Two large MAN diesel engines, each rated at 2170 HP and 470 RPM, are installed on the IXC submarine.  The engines are built for rugged service.  The peak and mean effective pressure per cylinder is high because of the installation of high speed superchargers.  Water jackets require particular care as the engine is cooled by salt water.  Furthermore, the engine must withstand high back pressures that are necessary when it is being used for producing exhaust gas for blowing ballast tanks.  
          The air induction and engine exhaust systems, as altered, were able to function for surface and submerged operations.  The installation for surface operation is more or less standard in nature.  For submerged operation a folding type snorkel was installed.  Its installation was accomplished without an excessive amount of change in either the engine or in any associated systems.  
July, 1946
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  1.  Introduction  
          No attempt will be made in this section to either describe in detail the diesel engine or comment on particular features of its design.  A six cylinder MAN engine, that has the same cylinders, pistons and attached pumps, and is, in most other respects, similar to the nine cylinder engine on the IXC, was installed on the XXI submarine and is to be reported on by the Engineering Experiment Station.  Furthermore, complete details, descriptions and operating instructions are contained in the "Description and Operating Instructions for the M9V 40/46C and N9V 40/46 CB Diesel Engines for U Boats Type IXC and IXD2, volumes I and II".  The major part of this report will be concerned with the adjuncts to the diesel engine that are peculiar to its installation on the submarine, in particular, the air induction and exhaust systems.  That part of the exhaust system related to the blowing of main ballast tanks is discussed in detail under the S49 section of this report.    
  2.  General Description  
          Two nine-cylinder, supercharged four-cycle MAN diesel engines with salt water cooling are installed on the IXC submarines.  On some vessels of this class the supercharger is driven by an exhaust turbine and on others it is driven mechanically.  The diesel engines provide power for propulsion or battery charging on the surface and submerges (running on snorkel) and provide exhaust gas for blowing main ballast tanks after surfacing.  (In lieu of low pressure air).  
          The snorkel operation was not contemplated in the original engine design as it was an alteration made after the vessel was placed in service.  The original diesel design had two sets of cams, one for ahead and one for astern operation.  When the snorkel was installed on the submarine the reverse cam on the diesel engine was removed and a "snorkel" cam was put in its place.  This special cam, by decreasing valve overlap, keeps the exhaust gas temperatures within allowable limits during periods of high back pressure.  The exhaust temperatures on the German MAN diesel run considerably higher than on the U.S. submarine diesels, maximum temperatures are 1022°F and 770°F respectively.  
          A vibration damper and vibration indicator are on the forward end of the crankshaft.  The damper is a mass inertia flywheel type with eight individual sets of leaf springs serving as the coupling agent.  Information on the indicator is contained in the S65 section of the IXC report.  
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          Each diesel engine normally has a 200 liter (7.06 cu. ft.) starting air flask.  The flasks provided are filled from the high pressure air line through a reducing valve set at 1060 psi.  A second reducing valve is inserted in the line to the diesel engine to decrease the pressure to 426 psi.  
          Two main air induction lines (port and starboard) run from immediately aft of the conning tower, along the outboard side of the pressure hull, and into the engine room at the forward end of the diesels.  The head valve lies just beneath the bridge deck.  To give added spray protection an air intake chute leads from this valve up through the conning tower fairwater.  The air induction pipe has an inside diameter of 20 in. and wall thickness of .236 in., and is tested to 355 psi so as to be able to take some additional depth charge pressure over and above submergence pressure.  Flanged construction is used throughout; all parts are made from medium steel and are galvanized.  Inboard of the hull valve the air passes through a flat rectangular duct (55 in. x 8.9 in.) that follows the contour of the hull to a point below the floor plates.  This serves to direct all water spray into the bilges.  
          The head valve and hull valve are very similar in construction.  They both close with pressure and have a sloping seat.  Dovetail type rubber gaskets are used on the disc; a special brass composition (similar to U.S. manganese bronze) is used for the valve seat.  Both valves are hand operated and have built-in locking features.  The valve bonnet for the head valve is part of a short bell-shaped casting that is secured to the induction piping and contains the valve seat.  A drain connection leads inboard from a point directly below this valve.  
          The valve bonnet for the hull valve is part of a casting to which the inboard duct is secured.  The casting serves to bring about a change in direction to the air as it comes inboard and passes into the rectangular duct.  The valve housings, discs, etc., are made from a good grade of medium steel.  
