Page 50 - Piston Engine-Based Power Plants

42 Piston Engine-Based Power Plants

The compression ratio of an engine is the ratio of the volume of the
cylinder at BDC divided by the volume at TDC, or the amount by
which the fuel air mixture has been compressed. Higher compression
ratios lead to greater power from the engine but also lead to a higher
gas temperature in the cylinder before ignition and this can cause spon-
taneous ignition or knocking. The compression ratio of a spark igni-
tion engine (the amount by which the air fuel mixture is compressed
within the cylinder) is normally limited to a maximum of between
9:1 and 12:1 to avoid knocking. Lean natural gas air mixtures have a
much higher resistance to knocking than stoichiometric mixtures and
can therefore tolerate higher compression ratios than gasoline.

Timing can also be used to help alleviate knocking. If the timing is
advanced, then engine ignition can take place before the mixture in the
cylinder reaches the temperature at which it will knock. In older
engines the timing was fixed mechanically but in modern engines this
can also be controlled electronically.

NATURAL GAS-FIRED SPARK IGNITION ENGINES

The natural gas-fired spark ignition engine has become popular for dis-
tributed generation and for a range of other power generation duties in
recent years. The primary reason for this is its low emission perfor-
mance compared to both gasoline engines and diesel engines.

As a fuel for a spark ignition engine, natural gas has an octane rat-
ing of around 120 130 depending on the source and composition.
This allows natural gas-fired engines to operate at higher compression
ratios than conventional gasoline engines and compression ratios of
between 12 and 15 are popular. A higher compression ratio allows
higher power to be delivered from an engine of a fixed size and this
can lead to higher efficiency.

In addition to the high compression ratio, natural gas engines can
be operated with much leaner fuel to air mixtures than gasoline
engines. The leaner mixture leads to a lower combustion temperature
which reduces efficiency compared to a gasoline engine but this is com-
pensated for by the higher compression ratio. More importantly, the
lower temperature can lead to lower production of nitrogen oxides and
the excess air and oxygen in the mixture means that it is easier to