Internal Combustion Simplified

The two-stroke engine, as found in chain saws, mopeds and boats, is a compact and efficient version of the four-stroke engine used in cars and trucks. (For an understanding of how the four stoke engine works, read “How Car Engines Work.”)

Two-stoke and four-stroke engines follow the same basic operating principles: a mixture of gas and air is drawn into a cylinder and compressed. The mixture is then ignited by a spark and expands rapidly as it burns. This pushes a piston downwards, turning the crankshaft to which it is connected. However, whereas a four stroke engine has one power stroke every two revolutions of the crankshaft, a two-stroke engine has one power stroke per revolution. This means that the drawing-in of a fresh charge of fuel and the expulsion of exhaust gases has to take place during the power and exhaust strokes of the piston.

To make this happen, the intake and exhaust valves used in the four-stroke engine are replaced with openings or “ports” in the wall of the cylinder. These are covered and uncovered by the piston as it moves up and down, so allowing gases to enter and leave. To get a better understanding of how a two-stoke engine works, let’s go through the complete combustion cycle, beginning with the compression of the fuel mixture.

The space above the piston is filled with a mixture of gas and air which is compressed as the piston rises in the cylinder. When the piston reaches the top of its stroke a spark is created at the spark plug, and this ignites the mixture. As the fuel burns, the gas expands rapidly and pushes down on the piston, which moves downwards, turning the crankshaft as it goes. This is termed the “power stroke.”

As the piston descends it uncovers the exhaust port and the burnt gas moves out into the exhaust system. At the same time, the piston reduces the space available in the crankcase (the area below the piston where the crankshaft is located,) which means the air there is compressed. A “transfer port” in the side of the cylinder bore (really little more than a large vertical groove,) allows this air to move up the side of the piston and into the combustion chamber above the piston, where it pushes out the exhaust gases.

However, this is not really air but the same gas and air mixture that is compressed and burnt to create the power stroke, so the question is, how did it get into the crankcase?

The answer is that there is a second opening in the cylinder, on the opposite side to the exhaust port and a little lower. This is the inlet port. When the piston is low enough to uncover the exhaust port it blocks off the inlet, which is why the compressed mixture in the crankcase moves up into the combustion chamber. As the piston moves up it closes the exhaust port but uncovers the inlet. The upwards motion of the piston also increases the volume in the crankcase, which sucks in the gas and air mixture. Simultaneously, the piston closes the transfer port and compresses the mixture above, preparing it for the ignition spark. From this point the cycle repeats.

Spark timing is controlled by magnets on the crankshaft. As the crank rotates these move past a sensor that generates a high voltage pulse for the spark.

The two-stroke engine is an excellent means of producing a lot of power in a small space, and its lack of valves makes it reliable and inexpensive to produce. Unfortunately though, its exhaust emissions are far worse than those from a four-stroke engine, and for this reason the two-stroke will probably be found in fewer applications in the future.