13 Air standard cycles

It will be clear from the foregoing sections that the real processes in the diesel engine cylinder, particularly those of fuel preparation, combustion and gas exchange are extremely complex and require sophisticated computational techniques which are discussed in a number of specialist texts.',2-3

Air standard cycles which are discussed in most elementary textbooks, provide a useful basis for comparing actual engine performance expressed in terms of indicated mean effective pressure (pjnd, eqn (1.1) and indicated thermal efficiency (r)„ eqn (1,4a) with corresponding values for highly idealized cycles, based on certain drastic simplifying assumptions as follows:

(a) the mass of working fluid remains constant throughout the cycle, i.e. gas exchange and fuel addition are ignored;

(b) the working fluid throughout the cycle is pure air treated as a perfect gas;

(c) the combustion and gas exchange processes are replaced by external heat transfer to or from the working fluid under idealized, e.g. constant volume or constant pressure conditions;

(d) compression and expansion processes are treated as adiabatic and reversible, i.e. heat transfer and friction effects are completely neglected;

(e) at any point of the working cycle, cylinder charge pressure and temperature are completely uniform, i.e. spatial variations in their values as for instance during combustion or scavenging, are completely neglected.

The most commonly used air-standard cycles are as follows (Figures 1.7a, b and c):

(a) The constant pressure or diesel cycle (Figure 1.7a)

Here combustion is simulated by constant pressure heat addition (2-3), and blowdown, followed by scavenge, by constant volume heat rejection 4-1. Compression 1-2 and expansion 3-4 follow the isentropic state relationships for a perfect gas. This particular cycle has, in the past, been used as a reference cycle for the 'classical' Diesel engine with air blast injection giving a rather long injection and hence heat release period, corresponding to 2-3. It has, however, little relevance to the modern diesel cycle.

(b) The constant volume or Otto cycle (Figure 1.7b)

Here combustion is simulated by constant volume heat release

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