Design and Simulation of Two Stroke Engines

Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
tinually search for new materials, lubricants and methods to further reduce the level of lubricant
consumption, for it is this factor that influences the exhaust smoke output from a twostroke
engine at cold start-up and at light load. In this context, the papers by Fog et al. [7.22]
and by Sugiura and Kagaya [7.25] should be studied as they contain much practical information.
One of the least understood design options is the bore-stroke ratio. Designers, like the rest
of the human species, are prone to fads and fashions. The in-fashion of today is for oversquare
engines for any application, and this approach is probably based on the success of oversquare
engines in the racing field. Logically speaking, it does not automatically follow that it is the
correct cylinder layout for an engine-driven, portable, electricity-generating set which will
never exceed 3000 or 3600 rpm.
If the application of the engine calls for its extensive use at light loads and speeds, such as
a motorcycle in urban traffic or trolling for fish with an outboard motor, then a vitally important
factor is the maintenance of the engine in a "two-stroke" firing mode, as distinct from a
"four-stroke" firing mode. This matter is introduced in Sec. 4.1.3. Should the engine skip-fire
in the manner described, there is a very large increase in exhaust hydrocarbon emissions. This
situation can be greatly improved by careful attention during the development phase to combustion
chamber design, spark plug location and spark timing. Even further gains can be
made by exhaust port timing and area control, perhaps leading to the incorporation of "active
radical" combustion to solve this particular problem, and this option should never be neglected
by the designer [4.34, 7.27].
In summary, the optimization of the simple two-stroke engine to meet performance and
exhaust emission targets can be subdivided as follows:
(i) Optimize the bore-stroke ratio.
(ii) Optimize the scavenging process.
(iii) Optimize the delivery ratio.
(iv) Optimize the port timings and areas.
(v) Optimize the air-to-fuel ratio.
(vi) Optimize the combustion process.
(vii) Optimize unsteady gas-dynamic tuning.
(viii) Optimize the lubrication requirements.
(ix) Optimize the pumping and mechanical losses.
To satisfy many of the design needs outlined above, the use of a computer-based simulation
of the engine is ideal. The engine models presented in Chapter 5 will be used in succeeding
sections to illustrate many of the points made above and to provide an example for the
designer that such models are not primarily, or solely, aimed at design for peak specific power
performance.
7.2.1 Typical performance characteristics of simple engines
Before embarking on the improvement of the exhaust emission and fuel economy characteristics
of the simple two-stroke engine, it is important to present and discuss some typical
measured data for such engines.
In Chapter 4, on combustion, the discussion focuses on the origins of the emissions of
carbon monoxide, nitric oxide, oxygen, carbon dioxide, and hydrogen, as created by that
471