Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The position of the peak of pre-mixed burning is at z° btdc, where:
The value of the coefficient, k\, is recommended as:
1251
z° = 4.1206 - —5- rpm (4.3.44)
1(T
„„«„« 1.0618 6.667 2
ki = 0.81255 + r-rpm o-rpm 2 (4.3.45)
10 4 10 8
The value of the coefficient, k2, is recommended as:
,,„„„ 8.5053 1.6167 2
k2 = 1.6377 3— rpm + =- rpm 2 (4.3.46)
10 4 10 7
It will be observed from Fig. 4.8(a) that the intervals for each period are fixed at 3°, 2°,
10° and 20°, i.e., a total burn period, b°, of just 35°, which is rapid indeed. In other words, the
phasing of the entire diagram is controlled by the value of z° with respect to the tdc position.
The combustion in DI engines is heavily influenced by swirl of the air with respect to the
injected fuel spray and droplets. In opposed piston uniflow-scavenged engines, such swirl is
easily arranged; indeed, too easily arranged and often overdone. In loop-scavenged units, it
cannot, for this would deteriorate the quality of the scavenging process. However, when the
fuel injection rate is optimized for the particular air flow pattern within a particular combustion
chamber geometry, the optimized heat release rate curve tends to be very similar to that
shown in Fig. 4.8(a), after the theory in Eqs. 4.3.44-46 has been applied.
4.3.7.2 The indirect injection (IDI) diesel engine
A sketch of a typical combustion chamber for such an engine is shown in Fig. 4.9. This
shape, and particularly the cut-out on the piston crown, is often referred to as a Ricardo
Comet design. In practice, many other IDI designs are employed [4.32].
Combustion in IDI diesel engines is characterized by less rapid burning around the tdc
position, by comparison with the DI design, and a sketch of a typical heat release rate curve is
shown in Fig. 4.8(b). As both Figs. 4.8(a) and (b) are drawn to scale, the slower combustion in
the IDI engine can be seen clearly. By definition, this deviation away from constant volume
combustion is less efficient, in practice by some 10% of the thermal efficiency of the DI unit.
It is also less noisy as the rates of pressure rise are reduced. It is also less efficient because of
the air pumping loss engendered by the piston motion into, and by higher pressure burned
charge post combustion from, the combustion chamber where the fuel is injected. Due to the
pressure drop, and hence temperature drop, into the combustion chamber during compression,
it is necessary to employ a glow plug (heater) to assist in starting the engine.
You may well ask the purpose of this engine, if its thermal efficiency is so compromised.
The answer lies in the high-speed swirl created through the throat of area, At, into the side
chamber. The rotational speed of swirl, Nsw, is designed to be some 20 to 25 times the rotational
speed of the engine and is ensured by designing the area of the throat, At, to be some 1.0
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