Connecting rod; 8 — crankshaft; 9 — spark plug.

The first stroke is the intake stroke. The piston moves down the cylinder and creates a partial vacuum in the cylinder. A mixture of air and fuel is forced through the inlet valve into the cylinder by atmospheric pressure, now greater than the pressure in the cylinder. During this stroke, the exhaust valve stays closed. The second stroke is the compression stroke. The piston moves up in the cylinder with both valves closed. The air and rue J mixture is compressed and the pressure rises.

The third stroke is the power stroke. Near the end of the compression stroke, the air I and fuel mixture is ignitedby an electric spark from the spark plug. The combustion that occurs causes a rise in temperature and enough pressure to force the piston down again.

Finally_, on the fourth stroke, or exhaust stroke, the piston moves up again and forc­es the burned gases out of the cylinder and into the exhaust system. This cycle repeats itself the entire time theengine is running.

The combustion chamber is where the air-fuel mixture is burned. The location of the combustion chamber is the area between the top of the piston at what is known as TDC (top dead center) and the cylinder head. TDC is the piston's position when it has cached the top of the cylinder, and the center line of the connecting rod is parallel to the cylinder walls. The two most commonly used types of combustion chamber are the hemispherical and the wedge shape combustion chambers. The hemispherical type is so named because it resembles a hemisphere. It is compact and allows high compression with a minimum of detonation. The valves are placed on two planes, enabling the use of larger valves. This improves «breathing» in the combustion chamber. This type of chamber loses a little less heat than other types. Because the hemispherical combustion chamber is so efficient, it is often used, even though it costs more to produce.

The wedge type combustion chamber resembles a wedge in shape. It is part of the cylinder head. It is also very efficient, and more easily and cheaply produced than the hemispherical type.

Engine Configurations. The V-type of engine has two rows of cylinders at (usu­ally) a ninety degree angle to each other. Its advantages are its short length, the great rigidity of the block, its heavy crankshaft, and attractive low profile (for a car with a low hood). This type of engine lends itself to very high compression ratios without block distortion under load, resistance to torsional vibration, and a shorter car length without losing passenger room.

Rotary Engine. One alternative to conventional automobile power is the rotary (or

Wankel) engine. It is widely known that Felix Wankel built a rotary engine in 1955.