Q. When should the exhaust valve close?

A. The exhaust valve is usually allowed to lag about 10 degrees after the piston reaches the end of its scavenging stroke.

Q. What is the act

ual duration of inlet stroke?

A. The period during which the inlet valve remains open varies according to the lag at the time of opening and closing. If the valve opens 15 degrees late and closes 35 degrees after the piston has started up on its compression stroke it has remained open a period equal to 200 degrees crankshaft travel. If it opens promptly on top center and closes on bottom center it would only remain open a period equal to 180 degrees crank travel or half a revolution of the flywheel.

Q. What is the act

ual duration of exhaust Fig. 34.-End View of Cylinder Employing Darracq Rotary Valve. stroke?

A. The exhaust valve usually remains open a period equal to a crank travel of 235 degrees.

Q. Why is exhaust valve opened early?

A. The exhaust valve is opened before the piston reaches the bottom of its power stroke because the gas contained in the cylinder has considerable pressure and will tend to fly out of the cylinder

Fig. 35.-Outlining Action of Mead Rotary Valve Motor. A-Exhaust Valve Just Closed, Intake Valve Just Opening, Piston at Beginning of Suction Stroke. B-Piston at Bottom of Stroke, Inlet Passage Just Closing. C-Piston at End of Compression Stroke, Both Valves Closed. D-Piston at Bottom of Explosion Stroke, Exhaust Passage Fully Opened.

because of its pressure even before the piston starts going up on a scavenging stroke. It is necessary to give this lead to exhaust on the high speed engines used as automobile power plants in order to insure thorough clearing of the cylinder by taking advantage of the pressure of the exhaust gases.

Q. Why is valve lag desirable?

A. When the piston reaches the top or bottom of its stroke, there is an appreciable period measured in degrees as the crank swings over its center position where the piston movement relative to the crank travel is comparatively small. If the valves are kept open during this period, more gas can enter or leave the cylinder than would be the case if the valve closed promptly when the piston reached the top of its stroke. The rapidly moving gas acquires a certain momentum which is taken advantage of by keeping the valve open as long as possible.

Fig. 36.-Outlining Construction of Piston and Slide Valve Motors of European Origin. A-Reciprocating Piston Controls Inlet and Exhaust Passages. Note Peculiar Valve Actuating Mechanism. B-Slide Valve Operated by Face Cam Driven From Motor Crank Shaft by Bevel Gears.

Q. How can one time valves easily?

A. A very simple method of valve timing is outlined at Fig. 39 which shows the manner in which the various points at which valves open and close are indicated on the face of the flywheel rim. All circles may be divided into 360 parts. Each of these parts is called

Fig. 3.-Sectional View of Cylinder of Sphinx Motor, which Utilizes Sliding Ring Valve.

a degree. If the diameter of the flywheel is known its circumference can be obtained by multiplying the diameter in inches by 3.1416. If the circumference is then divided by 360 the distance or portion. of the flywheel circumference equivalent to one degree may be easily computed. The first operation is to bring the piston in one of the

cylinders, usually that of the first or fourth, to its uppermost position at the end of the compression stroke. A point is then indicated on the flywheel which will register with the vertical center line of the engine. This indicates the firing point for two of the cylinders of a four cylinder engine, though obviously one fires during one revolution of the flywheel, while the other explodes a complete revolution after. In the example indicated the flywheel is 18" in diameter. A crank travel of 15 degrees is equal to 23" measured on the flywheel circumference. Forty-five degrees, which is the lead of the exhaust valve, is equivalent to a distance of 71%". The lag of inlet closing or 35 degrees is equal to a distance of 512' while the lag of exhaust closing or 10 degreco te It will be evident