4. The cleansing, or exhaust stroke, during which the burnt gas and smoke is forced out of the barrel.
For this reason, a gas engine which works on this principle is called a Four-Stroke Cycle Engine. It requires four strokes to complete the entire operation and bring it back to the beginning ready to start over again.
THE CYLINDER
So far we have confined ourselves to the parts of a cannon, but now that we are going to take up the study of the motor in its details let us call them by their regular names. The barrel of the cannon we will call a cylinder. In an actual motor a cylinder is made out of cast iron, carefully bored out inside, so that the hole is perfectly round, and the sides of the wall as smooth as possible.
Fig. 9.
You will realize that this is necessary as we want to reduce, as much as possible, any rubbing or friction, as it is called, between the piston and cylinder walls. Next we must provide some means of cooling these walls, as you know that the continuous firing would soon make them very hot. This is done by surrounding the cylinder with what is known as a water jacket through which water can be circulated, thereby carrying off the heat, and keeping the iron from getting red hot. We must also cut two holes in the side of the cylinder to make places for the valves and a place for the spark plug.
Fig. 10—End view of Horizontal One-cylinder Motor, showing piston, valves and valve mechanism.
A cylinder is generally mounted on its side in a one cylinder engine, and is set up on end when it is desired to use more than one. Therefore, in a one cylinder motor you will notice that the piston moves back and forth, whereas in a two-cylinder, four-cylinder or six-cylinder type, the pistons move up and down. As far as the action of the parts is concerned they work in exactly the same way, only that the valve mechanism has to be changed somewhat.