Basic Construction of an Engine

FIGURE 9-A simplified drawing of part of an engine is shown here. Note the locations of the cylinder, piston, connecting rod, crankshaft, combustion chamber, cylinder head, and spark plug.

Figure 9 shows a simplified drawing of part of an engine. In this illustration, note the round cylinder with the piston positioned inside it. The cylinder is a hollow metal tube that's drilled into the metal engine block. The piston is a can-shaped metal component that can move up and down inside the cylinder. The piston is the main moving part in an engine. In an actual engine, the top of the cylinder is sealed by a metal cover that's called the cylinder head. The cylinder head is bolted onto the top of the cylinder.

Note that when the piston is positioned at the very top of the cylinder, a small amount of open space is still left between the top of the piston and the cylinder head. This small, open space above the piston is called the combustion chamber. In the combustion chamber, a mixture of air and gasoline is burned to produce power. When the air-and-fuel mixture burns in the combustion chamber, it produces a small, contained explosion. The air expansion produced by this explosion is strong enough to force the piston downward in the cylinder.

Above the combustion chamber area, note the component called the spark plug. The spark plug is screwed into a threaded hole in the cylinder head. The end of the spark plug protrudes right through the cylinder head and into the combustion chamber. The spark plug is used to make sparks that will ignite the air-and-fuel mixture in the cylinder and cause it to burn. The sparking action of the spark plug is controlled by the engine's ignition system, which we'll discuss in detail in a later study unit.


FIGURE 10-The bottom end of the piston is connected to a rod and crankshaft assembly, as shown in this exploded diagram.

The bottom end of the piston is connected to a rod and crankshaft assembly (Figure 10). When the piston is forced downward in the cylinder, the piston's downward motion is transferred to the rod and crankshaft. The rod and crankshaft then convert the up-and-down motion of the piston into rotary motion (circular motion).

This conversion of up-and-down motion to rotary motion can be compared to the motion produced by a regular bicycle. When you pedal a bike, the up-and-down motion of your feet on the pedals is converted into circular motion in the rear wheel. The same principle applies to an engine. The up-and-down motion of the piston is converted to circular motion in the crankshaft that can be used to power a piece of equipment.

Thus, during the operation of an engine, the piston continually moves up and down inside the cylinder (Figure 11). Some special terms are used to describe the exact position of the piston in the cylinder at any one time. When a piston is at its lowest position in the cylinder, it's said to be at bottom dead center (BDC). When the piston is at its highest position in the cylinder, it's said to be at top dead center (TDC). The total distance that the piston moves from the top of the cylinder to the bottom of the cylinder is called the stroke.

FIGURE 11-A simplified drawing of the cylinder and piston is shown here. During the operation of an engine, the piston continually moves up and down inside the cylinder. Note the position of the cylinder at bottom dead center (BDC) and top dead center (TDC).

Note that the outside surface of a piston has several horizontal grooves cut into it. Each of these grooves holds a metal ring called a piston ring. A piston ring is a circular piece of metal that's split at one point (Figure 12A). Piston rings are designed to be springy so that they can stretch to fit over the outside of the piston and then snap back into their grooves (Figure 12B).

Once they're in place, the piston rings stick out like ridges on the surface of the piston. When a piston is in place inside a cylinder, the piston rings will press outward against the walls of the cylinder. This action helps form a tight seal between the piston and the cylinder, which is necessary for proper engine operation.

FIGURE 12-Figure 12A shows a piston ring, and Figure 12B shows the ring grooves on a piston. The rings will stretch over the outside of the piston and snap into place in the grooves.