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Q. Name simple rules for size of exhaust ports.
A. Usually the diameter of the exhaust port equals that of the piston multiplied by .35 on low speed engines and .50 on high speed engines. The area of the exhaust passage should equal that of the piston multiplied by the speed and divided by 5000 as a minimum.
Q. When should the inlet valve open ?
A. The inlet valve Intate
should open from ten to fifteen degrees crank travel after the piston
leaves its innermost Piston
position in the cylinder.
Q. When should the inlet valve close ?
A. The inlet valve closes from fifteen to thirty-five degrees after the piston starts to go up on its compression stroke.
Q. What is meant by valve lead?
A. If a valve opens Fig. 33.—Sectional View of Cylinder Utiliz- before the piston starts ing Mead Rotary Valve.
on the stroke corresponding to valve opening, the valve is said to open early or to have lead.
Q. What is meant by valve lag?
A. If a valve opens late or remains open after it is theoretically supposed to close, it is said to have lag.
Q. When should exhaust valve open?
A. The exhaust valve usually opens about 45 degrees as indicated by crank travel before the piston reaches the end of its power stroke. In this case the exhaust valve has a lead of 45 degrees.
Q. When should the exhaust valve ciose?
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
Intäke 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 upon its compression stroke it Exhaust
Valve 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 actual 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 Begin-
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
Shaft by Bevel Gears.
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