When the cylinder approaches within 20 degrees of the end of the inlet half-revolution a series of small inlet ports all around the circumference of the cylinder wall is uncovered by the top edge of the piston, whereby the combustion chamber is placed in communication with the crank chamber. As the pressure in the crank chamber is substantially atmospheric and that in the combustion chamber is below atmospheric, there results a suction effect which causes the air from the crank chamber to flow into the combustion chamber. The air in the crank chamber is heavily charged with gasoline vapor, which is due to the fact that a spray nozzle connected with the gasoline supply tank is located inside the chamber. The proportion of gasoline vapor in the air in the crank chamber is several times as great as in the ordinary combustible mixture drawn from a carburetor into the cylinder. This extra-rich mixture is diluted in the combustion chamber with the air which entered it through the exhaust valve during the first part of the inlet stroke, thus forming a mixture of the proper proportion for complete combustion.
The inlet ports in the cylinder wall remain open until 20 degrees of the compression half-revolution has been completed, and from that moment to near the end of the compression stroke the gases are compressed in the cylinder. Near the end of the stroke ignition takes place and this completes the cycle.
Fig. 111.—Timing Diagram Showing Peculiar Valve Timing of Gnome “Monosoupape” Rotary Motor.
The exact timing of the different phases of the cycle is shown in the diagram at [Fig. 111]. It will be seen that ignition occurs substantially 20 degrees ahead of the outer dead center, and expansion of the burning gases continues until 85 degrees past the outer dead center, when the piston is a little past half-stroke. Then the exhaust-valve opens and remains open for somewhat more than a complete revolution of the cylinders, or, to be exact, for 390 degrees of cylinder travel, until 115 degrees past the top dead center on the second revolution. Then for 45 degrees of travel the charge within the cylinder is expanded, whereupon the inlet ports are uncovered and remain open for 40 degrees of cylinder travel, 20 degrees on each side of the inward dead center position.
SPRINGLESS VALVES
Springless valves are the latest development on French racing car engines, and it is possible that the positively-operated types will be introduced on aviation engines also. Two makes of positively-actuated valves are shown at [Fig. 112]. The positive-valve motor differs from the conventional form by having no necessity for valve-springs, as a cam not only assures the opening of the valve, but also causes it to return to the valve-seat. In this respect it is much like the sleeve-valve motor, where the uncovering of the ports is absolutely positive. The cars equipped with these valves were a success in long-distance auto races. Claims made for this type of valve mechanism include the possibility of a higher number of revolutions and consequently greater engine power. With the spring-controlled, single-cam operated valve a point is reached where the spring is not capable of returning the valve to its seat before the cam has again begun its opening movement. It is possible to extend the limits considerably by using a light valve on a strong spring, but the valve still remains a limiting factor in the speed of the motor.
