11-10-2012, 10:51 AM
Computer simulation of a four stroke spark ignition engine
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Abstract
This paper introduces the preliminary simulation of a four stroke spark ignition engine. An arbitrary
heat release formula was used to predict the cylinder pressure, which was used to find the indicated work
done. The heat transfer from the cylinder, friction and pumping losses also were taken into account to
predict the brake mean effective pressure, brake thermal efficiency and brake specific fuel consumption.
Most of the parameters that can affect the performance of four stroke spark ignition engines, such as
equivalence ratio, spark timing, heat release rate, compression ratio, compression index and expansion
index are studied. The use of a real combustion curve has a profound influence on the similarity of the
pressure–volume profile to that seen for the real engine. The modeling process is obviously getting closer to
reality and is now worth pursuing as a design aid.
Introduction
It is very clear that the ideal Otto cycle is ineffective in simulating combustion in a spark
ignition engine, compared to data measured in a real engine. The problem is simply that an engine
cannot conduct combustion at constant volume, i.e. instantaneously at TDC, because a real
burning process takes time, the piston keeps moving and the cylinder volume changes. If this latter
problem could be remedied by keeping the piston stationary at TDC while combustion took place
and then moving it down the power stroke when all is burned, the imep and power would increase
by some 50%.
Total friction work for SI engine
The total friction work consists of three major components. These components are the pumping
work, Wp, which is net work per cycle done by the piston on the in-cylinder gases during the inlet
and exhaust strokes, rubbing friction work, Wrf , which is the work per cycle dissipated in overcoming
the friction due to relative motion of adjacent components within the engine, and accessory
work, Wa, which is the work per cycle required to drive the engine accessories, e.g., pumps,
fan, generator etc.
Effects of spark timing
Spark timing is the major operating variable that affects spark ignition engine performance,
efficiency and emissions at any given load and speed. The pressure versus crank angle curves
shown in Fig. 15 allow us to understand why engine torque (at given speed and intake manifold
conditions) varies as spark timing is varied relative to TDC. If combustion starts too early in the
cycle, the work transfer from the piston to the gases in the cylinder at the end of the compression
stroke is too large. If the combustion starts too late, the peak cylinder pressure is reduced, and the
expansion stroke work transfer from the gas to the piston decreases. Thus, there exists a particular
spark timing that gives maximum engine torque at fixed speed, mixture composition and flow
rate. It is referred to as MBT or maximum brake torque timing.
Effects of combustion duration
Increased turbulence in the unburned mixture at the time of combustion increases the burning
rate. Turbulence is usually increased by generating swirl during the induction process. Previous
work indicates that both the durations of the early stage and main stage of the burning process
decrease when the turbulent velocity at the start of combustion increased. The faster combustion
process comes primarily from the higher turbulence intensity, however, the decreases, characteristic
of the turbulence scale, that accompany the increases in turbulence are also significant, since they
result in a shorter characteristic burning time. It is important to note that the fuel conversion efficiency
of higher turbulence chambers at the same operating conditions can be lower than for normal
chambers, despite the faster burn rates, due to the higher heat transfer that accompanies the higher
in-cylinder velocities. The pressure versus crank angle curves for different values of combustion
duration are shown in Fig. 19. Figs. 20–22 show the effects of variations in combustion duration on
indicated and brake mean effective pressures, thermal efficiencies and specific fuel consumptions,
respectively. It can be seen that the optimum value of the combustion duration is around 30.
Conclusion
The present work introduces and simulates the combustion process of a four stroke, naturally
aspirated, spark ignition engine that results from the analysis of measured cylinder pressure data
and that relates the happenings in the combustion chamber to the burning of a real fuel with
respect to time. It can be seen that the simulation of the ideal Otto cycle, the cylinder pressure
diagrams and the predicted overall performance parameters begin to coincide with measured data
more closely.