past, several imported vehicles have used 2-stroke engines. As the name
implies, this engine requires only two strokes of the piston to complete
all four operations: intake, compression, power and exhaust.
Movement of the piston from BDC to TDC completes both intake and
When the piston nears TDC, the compressed air/fuel mixture is ignited,
causing an expansion of the gases. Note that the reed valve is closed
and the piston is blocking the intake port.
Expanding gases in the cylinder force the piston down, rotating the
With the piston at BDC, the intake and exhaust ports are both open,
allowing exhaust fases to leave the cylinder and air/fuel mixture to
Facts about 2-Stroke Engine
Although the 2-stroke-cycle engine is simple in
design and lightweight because it lacks a valve train, it has not been
widely used in automobiles. They tend to be less fuel efficient and have
dirtier exhaust than 4-stroke engines. Oil is often in the exhaust
stream because these engines require constant oil delivery to the
cylinders to keep the piston lubricated. Some of these engines require a
certain amount of oil to be mixed with the fuel.
In recent years, however, thanks to a revolutionary
pneumatic fuel injection system, there has been increased interest in
the 2-stroke engine. The injection system, which works something like a
spray paint gun, uses compressed air to flow highly atomized fuel
directly into the top of the combustion chamber. The system becomes the
long sought0sfter answer to the fuel economy and emissions problems of
the conventional 2-stroke engine. This fuel injection system is the
basis for orbital 2-stroke direct injection piston engine, which may be
used in cars in the future.
A small 2-stroke engine can deliver as much
horsepower, with less fuel, as a larger displacement 4-stroke engine
because in a 2-stroke engine, combustion occurs every crankshaft
revolution rather than every other revolution. This fact explains why
the orbital engines has gained promise for the future. The improvement
in fuel economy the orbital engine has achieved is due in part to a
number of mechanical design features in addition to its pneumatic direct
injection fuel system.