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. 37.-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 7g". The lag of inlet closing or 35 degrees is equal to a distance of 51/2" while the lag of exhaust closing or 10 degrees is equal to 1". It will be evident Fig. 38.-Side Sectional View of Sphinx Motor Showing Peculiar Annular Exhaust and Intake Passages and Ring Valve Controlling the Admission and Expulsion of Gas. Fig. 39.-Valve Timing Diagram Showing Spaces Laid Out on Flywheel, the Dimensions Given Cor- 78 that the valves may be timed to open and close very accurately if the engine flywheel is provided with the proper marks and a suitable trammel or other indicator is placed on the engine base to indicate the center line of the engine. Q. What is the camshaft? A. The small auxiliary shaft that carries the valve lifting cams and usually runs parallel with the crankshaft and which is driven by that member is called the camshaft. Some engines have but one camshaft, which carries the cams utilized in operating both inlet and exhaust valves. The "L" type cylinder engine needs but one camshaft while the power plant provided with "T" head cylinders needs two camshafts, one at each side of the motor. (Figs. 23 and 24.) INLET VALVE OPENS. INLET VALVE CLOSES. Fig. 40.-Showing the Time of Opening and Closing of Inlet Valve Relative to Crankshaft Travel. |