Figure 3-50.Valve shapes.
mates with a crowned tappet face. The camshaft lobe
does not meet the tappet in the center of its face. The
design causes the tappet face to rotate on the cam lobe,
rather than slide. This greatly increases component life.
Valve and Valve Seats
Each cylinder in a four-stroke-cycle engine must
have one intake and one exhaust valve. The valves that
are commonly used are of the poppet design. The word
poppet is derived from the popping action of the valve.
Poppet-type valves are made in the following three
basic shapes: semitulip, tulip, and mushroom (fig.
3-50). The valve shape used in a given engine depends
on requirements and shape of the combustion chamber.
Construction and design considerations are very
different for intake and exhaust valves. The difference
is based on their temperature operating ranges. Intake
valves are kept cool by the incoming intake mixture.
Exhaust valves are subject to intense heat from the burnt
gases that pass by it. The temperature of an exhaust
valve can be in excess of 1300°F. Intake valves are
made of nickel chromium alloy. Whereas, exhaust
valves are made from silichrome alloy. In certain
heavy-duty and most air-cooled engines, the exhaust
valves are sodium filled. During engine operation, the
sodium inside the hollow valve melts. When the valve
opens, the sodium splashes down into the valve head
and collects heat. Then, when the valve closes, the
sodium splashes up into the valve stem. Heat transfers
out of the sodium, into the stem, valve guide, andengine
coolant. In this way, the valve is cooled. Sodium-filled
valves are light and allow high engine rpm for
In vehicles that use unleaded fuel, a stellite valve is
preferred. A stellite valve has a special hard metal
coating on its face. Lead additives in gasoline, other
than increasing octane, act as a lubricant. The lead coats
the valve face and seat to reduce wear. With unleaded
fuel, the wear of the valve seat and valve face is
accelerated. To prevent this and prolong valve service
life, use a stellite valve.
Valve seats are important, as they must match the
face of the valve head to form a perfect seal. The seats
are made so they are concentric with the valve guides;
that is, the surface of the seat is an equal distance from
the center of the guide all around Although some
earlier engines were designed with flat contact surface
for the valve and valve seat, most are now designed with
valve seat angles of 30 to 45 degrees, as shown in figure
3-51. This angle helps prevent excessive accumulation
Figure 3-51.Valve-to-valve seat relationship.