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 damage.


Figure 6-1. Port direct scavenging.
Figure 6-1. Port direct scavenging.

Figure 6-2. Port loop scavenging.
Figure 6-2. Port loop scavenging.


Figure 6-3. Valve uniflow scavenging.
Figure 6-3. Valve uniflow scavenging.

Figure 6-4. Cross section of F-M cylinder with
uniflow port 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 system.

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.
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.
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 air.   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.
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 be flooded.

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 tight seal.

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 6-7. Cutaway of blower assembly, GM.

Figure 6-8. Front view of blower, 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 end plates.

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 6-9. Air silencer.

Figure 6-10. Cutaway of typical 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 blower.

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.

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
air blower.
Figure 6-11. Cross section through F-M scavenging air blower.

Figure 6-12. Blower impellers and timing gears, F-M.
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 the housing.

Figure 6-13. Blower assembly, timing gear end, F-M.
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 needs cleaning.

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