How Does My Engine Work?

A car’s engine is a mechanical marvel.  Every time we have to replace a head gasket I get to see some of the insides or an engine, and I’m always impressed.  The type of engine you have in your car was developed over 100 years ago.  The basic principles apply which applied then, apply to today.    However, the application of these principles has significantly increased for the better.  Better gas mileage, more power, and less polluting.

An engine converts heat energy, created from burning air and gasoline, into mechanical energy (used to move your car). The car has a piston which moves up and down in the engine, and valves which open and close.  It is a well choreographed machine which produces a lot of power.  A piston goes up and down four times in order to produce one stroke of power.  I could start the explanation anywhere along the path of the piston.  I decided to start with the intake stroke. 

Intake Stroke:  This stroke starts with the piston at the top of its stroke.  The intake valve opens as the piston starts to move from the top of its stroke to the bottom of its stroke.  As the piston moves downward, it pulls in air and fuel.  At the bottom of the stroke the intake valve closes.

Compression Stroke: With both valves closed, the piston moves up the cylinder compressing the air and fuel.  Most cars have an 8 to 1 compression ratio.  That means at the top of the stroke the air-fuel mixture is one-eight its original size.  On a side note, as the piston compresses the air and fuel the temperature of the mixture increases.

Power Stroke:  At the top of the compression stroke, with both valves closed, an electrical spark jumps the spark plug gap igniting the air and fuel.  As the mixture burns, pressure builds up inside the cylinder.  This pressure, which pushes the piston downward, creates the power to move your car.  This is where heat energy is converted into mechanical energy.

Exhaust Stroke:  When the piston is at the bottom of the cylinder the exhaust valve opens.  As the piston moves up the cylinder it pushed the hot exhaust gases out of the engine.  When the piston is at the top of its stroke, the exhaust valve closes and the intake valve opens.  And the cycle starts again.

This is called a “4 Stroke” engine because it takes 4 strokes to produce one power stroke.  A 4 cylinder engine produces one power stroke per one complete revolution.  While one piston is in the “Power Stroke” the other four engines are in one of the other three strokes.  With an 8 cylinder engine, two cylinders produce power with each complete revolution.  This is why, everything else being equal, the more cylinders you have the more powerful the engine.