The cut shows the action of the so-called “four-cycle” motor. Four strokes are required to produce an impulse on the piston and return the parts to their original positions. On the first, or suction stroke, the combustible mixture is drawn into the cylinder, the inlet valve being open and the outlet valve closed. On the second stroke, both valves are closed and the mixture is highly compressed. At about the end of this stroke, a spark ignites the charge, a still greater pressure is produced in consequence, and the energy of the gas now forces the piston outward on its third or “working” stroke, the valves remaining closed. Finally, the outlet valve is opened and a fourth stroke sweeps the burnt gas out of the cylinder.
Action of the Four-Cycle Engine
In the “two-cycle” engine, the piston first moves to the left, compressing a charge already present in the cylinder at F, and meanwhile drawing a fresh supply through the valve A and passages C to the space D. On the return stroke, the exploded gas in F expands, doing its work, while that in D is slightly compressed, the valve A being now closed. When the piston, moving toward the right, opens the passage E, the burnt gas rushes out. A little later, when the passage I is exposed, the fresh compressed gas in D rushes through C, B, and I to F. The operation may now be repeated. Only two strokes have been necessary. The cylinder develops power twice as rapidly as before: but at the cost of some waste of gas, since the inlet (I) and outlet (E) passages are for a brief interval both open at once: a condition not altogether remedied by the use of a deflector at G. A two-cycle cylinder should give nearly twice the power of a four-cycle cylinder of the same size, and the two-cycle engine should weigh less, per horse-power; but it requires from 10 to 30% more fuel, and fuel also counts in the total weight.
Action of Two-Cycle Engine
The high temperatures in the cylinder would soon make the cast-iron walls red-hot, unless the latter where artificially cooled. The usual method of cooling is to make the walls hollow and circulate water through them. This involves a pump, a quantity of water, and a “radiator” (cooling machine) so that the water can be used over and over again. To cool by air blowing over the surface of the cylinder is relatively ineffective: but has been made possible in automobiles by building fins on the cylinders so as to increase the amount of cooling surface. When the motors are worked at high capacity, or when two-cycle motors are used, the heat is generated so rapidly that this method of cooling is regarded as inapplicable. By rapidly rotating the cylinders themselves through the air, as in motors like the Gnome, air cooling is made sufficiently adequate, but the expenditure of power in producing this rotation has perhaps not been sufficiently regarded.
Motor and Propeller
(Detroit Aeronautic Construction Co.)