INTAKE AND EXHAUST SYSTEMS
6A1. Intake systems. All of our modern submarine diesel engines are of the 2-stroke cycle
type. The purpose of the intake systems in these
engines is to force out the exhaust gases of combustion as effectively as possible and to recharge
the cylinder with fresh air in order to support
combustion for the next succeeding cycle. The
supply of air must be in excess of that required
to just support combustion since the fuel is
thoroughly mixed with only part of the air compressed within the cylinder. The ratio of air to
fuel in most diesel engines is approximately 20 to 1 at full load.
6A2. Scavenging. The term scavenging is
used to describe the process of ridding the cylinder of burned exhaust gases during the latter
part of the expansion stroke and the early part
of the compression stroke of the 2-stroke cycle
engine. Scavenging is accomplished by admitting
fresh air under a pressure of about 1 to 5 psi
into the cylinder while the exhaust valves or
ports are open. This pressure usually is developed by means of a scavenging air blower. These
blowers are driven from the engines themselves
and generally are of the lobed rotor type, the
rotors revolving together in closely fitting housings. The process of scavenging must be carried
out in an extremely short period of time, depending upon the speed of the engine. The
burned gases must be blown out of the cylinder
and a fresh charge of air admitted during the
time that the ports or valves are open. For example, in an engine making 750 rpm with the
exhaust ports open for 140 degrees of crank
angle, the elapsed time the ports are open each
revolution is only (140/360) x (60/750) or approximately
1/32 of a second.
The scavenging air must be so directed
as to remove the burned gases from the remote
parts of the cylinder. The methods used may be
classified as follows: port scavenging (direct,
loop, and uniflow), and valve scavenging (uniflow ).
These methods are illustrated in Figures
6-1 to 6-4. In port direct scavenging, the exhaust
ports are on one side of the cylinder and the
scavenging ports on the other. In port loop scavenging, the exhaust and scavenging ports are
on the same side of the cylinder. In uniflow port
scavenging, the air enters at ports at the lower
end of the cylinder and passes out through ports
in the upper end of the cylinder.
In valve uniflow scavenging, air enters the
cylinder through ports in the bottom and passes
out through exhaust valves in the cylinder head,
carrying the burned exhaust gases with it.
The ports used for the inlet of scavenging
air are usually constructed so as to give the air a
whirling motion or turbulence to clear out all
possible exhaust gases and fill the entire cylinder
with a charge of fresh air.
In scavenging air systems, it is possible to
supercharge the cylinder during the air intake.
This is done by closing the exhaust ports or
valves slightly ahead of the inlet port closure.
This allows the air pressure in the cylinder to
build up to scavenging air pressure, increasing
the amount of air, the air-fuel ratio, and the
combustion efficiency. If the amount of fuel injected is increased to give the same air-fuel ratio
as before supercharging, the effect of supercharging is to give more power output to the
cylinder. In the present submarine type engines,
the F-M engine is supercharged, but the GM
engine is not.
6A3. Intake system components. The intake systems consist of the following parts:
a. Air intake silencers and strainers. Intake
air for submarine engines is drawn from the engine room compartments by the scavenging air
blower through air silencers and strainers. If
some type of air silencer were not used, the
noise of the intake air would be almost unbearable because of its high-pitched whistling sound.
Strainers are installed to remove any dirt or
other foreign matter that would otherwise enter
the scavenging blower or engine and cause
Figure 6-1. Port direct scavenging.
Figure 6-2. Port loop scavenging.
Figure 6-3. Valve uniflow scavenging.
Figure 6-4. Cross section of F-M cylinder with
uniflow port scavenging.
b. Scavenging air blower. The scavenging
air blower furnishes air under pressure to the
intake headers and receivers and eventually to
the cylinder inlet ports.
c. Air intake headers, receivers, and necessary piping. The air headers and receivers carry
the air from the scavenging air blowers to the
inlet ports of the cylinders. In most installations,
scavenging air headers and receivers are built
into the cylinder block. Drains are placed in the
scavenging air headers to drain off any liquids
that may have accumulated. Spring-loaded covers are also furnished in the scavenging air
header to allow the venting of excess pressure
in case of emergency.
d. Intake air ports. The intake air ports are
in the cylinder liner and permit the scavenging
air to pass from the scavenging air receivers into
the cylinder when the ports are open. The ports
are usually tangentially constructed so as to
give the air a whirling motion as it enters the
cylinders. They are usually opened and closed
by the reciprocating motion of the piston.
6A4. Exhaust systems. The purpose of the
exhaust system is to convey the burned exhaust
gases of combustion from the cylinders to the
atmosphere as silently as possible. The system
includes exhaust valves and ports, headers and
pipes, main inboard and outboard exhaust
valves, and engine mufflers.
