Page 60 - Piston Engine-Based Power Plants

52 Piston Engine-Based Power Plants

A supercharger, the alternative to a turbocharger, also acts to pump
air into the engine. However instead of being driven from the exhaust
gases it takes its power directly from the engine crankshaft.

Compression of air with a turbocharger or supercharger will raise
its temperature and this can affect the efficiency of the engine by low-
ering the density of the mixture entering the combustion chamber. In
order to avoid this a cooler, often called the aftercooler or intercooler,
is used to cool the air, which often exits the turbocharger at 200 C, to

around 30 C before it enters the engine cylinders. The intercooler often

has two stages, particularly if engine heat is being captured for cogene-
ration. This allows the first stage to provide heat at a relatively high
temperature.

The temperature inside the cylinder of a diesel engine during igni-
tion rises much higher than the temperature in a spark ignition engine.
As a consequence the production of NO x is much higher. Typical
levels are 450 1800 ppmV or 10 times higher than for the equivalent
spark ignition engine. Diesel engines therefore require extensive emis-
sion control systems if they are to comply with air quality regulations,
particularly when the units are operating in an urban environment.
Against this, diesel engines are capable of burning bio-diesel, a carbon
dioxide emission neutral fuel. This can be attractive in some situations.

The practical efficiency of the diesel engine ranges from 30%
2
(HHV ) for small engines to 48% (HHV) for the largest engines. Some
very large two-stroke diesel engines can reach higher efficiencies still
with the addition of a steam turbine bottoming cycle. Higher pressures
and temperatures could conceivably produce yet higher efficiency but
as with other types of thermodynamic engine, materials are the limiting
factor. Very high combustion temperatures and pressures require
exotic materials that are able to withstand them without deforming.
These are much more expensive than traditional steels and increase the
cost of the engine significantly, undercutting its economic advantage.

2
The energy content of a fuel may be expressed as either the higher heating value (HHV) or the
lower heating value (LHV). The HHV represents the energy released when the fuel is burned and
all the products of the combustion process are then cooled to 25 C. This energy then includes the

latent heat of vaporisation released when any water produced by combustion of, for example, nat-
ural gas is condensed to room temperature. The LHV does not include this latent heat and is
hence around 10% lower than the HHV in the case of natural gas.