In the first place, it improves the quality of the charge, and makes it burn very much better. When the charge enters the cylinder, the fuel vapor and air are not thoroughly mixed; much of the fuel is not turned into vapor. By compressing the charge it becomes heated; this vaporizes the fuel, and vapor and air become thoroughly mixed. Compression also increases the power. Suppose that the cylinder contains a quart of mixture which, when heated, will expand to a gallon. If this quart of mixture is compressed to a half pint, it will not lose its ability to expand to a gallon, and will exert more pressure in expanding from a half pint to a gallon than from a quart to a gallon. A leaky cylinder will cause a further loss of power because some of the charge will escape during the compression stroke, which will leave less to be burned and to develop power. =Ignition.=—Setting fire to the charge of mixture is called the _ignition_ of the charge, and it takes place close to the end of the compression stroke. To get the greatest power, all of the mixture should be on fire and heated most intensely as the piston begins the power stroke. When the mixture is set on fire, it does not explode like gunpowder, but burns comparatively slowly; the charge is ignited by an electric spark, and the flame spreads from that point until it is all on fire. In order to give the flame time to spread, the spark passes sufficiently before the end of the compression stroke to have the entire charge on fire as the power stroke begins. This is called the _advance_ of the ignition. The flame takes the same time to spread through the charge when the engine is running fast as when it is running slow. Therefore if the engine is speeded up, the spark must be advanced, for otherwise the piston would be on the power stroke before the flame would have time to spread all through the mixture. When the engine is slowed down, the spark must have less advance, or must be _retarded_, for, if it were not, the charge would all be in flame, and exerting its full pressure, before the piston reached the end of its compression stroke. The subject of ignition, which is of great importance, is covered more fully in Chapter VI. =Power Stroke.=—During the power stroke (No. 3, Fig. 1) the piston moves outward, and both valves are closed. As it begins, the mixture is all on fire, and great pressure is exerted against the piston. As the piston moves outward the combustion space becomes larger, and the gases obtain the room for expansion that they seek. As they expand, the pressure that they exert becomes less. By the time the piston is three quarters the way down the power stroke, the pressure is so reduced that it has little or no effect; the gases are still trying to expand, however, so the exhaust valve is opened at that point, and they begin to escape.