Page 381 - Advanced thermodynamics for engineers

370 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

Table 16.4 Equivalence Ratios
Equivalence ratio 1 l ¼ 0.6 (lean) Airefuel ratio, ε ¼ 25.20
Equivalence ratio 2 l ¼ 0.7 (lean) Airefuel ratio, ε ¼ 21.60
Equivalence ratio 3 l ¼ 0.8 (lean) Airefuel ratio, ε ¼ 18.90
Equivalence ratio 4 l ¼ 0.9 (lean) Airefuel ratio, ε ¼ 16.80
Baseline equivalence ratio l [ 1.0 (stoichiometric) Airefuel ratio, ε [ 15.12
Equivalence ratio 5 l ¼ 1.1 (rich) Airefuel ratio, ε ¼ 13.75
Equivalence ratio 6 l ¼ 1.2 (rich) Airefuel ratio, ε ¼ 12.60
Equivalence ratio 7 l ¼ 1.3 (rich) Airefuel ratio, ε ¼ 11.63

It is possible that future, lean-burn spark-ignition engines (GDI) will employ a combination of
quantitative and qualitative governing. Advantages of operating spark-ignition engine in the lean-burn
mode are:
• lower pumping losses at low load, giving improved low-load fuel economy;
• lower NO x emissions.

The effect of air–fuel ratio on flame speed was discussed in Chapter 15. Its influence on engine
performance, defined by the equivalence ratio (l ¼ ε stoic /ε), has been investigated over the range
shown in Table 16.4. The range of equivalence ratios goes from 0.6 < l < 1.3, where 0.6 is a weak
mixture with an air–fuel ratio of 25.2, and 1.3 is a rich mixture with an air–fuel ratio of 11.63. The
weakest mixture is just about being achieved in modern vehicles, but requires a high level of air motion
in the cylinder to achieve complete combustion and to avoid the possibility of misfire.
It can be seen from Fig. 16.15 that equivalence ratio has a large effect on the pressure–crank angle
diagram for the engine cycles. The baseline data achieved a peak pressure of 91.76 bar whilst the
slightly richer cycle, with l ¼ 1.1, achieved 93.73 bar, and can be seen to produce more work output.
This aligns with experimental experience, where the maximum power output of a spark-ignition en-
gine occurs slightly to the rich side of stoichiometric. Examination of Fig. 16.16 shows that the
maximum temperature reached with this cycle is almost the same as the stoichiometric one. As the
mixture is made richer the maximum pressure achieved in the cycle decreases, and the combustion
period increases. Both these effects are detrimental to the fuel economy of the engine, in addition to the
inability of the air to fully oxidise all the fuel in the charge.
When the mixture is made weaker the peak pressure achieved in the cycle reduces, and so does the
area of the p–V diagram. Hence, the weaker cycles are producing less power. This means that the
power output of the engine can be controlled by the air–fuel ratio rather than using the throttle. There
are other advantages in this approach, because the peak cycle temperature is reduced significantly at
high air–fuel ratios, and this will have a beneficial effect on the emissions of NO x . Hence, a spark-
ignition engine operating in the lean-burn regime looks an attractive proposition for meeting emis-
sions legislation at low load. There is, however, a problem with running engines in the lean region, and
this is the significant reduction in flame speed that occurs. The flame speed is highest at around
stoichiometric mixture strength (see Figs 15.5 and 15.6), but does not change much within a small
band of l on either side. However, by the time l has reduced to 0.8 the flame speed will have gone
down considerably, and when l is 0.7 and less the flame does not travel quickly enough to burn all the