Page 379 - Advanced thermodynamics for engineers

368 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

3000

2500
Temperature (K) 2000

1500
burned, r = 9.6
burned, r = 10.6
1000 burned, r = 8.6
unburned, r = 9.6
500 unburned, r = 10.6
unburned, r = 8.6

-40 -20 0 20 40 60 80 100 120 140
Crankangle (deg atdc)
FIGURE 16.13
Variation of gas temperatures with compression ratio.

deduced that a higher compression ratio increases the efficiency of the cycle, as would be expected from
the Otto cycle analysis; increases the emissions of NO x , because of the higher combustion temperature;
and increases the mechanical loading on the engine structure. Also, the conditions of the gas at the end
of the combustion process (the end gas, which is trapped around the periphery of the combustion
chamber) will have a greater tendency to spontaneously ignite (detonate) at higher compression ratios
because both the pressure and temperature are higher at the crucial time.

16.6.2 EFFECT OF ENGINE SPEED ON COMBUSTION
Three values of engine speed have been considered, as shown in Table 16.3. The only parameter
changed in this case was the engine speed, and this means that the ignition timing is not the optimum
for all the speeds.
It can readily be seen, from Fig. 16.14, that the spark timing is over-advanced for the lower speed of
2500 rev/min, because the peak pressure occurs at tdc. However, the ignition timing would have to be
advanced for the engine speed of 4500 rev/min to improve the efficiency of the cycle. The effect of not
having the optimum timing is that the peak temperature is highest at 2500 rev/min, and this will
produce excessive NO x and give a tendency to detonation. The reason for this change in pressure and

Table 16.3 Engine Speeds
Lower engine speed (rev/min) 2500
Baseline engine speed (rev/min) 3500
Higher engine speed (rev/min) 4500