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.
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.Continue Reading