The exhaust manifold (fig. 4-47) connects all the
engine cylinders to the exhaust system. It is usually
made of cast iron. If the exhaust manifold is properly
formed, it can create a scavenging action that will cause
all of the cylinders to help each other get rid of exhaust
gases. Back pressure (the force that the pistons must
exert to push out the exhaust gases) can be reduced by
making the manifold with smooth walls and without
any sharp bends. All these factors are taken into
consideration when the exhaust manifold is designed,
and the best possible manifold is manufactured to fit
into the confines of the engine compartment.
MANIFOLD HEAT CONTROL VALVE
On some gasoline engines, a valve is placed in the
exhaust manifold to deflect exhaust gases toward a hot
spot in the intake manifold until the engine reaches
operating temperature (fig. 4-48). This valve is a flat
metal plate that is the same shape as the opening that
controls it. It pivots on a shaft and is operated by a
thermostatic coil spring. The spring pulls the valve
closed against a counterweight before warm-up. The
spring expands as the engine warms up, and the
counterweight pulls the valve open.
The muffler (fig. 4-49) reduces the acoustic
pressure of exhaust gases and discharges them to the
atmosphere with a minimum of noise. The muffler
usually is located at a point about halfway in the vehicle
with the exhaust pipe between it and the exhaust
manifold and the tailpipe leading from the muffler to the
rear of the vehicle.
The inlet and outlet of the muffler usually is slightly
larger than their connecting pipes, so that it may hook up
by slipping over them. The muffler is then secured to
the exhaust pipe and tailpipe by clamps.
A typical muffler has several concentric chambers
with openings between them. The gas enters the inner
chamber and expands, as it works its way through a
series of holes in the other chambers and finally to the
atmosphere. They must be designed also to quiet
exhaust noise while creating minimum back pressure.
High back pressure could cause loss of engine power
and economy and also cause overheating.
Figure 4-47.Exhaust manifold.