The exhaust valves or ports, as the case
may be, are properly timed so as to permit the
gases of combustion to escape from the cylinder
at the correct point of the cycle. In the GM engine, this is accomplished by means of exhaust
valves; in the F-M engine, by means of exhaust
ports. Due to the heat that must pass through
these exhaust valves or ports, they must be
made of special material or be thoroughly cooled
to prevent distortion and pitting. Valves are
usually made of a high silicon heat-resistant
alloy steel. In some large engine installations,
the exhaust valves may be water or sodium
cooled. A thermocouple is usually placed at the
exhaust elbow to measure the exhaust temperature of each cylinder. When exhaust valves are
used, they are opened and closed by means of
rocker arm and camshaft assemblies. The exhaust ports, if used, are opened and closed by
the reciprocating motion of the pistons.
The exhaust headers or belts conduct the
exhaust gases from the exhaust valves or ports
to the atmosphere through an inboard and an
outboard exhaust valve and muffler. The exhaust manifold and exhaust elbows (if used)
are usually water jacketed to permit cooling of
the piping and manifolds. The cooling water
normally comes from the engine fresh water system. Cooling of these parts keeps down the temperature of the metal, thus prolonging its life
and reducing its expansion to a minimum. In
most exhaust systems, drains are provided to
allow drainage of any accumulated liquids from
the exhaust belts.
In submarine installations, the gases of
combustion are piped from the exhaust headers
to the outside of the submarine through an in
board and outboard main engine exhaust valve
and muffler. The inboard exhaust valve is inside
the pressure hull of the submarine and is hand
operated. The outboard exhaust valve is located
outside the pressure hull and is operated either
by hand or by hydraulic power, the controls for
the valve being at the throttleman's station at
the engine. Both inboard and outboard exhaust
valves are water cooled, the former usually by
water from the engine fresh water system, the
latter by water from the engine salt water
Mufflers are placed in the exhaust system.
This is necessary, because in a 2-stroke cycle
engine the uncovering of the exhaust ports releases
a pressure of 20 to 40 psi in the exhaust
system and this produces a noise that can be
heard for miles if not muffled by some form of
silencer. These mufflers are usually of cast or
sheet iron construction with a system of baffles
that break up the noise without producing back
pressure. There are two general types of mufflers
in use, the wet type and the dry type. In both
types, circulating water is used to reduce the
temperature of the exhaust gases as much as
possible. The difference between the two is that
in the dry type the exhaust gases do not come
in contact with the cooling water, whereas in
the wet type the gases are expanded in the
muffler in the presence of a water spray. The
exhaust gases in passing through the water spray
are cooled, condensed, decreased in volume, and
Figure 6-5. Typical exhaust system piping.
effectively silenced. Under normal operation, the
smoke is also eliminated. Submarine installations use the wet type of muffler. From the
muffler, the exhaust gases are passed out into
the atmosphere through a section of piping
known as the tail pipe.
B. GENERAL MOTORS INTAKE AND EXHAUST SYSTEM|
6B1. General description. The General
Motors engine employs the uniflow valve
method of scavenging. The blower, mounted at
the forward end of the engine crankcase and
driven by the engine, takes air from the atmosphere through an attached silencer and forces
it under pressure into the air box. The air box
consists of the frame space in the engine included between the two legs of the V-construction and the open space between the upper and
lower deckplates of each bank. The air from the
air box goes through the cylinder inlet ports
whenever the individual pistons uncover the
ports at the end of the expansion or power
stroke. This scavenging air forces out the exhaust gases and charges the cylinder with fresh
The exhaust gases are released from the
cylinder when the exhaust valves are opened by
action of the camshaft and rocker arm assembly.
The exhaust valves are opened ahead of the inlet ports to allow the pressure of the exhaust
gases to be partially released before the low-pressure scavenging air is admitted to the cylinder. The exhaust gases pass through the exhaust
valves into the water-cooled cylinder head and
thence into the exhaust elbow connecting each
cylinder head with the main exhaust manifold.
This manifold extends longitudinally along the
top centerline of the engine with elbow connections into each cylinder head. Thermocouples
for measuring the temperature of the exhaust
gases for each cylinder are located in each exhaust elbow. Both exhaust elbows and exhaust
Figure 6-6. GM cylinder intake and exhaust.
manifold are water jacketed for cooling purposes. From the main exhaust manifold, the
gases pass into a vertical pipe which leads to
the inboard exhaust valve. From this valve, the
gases pass outside the pressure hull, through exhaust piping which leads to the hydraulically
operated main engine outboard exhaust valve,
and thence to the atmosphere by way of the
muffler and tail pipe. The inboard exhaust valve
is cooled by water from the engine fresh water
system, while the outboard valve is cooled by
the engine salt water system.
