Page 62 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 41

includes a crankcase heater to keep the engine warm. A crankcase heater is considered
mandatory for a CHP system serving as an emergency power system.
Reciprocating engines are classified into three speed categories, low, medium, and
high speed, that see engine speeds range from 60 to 275 rpm for low-speed engines
and 1000 to 3600 rpm for high-speed engines. Spark ignition engine generators are
designed to operate on fuels such as natural gas or fuel oils but can burn other fuels.
One other fuel is methane fuel derived from decomposition of organic matter as
mentioned earlier. In Brazil, engines burn 100 percent alcohol. In the United States, a
gasoline-alcohol mixture is sometimes used (see Chap. 25). During World War II, the
Japanese powered their auto engines with coking gas derived from heating coal or
wood in an airless chamber and driving off gases which were piped to the engine fuel-
air intake. In short, almost any gaseous fuel can be burned; however, some fuels will
damage the engine and scrubbers/filters may be required to meet engine manufac-
turer warranty requirements.
Spark ignition engines get their name from the electrical charge, or spark, that is
added at the end of the compression stroke that ignites the fuel-air mixture to start the
power stroke.
Compression ignition engines differ from spark ignition engines, in that there is no
spark added to the air-fuel mixture to start combustion. Instead, the intake air is com-
pressed by the piston’s motion inside the cylinder. A compression ignition engine uses
very high compression ratio which heats the air in the chamber to a point high enough
to ignite the fuel. At the top of the compression stroke fuel is injected into the hot com-
pressed air in the combustion chamber and spontaneous ignition occurs. The heat of
combustion develops very high pressure which drives the piston in an expansion stroke.
When a compression engine is first started a glow plug is heated by an electric source.
Once the engine is running and hot, the glow plug is no longer needed. Compression
ignition engines most commonly use diesel fuel (No. 2 oil), but can also be fueled by a
wide range of petroleum products (up to No. 6 oil). Compression ignition engines (also
referred to as diesel cycle engines) can also be fired with gaseous fuel in combination
with liquid fuel, called pilot oil, used as the ignition agent in dual fuel engines. The fuel
in a compression cycle engine needs to have a fairly high flash point to prevent ignition
until full compression is achieved at the top of the piston stroke.

Turbo- or Supercharger Power Boosters
Both spark ignition and compression ignition engines can be outfitted with turbo- or
superchargers to increase power output and, often times, to improve efficiency. As
noted above, a turbocharger is a relatively small compressor that is mounted on a com-
mon shaft with a small turbine. As hot exhaust gas enters the turbine and expands, the
turbine spins, spinning the compressor impeller at the same time. Engine intake air is
routed through the compressor to precompress the combustion air, creating a denser air
charge to enter the cylinder. A supercharger is another type of compressor that also
works to precompress the air; however, it does not rely on exhaust gas to drive the com-
pressor. Instead, superchargers are belt- or gear-driven from the engine’s crankshaft,
using a small amount of engine output power to yield a greater overall power output.
Turbochargers are the most commonly utilized precompression tools in CHP today.

Rich Burn versus Lean Burn Engines
Naturally aspirated engines must have a fuel-air ratio that is high enough to ignite in
the engine without additional heat of compression. A “rich” mixture can have too
little oxygen for complete combustion and some of the fuel energy is wasted. Also,