Gnome “Monosoupape” Valve Timing
In the present design of the Gnome motor, a cycle of operations somewhat different from that employed in the ordinary four-cycle engine is made use of, says a writer in “The Automobile,” in describing the action of this power-plant. This cycle does away with the need for the usual inlet valve and makes the engine operable with only a single valve, hence the name monosoupape, or “single-valve.” The cycle is as follows: A charge being compressed in the outer end of the cylinder or combustion chamber, it is ignited by a spark produced by the spark-plug located in the side of this chamber, and the burning charge expands as the piston moves down in the cylinder while the latter revolves around the crank-shaft. When the piston is about half-way down on the power stroke, the exhaust valve, which is located in the center of the cylinder-head, is mechanically opened, and during the following upstroke of the piston the burnt gases are expelled from the cylinder through the exhaust valve directly into the atmosphere.
Instead of closing at the end of the exhaust stroke, or a few degrees thereafter, the exhaust valve is held open for about two-thirds of the following inlet stroke of the piston, with the result that fresh air is drawn through the exhaust valve into the cylinder. When the cylinder is still 65 degrees from the end of the inlet half-revolution, the exhaust valve closes. As no more air can get into the cylinder, and as the piston continues to move inwardly, it is obvious that a partial vacuum is formed.
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.
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.