Page 59 - Piston Engine-Based Power Plants

Diesel Engines 51

Table 5.1 Four-Stroke Engine Performance Parameters
Diesel Engine Spark Ignition Engine
Typical size range 1 kW to 80 MW 1 kW to 6.5 MW
Efficiency 30% 48% 28% 42%
Compression ratio 14:1 to 25:1 8:1 to 12:1

typically between 14:1 and 25:1 (Table 5.1). In order to withstand the
higher pressure, the engine components in a diesel engine must be
stronger than in a spark ignition engine. This makes the engines
heavier and more expensive than gasoline engines. However it also
leads to higher efficiency so that a diesel engine can achieve 50% fuel
to energy conversion efficiency, significantly higher than a gasoline
engine.
Another ramification of the higher compression ratio is that the
combustion of fuel leads to a higher temperature in the cylinders that
would be found in a spark ignition engine. This means that a highly
efficient cooling system is required.
Fuel is introduced into the cylinders of a diesel engine using injec-
tors. These are precision components that must be able to accurately
control the amount of fuel that is injected during each cycle of the
engine. Control is important because it is the quantity of fuel that
determines the speed at which the engine runs. Fuel within the fuel sys-
tem also acts as a coolant for the fuel system and more fuel is generally
pumped around the fuel system than is needed to supply the engine.

In order to gain the highest efficiency and to ensure that the high
volumetric air requirements of the diesel engine can be met, many die-
sel engines are turbocharged or supercharged. A turbocharger is a
pump that draws air into the engine and pressurises it. The pump is
driven by an impeller that is placed in the exhaust gas stream of the
engine. This essentially captures waste energy from the engine exhaust
and uses it to boost the air input. The higher pressure air helps supply
the cylinders with the air they need. Some diesel engines have an air
input port part-way down the cylinder in addition a valve at the top of
the cylinder. This is blocked by the piston during part of the cycle but
opens as the piston falls towards BDC. Air that is swept into the cylin-
der through this port can help sweep out (scavenge) the combustion
gases. Higher pressure intake air makes this more efficient.