression ratio for a given engine. However,
as mentioned in Chapter 4, the selection of the optimum compression ratio is conditioned
by the absolute necessity to minimize the potential of the engine to detonate. Further,
as higher compression ratios lead to higher cylinder temperatures, and the emission of oxides
of nitrogen are linked to such temperatures, it is self-evident that the selection of the compression
ratio for an engine becomes a compromise between all of these factors, namely, power,
fuel consumption, detonation and exhaust emissions. The subject is not one which is amenable
to empiricism, other than the (ridiculously) simplistic statement that trapped compression
ratios, CRt, of less than 7, operating on a gasoline of better than 90 octane, rarely give
rise to detonation.
The correct approach is one using computer simulation, and in Appendix A7.1 you will
find a comprehensive discussion of the subject, using the "standard" chainsaw engine as the
background input data to a computer simulation with a two-zone combustion and emissions
model, as previously described in the Appendices to Chapter 4.
Active radical combustion
One aspect, active radical (AR) combustion, is described briefly in Sec. 4.1.3. It deserves
further amplification as it will have great relevance for the optimization of the simple twostroke
engine to meet emissions legislation at light load and low engine speed, including the
idle (no load) condition. The first paper on this topic is by Onishi [4.33] and the most recent
is by Ishibashi [4.34]. The combustion process is provided by the retention of a large propor-
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