What you see above in the front half of the picture is not the engine of a motor car, but a diagram of a small part of an engine. A motor engine has at least four cylinders, and the newer cars have six, eight, or even more. Our diagram shows only one cylinder, much enlarged, and omits the cooling and the oiling systems and dozens of other important parts, but it does show the most important fact – that is, what makes a motor engine run — better than any picture of a whole engine could do.
Let us learn some of the necessary parts shown upon our diagram. The intake valve A is shown open so that gas may be drawn into the cylinder; B is the exhaust valve through which the used-up gas escapes; C is a spark plug; D is the intake pipe through which gas passed on its way to the cylinder; E is the piston which fits closely within the cylinder walls. It is joined by the connecting rod J to the crankshaft I which is fastened to the flywheel H. The brake drums on the inner sides of the wheels are marked S; the foot-brake pedal is marked G, and R shows the hand brake. The names of some of the other necessary parts which you see lettered will be told under the diagrams which follow this.
Now let us see what happens when the starter button, (which may be on the instrument board or upon the floor), is pushed. A current of electricity runs from the battery under the floor to the electric starter (not shown). The starter makes the flywheel H and the crankshaft I turn, and the piston E is pulled toward the bottom of the cylinder, thus creating a partial vacuum. Gasoline, which has passed through a pipe from the tank at the back into the carburetor F, is broken up into a fine spray as air rushes in to fill the vacuum in the cylinder. The spray is mixed with air in the proportion of about one part to fifteen of air.
This mixture, which is a very explosive vapor, is drawn through the pipe D and the valve A into the cylinder. When the piston E has reached the bottom, both valves A and B are closed, and the flywheel, as it continues to rotate, pushes up the piston. As the piston fits very closely within the cylinder, the vapor is compressed into a much smaller space. Smooth but elastic metal rings encircle the piston and press against the smooth wall of the cylinder.
As the piston E nears the top of the cylinder, and the gas has been compressed into a very small space, the spark plug C from which a wire runs to the ignition coil (not shown), and to the battery, is arranged to give off an electric spark inside the cylinder. The compressed mixture of gasoline vapor and air instantly explodes, and drives the piston down with tremendous force. This explosion furnishes the energy which drives the car. The connecting rod and the crank shaft make the flywheel spin swiftly, and as it rotates it pushes the piston up again.
Exactly the same things are going on in the other cylinders of the car, but the explosions are arranged to occur at different times, so that the flywheel may turn smoothly and with uniform speed. When the car is running rapidly the explosions take place a very tiny fraction of a second apart. The generator (not shown) is a small dynamo, which is constantly furnishing electricity to the battery while the engine is running, and the spark plugs are described on the next page. Without the flywheel the piston might stop at the bottom of the cylinder after the first explosion. You can see the great flywheels on steam engines illustrated on other pages of our book.
As the piston starts upward after the explosion the exhaust valve B opens and the used-up gas is forced out through the exhaust pipe K, and the muffler which softens the noise of the explosion. As the flywheel continues to turn the exhaust valve B closes, the piston starts downward, the valve A opens, more gas is sucked into the cylinder as described under the first picture, and the whole process is repeated.
The crankshaft and flywheel can run without moving the car, or can be connected with the drive-shaft L in the transmission Q. The drive-shaft L by means of the cog-wheels M and N turns the rear axle O, and makes the car move. Whether the car stands still while the engine is running, goes backward, or moves forward at low, intermediate, or high speed is governed by the gear shift lever P which moves cog-wheels inside of Q. The engine can also be disconnected from the transmission by pressing upon the clutch pedal T.
An engine like this is called a four cycle motor, because the piston moves up or down four times for every complete explosion.