In order that a gas engine may run properly, the mixture must be set on fire, or _ignited_, at exactly the right time; if ignition occurs too early or too late, there will be a loss of power. The greatest pressure will be obtained at the instant when all of the mixture is burning, and this should take place just as the piston begins to move outward on the power stroke. A little time is required for the mixture to burn; there is a brief interval between the instant when it is set on fire and the instant when it is all in flame. Thus it is clear that if the mixture is all to be burning as the piston starts the power stroke, it must be set on fire before that time, or, in other words, toward the end of the compression stroke. The point at which ignition should occur depends on the speed of the engine and should change when the speed changes. The time required for the flame to spread throughout the mixture does not change; let us say that, with the engine running at 1200 revolutions a minute, the mixture can be ignited when the piston is ¼ inch from the end of the compression stroke, and will all be in flame by the time the piston starts on the power stroke. If the engine is slowed down to 600 revolutions a minute and no change is made in the ignition, the mixture will all be in flame before the piston reaches the end of the compression stroke; pressure will then be produced before the piston is in position to perform the power stroke. The pressure will try to make the engine run backwards; it will sometimes be sufficient to make the engine stop. If the momentum of the flywheel is sufficient to force the piston to the end of the stroke against the pressure, this condition will cause a loss of power. This is called _preignition_, or ignition that occurs too soon. One effect of it is to produce a hard, metallic knocking, due to the oil being squeezed out of the bearings by the great pressure, which permits the bearing and shaft to strike. The remedy is to make ignition occur later in the stroke. If the engine is speeded up above 1200 revolutions, the piston will have had time to move some distance on the power stroke before the mixture is all in flame; the combustion space will then be too large to permit the mixture to produce its greatest pressure, and again there will be a loss of power. The remedy in this case is to make ignition occur earlier in the compression stroke. When ignition is made to occur early in the compression stroke, it is said to be _advanced_; when it is made to occur late in the stroke, it is said to be _retarded_. To get the best results, the engine should be run with ignition advanced as far as is possible without causing knocking. The charge of mixture is always set on fire by an electric spark, and the parts that produce and control this spark are called the _ignition system_.