There is a two-cycle type of motor in which the burnt gases are removed from the cylinder, not by the piston, but by the injection of the unburnt fuel mixture. On the upward or compression stroke the piston not only compresses the mixture in the cylinder, but draws in a fresh charge of fuel into the crank case, and when the piston is driven down by the combustion of the fuel in the cylinder, the charge in the crank case is compressed until, near the end of its stroke, the piston uncovers a port in the cylinder through which the fuel from the crank case is forced in. (See Figure 51.) Just before this occurs the piston uncovers an exhaust port in the opposite side of the cylinder, and the burnt gases start to flow out before the fresh charge of fuel pours into the cylinder. The incoming gas is directed upward so as to completely scavenge the cylinder of all burnt gases. There are no bothersome poppet valves in this engine. The piston itself acts as a valve, opening and closing the inlet and exhaust ports as it slides by them. Every other stroke of the piston is a power stroke, so that the engine acts like a one-pedal bicycle, which receives a push every time the pedal comes around. Unfortunately, the burnt gases are never completely scavenged, and the fresh charge of fuel is always more or less diluted by the product of combustion remaining from the previous charge. This represents just so much loss of power, and may interfere with the ignition of the charge. For this reason the four-cycle engine is generally considered more reliable and efficient, and is far more generally used, particularly on motor vehicles. To overcome the intermittent character of the internal combustion engine a number of cylinders are used, which come into play successively. In the four-cylinder motor one piston is always on the power stroke. In recent years the number of cylinders has progressively increased from four to six, eight, and twelve cylinders, while in racing power boats the number of cylinders has gone up as high as twenty-four. COOLING THE CYLINDERS