THE GASOLINE AUTOMOBILE ENGINE The automobile engine transforms the chemical energy of a mixture of air and fuel (petrol or gasoline) into mechanical energy which can do work, i.e., move the car. The engine is often also called the power plant. Essen tially the engine consists of : (I) an enclosed cylinder in which a gaseous mixture of air and fuel is burned ; (2) a piston which moves back and forth in the cylinder; (3) a connecting rod which links the piston to a crank shaft; and (4) a crankshaft which changes the reciprocating motion of the piston into rotary motion. This is shown in fig. 1. Other parts needed for the con ventional gasoline engine are : (1) A carburettor for mixing air and fuel, with an intake pipe to con duct the mixture up to the inlet valve; (2) inlet valve, which, upon being opened, admits the mixture into the cylinder, and upon being closed, prevents this charge from escaping ; (3) a - spark-plug, which ignites the charge at the proper time ; and (4) an exhaust valve, which opens to allow free exit to the charge after it has delivered part of its energy to the engine. Practically
all automobile engines operate on what is called the four-stroke cycle. The stroke refers to the travel of the piston in the cylinder, from top to bottom for one stroke, and back again for another stroke. Four strokes complete one cycle.
The following events take place in the engine during one cycle: (1) Intake stroke: The inlet valve is open. The piston travels down the cylinder, and, acting like a pump, draws in a charge of air and fuel through the inlet valve. Strictly speaking, atmospheric pressure forces the mixture of air and fuel into the cylinder to fill up the space left vacant and partially evacuated by the reced ing piston. (2) Compression stroke: The valves are both closed. The cylinder is full of the air-fuel mixture. The piston travels up the cylinder, and compresses this mixture into a very small space at the top of the cylinder. Shortly before the piston reaches the