Design and Simulation of Two Stroke Engines

Chapter 5 - Computer Modeling of Engines
A three-cylinder engine
The illustration is for the engine designed by Thornhill [5.24] and it is a V6 3.2-liter
engine with a 90 mm bore and a 82 mm stroke. One bank of three cylinders is simulated, each
one firing at 120° intervals, with a 1-3-2 firing order. The supercharger included within the
simulation is a Roots blower, geared at 1.5 times off the engine crankshaft, and the full map of
its characteristics for mass flow rate, air temperature rise, and isentropic efficiency, as a function
of pressure ratio and rotational speed, is incorporated into the simulation. The physical
geometry of the engine is given in some detail by Thornhill [5.24] and the salient points of
that geometry are repeated here so as to give meaning to the discussion.
The exhaust port opens at 95° atdc and the scavenge ports at 120° atdc. The exhaust port
is simulated as a 55-mm-wide port with 16 mm corner radii. The scavenge ports are simulated
as four ports with a total effective width of 74 mm and with 4 mm corner radii. The trapped
compression ratio is 7.0. The exhaust system is as sketched in Fig. 5.8(b) with the lengths, L\
and L3, being 200 mm, and length, L2, being 100 mm; the diameters, di, 62, and d3, are each
50 mm. The downpipe from the manifold, 64, is 55 mm in diameter and the length, L4, is 1 m
before the 10-liter expansion box, i.e., volume, Veb.
An intercooler is employed between the blower and the engine. The scavenging model is
assumed to be that appropriate to cylinder YAM1, as shown in Fig. 3.16. The ignition timing
is at 25° btdc, with an air-fuel ratio of 14.5 on unleaded gasoline, for a simulation conducted
at 3500 rpm to show the effects of the compact manifold on the charging characteristics of the
engine. The summary of the overall simulation for the 3.2-liter V6 engine at 3500 rpm shows
the performance characteristics to be: power 136 kW, bmep 7.47 bar, bsfc 245 g/kWh, DR
0.80, SE 0.877, TE 0.77, CE 0.615, rjm 0.88, air flow 626 kg/h, fuel flow 33.4 kg/h, fuel
injected per shot 26.5 mg, peak cylinder pressure 46 bar, peak cylinder temperature 2560 K,
position of peak pressure 18° atdc, mean blower supply pressure 1.23 bar, and mean air supply
temperature 38°C. Although they are not the primary objective of the discussion, the high
specific output and the low specific fuel consumption of the engine are quite evident. The
discussion in Chapter 7 focuses more clearly on the benefits of direct in-cylinder fuel injection
to reduce hydrocarbon emissions and fuel consumption in two-stroke engines.
The principal objective is to show the influence of the compact manifold on the charging
characteristics of the engine, and some of the fundamental design requirements of a blowerscavenged
engine by comparison with that generated by a crankcase compression pump. This
is summarized in Figs. 5.36(a)-(c) for the pressures in the cylinder during the open cycle, the
pressures at the cylinder in the exhaust and scavenge ports, and the charging efficiency which
this produces. The three sub-figures show these effects for cylinders 1-3, respectively.
The effect of blower scavenging
The principal difference between blower scavenging and crankcase compression scavenging
is that blower scavenging normally lasts for the majority of the duration that the scavenge
ports are open. This can seen clearly in Fig. 5.36(a), where the scavenge port pressure is
virtually constant and almost always above the cylinder pressure. The charging efficiency
rises to a plateau by 210° atdc, which is then held to the trapping point. This should be
contrasted with the chainsaw in Fig. 5.20, where the sharper scavenging process reaches its
ceiling at this point and then spills nearly 10% of its contents out of the cylinder.
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