Design and Simulation of Two Stroke Engines

Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
The results of the simulation, either in magnitude or in profile, are sufficiently close to
those measured that the simulation can be employed in the design mode with some considerable
degree of confidence that its predictions are suitably relevant and accurate.
The energy content in exhaust gas emissions
Exhaust gas which contains carbon monoxide and hydrocarbons is transmitting energy
originally contained within the fuel into the exhaust system and the atmosphere. This energy
content, Qex, is determined from the power output, W, the specific hydrocarbon emission
rate, bsHC, the specific carbon monoxide emission rate, bsCO, the calorific value of the fuel,
CfiHC> and the calorific value of carbon monoxide, Cfico They are related thus, in conventionally
employed units, where the units of power output are kW, bsCO and bsHC are g/kWh,
and the calorific values of carbon monoxide and the fuel are expressed as MJ/kg:
W
Qex = — (Cfico x bsCO + CflHcbsHC) W (7.2.1)
3.6
As a typical example of the simple two-stroke engine, consider an engine with a power
output of 4 kW, specific carbon monoxide and hydrocarbon emission rates of 160 and 120
g/kWh, respectively, and with fuel and carbon monoxide calorific values of 43 and 10 MJ/kg,
respectively. The exhaust energy content is found by:
W
Qex = —(CflCO x bsCO + CflHcbsHC)
3.6
= —(10x160 + 43x120) = 7511 W
3.6
Observe that this amounts to 7.5 kW, or nearly twice the power output of the engine. The
energy being "thrown away" in this fashion is insupportable in the environmental context.
Should this energy be realized in the exhaust system, either by a reactor or by a catalyst,
the very considerable heat output would raise the exhaust gas temperature by many hundreds
of degrees.
7.3 Optimizing fuel economy and emissions for the simple two-stroke engine
In Sec. 7.2, the problems inherent in the design of the simple two-stroke engine are introduced
and typical performance characteristics are presented. Thus, you are now aware of the
difficulty of the task which is faced, for even with the best technology the engine is not going
to be competitive with a four-stroke engine in terms of hydrocarbon emission. In all other
respects, be it specific power, specific bulk, specific weight, maneuverability, manufacturing
cost, ease of maintenance, durability, fuel consumption, or CO and NO emissions, the simple
two-stroke engine is equal, and in some respects superior, to its four-stroke competitor. There
may be those who will be surprised to see fuel consumption in that list, but investigation
shows that small-capacity four-stroke engines are not particularly thermally efficient. The
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