Page 385 - Advanced thermodynamics for engineers

374 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

120
Flame factor, ff =4.0
100
Flame factor, ff =5.0
Flame factor, ff =3.0
80
Pressure (bar) 60 40

20

-50 0 50 100 150
Crankangle (deg atdc)
FIGURE 16.19
The variation of cylinder pressure with flame speed factor, f f .

combustion have come about by the increase in the value of f f through the use of cylinder head and
valve arrangements which enhance the turbulence in the cylinder during combustion. Such an
improvement in the value of f f is particularly necessary to achieve good combustion with lean mixtures
and high levels of egr (see section 16.6.6).

16.6.6 EFFECT OF RESIDUALS FRACTION (OR EGR) ON ENGINE COMBUSTION
The residuals, the products of combustion from a previous cycle, which exist inside an engine cylinder
when the inlet valve closes can come from the following sources:
• incomplete scavenge of the engine cylinder during the gas exchange process;
• back flow through the exhaust valve at the end of the gas exchange process; and
• exhaust gas recirculated from the exhaust manifold to the inlet with the express purpose of
controlling emissions.

In the first two cases, the exhaust gases will be hot because they will not have undergone any form
of cooling or heat transfer. In the last case the gases could be cold, having passed through a heat
exchanger on their way from the exhaust pipe to the inlet manifold.
The effect of egr is to change the composition of the charge before combustion. The exhaust gases
primarily consist of CO 2 ,H 2 O, and N 2 , and this mixture contains a significant quantity of triatomic gases.
These have a higher value of c p and a lower value of k, which means that the index of compression is
reduced and the temperature at the end of compression is also decreased. This has the overall effect of
reducing the temperatures achieved during combustion, and has a significant effect on the production of
NO x . The effect of increasing the cold residual fraction in the cylinder at ivc is shown in Figs 16.20 and
16.21. The residual fraction was increased from 5% (0.05) to 15% (0.15) for the cases shown.
Figure 16.20 shows how the increase in residual fraction reduces the pressures achieved in the cylinder,
and the impact this will have on power output. It can be seen from Fig. 16.21 that the egr has reduced the
peak temperature by about 250 K (from 2885.8 to 2643.7 K), and has resulted in a significant reduction
in the period the temperature is above 2000 K – an important factor in the production of NO x .
These effects have come about for two reasons:
• the addition of exhaust gas to the charge has changed the gas properties, as discussed above;
• the egr has effectively added more inert gas to the charge, and reduced the amount of fuel that can
be burned to maintain the stoichiometric air–fuel ratio (based on the oxygen in the charge).