Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The population of the world has increased alarmingly, due in no small way to a more
efficient agriculture which will feed these billions of humans. That agricultural system, and
the transportation systems which back it up, are largely efficient due to the use of internal
combustion engine-driven machinery of every conceivable type. This widespread use of internal-combustion
engines has drawn attention to the exhaust emissions from its employment,
and in particular to those emissions that are harmful to the environment and the human
species. For example, carbon monoxide is toxic to humans and animals. The combination of
unburned hydrocarbons and nitrogen oxides, particularly in sunlight, produces a visible smog
which is harmful to the lungs and the eyes. The nitrogen oxides are blamed for the increased
proportion of the rainfall containing acids which have a debilitating effect on trees and plant
growth in rivers and lakes. Unburned hydrocarbons from marine engines are thought to concentrate
on the beds of deep lakes, affecting in a negative way the natural development of
marine life. The nitrogen oxides are said to contribute to the depletion of the ozone layer in
the upper atmosphere, which potentially alters the absorption characteristics of ultraviolet
light in the stratosphere and increases the radiation hazard on the earth's surface. There are
legitimate concerns that the accumulation of carbon dioxide and hydrocarbon gases in the
atmosphere increases the threat of a "greenhouse effect" changing the climate of the Earth.
One is tempted to ask why it is the important topic of today and not yesterday. The answer
is that the engine population is increasing faster than people, and so too is the volume of their
exhaust products. All power units are included in this critique, not just those employing reciprocating
IC engines, and must also encompass gas turbine engines in aircraft and fossil fuelburning,
electricity-generating stations. Actually, the latter are the largest single source of
exhaust gases into the atmosphere.
The discussion in this chapter will be in two main segments. The first concentrates on the
reduction of fuel consumption and emissions from the simple, or conventional, two-stroke
engine which is found in so many applications requiring an inexpensive but high specific
output powerplant such as motorcycles, outboard motors and chainsaws. There will always
be a need for such an engine and it behooves the designer to understand the methodology of
acquiring the requisite performance without an excessive fuel consumption rate and pollutant
exhaust emissions. The second part of this chapter will focus on the design of engines with
fuel consumption and exhaust pollutant levels greatly improved over that available from the
"simple" engine. Needless to add, this involves some further mechanical complexity or the
use of expensive components, otherwise it would be employed on the "simple" engine. As
remarked in Chapter 1, the two-stroke engine, either compression or spark ignition, has an
inherently low level of exhaust emission of nitrogen oxides, and this makes it fundamentally
attractive for future automobile engines provided that the extra complexity and expense involved
does not make the two-stroke powerplant non-competitive with the four-stroke engine.
Before embarking on the discussion regarding engine design, it is necessary to expand on
the information presented in Chapter 4 on combustion, particularly relating to the fundamental
effects of air-fuel ratio on pollutant levels and to the basic differences inherent in homogeneous
and stratified charging, and homogeneous and stratified combustion.
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