Page 380 - Advanced thermodynamics for engineers

16.6 SIMULATION OF COMBUSTION IN SPARK-IGNITION ENGINES 369

100
Engine speed
80 N = 3500 rev/min
Pressure (bar) 60 N = 4500 rev/min
N = 2500 rev/min

40

20

-40 -20 0 20 40 60 80 100 120 140
Crankangle (deg atdc)
FIGURE 16.14
The effect of engine speed on the pressure–crank angle diagram.

temperature diagrams is how the flame speed varies, in terms of crank angle, with engine speed. The
actual variation of flame speed with time will be quite small, but the effect of increasing the engine
speed is to spread the heat release period over a longer crank angle interval, and hence further down the
expansion stroke. This will reduce the efficiency of the engine cycle at high speeds because the
effective expansion ratio of the gas is reduced. In this example, the flame speed factor, f f , was
maintained constant although in a real engine this might change as the speed varies. It has been shown
that the turbulence levels in an engine do increase with speed, but less than proportionally, which
means that f f will decrease slightly as engine speed increases. Because the combustion period, in terms
of crank angle, changes with engine speed the ignition timing of spark-ignition engines is related to
engine speed, either by a mechanical (fly-weight) system or by the electronic engine management
system. The timing is advanced (moved farther before tdc (btdc)) as the engine speed increases.

16.6.3 THE EFFECT OF AIR–FUEL RATIO ON ENGINE COMBUSTION
The power output ofa spark-ignitionengineisbasically controlledbythe quantity of charge taken into the
engine cylinder. This is achieved by opening and closing a throttle valve in the intake system, or by
directly changing the trapped air–fuel ratio if the engine is a GDI one. The effect of the throttle valve is to
produce a depression in the intake manifold, and reduce the density of the charge trapped in the cylinder
at inlet valve closure (ivc). The power output of a diesel engine (and a GDI one, to some extent) is
controlled by changing the amount of fuel delivered to the engine cylinder while maintaining the
quantity of air trapped in the cylinder essentially constant: it is controlled by changing the quality of the
charge in the engine cylinder. A major advantage of the diesel engine when operating at low load is that
the pumping work required to pass air through the engine is not increased, whereas the spark-ignition
engine has to ‘pump’ air from a low pressure in the inlet manifold to atmospheric pressure in the exhaust.