Engine Structure:

A car usually has a piston engine.  It consists of several moving parts: pistons, connecting rods, crankshaft, camshaft, valve lifters, intake valves and exhaust valves.  The cast iron or aluminum engine block holds the moving parts.  The engine block has a series of holes which are called cylinders.  The cylinders can be arranged in line or in a V-shape and in the upper part of the block.  Each cylinder holds a piston and connecting rod.  A circular ring is used to seal the small gap between the piston and the cylinder wall.  The lower part of the block is called crankcase which holds the crankshaft with bearing mounts.  Pistons are connected to the crankshaft by connecting rods.

The cylinder heads are the top covers of the cylinders which are tightly bolted to the top of the block.  The cylinder heads contain combustion chambers.  Each combustion chamber contains at least one intake and exhaust valve and one spark plug per cylinder.  The valves are opened and closed in a specific sequence with valve lifters controlled by the camshaft.  The camshaft is connected to the crankshaft through a time belt.

Engine Operation:

In each cylinder, a complete cycle of four strokes takes place while the crankshaft makes two complete revolutions.  These four strokes are intake, compression, ignition (power) and exhaust.

During the intake stroke, the exhaust valve is closed and the intake valve is open.  The air-fuel mixture is drawn or injected into the cylinder when the piston goes down.  During the compression stroke, the piston goes up and compresses the fuel mixture at the top of the cylinder.  This increases the temperature and pressure.  At this point, the ignition stroke begins.  The spark plug ignites the fuel mixture.  The piston is forced downward by the expansion of burning fuel mixture.  When the piston goes up again, the exhaust stroke starts.  The exhaust gases are forced out through the opening exhaust valve.  Another cycle then follows.  The connecting rod converts the up and down motion of the piston into the rotary motion of the crankshaft.  The crankshaft motion is then transferred to the wheels through the transmission and drive shafts.