Page 60 - Advanced thermodynamics for engineers

44 CHAPTER 3 ENGINE CYCLES AND THEIR EFFICIENCIES

However, it is possible to define ‘engines’ which can be analysed by endoreversible cycles: these
‘engines’ replace the energy flows brought about by gas flows and combustion by heat transfer pro-
cesses. Such cycles will be described below.
There are three air-standard cycles:
• constant volume ‘combustion’ (Otto),
• constant pressure ‘combustion’ (Diesel) and
• dual ‘combustion’ – this is a combination of constant volume and constant pressure
combustion, and results in a slightly more realistic cycle.
These are the heat engine equivalent of the reciprocating engine and different from the actual
engine cycle because:
• the cycle is a closed one with heat transfer;
• the working fluid does not change composition;
• the energy addition obeys closely defined rules, e.g. constant volume energy addition;
• the rates of heat release (energy addition) are unrealistic;
• indicated work outputs are evaluated.
The effect of two differences between ideal and real cycles will be examined in Chapter 16:

1. frictional losses and
2. the finite rate of heat release.

3.2.1 OTTO CYCLE
The Otto cycle is an air-standard cycle which approximates the processes in petrol or diesel engines. It
is based on constant volume heat addition (combustion) and heat rejection processes, and isentropic
compression and expansion. The diagram is shown in Fig. 3.10, where it is superimposed on an actual
p–V diagram for a diesel engine.
The actual p–V diagram for an engine has rounded corners because of the processes of combustion
take place at a finite rate. The Otto cycle has sharp corners because the ‘combustion’ is switched on and
off instantaneously. It can be seen from Fig. 3.10 that the area of the Otto cycle is larger than that of the
actual cycle, and this has to be taken into account when analysing engine cycles – the actual engine
cycle will always produce less work output than the Otto cycle.
A typical engine ‘cycle’ is defined in Fig. 3.11. It consists of a compression stroke (Fig. 3.11(a)),
followed by a period of combustion close to top dead centre (tdc) (Fig. 3.11(b)) and then by expansion
(Fig. 3.11(c)). These two strokes form the power producing processes, but afterwards the products of
combustion have to be replaced by fresh air. This is symbolised in Fig. 3.11(d), where the exhaust
valve is open at the beginning of the exhaust stroke. In a four-stroke engine the piston executes two
complete revolutions of the crankshaft, and uses two strokes while the gas is pushed out by the piston
on the up stroke, and then the intake valve is opened to enable air to be induced. In a two-stroke engine
the intake and exhaust strokes occur at the end and the beginning of the expansion and compression
strokes, respectively. These processes are called the gas exchange processes, and are one of the main
reasons why real engines are not heat engines. The other reason is the combustion process, when the air
is used to burn the fuel. This process of combustion means that the fluid in the engine cannot undergo a
cycle.