A Captured Japanese Diesel

A small Diesel engine removed from a captured Japanese landing boat was recently tested at the U. S. Naval Engineering Experiment Station. The engine was operated extensively on a dynamometer test-stand and was completely disassembled and critically analyzed with regard to fea­tures of design and construction. It was found to be a good, sturdy, low-output en­gine, extremely simple in design and construction, and requiring a minimum of first- class workmanship in manufacture.

The most similar American engine, al­though not generally used in landing craft, is probably the Navy type DD Diesel boat engine. As a method for appraising the Japa­nese engine, therefore, we shall compare it to the Navy type DD engine. Comparative data for the two engines are presented in Table 1.*

Table 1 shows that the two engines are closely similar in general type and have nearly equal displacements, that the respec­tive specific fuel consumptions differ by only 2 per cent. The Japanese engine, however, is 33 per cent heavier than the Navy type DD engine on a weight-per-horsepower basis. This greater specific weight might suggest that the Japanese engine is more reliable, but more expensive. Before discussing the factors of reliability and cost of production, however, let us present a condensed description of this Japanese Diesel. We shall consider in turn the four engine systems and the major parts and assemblies.

Lubricating oil system.—The gear-type lubricating oil pump was mounted on the No. 7 main bearing cap and was gear driven from the crankshaft. Suction was taken di­rectly from the crankcase sump through a single strainer of 34 mesh. Lubricating oil from the pump was discharged through a jumper line to a drilled passage in the cylin­der block leading to an external lubricating oil header running the length of the engine. This header connected with drilled passages leading to the main bearings and with an external line leading to the lubricating oil cooler. The crankshaft was drilled for lubri­cation of the crankpins; the connecting rods were not drilled for pressure lubrication of the piston pins.

Lubricating oil pressure was regulated by a spring-loaded by-pass valve in the oil cooler, which returned the by-passed oil to the crankcase sump. Only by-passed oil flowed through the oil cooler. There was also a manual by-pass valve in the oil cooler by which the by-passed oil could be returned directly to the sump without passing through the cooling tubes of the oil cooler. The main gear train bearings and the No. 6 camshaft bearing were lubricated through drilled passages in the cylinder block and cylinder head. At the No. 6 camshaft bearing oil entered the drilled camshaft and was dis­charged through drilled holes at the camshaft bearings and in the base circle sectors of the cams. There were oil accumulator pans under the respective cams, and the rocker arm shafts were splash lubricated. Oil drained from the head to the sump over the main gear train. The pistons and cylinder walls were splash lubricated.

The wrist pin bearing surfaces in the piston received oil through drilled passages in the piston from the groove below the oil scraper ring. The connecting rod-wrist pin bearing surface received splashed oil through a coun­tersunk hole in the top of the connecting rod eye. The lubricating oil cooler contained U- shaped copper tubes, through which the oil passed; the sea water flow was perpendicular to the oil flow, with suitable baffles provided to obtain a two-pass flow. This design of oil cooler probably has a somewhat lower cool­ing efficiency than current American types.

Table 1

Comparative Data: Japanese Landing Boat Diesel Engine and U. S. Navy Type DD Diesel Engine

	Japanese Engine	Navy Type DD Engine
Number of Cylinders	6	6
Operating Cycle	4-stroke	4-stroke
Arrangement of Cylinders	Vertical, in line	Vertical, in line
Bore, in.	4.33 (110 mm)	4.25
Stroke, in.	5.52 (140 mm)	5.50
Combustion Chamber, type	Open	Lanova
Type of Injection	Solid	Solid
Rated Horsepower	65	105
Rated Speed	1600	1885
BMEP, psi	66.0	94.2
Weight, Dry, lb	1490	1810
Weight, lb. per rated hp.	22.9	17.2
Fuel Cons, at rated hp, lb/bhp-hr.	0.506	0.495
Exhaust Temperature, °F	1113	1140
Cooling System, type of	Sea Water	Fresh Water
Number of Intake Valves per Cylinder	1	1
Number of Exhaust Valves per Cylinder	1	1
Fuel Pump, type	Similar to Bosch APE	Ex-cell-o
Injection Nozzle, type	Open	Closed
Nozzle Tip, number of orifices	2	Pintle
Nozzle Spray Holes, diameter, in.	0.008	—
Nozzle Tubing, outside diameter, in.	0.239	0.250
Nozzle Tubing, inside diameter, in.	0.068	0.094
Camshaft, Location of	Overhead	Cylinder Block
Number of Cams per Cylinder	1	2
Main Bearings, number of	7	7
diameter of, in.	3 ¼	3
length of thrust (No. 1)	1 ¾	2 1/16
length of intermediate	1 9/32	1 ¼
length of No. 7	1 5/6	2 ¼
Connecting Rod Bearings, diameter of	2 7/8	2 ¼
length of	2 3/32	1 15/32
Valve Timing: Intake Opens	20°BTC	12°BTC
Intake Closes	37°ABC	3 6° ABC
Exhaust Opens	31°BBC	35°BBC
Exhaust Closes	18°ATC	6°ATC

