Internal Combustion Engines

Página 1

For a given size engine operating at a particular speed, the

two-stroke engine will be more powerful than a four-stroke

engine since the two-stroke engine has twice as many power

strokes per unit time. Unfortunately the efficiency of a two-

stroke engine is likely to be lower than that of a four-stroke

engine.

The problem with two-stroke engines is ensuring that the induction and exhaust processes occur efficiently, without

suffering charge dilution by the exhaust gas residuals. The

spark ignition engine is particularly troublesome, since at part

throttle operation the crankcase pressure can be less than

atmospheric pressure. This leads to poor scavenging of the

exhaust gases, and a rich air/fuel mixture becomes necessary

for all conditions, with an ensuing low efficiency (see chapter

4, section 4.1).

These problems can be overcome in two-stroke direct

injection engines by supercharging, so that the air pressure at

inlet to the crankcase is greater than the exhaust back-

pressure. This ensures that when the transfer port is opened,

efficient scavenging occurs; if some air passes straight through

the engine, it does not lower the efficiency since no fuel has so

far been injected.