The Four-Stroke Cycle

What really makes a car go?  There are many parts to an engine that are visible, like an alternator, starter, hoses, wires and a bunch of other things.  But what is in the inside and how does it work? The process is really just a mixture of parts, moving in sync in a distinct pattern called the four-stoke cycle.

To understand the components of a four-stroke engine better, a little background information may be needed.    "In a gasoline engine, exhaust and intake manifolds (tunnels) and valve ports(inlets or outlets) are needed to supply air and fuel on the intake (first down) stroke of the cylinders, and to expel burned gases on the exhaust(second up) stroke.  The ports are opened and closed at the proper times by the intake and exhaust valves, which close against the sides of the ports"  (Doyle 128).  The controlling of the valves start with a cam shaft.  A cam shaft has lobes on it which are all shaped differently, but resemble an oval.  On a camshaft lobe there rests a lifter.  A lifter is cylinder shaped and pushes up on a push rod when the lobes are rotating.  A push rod, which is connected to a rocker arm, then "rocks" down on a spring that in turn pushes open a valve.  This all happens due to the rotation of the camshaft which is geared to the crankshaft (which is connected to the pistons) which is all part of the four stroke cycle.

The four-stroke cycle starts with the intake stroke.  On the intake stroke, the piston moves down from its top dead center or the farthest "up" position creating a partial vacuum which draws in the fresh air-fuel mixture from the open intake valve.  Think of the vacuum like a syringe with no needle.  "If  you put your finger over the end of the tube and pull back the plunger, then pop your finger off the tube, the suction will draw fresh air into the tube" (Freiburger 76).  As the piston is moving towards its bottom dead center or the farthest "down" position, "the intake valve begins to close.  The valve is kept open as long as possible to get as much air-fuel mixture into the chamber as it can"  (Doyle 128).  But as soon as the piston passes bottom dead center the valve closes.  The exhaust valve is closed at this time (Doyle 129).   So the cycle goes onto the next stroke, the compression stroke.

The compression stroke is not as complicated as the intake stroke.  On this stroke, all of the valves remain closed as the piston is driven up the cylinder wall by the crankshaft.  And while moving up, the piston compresses and traps the air-fuel mixture, hence the name "compression" stroke.  "Compressing the mixture is the key to power because it forces fuel molecules together to aid combustion by releasing more energy from the gasoline" (Freiburger 76).  And  "since compression and power are directly related, both valves must be tightly sealed, for any leakage leads to a decrease of compression and loss of power"  (Doyle 129).  And with the air-fuel mixture compressing, the cycle moves to the next stroke called the power or ignition stroke.

The power stroke starts before the piston reaches top dead center on the compression stroke.  The fuel is then ignited by a spark-plug and the combustion causes the gases to expand creating a high pressure.  That high pressure forces the piston down, which is the power stroke.  During the power stroke, the valves remain fully closed to seal the 750 pounds of pressure per square inch (Doyle 129).

With the piston on the way down, there is still some pressure, but it is of no use as far as engine power is concerned.  The exhaust valve is then opened releasing the pressure that would resist the piston on the exhaust stroke (Doyle 129).

The exhaust stroke begins "with the exhaust valve open and the intake valve still closed.  The piston then moves upward forcing hot gases out through the exhaust valve port"  (Doyle 129).  The exhaust valve remains open throughout the exhaust stroke and partially into the intake stroke to assure that none of the burnt gases still linger.  

And that is the four stroke cycle.  It keeps repeating and repeating until the engine is shut down.  "At a typical freeway engine speed, about 2200 rpm, the entire four-stroke process is completed at a rate of over eighteen times a second in each cylinder"  (Freiburger 76).  All of the parts of an engine have to cooperate in order for the engine to run smoothly.  If one thing is off, the engine just will not run at its peak.        

Works Cited

Doyle, John.  The Auto Repair Book.  Chicago: J. G. Ferguson Publishing Company, 1983.

Freiburger, David.  "How It Works: The Four-Stroke Cycle."  Hot Rod Jan. 1993: 74-76.