Fuel oil system.—The fuel transfer pump contained a 55-mesh strainer and was of the conventional diaphragm type, operated by an elliptical cam on the engine camshaft. Fuel from the transfer pump flowed through a strainer consisting of two wire screens of 59 by 41 and 85 by 57 mesh, respectively, and then to the fuel injection pump, which de­livered fuel to the six injection nozzles through steel tubes of equal length. Each nozzle contained an edge-type strainer simi­lar to those used in some Bosch nozzles. Fuel transfer pressure was regulated by a spring-loaded by-pass valve in the fuel in­jection pump body; the by-passed fuel was returned to the transfer pump suction con­nection.

Cooling water system.—The engine was di­rect sea water cooled. From the gear-type water pump the sea water was discharged via the oil cooler into the cylinder block; it then flowed up through cored passages to the cylin­der head, from there entered the exhaust manifold water jacket, and was then dis­charged from the engine. No cooling water thermostat was provided, but there were provisions for a manual by-pass arrangement whereby the cooling water could be recircu­lated.

Air system.—Air was inducted on the port side of the engine through three intake si­lencers, each containing a single screen of 8 mesh, provided with sliding top covers ap­parently for the purpose of burning fuel- soaked waste in the silencers as a cold starting aid. The exhaust was discharged into a water-jacketed exhaust manifold mounted on the starboard side of the cylinder head.

Cylinder head.—The cylinder head was a single iron casting, conventional in design. The open type combustion chamber was contained entirely in the cylinder head. Each cylinder was provided with a manual screw- type compression release plug. The head car­ried both intake and exhaust manifolds, the injection nozzles, the fuel transfer pump, and supported the engine camshaft.

Cylinder block.—The cylinder block was a well-designed conventional iron casting with ample cooling water passages. There were full length water jacket clean-out openings on both sides of the block. The upper half of the flywheel housing was cast integral with the block. Cylinder liners were not used. Well-braced and heavily constructed main bearing saddles supported the crankshaft. An external breather line connected the crank­case to the center air silencer.

Bearings.—The main and connecting rod bearings were of the steel-backed precision insert type, apparently of copper-lead alloy composition. Thrust was taken by the for­ward main bearing. The piston pin-connect­ing rod bearings and the camshaft bearings were of bronze composition.

Pistons.—The pistons were of aluminum alloy composition and had an excellent sur­face finish. The three compression rings and one oil ring were of the angle-joint type and were located above the piston pin. There was a circumferential oil groove at the center of the wrist pin bosses. The wrist pin was retained by conventional snap rings and lubri­cated through drilled passages connecting the wrist pin bosses and a groove just below the oil ring. The pistons were not oil cooled.

Connecting rods.—The connecting rods were of conventional design and rather heavy forged steel construction. The bearing caps were held by four bolts, and the upper bear­ing shells were located by dowel pins. The rods were not drilled for pressure lubrication of the piston pins, which received oil by splash through countersunk holes in the tops of the connecting rod eyes.

Crankshaft and flywheel.—The crankshaft was of conventional design and was drilled for pressure lubrication of the crankpin journals. The flywheel, located at the for­ward end of the engine, was of composite construction, consisting of a heavy flange on which the starting gear teeth were cut, to which was bolted a heavy rim to mate with the cone clutch of a power take-off. The fly­wheel was graduated in degrees over appro­priate sectors and was marked for the respec­tive top and bottom center positions of all cylinders.