          Independent port and starboard exhaust systems are installed.  The exhaust from each engine passes through an inboard and outboard exhaust valve, through the muffler, a damper, an exhaust elbow, spark arrestor, and then overboard through openings just below the main deck level.  All units are waterjacketed.  
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  A galvanized steel line from the exhaust gas elbow between the inner and outer exhaust valves leads to the main ballast tank blow manifold and snorkel mast.  A cutout valve is installed on the line where it leaves the elbow.  The port and starboard lines are joined before they reach the main blow manifold.  The line to the snorkel mast is connected to this common line; a stop valve is installed to permit securing the exhaust to the snorkel when it is not in use.  
          The exhaust valves, the piping between them and the waterjacket on the inboard valve are tested at 284 psi.  The exhaust line between the exhaust elbow and the snorkel mast exhaust valve and main blow manifold is tested at 142 psi for tightness and 213 psi for strength.  The exhaust piping and fittings aft of the outboard exhaust valve, and all external circulating water piping and water jackets are tested at 28.4 psi.  
          The inboard exhaust valve is made up in two sections.  The lower section contains the valve seat and disc.  The upper section acts as a housing for the valve disc when the valve is opened.  The valve operates on a counterweighted hinged principle; a lever that is keyed to the hinge shaft is used to open and close the valve.  Provision is made for rotating the valve disc on its seat, the rotation serving to clean the carbon from the seat and to provide a slight amount of grinding in.  A worm keyed to an inner hinge shaft meshes with the geared periphery of the valve disc.  A ratchet arrangement on the same hinge shaft provides the means for giving the circular rotation to the disc.  A special tightening arrangement is also provided for the valve when it is in the closed position.  Leverage is applied to a bell crank, that is keyed to the outer hinged shaft, through a hand-wheel and its threaded shaft.  
          The outboard exhaust valve also works on the hinged principle and has a similar grinding in arrangement.  The main difference between the two valves lies only in the method of transmitting the motion to them for opening and closing as well as for rotating.  The outboard valve is opened by a handwheel that transmits its rotation through a gear-and-crank arrangement to the hinged shaft of the disc.  Tightening of the valve on its seat is accomplished directly by the handwheel in this case.  When the valve is opened the disc is housed in a recess.  In lieu of a ratchet arrangement to provide rotation to the disc a handwheel arrangement is used.  The handwheel  
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  is geared directly to the inner hinged shaft on which the worm that rotates the disc is located.  A spring is attached to the hinged shaft to take the weight of the disc when it is being closed.  
          All cutout valves in the exhaust line with the exception of those in the main blow manifold have provision for rotation of the valve disc on its seat.  It is accomplished on the smaller valves by an inner shaft that is keyed to the valve disc and which passes through the center of the valve handwheel.  Small drain and vent valves as well as the direct exhaust valves have this feature.  
          A dry type muffler was designed for installation on the IXC and IXD2 vessels.  The muffler is built in two circular sections.  The inner section consists of three separate cylinders spaced longitudinally and joined by small radial fins.  The outer section is the muffler shell proper and its water jacket.  In cross-section the inner cylinder and the shell form concentric circles with approximately equal gas volumes within the inner and outer gas space.  Zincs are installed to reduce corrosion of the muffler.  
          All expansion in the exhaust piping, both before and after the muffler, is taken up in the muffler.  The foundation for the latter is rigidly attached to the outer waterjacket.  Expansion between the waterjacket, muffler shell and inner inserted cylinders is provided at the after end by a packing gland, and between the inner and outer shell by another gland at the forward end.  Packing glands are also provided at the forward and after ends of the muffler where the main exhaust piping leads into it.  Means for ready flooding on diving and for venting and draining the waterjackets is also provided.  
          The exhaust damper is installed primarily for use when blowing ballast tanks.  It permits a ready regulation of both the volume and pressure of the exhaust gases that pass to the blow manifold and thus minimizes the variation necessary in engine output.  Without the damper, wire drawing through one of the main exhaust valves would be necessary which would, in turn, soon ruin the seat.  The damper is controlled by a handwheel in the engine room which imparts rotary motion to it through a gearing and crank arrangement.  
          The spark arrestor is a simple device for causing the sparks in the exhaust to be deposited in a pool of  
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  water.  Circular baffles direct the gasses entering the arrestor into the bottom of the large casing in which the water level is kept essentially constant.  Baffles on the outboard side of the arrestor direct the gas flow over the side.  
