Page 51 - Piston Engine-Based Power Plants

Spark Ignition Engines 43

achieve complete combustion. This reduces the quantities of unburnt
hydrocarbons and carbon monoxide, both of which are products of
incomplete combustion.

Since natural gas is a gas, effective mixing with airismucheasier to
achieve than with a high vapour pressure liquid fuel such as gasoline. This
allows for smoother combustion. However problems will arise if the ratio
of air to natural gas becomes too high because then the mixture may not
ignite, or may only ignites partially, leaving unburnt gas in the exhaust.

DUAL FUEL ENGINES

Diesel engines can operate with much higher compression ratios than
spark ignition engines and this allows them to achieve higher efficien-
cies. The large disparity in efficiency between a spark ignition engine
and a diesel engine has prompted engine developers to search for a
way of achieving the efficiency of a diesel engine in spark ignition
engine. This is the origin of the dual-fuel engine which has been the
most successful of these hybrids (Fig. 4.4).
A dual fuel engine is an engine designed to burn predominantly nat-
ural gas but with a small percentage of diesel as a pilot fuel to start
ignition. The engines operate using a hybrid of the diesel and the Otto
cycles. In operation, a natural gas air mixture is admitted to the cylin-
der during the intake stroke, then compressed during the compression
stroke. At the top of the compression stoke the pilot diesel fuel is
admitted and ignites spontaneously, igniting the gas air mixture to
create the power expansion. Care has to be taken to avoid spontaneous
ignition of the natural gas air mixture, but with careful design the
engine can operate at close to the compression conditions of a
diesel engine, with a high-power output and high efficiency, yet with
the emissions close to those of a gas-fired spark ignition engine.
However efficiency tends to fall and emissions of unburnt hydrocar-
bons and carbon monoxide rise at part load.

Typical dual fuel engines operate with between 1% and 15% diesel
fuel. Since a dual fuel engine must be equipped with diesel injectors,
exactly as if it were a diesel engine, a dual fuel engine can also burn
100% diesel if necessary, though with the penalty of much higher
emissions.