Page 37 - Piston Engine-Based Power Plants

Types of Reciprocating Engine 29

piston acts to close off or open ports through which fuel enters and
exhaust gases exit the engine.

The case surrounding the engine and crankshaft (the crankcase) is
often used to deliver the fuel to cylinder during each cycle. During the
power stroke, ignition takes place while the piston is at the top of the
cylinder and the air and fuel mixture is fully compressed. While in this
position, more fuel and air can enter the engine casing through a port in
the side. As the piston begins the power stroke this port is closed by the
piston which simultaneously begins to put the mixture in the casing
under pressure. Part way through the power stroke two new ports are
opened into the cylinder, one is the exhaust port and the other is a port
to admit fuel and air. When the piston reaches the bottom of the cycle
and begins to return the fuel air mixture, now pressurised compared to
the combustion gases, enter the cylinder and help force the exhaust
through the exhaust gas port. The exhaust and fuel ports close as the
piston rises within the cylinder and the cycle repeats. Two-stroke engines
are simpler than four-stroke engines because they do not require valves
as the ports are controlled by the piston. However, some engines do use
a small reed valve to control the fuel air mixture entering the engine,
with the reed opening when the pressure inside the crank case or cylin-
der is low compared to the feed pressure. There are also variations in
the way the exhaust gases are forced out of the engine (scavenging),
depending upon the gas flows and the shape of the piston head. The
engine offers a better power to weight ratio than a four-stroke engine
because power is delivered once for every cycle of the engine.
Lubricating oil for the engine is often mixed with the fuel, so orientation
of the engine is not critical for its operation. In consequence these
engines can operate at any angle, making them useful for small devices
such as chain saws. The design of larger two-stroke engines is more
sophisticated and lubrication is maintained from within the engine.
Small versions of the engine tend to be spark ignition while larger
versions, often designed for high efficiency, are diesel engines. Two-
stroke engines, particularly the smaller versions, have a narrow operat-
ing speed band so they are less flexible than four-stroke engines.

Six-Stroke Engines
In addition to the two main types of engine cycle there are also six-
stroke engine cycles. The aim of these is to increase efficiency or reduce
complexity. In the single piston six-stroke engine cycle, the normal four