          Two inboard grease manifolds are installed for use with the operating gear in the exhaust gas system.  One manifold services all bearings, gears, shafts etc. forward of the mufflers and the other services those parts aft of the muffler.  The piping and fittings are tested to 284 psi.  
  3.  Snorkel Installation  
          Considerable information on snorkel developments and installations is contained in NavTechMisEu Technical Report 517-45.  Also much test data and information is being gathered at the Engineering Experiment Station.  
          A number of different types of snorkel installations have been installed on the type IXC submarines.  All of this class in U.S. custody had the folding type snorkel incorporating original design features.  These features included the float type of head valve and the air induction outlet from the mast at the bridge level, necessitating a gasket fit between the mast and the external air piping with the mast in the raised position.  The snorkel mast is located on the starboard side of the vessel abreast the after periscope.  On this class the mast rotates about a transverse axis at its base and houses in the forward superstructure.  The mast is approximately 28 ft. long and has a teardrop cross-section, 13" x 27" (approx.).  The forward cylindrical part is for air intake and the remaining after section is for exhaust.  The air intake valve is at the top and functions on a simple ball float and lever arrangement; i.e., when the float is submerged its buoyancy closes the valve, and when it is in the air its weight opens the valve.  The exposed head is approximately 25" x 45" in horizontal cross section.  A small radar detecting antenna is attached to the head fitting and projects approximately 12" above the top of the mast.  The exhaust outlet is about 51 inches below the air intake and is fitted with a head to deflect the gases downward and distribute them as widely as possible.  The air outlet is located at a point on the mast just below the bridge level.  A rubber gasket on the flanged face of the outlet seats against a raised portion on the stationary air induction pipe flange that it meets when the mast is raised.  The mast is hinged at the bottom and rotates  
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  on two bearings.  The short shaft that is necessary at the bottom of the mast is hollow, and forms the inlet to the mast for the exhaust gases.  A stuffing gland is formed between the rotary and stationary sections of the exhaust piping.  
          A hydraulic piston and lever arrangement is used for raising and lowering the mast.  A lever arm is attached to the center and at the front of the lower housing; it has an effective length of 26.4 ins. and swings through an arc of 94" on raising the mast.  The hydraulic cylinder has a bore of 9.85 ins. and is approximately 5 ft. overall in length.  The piston has a stroke of 37.2 ins.  The piston packing as well as the piston rod packing consists of two sets of "neoprene" chevron type gaskets (single lip); the sets are opposed and therefore seal from both directions.  A small drain off connection leads from the center of the rod seal to inside the pressure hull and acts to limit salt water contamination of the hydraulic oil.  A cross-head and crank are attached to the piston rod to give the necessary motion to the snorkel mast lever arm.  
          The air induction line that leads from the mast is connected into the top of the ship's outboard ventilation valve.  A separate valve and strainer are attached to this line for use in flooding the mast prior to lowering it.  A separate crossover line leads from the ventilation air induction line to the engine air induction line.  Thus, when operating the system it is necessary, with this setup, to open both the outboard ventilation induction valve and either the inboard engine or ventilation air induction hull valves.  
          The exhaust line from the mast directly to the snorkel exhaust gas valve previously mentioned.  
          During operation the ship should e submerged to the depth that brings the waterline between the air inlet and exhaust openings so that the gases will exhaust underwater.  This will provide constant cooling of all ducts carrying the hot exhaust gases.  
          An improvement in snorkel mast design on other vessels is incorporated in the elimination of the gasketed joint at the air outlet.  The air outlet was put in the bottom swivel joint opposite that of the exhaust; a similar packing gland to that on the exhaust side was installed between the rotary and stationary parts.  
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          The float type valve proved dangerous and did not provide the rapidity of valve action desired.  Whenever an appreciable vacuum built up in the vessel the external air pressure would tend to seat the valve.  This would accentuate an already bad condition as the engines would be forced to draw more air from the vessel and thus further increase the vacuum.  
          Snorkel operation was carried on in one of three normal ways as follows:  
          (a)  Running one engine and its motor as a generator.  The power supplied was used to carry a float on the battery and at the same time to run the motor on the opposite shaft for propulsion.  
          (b)  Running one or both engines for propulsion alone.  
          (c)  Running one engine for battery charge and the other engine for propulsion.  
