The simple two-stroke engine described at the beginning of this chapter is often constructed in such a form that no automatic inlet valve is required on the crankchamber.
In this case the induction pipe is connected to a third set of ports just below and a little to one side of the inlet ports to the working cylinder, and these are uncovered by the piston towards the completion of its upstroke, thus allowing the carburetted air to enter the crankchamber. Such an arrangement constitutes a three-port two-stroke engine, which is of course less efficient than a two-port engine with automatic valve, but has the great merit that it is entirely valveless, and therefore extremely simple and cheap to manufacture. It is much used for motor boat work, both in this country and in America, on account of its relatively low speed of rotation.
Fig. 66.—Twin-cylinder Two-stroke Engine with Crankchamber Compression.
[CHAPTER XI]
HORSE POWER AND THE INDICATOR DIAGRAM
A book on “The Petrol Engine” would hardly be complete without some reference to horse-power and the indicator diagram. The following definitions must be carefully studied.
Work.—A force is said to do mechanical work when it overcomes a resistance in its own line of action. The line of action of a force is a line indicating the direction in which the force acts. Engineers measure work in foot-pound units. The product obtained when we multiply the magnitude of the force or resistance (in pounds) by the distance through which it has acted or been overcome (expressed in feet) gives the quantity of work done in foot-pounds.
Example:—A force of 50 lbs. is exerted in overcoming a resistance through a distance of 12 feet. Find the work done.