The parts of engine air systems considered so far have provided a passage for air into the cylinders and for the release of gases from the cylinders after combustion. The relationship of blowers, or turbochargers, to both the intake and exhaust systems has also been pointed out.

The system that functions primarily to carry gases away from the cylinders of an engine is called the EXHAUST SYSTEM. In addition to this principal function, an exhaust system may be designed to perform one or more of the follow-ing functions: muffle exhaust noise, quench sparks, remove solid material from exhaust gases, and furnish energy to a turbine-driven super-charger. In the following sections, we will discuss the principal parts that may be used in combination to accomplish the functions of an engine exhaust system.

EXHAUST MANIFOLDS

When the gases of combustion are forced from the cylinders of an engine, the gases enter a unit that is generally referred to as the MANIFOLD. The EXHAUST MANIFOLD for a large diesel is shown in figure 6-14. This exhaust manifold is made up of sections called chamber assemblies along with expansion joints and adapter assem-blies. The expansion joints, which are used be-tween chamber assemblies, provide the necessary flexibility to compensate for expansion and contraction of the manifold resulting from temperature changes. Some manifolds are made of steel plate with welded joints and branch elbows of steel castings, while others are made of aluminum. The exhaust manifolds of most marine engines are generally cooled by water flowing through a water jacket surrounding the manifold. The exhaust manifold shown in figure 6-14 is for the passage of gases from the combustion spaces to the exhaust inlet of the turbocharger. Thus, the turbine end of the turbocharger is considered to be part of the exhaust system since it forms part of the passageway for the escape of gases to the exhaust outlet.

After passing through the turbine end of a turbocharged engine or the exhaust manifold of a naturally aspirated 4-stroke cycle engine or being discharged from the exhaust manifold of a 2-stroke cycle engine, the gases pass through the exhaust pipe (flexible or rigid) to the silencer, or muffler.

Silencers, or mufflers, are placed on internal combustion engines mainly to reduce the noise created by the exhaust gases as the exhaust valves or ports open. In addition to acting as silencers, most mufflers also act as spark arresters that trap the burning carbon particles and soot from the mufflers as the exhaust gases are directed to and discharged into the atmosphere.