(4) Valves. There are two valves (Figure 15) at the top of the
cylinder. A valve is an accurately machined plug that fits into a machined
opening at the top of the cylinder. When the valve is resting in this
position, it is said to be seated. When a valve is so positioned, it is closed
and the opening is sealed off. When the valve is pushed off its seat, it is
opened. The valves in the cylinder are closed part of the time and opened part
of the time. One of the valves, called the intake valve, opens to admit a
mixture of fuel and air into the cylinder. The other valve, called the exhaust
valve, opens to allow the escape of burned gases after the fuel and air mixture
has burned. Valves are opened by the camshaft rotation and closed by a spring.
Figure 15. Intake and Exhaust Valves.
(5) Camshaft. The camshaft is designed to open the intake valve at the
beginning of the intake stroke and hold it open long enough to obtain the most
efficient filling of the cylinder. Likewise, the camshaft opens the exhaust
valve at the beginning of the exhaust stroke and holds it open long enough to
obtain the most efficient emptying of the cylinder. In most engines, the lobes
on the camshaft are designed to open the valves smoothly and gradually. This
avoids shock to the valves and allows quietness of operation.
The final design is usually a compromise between efficiency and quiet
operation. When the cam lobe rotates out of the way, the valve is closed and
held closed by spring pressure. In fourstroke engines, each valve is opened
every other revolution of the crankshaft, so the camshaft is geared to run at
onehalf the crankshaft speed.