C H A P T E R 6
Perhaps the best-known engine in the world is the reciprocating internal combustion (IC) engine. Virtually every person who has driven an automobile or pushed a power lawnmower has used one. By far the most widely used IC engine is the spark-ignition gasoline engine, which takes us to school and work and on pleasure jaunts. Although others had made significant contributions, Niklaus Otto is generally credited with the invention of the engine and with the statement of its theoretical cycle. Another important engine is the reciprocating engine that made the name of Rudolf Diesel famous. The Diesel engine, the workhorse of the heavy truck industry, is widely used in industrial power and marine applications. It replaced the reciprocating steam engine in railroad locomotives about fifty years ago and remains dominant in that role today.
The piston, cylinder, crank, and connecting rod provide the geometric basis of the reciprocating engine. While two-stroke-cycle engines are in use and of continuing interest, the discussion here will emphasize the more widely applied four-stroke-cycle engine. In this engine the piston undergoes two mechanical cycles for each thermodynamic cycle. The intake and compression processes occur in the first two strokes, and the power and exhaust processes in the last two. These processes are made possible by the crank-slider mechanism, discussed next.
6.2 The Crank-Slider Mechanism
Common to most reciprocating engines is a linkage known as a crank-slider mechanism. Diagramed in Figure 6.1, this mechanism is one of several capable of producing the straight-line, backward-and-forward motion known as reciprocating. Fundamentally, the crank-slider converts rotational motion into linear motion, or vice-versa. With a piston as the slider moving inside a fixed cylinder, the mechanism provides the vital capability of a gas engine: the ability to compress and expand a gas. Before delving into this aspect of the engine, however, let us examine the crank-slider mechanism more closely.
It is evident from Figure 6.2 that, while the crank arm rotates through 180°, the piston moves from the position known as top-center (TC) to the other extreme, called bottom-center (BC). During this period the piston travels a distance, S, called the stroke, that is twice the length of the crank.
For an angular velocity of the crank, , the crank pin A has a tangential velocity component S/2. It is evident that, at TC and at BC, the crank pin velocity component in the piston direction, and hence the piston velocity, is zero. At these points, corresponding to crank angle = 0° and 180°, the piston reverses direction. Thus as varies from 0° to 180°, the piston velocity accelerates from 0 to a maximum and then returns to 0. A similar behavior exists between 180° and 360°. The connecting rod is a two-force member; hence it is evident that there are both axial and lateral forces on the piston at crank angles other than 0° and 180°. These lateral forces are, of course, opposed by the cylinder walls. The resulting lateral force component normal to the cylinder wall gives rise to frictional forces between the piston rings and cylinder. It is evident that the normal force, and thus the frictional force, alternates from one side of the piston to the other during each cycle. Thus the piston motion presents a challenging lubrication problem for the control and reduction of both wear and energy loss.
The position of the piston with respect to the crank centerline is given by x = (S/2)cos + Lcos [ft | m] (6.1)
where yA = (S/2)sin = Lsin can be used to eliminate to obtain x/L = (S/2L)cos + [1 (S/2L)2 sin2 ]½ [dl] (6.2)
Thus, while the axial component of the motion of the crank pin is simple harmonic, xA = (S/2)cos, the motion of the piston and piston pin is more complex. It may be 226
seen from Equation (6.2), however, that as S/L...
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