Nomenclature of engine parts

Pistons [84]. Pistons receive the energy or force resulting from the combustion of fuel within the cylinders. As the pistons move downward, they transmit this energy through a connecting rod to the crankshaft.

Piston Rings [85]. Piston rings are used to maintain a pressure-tight seal between the moving piston and the cylinder wall. Piston rings also provide a means of conducting heat away from the head of the piston, and they are designed to prevent oil from entering the combustion chamber. They generally are made of cast gray iron, although steel is sometimes used. Most piston rings are classified as either “compression” or “oil-control” rings. The number and type of piston rings used is determined by the requirements of a particular engine.

Piston Pins [86]. A piston pin, sometimes called the wrist pin, connects the piston to the upper end of a connecting rod. The piston pin is fitted into accurately bored holes located in the piston bosses. The upper end of the connecting rod rides on the central portion of the pin between the two piston bosses. The piston pin rides on bearing surfaces located either in the piston bosses, the connecting rod, or both. Several types of retaining devices are employed to prevent endwise movement of the piston pin.

Connecting Rod [87]. A connecting rod, attached to the piston by means of the piston pin, converts the reciprocating (up and down) motion of the piston to a rotary motion of the crankshaft. Connecting rods are usually drop-forged [88] from alloy steels and are made with an I-beam [89] cross section. The upper or small end of the connecting rod usually contains a bushing or clamp or the piston pin. The lower end of the connecting rod is split to permit assembly to the crankshaft and contains a journal bearing.

Crankshaft [90]. The crankshaft transforms the power it receives from the pistons and connecting rods into a rotary motion, returning the piston to the top of the cylinder. The crankshaft is provided with journals which rotate in bearings located in the engine crankcase. The crankshaft has one or more crank arms along its length, the number depending upon the design of the engine and the number of cylinders. The journals, between the crank arms provide bearing surfaces for the large, split end of the connecting rod. Crankshafts are either forged or cast from alloy steels and often have counterbalances located opposite the crank arms to assist in reducing main-bearing loads and to improve engine smoothness. A flywheel bolted to a flange on the crankshaft serves to smooth out the flow of power from the engine.

Engine bearings [91]. The rotating parts of an engine generally are supported in plain bearings, the journals turning within a bearing of antifriction metal. The antifriction metal employed in engine bearings is an alloy such as babbitt [92], copper-lead, cadmium-silver, and others. Bearing metals are selected for their low coefficient of friction and their ability to withstand heavy bearing loads, high surface speeds, and high temperatures without seizure and excessive wear of the crankshaft. Engine bearings are either replaceable or cast directly in the crankcase or connecting rods. Replaceable bearings usually are composed of thin steel shells lined with a thin layer of bearing metal. To provide ease in assembly and replacement, the main and connecting rod bearings are usually the “split” or two-piece type. On some types of engines, ball or roller bearings are employed for main and connecting rod bearings.

Valves [93]. In most engines, intake and exhaust valves of the poppet type are employed to open and close openings or ports through which the gases enter and leave the cylinders. Each cylinder in the four-stroke-cycle engine has at least one intake and one exhaust valve. The valves are located either in the cylinder block or in the cylinder head and are supported in valve guides. A camshaft opens each valve at the proper time and a valve spring closes the valve.

In two-stroke-cycle gasoline engines, the fuel mixture is admitted and the exhaust gases expelled through ports in each side of -the cylinder, the ports being opened and closed by the action of the piston. Two-stroke-cycle diesel engines generally have one port opening in the cylinder and one cam-actuated poppet valve through which the air is admitted into the cylinder and the exhaust gases expelled.

Camshaft [94]. A camshaft opens the valves against the tension of the valve springs at the proper time and holds them open for the required interval. A separate cam is provided on the camshaft for the operation of each valve. Some opposed engines have each intake cam operate two intake valves. The camshaft is driven from the crankshaft through timing gears, or a timing chain and sprockets.

In four stroke-cycle engines, the camshaft revolves at one-half crankshaft speed, and each valve opens and closes once every two, revolutions of the crankshaft. In a two-stroke-cycle diesel engine, the camshaft revolves at crankshaft speed, and each valve opens and closes with each revolution of the crankshaft,

Valve Lifters [95]. Valve lifters or tappets are employed between the camshaft and the valve stem to open the valves. Valve stems expand when they become heated; and in most, engines a definite clearance must be provided between the valve stem and the valve lifter. In some engines, valve lifters are provided with adjusting screws to regulate the clearance. Some engines are equipped with self-adjusting hydraulic, valve lifters which operate with no clearance between the valve stem and valve lifter.

Manifolds [96]. Manifolds are employed to conduct the gases into and out of the cylinders. An intake manifold is connected between the carburetor and the intake valve ports leading into the cylinders. The exhaust manifold connects the exhaust ports to the exhaust system. The intake and exhaust manifolds may be separate castings bolted together, or both may be cast together. Exhaust gas usually is utilized to heat the intake manifold, thus assisting in vaporizing the incoming fuel charge.

In diesel engines, the intake manifold conducts air to the cylinders, the fuel oil being sprayed directly into the cylinder at the proper time by a fuel injector. Two-stroke-cycle gasoline engines have no intake manifold. The crankcase is utilized as a receiver for the fuel mixture.