Читать книгу Internal Combustion Engines - Allan T. Kirkpatrick - Страница 29

The performance characteristics of three different diesel engines are compared in Table 1.1. The engines are a four‐cylinder 1.9 L automobile engine, a six‐cylinder 5.9 L truck engine, and a six‐cylinder 7.2 L military engine. Comparison of the data in the table indicates that the performance characteristics of piston engines are remarkably similar when scaled to be size independent. As Table 1.1 illustrates, the mean piston speed is about 12 m/s, the bmep is about 15 bar, the power/volume is about 40 kW/L, and the power/mass about 0.5 kW/kg for the three engines.

Table 1.1 Performance Comparison of Three Different Four‐Stroke Turbocharged Diesel Engines

	1.9 L	5.9 L	7.2 L
Parameter	Automobile	Truck	Military
# Cylinders	4	6	6
Bore (mm)	82	102	110
Stroke (mm)	90	120	127
Displacement per cylinder (L)		0.983	1.20
Power (kW)	110	242	222
Mass (kg)	200	522	647
Engine speed (rpm)	4000	3200	2400
Mean piston speed (m/s)	12.05	12.78	10.16
Bmep (bar)	17.3	15.4	15.4
Power/Volume (kW/L)	57.9	41.0	30.8
Mass/Volume (kg/L)	105	88	90
Power/Mass (kW/kg)	0.55	0.46	0.35

There is good reason for this; all engines in a given era tend to be made from similar materials. The small differences noted could be attributed to different service criteria for which the engine was designed. Advances in engine technology have allowed manufacturers to continue to increase the power/mass. The iron in engine blocks and cylinder heads has been replaced by aluminum, which has half the weight of iron, and intake manifolds are now made of composite materials. With turbocharging, engines for vehicles have also become smaller, with four‐ and six‐cylinder engines replacing six‐ and eight‐cylinder engines, respectively.

Since material stresses in an engine depend to a first order only on the bmep and mean piston speed, it follows that for the same stress limit imposed by the material, all engines should have the same bmep and mean piston speed. Finally, since the engines geometrically resemble one another independent of size, the mass per unit displacement volume is more or less independent of engine size.