Page 39 - Piston Engine-Based Power Plants

Types of Reciprocating Engine 31

combustion chamber. When combustion takes place, the two pistons
are forced in opposite directions and they are then returned by bounce
chambers. Operation of this type engine requires the two pistons to be
synchronised mechanically so that it remains balanced. This type of
engine has been used in gas generators. The symmetrical design makes
the engine vibration free.

Free piston engines are potentially more efficient than conventional
reciprocating engines but practical engines cannot yet compete. There
may be applications for these engines in hybrid vehicles and in the power
generation industry if successful implementations can be developed.

ROTARY ENGINES

There are a number of engine designs that have been called rotary
engines. One, often called a radial engine, has conventional four-stroke
cylinders but the cylinders and pistons are arranged radially around
the crankshaft. These engines always have an odd number of cylinders
driving the shaft.

A second rotary engine is essentially the same as the radial engine
but in this case it is not the crankshaft that rotates but the cylinders
and pistons and the crankcase. The engines were popular in aircraft
during the early decades of the 20th century, with the aircraft propeller
being bolted directly to the crankcase.

A final variant is the Wankel engine. This uses an eccentric, trian-
gular rotary piston or rotor inside an oval chamber, as shown in
Fig. 3.5. The rotor moves within the chamber in such a way that it
creates three individual combustion chambers. As the rotor turns, the
volume of each changes. Each individual chamber is filled with a fuel
and air mixture, the chamber becomes compressed and then the
fuel air mixture is ignited, and the expansion of the hot gases turns
the rotor to allow this chamber to expand again and expel the exhaust
gases. The engine has been used in motor vehicles and in some other
applications but has never been widely adopted.

ENGINE SIZE AND ENGINE SPEED

The speed at which a piston engine operates will usually depend on its
size. In general small units operate at the highest shaft rotational speed