Page 47 - Piston Engine-Based Power Plants

Spark Ignition Engines 39

The composition of the fuel air mixture in the cylinder may be close
to the stoichiometric ratio required for complete combustion of the fuel
in air but more often it will contain a significant excess of air. In com-
mon with all thermodynamic heat engines, the efficiency that a recipro-
cating engine can achieve increases with the temperature of the working
fluid, in this case the combustion gases in the cylinder. For a spark igni-
tion engine the highest cylinder temperature is reached when the air to
fuel ratio is around 15:1, the ratio at which a stoichiometric amount of
oxygen is available to react with all the carbon and hydrogen within the
fuel. An engine which operates with this air to fuel mixture is described
as a rich-burn engine. A rich mixture leads to the highest temperature,
and the highest engine power, but also leads to the greatest formation of
nitrogen oxides (NO x ) as well a significant amounts of carbon monox-
ide and unburnt hydrocarbon particles as a result incomplete combus-
tion of some of the fuel. Under most circumstances therefore, engines
operating continuously on a rich mixture will require emission control
systems to limit release of these potential pollutants.
If engine emissions are to be reduced during combustion, then the
combustion temperature must be lowered and a greater amount of oxy-
gen introduced to ensure complete combustion of the fuel. Such engines
are described as lean-burn engines and can operate with an air to fuel
ratio of between 20:1 and 50:1 depending upon the fuel, significantly
higher than in the rich-burn engine. The greater proportion of air lowers
the overall combustion temperature (there will be relatively less fuel
entering the combustion chamber in the lean mixture), reducing the pro-
duction of nitrogen oxides from nitrogen in air and providing the condi-
tions for much more complete combustion of the fuel. This will reduce
the amounts of carbon monoxide and unburnt hydrocarbons in the
exhaust gases. Against this, the lower temperature reduces overall effi-
ciency and power. Lean-burn engines achieve a typical efficiency of
1
only 28% (LHV), compared to up to 42% (LHV) for a rich-burn
engine. An engine tuned for maximum efficiency will produce roughly
twice as much NO x as one tuned for low emissions. Typical NO x emis-
sion levels for spark ignition engines are 45 150 ppmV.

1
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.