Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
they illustrate the potential attractiveness of an optimized two-stroke engine to the automotive
industry.
To return to the discussion regarding simple two-stroke engines, Figs. 7.3-7.8 should be
examined carefully in light of the discussion in Sec. 7.1.1 regarding the influence of air-fuel
ratio on exhaust pollutant levels. As carbon monoxide is the one exhaust gas emission not
distorted in level by the scavenging process, it is interesting to note that the theoretical predictions
provided by the equations in Sec. 4.3 for stoichiometric, rich, and lean air-fuel ratios are
quite precise. In Fig. 7.5 the CO level falls linearly with increasing air-fuel ratio and it levels
out at the stoichiometric value. At one-tenth throttle in Fig. 7.8, exactly the same trend occurs.
The theoretical postulations in terms of the shape of the oxygen curve are also observed
to be borne out. In Fig. 7.5 the oxygen profile is flat until the stoichiometric air-fuel ratio, and
increases linearly after that point. The same trend occurs at one-tenth throttle in Fig. 7.8,
although the flat portion of the curve ends at an air-to-fuel ratio of 14 rather than at the
stoichiometric level of 15.
The brake specific fuel consumption and the brake specific hydrocarbon emission are
both minimized at, or very close to, the stoichiometric air-fuel ratio.
All of the theoretical predictions from the relatively simple chemistry described in Sec.
4.3 are shown to be relevant. In short, for the optimization of virtually any performance
characteristic, the simple two-stroke engine should be operated as close to the stoichiometric
air-fuel ratio as possible within the limits of the mechanical reliability of the components or
of the onset of detonation. The only exception is maximum power or torque, where the optimum
air-fuel ratio is observed to be at 13, which is about 13% rich of the stoichiometric level.
7.2.1.2 Typical performance maps for simple two-stroke engines
It is necessary to study the more complete performance characteristics for simple twostroke
engines so that you are aware of the typical characteristics of such engines over the
complete load and speed range. Such performance maps are presented in Figs. 7.9-7.11 from
the publication by Batoni [7.1] and in Fig. 7.12 from the paper by Sato and Nakayama [7.2].
The experimental data from Batoni [7.1]
In Figs. 7.9-7.11 the data are measured for a 200 cc motor scooter engine which has very
little exhaust tuning to assist with its charge trapping behavior. The engine is carburetted and
spark-ignited, and is that used in the familiar Vespa motor scooter. The units for brake mean
effective pressure, bmep, are presented as kg/cm 2 where 1 kg/cm 2 is equivalent to 0.981 bar.
The units of brake specific fuel consumption, bsfc, are presented as g/hp.hr where 1 g/hp.hr is
equivalent to 0.746 g/kWh.
The bmep from this engine has a peak of 4.6 bar at 3500 rpm. Observe that the best bsfc
occurs at 4000 rpm at about 50% of the peak torque and is a quite respectable 0.402 kg/kWh.
Below the 1 bar bmep level the bsfc deteriorates to 0.67 kg/kWh. The map has that general
profile which causes it to be referred to in the jargon as an "oyster" map.
The carbon monoxide emission map has a general level between 2 and 6%, which would
lead one to the conclusion, based on the evidence in Figs. 7.5 and 7.8, that the air-fuel ratio
used in these experimental tests was in the range of 12 to 13. By the standards of equivalent
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