After passing through the turbine end of the supercharger of a 4-stroke cycle engine or after being
discharged from the cylinders of a 2-stroke cycle engine, the gases-pass through the exhaust pipe, which may
be either flexible or rigid to the silencer or muffler. The gases are discharged from the silencer to the
atmosphere through the tail pipe or overboard discharge pipe. As the name implies, silencers or mufflers are
provided on internal-combustion engines mainly to reduce noise created by the exhaust as the exhaust valves or
ports open. This noise could be reduced by placing sound-absorbent, flameproof material in the exhaust
passages, but the resulting back pressure might prevent the engine from operating. Because of the serious
effects of back pressure upon the operation of the engine, the noise-reducing device must be so designed that a
minimum exhaust back pressure is created. Since seawater is readily available, most modern marine engines
use an exhaust silencer of the wet type. The muffler shown in Figure 9-10 is also of the wet type. Wet-type
mufflers are usually of cast or sheet iron construction, with a system of internal baffles which break up the
exhaust gas pulsation. Therefore, a silencing effect is obtained without producing a back pressure in the
system. The water used in wet-type silencers also aids in reducing noise. The water cools the exhaust gases,
causing the gases to contract. The decrease in volume reduces the velocity of the exhaust gases and thereby
reduces the exhaust noise. The water itself absorbs some of the sound. The silencer consists of a steel drum
which is divided into two compartments by a transverse baffle plate. The exhaust inlet pipe extends part way
through the inlet compartment so that the exhaust gases must circle back, passing through a stream of water,
before entering the pipes which extend through the baffle plate. In the outlet compartment, the exhaust gases
are again deflected before they enter the water outlet pipe. A second stream of water enters the tail pipe and
helps carry the gases to the overboard discharge. Some marine engines use a dry-type exhaust silencer. In both
the wet-and dry-type silencers of most marine installations, circulating water is used to reduce the temperature
of the exhaust gases. The principal difference between the two is that in the dry-type, the exhaust gases do not
come in contact with the cooling water. In other words, the water does not flow through the silencer
compartment but flows instead through a jacket around the silencer. In wet-type silencers, the gases are
expanded into the silencer and come in direct contact with the water. In passing through the baffles and
through the water, the gases are cooled, condensed, decreased in volume, and effectively silenced. In addition
to acting as silencers, most mufflers also function as spark arresters. The water in wet-type silencers serves as a
spark arrester. In some dry-type silencers, a device is incorporated to trap burning carbon particles and soot.
The interiors of all mufflers are subjected to moisture-condensation in the dry-type mufflers and the supplied
water in the wet-type mufflers. Because of this, most silencers are coated inside and out with a resistant
material.