Internal Combustion Engines. Allan T. Kirkpatrick

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СКАЧАТЬ href="#ulink_24f48dea-fe73-5ef2-a1dd-f2452451e540">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 The iron in engine blocks and cylinder heads has been replaced by aluminum, which has half the weight of iron, and intake manifolds are now made of composite materials.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.

1.5 Engine Configurations

      Internal combustion engines can be built in many different configurations. For a given engine, using a four‐ or two‐stroke Otto or Diesel cycle, the configurations are characterized by the piston‐cylinder geometry, the inlet and exhaust valve geometry, the use of super or turbochargers, the type of fuel delivery system, and the type of cooling system. The reciprocating piston‐cylinder combination remains the dominant configuration of the internal combustion engine.

Since the invention of the internal combustion engine, many different piston‐cylinder geometries have been designed, as shown in Figure 1.13. The choice of a given arrangement depends on a number of factors and constraints, such as engine balancing and available volume. The in‐line engine is the most prevalent because it is the simplest to manufacture and maintain. The V engine is formed from two in‐line banks of cylinders set at an angle to each other, forming the letter V. A horizontally opposed or flat engine is a V engine with 180 offset piston banks. The W engine is formed from three in‐line banks of cylinders set at an angle to each other, forming the letter W. A radial engine has all of the cylinders in one plane with equal spacing between cylinder axes. Radial engines are used in air‐cooled aircraft applications since each cylinder can be cooled equally. Since the cylinders are in a plane, a master connecting rod is used for one cylinder, and articulated rods are attached to the master rod. Alternatives to the reciprocating piston‐cylinder arrangement have also been developed, such as the rotary Wankel engine, in which a triangular shaped rotor rotates eccentrically in a housing to achieve compression, ignition, and expansion of a fuel–air mixture.

Figure 1.13 Various piston‐cylinder geometries. (Adapted from Obert 1950.)

      Intake and Exhaust Valve Arrangement

Gases are admitted and expelled from the cylinders by valves that open and close at the proper times, or by ports that are uncovered or covered by the piston. There are many design variations for the intake and exhaust valve type and location. Poppet valves (see Figure 1.14) are the primary valve type used in internal combustion engines since they have excellent sealing characteristics. As shown in the pushrod configuration of Figure 1.14, springs are used to return the valve to a closed position. Sleeve and rotary valves have also been used, but do not seal the combustion chamber as well as poppet valves.

      The poppet valves can be located either in the engine block or in the cylinder head, depending on airflow, cooling, and manufacturing considerations. Older engines and small four‐stroke engines have the inlet and exhaust valves located in the block parallel to the cylinders, a configuration termed under‐head or L‐head. This configuration provides good cooling to the valves from the engine block coolant, however with undersquare (bore stroke) engines the maximum valve diameter is limited, resulting in poor volumetric efficiency. The F‐head configuration positions the intake valve in the cylinder head just above the cylinder, increasing the volumetric efficiency, with the exhaust valve remaining on the side.

      Currently, most engines use valves located in the cylinder head, an overhead or I‐head configuration, as this configuration has allows increased valve diameter resulting in good inlet and exhaust flow characteristics. СКАЧАТЬ