Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(138) High Speed Engine Valve Timing.

The faster a motor turns, all other things being equal, the greater the amount of advance necessary with the valves, as the higher the speed the less the time required to fill or empty the cylinder. In a short stroke high speed motor the exhaust should close and the intake open as early as possible in order to admit the full charge. The exhaust should open early to allow of the full escape of the gases, as the time allowed for expulsion is extremely short when an engine runs 1,000 R.P.M. and the back pressure is liable to be considerable.

The inlet valve of high speed engines should remain open for a considerable period after the crank passes the outer dead center on the suction stroke, owing to the inertia of the gases which tends to fill the cylinder. Lengthening the period of opening of the inlet valve in multiple cylinder engines produces better carbureting conditions and reduces the variations of pressure in the manifold.

EXHAUST VALVES. The exhaust valve should begin to open 40° BEFORE the crank reaches the OUTER dead center on the working stroke, and should close 10° AFTER the crank has passed the inner dead center.

INLET VALVES. The inlet valve should open 15° AFTER the crank passes the inner dead center on the suction stroke, and should close 35° after the crank passes the outer dead center.

The inlet valve should never open before the exhaust valve closes, although this is done on several types of high speed aeronautical engines. The makers of these engines claim that this practice scavenges the combustion chamber more thoroughly and makes the mixture more effective owing to the inertia of the burnt gases forming a partial vacuum in the combustion chamber. The writer has never been able to get satisfactory results with this timing and doubts whether it can be accomplished successfully.

In timing an engine great care should be taken to get the crank exactly on the dead center.