Page 335 - Advanced thermodynamics for engineers

324 CHAPTER 15 COMBUSTION AND FLAMES

(the mixture was often enriched at high load to reduce combustion temperatures). In modern engines,
attempts are made to run the engines with weaker mixtures (i.e. lean-burn). At present, most spark-
ignition engines operate with a throttle to limit the air flow at low loads. This is known as quantita-
tive governing of the engine power because the quantity of charge entering the engine is controlled,
and this controls the power output. Throttling the engine reduces the pressure in the inlet manifold, and
effectively the engine has to pump air from the inlet manifold to the exhaust system, giving a negative
pumping work of up to 1 bar mean effective pressure (mep) (see Chapters 3 and 16). Nowadays many
spark-ignition engines operate over part of their power range with lean mixtures, and some petrol
engines operate with fuel injection directly into the cylinder but with ignition by a spark. The
advantage of lean-burn engines is that the load control can be partly by making the mixture leaner, and
partly by throttling. Engines are discussed in more detail in Chapter 16.
The power output of a diesel engine is controlled by changing the amount of fuel injected into
the cylinder – the air quantity is not controlled. This means that the overall air–fuel ratio of the
diesel engine changes with load, and the quality of the charge is controlled. This is referred to as
qualitative governing and has the benefit of not requiring throttling of the intake charge. The diesel
engine operates over a broad range of air–fuel ratios. However, the richest operating regime of a
diesel engine is usually at an air–fuel ratio of greater than 20:1, whereas the petrol engine can
achieve the stoichiometric ratio of about 15:1, or richer. This means that, even at the same engine
speed, the diesel engine will only produce about 70% of the power output of the petrol engine.
However, the lack of throttling, higher compression ratio and shorter combustion period mean that
the diesel engine has a higher thermal efficiency than the petrol engine. (Note: the efficiency of
engines should be compared on the basis of the specific fuel consumption (kg/kWh) rather than miles
per gallon because the calorific values (kJ/kg) of diesel fuel and petrol are approximately the same,
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but the calorific value of diesel fuel in kJ/m is about 11% greater than that of petrol because the fuel
density is about 11% greater. A diesel vehicle which achieves only 11% more miles/gallon than a
petrol engine is not more efficient!) Gas turbine combustion chambers and boilers also control their
output in a qualitative manner, by varying the air–fuel ratio: gas turbines are discussed in Chapter 17.
An important feature of a flame is its rate of progress through the mixture: referred to as the flame
speed. Flame speed is easiest understood when related to a premixed laminar flame, but the concept of
flame speed applies in all premixed combustion.

15.2 THERMODYNAMICS OF COMBUSTION

When fuel and air are mixed together they are in metastable equilibrium, as depicted schematically in
Fig. 15.1. This means that the Gibbs energy of the reactants is not at the minimum, or equilibrium,
value but is at a higher level. Fortunately it is still possible to apply the laws of thermodynamics to the
metastable state, and hence the energies of reactants can be calculated in the usual way. Even though
the reactants are not at the minimum Gibbs energy spontaneous change does not usually occur: it is
necessary to provide a certain amount of energy to the mixture to initiate the combustion – basically to
‘ionise’ the fuel constituents. Once the fuel constituents (usually carbon and hydrogen) have been
‘ionised’ they will combine with the oxygen in the air to form compounds with a lower Gibbs energy
than the reactants: these are called the products (usually carbon dioxide, water, etc.). In the case of the
spark-ignition engine the ionisation energy is provided by the spark, which ignites a small kernel of the