Fuel pump.—The fuel injection pump was similar in principle and design to the Bosch type APE pump except for a difference in plunger design. The Japanese plunger had a diagonally milled step in lieu of the Bosch helix, yet both plungers apparently produced the same characteristics of fuel delivery. The Japanese pump produced variable beginning and constant ending of injection, was conse­quently timed for port opening. The fuel pump cams were contoured to produce an ex­ceedingly rapid fuel delivery.

Injection nozzles.—The injection nozzles were of the open type and unusually simple in construction. A notch in the nozzle tip flange permitted combustion gases to contact nearly one inch of the tip and thus to preheat the fuel. The nozzles contained two spray holes of 0.008 inch diameter.

Valve gear.—The intake and exhaust valves were of the conventional poppet type, of equal size, and each was retained by two springs. The valve stems were splash lubri­cated, with double felt washers around the stems aiding the lubrication. Valve tappet clearance was adjusted by means of a flat end set screw working directly on the top of the valve stem. The overhead camshaft had one cam per cylinder, which operated directly on the slightly rounded rocker arm pads.

Miscellaneous.—The governor was of the variable speed mechanical type and was gear driven from the crankshaft. It was designed to contain one spring in the governor housing and one external spring in the fuel pump rack linkage. The electrical equipment was de­signed for 24 volts, was equipped with both ball and sleeve-type bearings. The starting motor was similar to the Bosch design in which the entire armature moves axially to engage the starting motor pinion with the flywheel gear. The reverse gear was of the conventional planetary type, with bevel gears; it provided no reduction of engine speed in either forward or reverse drive.

General.—The engine parts were well num­bered, for purposes of assembly location, with conventional Arabic numerals. The letters “NSK” were stamped on the ball bearing races, followed by size numbers iden­tical with American size designations. The cylinders were numbered starting at the for­ward end of the engine. The generous use of brass and bronze was evident in many parts of the engine. The metric system of measure­ment was apparently used throughout the engine design.

We are now in a position to discuss the pro­duction cost and reliability in operation of the Japanese engine.

Cost of production.—Although we might have at first expected the low-output Japa­nese engine to have a higher cost-per-horse- power than the high-output Navy type DD engine, we see that there are a number of features in the Japanese design which— whether good engineering practice or not— tend to lower its specific cost. A list of these low-cost design features—as compared to our more conventional Navy type DD engine— would include the following:

1. No removable cylinder liners.
2. Overhead camshaft. (No separate cam fol­lowers, no pushrods, no pushrod passages in block.)
3. Only one cam per cylinder.
4. Direct sea water cooling system. (No fresh water pump, no fresh water heat exchanger.)
5. No reduction gear.
6. Connecting rods not drilled for pressure lubrication of wrist pins.
7. The lack of filtering equipment. (No lubri­cating oil filters, no fuel oil filters.)
8. Simpler design of fuel pump plunger. (No helix as in Bosch plunger.)
9. Injection nozzles of simpler design.
10. No separate timing gears on cooling water pump.

Assuming for the purpose of comparison that costs of labor and materials are equal in the United States and Japan and that equal benefits are derived from production meth­ods, we estimate the cost of producing the Japanese engine to be scarcely more than half the cost of the Navy type DD Diesel engine. This represents a closely comparable cost on a dollars-per-horsepower basis.

Reliability.—Some of the low-cost features of the Japanese engine, however, lessen the reliability inherent in its low-output design. It is likely, for instance, that the direct sea water cooling system, in landing boat service, might cause sand deposits from beaches to foul the engine water jackets. Sand pumped with the sea water, moreover, could be ex­pected to cause rapid wear of the water pump bronze gear-impellers. Again, the lack of fil­tering equipment in the fuel and lubricating oil systems seems to be poor practice. All things considered, however, we believe that the design deficiencies of the engine are more or less balanced by its conservative speed and horsepower. We should expect the reli­ability of the Japanese engine in service oper­ation to be comparable to that of the U. S. Navy type DD Diesel engine.

Our appraisal of this Japanese landing boat Diesel engine can now be summarized in one sentence: It is a good, sturdy engine, unusually simple in design, comparable in re­liability, fuel consumption, and cost-per- horsepower to the U. S. Navy type DD Diesel engine, but one-third heavier for the horsepower developed.