All this sounds simple enough, but a great many things must be considered that make the construction of a successful working motor a difficult problem.

In the first place, the carbureter, which turns the oil into gas, must work automatically, the proper amount of oil being fed into the machine and the exact proportion of air admitted for the successful mixture.  Then the gas must be admitted to the cylinders in just the right quantity for the work to be done.  This is usually regulated automatically, and can also be controlled directly by the driver.  Since the explosion of gas in the cylinder drives the piston out only, and not, as in the case of the steam-engine, back and forward, some provision must be made to complete the cycle, to bring back the piston, exhaust the burned gas, and refill the cylinder with a new charge.

In the steam-engine the piston is forced backward and forward by the expansive power of the steam, the vapour being admitted alternately to the forward and rear ends of the cylinder.  The piston of the gasoline engine, however, working by the force of exploded gas, produces power when moving in one direction only—­the piston-head is pushed out by the force of the explosion, just as the plunger of a bicycle pump is sometimes forced out by the pressure of air behind it.  The piston is connected with the engine-crank and revolves the shaft, which is in turn connected with the driving-wheels.  The movement of the piston in the cylinder performs four functions:  first, the downward stroke, the result of the explosion of gas, produces the power; second, the returning up-stroke pushes out the burned gas; third, the next down-stroke sucks in a fresh supply of gas, which (fourth) is compressed by the following-up movement and is ready for the next explosion.  This is called a two-cycle motor, because two complete revolutions are necessary to accomplish all the operations.  Many machines are fitted with heavy fly-wheels, the swift revolution of which carries the impetus of the power stroke through the other three operations.

[Illustration:  A GIANT AUTOMOBILE MOWER-THRASHER This machine cuts a swath 35 feet wide and thrashes and sacks the grain as it moves along.  Seventy to 100 acres of grain a day are harvested by this machine, and 1,000 to 1,500 sacks are produced each working day.]

To keep a practically continuous forward movement on the driving-shaft, many motors are made with four cylinders, the piston of each being connected with the crank-shaft at a different angle, and each cylinder doing a different part of the work; for example, while No. 1 cylinder is doing the work from the force of the explosion, No. 2 is compressing, No. 3 is getting a fresh supply of gas, and No. 4 is cleaning out waste gas.  A four-cylinder motor is practically putting forth power continuously, since one of the four pistons is always at work.