Drains are provided in the piping between
the inboard and outboard exhaust valves so that
any salt water that may have leaked past the
outboard exhaust valve can be drained into the
engine room bilges. On a submarine it is extremely important that this space be drained
before starting an engine after surfacing from
submerged operations, otherwise the engine may
6B2. Scavenging air blower. The scavenging air blower is of the positive displacement
type consisting of a pair of rotors revolving together in a closely fitted housing. Each rotor has
three helical lobes which produce a continuous
and relatively uniform displacement of air. The
rotors do not touch each other or the surrounding housing. Air enters the housing at the top
and fills the spaces between the rotor lobes as
they roll apart. The air is carried around the
cylindrical sides of the housing, in the closed
spaces between the lobes and the housing. It is
forced under pressure to the bottom of the housing as the lobes roll together. Then the air passes
through the space between the inner and outer
wall of the blower housing and into the air box
around the cylinder liners.
Each rotor is carried on a tubular serrated
shaft. Endwise movement is prevented by two
taper pins. No gaskets are used between the end
plates and the housing due to the importance of
maintaining the correct rotor end clearance.
A fine silk thread around the housing and
inside the stud line, together with a thin coat of
nonhardening gasket compound, provides an air
Babbitted bearings in the end plates locate
the rotors in the two half-bores of the housing.
These bearings permit clearances to be held to
Figure 6-7. Cutaway of blower assembly, GM.
Figure 6-8. Front view of blower, GM.
a minimum between the rotor tips and the housing bores. Both ends of the rotor bearings have
thrust surfaces at the gear end of the blower.
The thrust surfaces locate the rotors endwise
and prevent contact between the rotors and the
The blower is driven from the crankshaft
through a quill shaft and through a train of
helical spur gears. The quill shaft is driven
through a serrated quill shaft coupling on the
crankshaft, and drives the main driving gear in
the train through a serrated connection in the
gear hub. The main drive gear transmits power
directly to the blower rotor driver gear. The
quill shaft coupling is fastened to the end of the
crankshaft and is driven through large dowel
pins. The rotor driver and driven gear are
closely fitted and rigidly attached to both rotor
shafts to prevent the rotors from touching as
they revolve. Each gear hub is pressed on the
serrated rotor shaft. A hexagon head lockscrew
in the rotor shaft holds a thrust collar as a
spacer between the gear hub and the end of the
rotor. The collar maintains clearance between
rotors and blower end plate.
The blower rotor gears are bolted to the
gear hub flanges and are located angularly by
dowel pins. Due to the importance of having the
rotors roll together without touching, yet with
the least possible clearance, it is necessary to
locate the dowel pins during assembly for a
given set of gears and hubs.
Oil passages in the end plates conduct lubricating oil under pressure to the blower bearings. Oil seals are provided at each bearing to
prevent oil from entering the rotor housing.
6B3. Intake silencer. The air is drawn into
the blower through an intake silencer mounted
on the blower intake adapter. The silencer is a
double sheet metal case with screened openings
at the top. Felt padding is cemented between
the double layers of metal at the top and sides
of the case. To minimize the noise caused by the
entering air, a perforated metal tube is welded
through the center of the case, and the upper
space between the outer shell and intake tube is
filled with sound-deadening material.
6B4. Air maze. A breather system is used to
prevent contamination of the engine room atmosphere by heated or fume-laden air which
Figure 6-9. Air silencer.
Figure 6-10. Cutaway of typical air silencer.
would otherwise escape from the crankcase.
This ventilation of the crankcase also reduces
the formation of sludge in the oil and prevents
any accumulation of combustible gases in the
crankcase and oil pan.
Atmospheric air for the breather system
enters the engine through the cylinder head
cover breathers. The blower suction draws air
from the crankcase through the air maze which
removes the oil from the vapor being drawn into
The air maze element consists of a number
of fine steel and copper wire screens that remove
the oil from the oil-laden air as it is drawn
through the air maze screens. The oil deposited
on the wire drips to the bottom of the air maze
housing, and then drains back to the sump tank
through a tube.
