Introduction: The History
Many people claimed the invention of the internal combustion engine in the 1860's, but only one has the patent on the four stroke operating sequence. In 1867, Nikolaus August Otto, a German engineer, developed the four-stroke "Otto" cycle, which is widely used in transportation even today. Otto developed the four-stroke internal combustion engine when he was 34 years old.
The Diesel Engine came about in 1892 by another German engineer, Rudolph Diesel. The Diesel engine is designed heavier and more powerful than gasoline engines and utilizes oil as fuel. Diesel engines are a commonly used in heavy machinery, locomotives, ships, and some automobiles.
It is important to mention that the basic operating principles of these engines have been around for more than a hundred years and they are still in place. Some people get discouraged when they look under the hood and cannot recognize a thing on their automobile. Rest assured that underneath all of those wires and sensors lies an engine with the same basic operating principles of that "Otto" engine over a century old.
The Internal Combustion Engine:
Before explaining the operation of the four-stroke engine, some of the internal parts must be identified. Refer to the drawing of the basic internal combustion engine. Throughout the presentation, these parts are mentioned, so an understanding of what they do should be helpful. The Intake Valve opens at a precise time to allow the air/fuel mixture to enter the cylinder. The Exhaust Valve opens at a precise time to allow the burned gases to leave the cylinder. The Spark Plug ignites the air/fuel mixture in the cylinder, which creates an explosion. The force of the explosion is transferred to the Piston. The piston travels up and down in a Reciprocation Motion. The force from the piston is then transferred to the Crankshaft through the Piston Rod (connecting rod). The piston rod converts the reciprocating motion of the piston, to the Rotating Motion of the crankshaft. Now that the basic parts are identified, lets go through the four strokes of the internal combustion engine, which are Intake, Compression, Power, and Exhaust.



The Intake Stroke:
On the intake stroke, the intake valve has opened. The piston is moving down, and a mixture of air and vaporized fuel is being pushed by atmospheric pressure into the cylinder through the intake valve port.


The Compression Stroke:
After the piston reaches the lower limit of its travel, it begins to move upward. As this happens, the intake valve closes. The exhaust valve is also closed, so the cylinder is sealed. As the piston moves upward, the air/fuel mixture is compressed. On some small high compression engines, by the time the piston reaches the top of its travel, the mixture is compressed to as little as one-tenth its original volume. Thus, the compression of the air/fuel mixture increases the pressure in the cylinder. The compression process also creates the air/fuel mixture to increase in temperature.


The Power Stroke:
As the piston reaches the top of its travel on the compression stroke, an electric spark is produced at the spark plug. The ignition system delivers a high voltage surge of electricity to the spark plug to create the spark. The spark ignites the air/fuel mixture. The mixture burns rapidly and cylinder pressure increases to as much as (600psi). All of this pressure against the piston forces it down in the cylinder. The power impulse is transmitted down through the piston, through the piston rod (connecting rod), and to the crankshaft. The crankshaft is rotated due to the force.


The Exhaust Stroke:
As the piston reaches the bottom of its travel, the exhaust valve opens. Now, as the piston moves up on the exhaust stroke, it forces the burned gases out of the cylinder through the exhaust port. When the piston reaches the top of its travel, the exhaust valve closes, and the intake valve opens. The cycle repeats again with the intake stroke. The four strokes are continuously repeated during the operation of the engine.


The Diesel Engine:
Not much is different in the Diesel engine. In the Diesel, the fuel is not mixed with the air entering the cylinder during the intake stroke. Air alone is compressed during the compression stroke. The Diesel fuel oil is injected or sprayed into the cylinder at the end of the compression stroke. In Diesel engines, compression ratios are as high as (22.5 to 1) and provide pressures of (500psi) at the end of the compression stroke. Through the compression process, the air can be heated up (1000 degrees F). This temperature is high enough to spontaneously ignite the fuel as it is injected into the cylinder. The high pressure of the explosion forces the piston down as in the gasoline engine.

Summary:
The four strokes of the internal combustion engine are as follows (and in order): Intake, Compression, Power, and Exhaust. These four strokes require two revolutions of the crankshaft. The process continuously repeats itself during the operation of the engine. So, if a four-cylinder engine requires two complete revolutions of the crankshaft to ignite all of its cylinders, how many revolutions does an eight-cylinder engine require. Hopefully you answered two. It only takes two revolutions of the crankshaft to fire all of the cylinders of any four-stroke engine. The connecting rod converts the reciprocating motion (up and down) of the piston to the rotating motion of the crankshaft. The Diesel engine differs from the gasoline engine in that the intake stroke only pulls in air, not air and fuel. The fuel is injected into the cylinder at the end of the compression stroke. The fuel burns immediately (without the use of a spark plug) because of the high temperature of air in the cylinder.