Page 62 - Piston Engine-Based Power Plants

54 Piston Engine-Based Power Plants

change, depending upon the engine load, engine temperature and other
factors. In order to maintain constant speed the engines are equipped
with a feedback system that uses the engine speed to help modulate the
fuel feed. In most diesel engines built before the 1980s this was carried
out by a mechanical device called a governor. Modern engines use an
electronic control unit which monitors and controls all engine
parameters.

TWO-STROKE DIESEL ENGINES

Diesel engines are normally classified by their speed. Slow-speed
engines operate at up to 275 rpm, medium-speed diesel engines have
speeds of 275 1000 rpm and high-speed diesel engines operate at
above 1000 rpm. These latter are the most common diesel engines and
can be found in many applications including vehicles and smaller
power generation applications. While the higher speed diesel engines
are usually four stroke, many of the very large, slow-speed engines are
two-stroke engines.
The two-stroke engine has a higher power to weight ratio than a
four stroke because it has one power stroke for every revolution of the
shaft rather than one for every two revolutions in the four-stroke
engine. In small engines the two-stroke cycle is usually considered
less efficient but the large slow-speed diesel engines are as efficient or
more efficient than their four-stroke cousins.

Large two-stroke engines are extremely heavy and this results in
their slow-speed engines often operate at 100 rpm or less. They have
tall cylinders and this permits a long piston stroke. A long stroke com-
bined with the slow-engine speed enables more precise control over
combustion and this in turn enables a more efficient engine to be built.
Very large two-stroke engines are also relatively more powerful than a
four stroke; a large two-stroke engine is roughly 80% more powerful
than a similarly sized four stroke. (This advantage reduces as the size
of the engine decreases.)

Many large two-stroke diesel engines are designed for ship propul-
sion. In this application the slow speed means that the engine crank-
shaft can be connected directly to the propeller. In power applications
the shaft is geared up to drive a generator.