C. FAIRBANKS-MORSE INTAKE AND EXHAUST SYSTEM|
6C1. General description. The inlet or scavenging air system supplies the fresh air that
blows the exhaust gases out of each cylinder at
the end of the power stroke and recharges and
supercharges the cylinder for the next compression stroke. The air is drawn from the engine
room into the scavenging blower through an air
intake silencer. From the scavenging air blower,
the air is forced into two exhaust belts and receivers, one extending along each side of the
engine. These receivers conduct the air up to the
cylinder block compartments which surround
the cylinder liners at their inlet ports. These
ports direct the scavenging air tangentially into
the cylinder when the upper piston uncovers the
scavenging air ports. This air clears out the exhaust gases of combustion and fills the cylinder
with a charge of fresh air. As the lower crank
leads the upper crank by 12 degrees, the exhaust
ports are uncovered by the lower piston before
the inlet ports are uncovered by the upper
piston. The delay allows most of the pressure
of the cylinder to escape through the exhaust
ports before the relatively low pressure of the
scavenging air is admitted. The lower crank lead
also causes the lower piston to cover the exhaust
ports before the upper piston has covered the inlet ports. This allows the inlet air to be built up
in the cylinder to the scavenging air pressure,
resulting in a certain degree of supercharging.
From the exhaust ports, the exhaust gases
pass into the exhaust belt which encloses the
lower part of each cylinder liner to a height
slightly above the liner exhaust ports. The gases
then pass into two exhaust manifolds, one on
each side of the engine, along the manifolds to
the control end of the engine, thence through
two exhaust nozzles or elbows to the exhaust
piping which leads the exhaust gases up to the
inboard exhaust valve. The exhaust belts and
exhaust nozzles are cooled by fresh water from
the engine fresh water system. From the inboard
exhaust valve, the gases pass outside the pressure hull through the outboard exhaust valve,
muffler, and tail pipe to the atmosphere. As in
the GM installation, a drain is placed in the exhaust piping between the outboard and inboard
exhaust valves. Both inboard and outboard exhaust valves are cooled by water from the engine salt water system.
6C2. Scavenging air blower. Scavenging
air is supplied to the cylinders under a pressure
of from 2 to 5 psi by a positive displacement
type blower. The blower consists of the housing
which has inlet and outlet passages and encloses two three-lobe spiral impellers. The impellers are interconnected by timing gears driven
by a gear drive from the upper crankshaft.
Scavenging air from the atmosphere is
drawn through the air silencer and enters the
inlet passage of the blower. It is moved by the
lobes along the walls of the blower housing and
forced through the outlet passages and through
piping to the air receiver compartments on each
side of the cylinder block.
Due to the design of the impeller lobes, the
scavenging air is discharged from the blower at
a uniform velocity. Efficient operation is possible due to the small clearances between the
impellers, the impellers and the blower housing,
and the impellers and the bearing plates. Oil
should never be allowed to leak into the blower
housing or the air receivers. To permit removal
of any water that may enter the blower air
passages through the air silencer, or indirectly
through the exhaust manifold, drain tubes and a
Figure 6-11. Cross section through F-M scavenging
Figure 6-12. Blower impellers and timing gears, F-M.
drain tube cock are provided. This cock should
be opened before starting the engine if any abnormal condition is suspected. Opening of this
cock will drain the outlet air passages of the
blower and the lowest part of the housing.
Each impeller is cast on a splined shaft.
Each shaft turns in two roller bearings, the outer
bearing taking the shaft thrust. The bearings
are held by retainer rings in the end plates
which also locate the impellers with radial relation to each other. The thrust bearings prevent
contact between the ends of the impellers and
Figure 6-13. Blower assembly, timing gear end, F-M.
Power to drive the blower is transmitted
from the upper crankshaft through a flexible
gear drive that meshes with a drive pinion. The
drive pinion drives the blower driving timing
gear, on the end of the lower impeller, which in
turn transmits power to the driven timing gear
on the end of the upper impeller. The flexible
drive gear and drive pinion on the upper crankshaft are lubricated by oil sprayed through
nozzles from the engine lubricating system. The
blower timing gears and the inner and outer
bearings are lubricated by oil through tubes
from the engine lubricating system. Oil is
collected between the end cover and the inner
housing of the blower and drained to the vertical
drive housing from which it returns to the engine oil sump. Gaskets between the bearing
plates and blower housing form an oiltight seal.
6C3. Intake silencer. The intake silencer,
through which air is drawn before entering the
scavenging air blower, is similar to the GM unit
in design and construction. It is mounted directly over the inlet opening of the blower.
6C4. Oil separator.The upper crankshaft
and the lower crankcase compartments are
vented by means of a pipe connected to the suction side of the blower. In the vertical drive
compartment this vent line passes through an
oil separator, in which a copper ribbon screen
prevents oil from being carried into the blower
with air from the crankcase. Any leakage of lubricating oil from the side covers of the lower
crankcase is an indication that the separator
Copyright (C) 2004 Maritime Park Association
All Rights Reserved
Version 1.10, 22 Oct 04