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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 2312" measured on the flywheel circumference. Forty-five degrees, which is the lead of the exhaust valve, is equivalent to a distance of 778". The lag of inlet closing or 35 degrees is equal to a distance of 512" while the lag of exhaust closing or 10 degrees is equal to 118". It will be evident
Fig. 39.—Valve Timing Diagram Showing Spaces Laid Out on Flywheel, the Dimensions Given Cor
responding to the Number of Degrees of Crank Shaft Movement. The Dimensions Indicated in Inches are Only Suitable for Flywheel 18 in. in Diameter, though the Timing in Degrees May Be Used as a Guide for Many Four-Cylinder Motors.
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.)
Fig. 40.-Showing the Time of Opening and Closing of Inlet Valve
Relative to Crankshaft Travel.
EXHAUST VALVE OPENS.
EXHAUST VALVE CLOSE S.
Fig. 41,-Outlining Position for Opening and Closing Exhaust Valves. Q. How is the camshaft driven?
A. The camshaft may be driven by means of gearing as shown at Fig. 42 or by silent chains as outlined at Fig. 43.
Q. At what speed is the camshaft driven?
A. The camshaft of a four-cycle engine is always driven at half the engine speed and always by positive gearing.
Q. What is a cam?
A. A cam is a cylinder of metal having a raised portion at one point on its periphery.
Q. What is the difference between inlet and exhaust cams?
A. The difference between inlet and exhaust cams is in the cam profile as the member intended to lift the exhaust valve has a longer dwell or larger and longer raised portion because the exhaust valve is kept open longer than the inlet member.