means of a central tube. Gas engines of this type are made in single units as large as 2,000 horsepower.
59. The two-cycle type of engine is extremely simple, as it has few moving parts. On the other hand, some power is unavoidably wasted, because the exhaust valve or port must be opened earlier than is necessary in the four-cycle engine, owing to the extremely short time in which the exhaust gases must be discharged. Moreover, there is some unavoidable mixing of the fresh charge with the burned gases, since they cannot be sharply separated, and it is quite possible in many cases for a portion of the fresh charge to escape from the exhaust port unburned. If this sort of loss is to be avoided, the alternative is to restrict somewhat the quantity of fresh charge transferred to the cylinder, so that a smaller quantity of the fresh mixture enters the cylinder than would be used in the four-cycle engine, and the power is correspondingly smaller. The mean effective pressure of a four-cycle engine is seldom less than 65 pounds per square inch, using gasoline, and is nearly always above 60 pounds on full load. An average figure for the mean effective pressure of a two-cycle engine would be from 35 to 45 pounds. Another source of loss is the compression of the charge in the crank-case. The power used in doing this is not recovered, as is the power used in compressing the charge in the cylinder, for the reason that the gas is simply released by uncovering the transfer port and does no work on the piston. For these and other reasons a two-cycle engine is not often used in small sizes where economy of fuel consumption is important; but its small first cost makes it popular for launches and similar use, in which fuel economy is of less consequence than simplicity of operation.
