Fig. F-9. Cross Section of Fairbanks-Morse Three Port—Two Stroke Cycle Engine, with Parts Named. ]
This is an advantage not possessed by the four stroke cycle engine of the ordinary type, as the cams and valve mechanism require reversal as well as a reversal of the ignition system. This relation between the valve action and rotation in a four stroke cycle engine may be illustrated by the following example. Consider the piston at the end of the compression stroke in an engine designed for right hand rotation. After ignition, under the proper conditions, the piston would descend turning the crank to the right until it reached the bottom of the stroke, at which point the exhaust valve would open and relieve the pressure in the cylinder.
Let us now consider an attempt at reversing the engine by causing the spark to occur before the piston reached the end of the compression stroke with the crank still inclined toward the left. In this case the piston would force the crank down in a left hand direction until it reached the end of the stroke. The exhaust valve would not open to relieve the pressure, as the exhaust cam would be moving away from the valve rod instead of toward it. Should the crank swing a little past the dead center, because of its momentum, the inlet valve would be opened instead of the exhaust, and the contents of the cylinder would shoot through the intake pipe and carburetor. This would bring matters to a close as far as rotation was concerned.
The opening of the inlet valve on the reversed working stroke would occur as the inlet valve closes one stroke, or one-half revolution, before the end of the compression stroke. As the engine turned backward one-half revolution, the inlet cam would again be brought into contact with the inlet valve rod, opening the valve and allowing the burned gases to pass through the carburetor. Should the pressure be sufficiently reduced by inlet valve to allow the piston to reach the end of the second stroke, it would start on the third stroke by inhaling a “charge” of burnt gas through the exhaust valve which would now be open.
As the piston does not sweep out all the cylinder volume because of the space left at the end of the cylinder for compression, more or less burned gas remains in the combustion chamber which dilutes the active mixture taken in on the suction stroke. Not only are the residual gases useless in generating heat but they also occupy a considerable space in the cylinder that might otherwise be filled with a heat producing mixture. Their diluting effect also prevents the complete combustion of a certain percent of the fuel actually taken into the cylinder for which the burnt gas is incapable of supporting combustion.