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346 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

16.2 FURTHER CONSIDERATIONS OF BASIC ENGINE CYCLES
However, before getting involved in these details it is necessary to expand on some of the ideas
introduced in Chapter 3.
In chapter 3, the compression ratio was defined as the ratio of volumes (V 1 /V 2 ), see Fig. 16.1: this is
normally called the nominal compression ratio of the engine. There are actually two compression
ratios applicable to engines: the nominal compression ratio and the effective compression ratio. The
nominal compression ratio is the volume of the cylinder at bottom dead centre (bdc) divided by
the volume of the cylinder at top dead centre (tdc). The volume of the cylinder at tdc (V 2 ) is called the
clearance volume (V cl ). The ‘size’ of an engine is defined by its swept volume, which is V 1 –V 2 for each
cylinder; the capacity of an engine is its total swept volume, i.e. a 2-L engine with four cylinders has a
3
swept volume of 500 cm /cylinder. The compression ratio, r, can then be defined as
V tdc V cl þ V s V s
r ¼ ¼ ¼ þ 1 (16.1)
V bdc V cl V 2
However, the more important parameter from the point of view of the engine operation is the
effective compression ratio. This is always less than the nominal value and is defined as the volume at
inlet valve closure (ivc) divided by the clearance volume, i.e.

V ivc
r eff ¼ (16.2)
V cl
The position at which the inlet valve closes is dependent on the engine and its application. High
speed, high performance engines will normally have the point of ivc well after bdc; this limits the
effective compression ratio but improves airflow through the engine (see Winterbone and Pearson
(1999)).

180 3
160
140 constant
volume
combustion
Pressure, p / (bar) 100 compression
120
Isentropic
80
and
60
40 2 expansion
20
4
0 1
0 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030
3
Volume, V / (m )
FIGURE 16.1
p–V diagrams for an ‘Otto’ cycle engine. A standard Otto cycle; an actual engine cycle.