          The operation of the plant when running on the snorkel is as follows:  
          (a)  The water in the air intake part of the mast is drained into the vessel.  
          (b)  The exhaust line between the inboard exhaust valve and the engine is drained.  
          (c)  The engine is started and the back pressure is allowed to build up to approximately 10 psi at which time the inboard exhaust valve and the snorkel valve near it are opened.  This will force any water in the exhaust elbow or horizontal line to the mast toward the snorkel foot valve.  When the pressure is re-established the foot valve is then cracked to permit the exhaust pressure to blow the water out of the exhaust side of the mast.  Full control on the operation is maintained with the foot valve.  If the head of water at this point is too great so as to require back pressures that will stall the engines the valve can be secured and the head pressure will be released from the engine.  Even though the engine may have stalled it will not flood because of the volume of exhaust  
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  gases in back of the water.  Unless the snorkel mast is well under water (10 ft. or more) the head pressure is not in excess of the back pressure that can be built up by the engine, so that under normal circumstances the water is ejected in several seconds and the engine is operating under low back pressures.  No trouble is encountered with the exhaust line thereafter unless the engine stops for some other reason as the water heads involved are slight compared to those present on starting.  As for the air intake line, once it is initially drained, the water that gets into it during operation is removed by the inboard drain on the water trap.  The most serious condition arises when the valve sticks open after it submerges in which case the overflow from the trap goes into the engine room bilges.  The size of the trap and its drain line however should be sufficient to handle most of the water.  Should the ship's ventilation blowers be taking a suction directly from the induction line they may receive a slug of water that will damage the impeller.  
  On some of the German submarines provisions are made for blowing the supply and (or) exhaust sides of the mast with air, and for draining the exhaust line inboard.  These give additional means for eliminating the water in the mast and the external piping and, thereby, insure easy starting of the engines for snorkel operation.  In practice, however, the normal procedure is that outlined above.  
  4.  Individual Components  
          (a)  Diesel Engine  
                  (1)  General characteristics  
Cylinder Diameter
15.75 in
18.1 in.
Displacement Volume
3530 cu. in.
Nominal Rating
2170 H.P.
Mean Turning Moment
24,200 ft. lbs.
Mean Piston Velocity
23.6 ft./sec.
Weight dry with blower
56,100 lbs.
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                  (2)  Ratings with Gas Turbine Supercharger  
lb/HP hr.
Highest 1/2 hr. -
Highest 2 hr. -
Highest Continuous -
3/4 Rated Power -
1/2     "         " -
1/4     "         " -
1/10   "         " -
Highest Continuous without supercharger -
                  (3)  Ratings with Mechanically Driven Supercharger  
lb/HP hr.
Highest 1/2 hr. -
Highest 2 hr. -
Highest Continuous -
                  (4)  Working Cylinder  
Compression volume - 253 cu. ins.
Compression ratio - 14.4/1
                  (5)  Valve timing  
Exhaust open
45° before bottom center 42° before bottom center
Exhaust close
55° after top center 21° after top center
Intake open
75° before  "    " 23° before "      "
Intake close
35° after bottom center 29° after bottom center
                  (6)  Exhaust Gas Turbine Supercharger  
Suction volume - 100 C.F.M.
Pressure Diff. - 3.84 psi
RPM (Normal) - 9400
  5.  Conclusions  
          The diesel engine selected for the IXC has several interesting compromises in design.  In order to obtain the high power desired in each of the two engines for the limited direct drive RPM it was necessary to highly supercharge the engine.  This was accomplished effectively by  
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  by the exhaust turbine supercharger.  However, in so doing, design limitations in engine size required the acceptance of high exhaust temperatures for a submarine installation.  The resultant mean effective pressure and power for each cylinder are high, and the fuel rate over a wide range of operation is low.  
          The practicability of the use of a snorkel with the diesel engine for submerged propulsion and battery charging was proved by the installations on this and other earlier types of submarines.  Although serious difficulties arose from the first installations, they were operated with considerable success and pointed the way to improvements in the snorkel design to make its use less hazardous.  It gives the submarine that has such an alteration a marked advantage in well patrolled enemy waters over one that is not so equipped.  
          The features of the design of both the exhaust gas and the air induction systems as they tie into the snorkel installation are of interest.  However, in general, the systems as used for surface propulsion are standard in nature and are of little exploitation value.  
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