Four-Cycle Engine

an internal combustion engine with an operating cycle consisting of four piston strokes that occur in the course of two revolutions of the crankshaft.

Figure 1. Operation of a carburetor-type four-cycle engine: (a) intake, (b) compression, (c) power, (d) exhaust; (p) cylinder pressure, (V) effective cylinder volume, (S) distance traveled by piston

The first two strokes, the intake and compression strokes, occur during the first revolution of the crankshaft. The intake of the fuel mixture in a carburetor engine or a spark-ignition engine or of the air in a diesel engine occurs as the piston moves from top dead center to bottom dead center. The compression of the fuel mixture or of the air is accomplished as the piston moves from bottom dead center to top dead center.

The power stroke, in which the piston moves as it does during intake, and the exhaust stroke, in which the piston moves as it does in compression, occur in the second revolution of the crankshaft. The power stroke, during which the fuel mixture undergoes combustion and expansion, results from the pressure of the combustion products on the piston. During the exhaust stroke, the final stroke, the combustion products are removed from the cylinder.

The intake, compression, and exhaust strokes are accomplished by means of the kinetic energy stored during the power stroke in the crankshaft and in the parts connected to it when the chemical energy of the fuel is converted into the mechanical energy of the moving parts of the engine.

Lasting for only a fraction of a second, combustion is accompanied by a rapid increase in the forces acting on the cylinder head, the piston, and other engine parts. Consequently, the weight of the moving parts must be increased in order to strengthen them. An intrinsic drawback of the piston engine is that this increase in weight is accompanied by an increase in the inertial force on the moving parts.

Figure 1 shows the dependence of the pressure p in the cylinder of a four-cycle engine on piston position in each stroke.

The power stroke in a one-cylinder four-cycle engine is accompanied by three nonpower strokes. In a two-cycle engine, however, there is only one nonpower stroke. Consequently, a one-cylinder four-cycle engine runs rougher than a one-cylinder two-cycle engine. For smoother running, four-cycle engines are manufactured with large flywheels or with several cylinders. The operating cycle of a four-stroke rotary-piston engine is analogous to the operating cycle of the four-cycle engine, with the exception that, in the rotary engine, the cycle is completed in one rotor revolution.