Engine Valve Train
Most engine subsystems can be implemented
using different technologies, and better technologies can improve the
performance of the engine. Let's look at all of the different subsystems used
in modern engines, beginning with the valve train.
The camshaft
The valve train consists of the valves and a
mechanism that opens and closes them. The opening and closing system is called
a camshaft. The camshaft has lobes on it that move the valves up and
down
Most modern engines have what are called
overhead cams. This means that the camshaft is located above the valves, as
you see in Figure 5. The cams on the shaft activate the valves directly or
through a very short linkage. Older engines used a camshaft located in the sump
near the crankshaft. Rods linked the cam below to valve
lifters above the valves. This approach has more moving parts and also
causes more lag between the cam's activation of the valve and the valve's
subsequent motion. A timing belt or timing chain links the
crankshaft to the camshaft so that the valves are in sync with the pistons. The
camshaft is geared to turn at one-half the rate of the crankshaft.
Many high-performance engines have four valves per cylinder (two for intake,
two for exhaust), and this arrangement requires two camshafts per bank of
cylinders, hence the phrase "dual overhead cams."
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