Multi-crankshafts opposed piston engines (1)
by François Dovat
(© François Dovat)
This type of engine has the major advantages of excellent uniflow scavenging and thermodynamic efficiency due to its double stroke which provides a better surface to volume ratio of the combustion chamber, and less heat loss to the coolant. Moreover, since it has neither cylinder head, head gasket nor valvetrain, its reliability is potentially higher. In the US Navy's nuclear submarines, opposed piston Fairbanks-Morse diesels provide the drive for the generators and emergency propulsion. Engines of the same type have equipped American diesel-electric submarines since World War II.
In 1929, when professor Hugo Junkers made public the trend of his research concerning opposed piston aircraft diesel engines, the first test flights had just been successfully achieved at Dessau. Three years later, the Jumo 204 entered regular airline service on a four-engined plane, the Junkers G 38. They had to be temporarily withdrawn from duty due to piston and cylinder liner problems, but from 1937 onwards they enabled the North and South transatlantic seaplane routes for Lufthansa.
The Junkers aircraft 2-stroke diesels comprised 6 in-line vertical cylinders, 12 pistons and 2 crankshafts coupled by a gear train, also used for the propeller reduction gear. The lower pistons opened the intake ports, the upper ones opening the exhaust ports. In order to get the optimum timing, the lower (intake) crankshaft was about 11° retarded relative to the upper exhaust one. So the latter received roughly 3/4 of the power and it was geared directly to the propeller shaft. Moreover, as the lower crankshaft drove the scavenge blower, the water and oil pumps as well as the generator, its net output transmitted to the propeller was reduced to below one quarter of the total. A torsionally elastic drive shaft absorbed the vibrations between the aforesaid crankshaft and all the accessories, in particular the double entry centrifugal compressor, driven at 8.49 times engine speed by a two stage step up gear.
The engine block was a splendid light alloy casting, absolutely symmetrical, into which wet liners were pressed and maintained in place by a ring-nut. The crankshaft bearing webs consisted of three dismountable parts, in order to allow the introduction of the liners.
Two camshafts, laid out symmetrically on each side of the block and driven by the central gear, operated 12 injection pumps by means of rocker levers. Each pump sent metered fuel to 2 injector-nozzles by means of short lines, with the result that each cylinder incorporated 4 injectors at 90° to each other. With each injector having 2 orifices, there were 8 fuel sprays in every combustion chamber. Two fuel delivery pumps driven from the rear end of each camshaft fed the injection pumps. All the fuel injection equipment was designed and made by Junkers, and was able to develop an injection pressure of up to 620 bars.
Figure 1 : Jumo 205 D
Figure 2 : Jumo 205 C at the Swiss Transportation Museum, Lucerne
Figure 3 : Jumo 205 C, fuel injection equipment
Figure 4 : Jumo 205 C, connecting rod assembly
Figure 5 : Jumo 205 C
Figure 6: Jumo 205 C
The intake and exhaust manifolds were also doubled and symmetrical, located on both sides of the engine, a feature which provided outstanding scavenging. Five rows of small circular intake ports bored tangentially through the liner wall at the bottom of the lower piston stroke, conferred to the air charge a spiral motion which became high velocity swirl by the time of the injection process.
A hydraulic damper and a coaxial torsion shaft placed inside the hollow propeller shaft and driving it from its front end decoupled the propeller from the gear train vibrations.
The engine was closed by front, upper and lower magnesium alloy casings.
The long pistons were not oil-jet cooled as they typically are nowadays, and the high thermal loading of the exhaust pistons was exacerbated by the blow-down that occurs as the ports are uncovered by the head ring and the hot combustion gasses escape past the piston headland. Junkers solved this problem with composite pistons made up of two main parts, and whose steel head imprisoned an L shaped peripheral ring over a Ni-resist ring in-between it and the light alloy piston skirt. The L-ring sealed the space between the cylinder liner wall and the crown of the piston thanks to a minimum clearance, which the firing pressure tended to reduce even more.
But of course, the effectiveness of such rings is affected if the cylinder bore diameter undergoes variations along the swept stroke. The temperature of the liners of the first Jumo 204 and 205 engines reached approximately 240°C in the zone of the combustion chamber, although the cooling fluid circulated normally at 83°C around them. The consequent thermal expansion of the liners imposed a considerable hoop load on the L rings which had to slightly dilate during each stroke towards IDC (Inner Dead Center). This defect was cured by milling helical channels around the liners in the critical zone, thus bringing the cooling fluid closer to the cylinder walls and increasing also the surface area for thermal transfer. The loss of rigidity of the liners due to these channels was compensated for by shrinking sleeves around them.
Thanks to these improvements, it was possible not only to increase the power at 2200 rpm from 600 hp (205 C) to 700 hp (205 D), but also to adopt ethylene-glycol cooling with a temperature of up to 130°C and thus to reduce the size and weight of the radiators.
The Jumo 204 had a cubic capacity of 28 liters and a stroke/bore ratio of 1.75. This ratio was decreased to 1.52 on the Jumo 205 of 16.6 liters, and to 1.23 on the Jumo 206 (25.5 liters by a bore increased to 130 mm) intended to replace the Jumo 204, obsolete by 1937. One had then to face a deterioration of the scavenging efficiency due to the fact that the air charge was centrifuged to the circumference of the cylinders, leaving in place a core of hot burnt gases. After much research and experiments, a new design for the intake ports ended this phenomenon and made it possible to decrease the volume of scavenge air from 1.6 to 1.3 times the cylinder capacity. If the 4 upper rows of ports were still tangentially drilled, the lower row ports were aimed straight at the center of the cylinder and projected a blast of compressed air, thus expelling the residual gases. The take-off power (one minute maximum) of the 205 D could therefore be increased to 880 hp at 3000 rpm, that is to say a mean piston speed of 16 m/sec – really amazing for a diesel engine. The Jumo 206 is said to have reached 1200 hp at 2600 rpm, but for reasons unknown to the author, this version never gave satisfaction and only a small number of prototypes were built. Nevertheless, its development continued until 1940 and gave birth to the Jumo 208. It was fed by a 1st stage turbocharger, together with a mechanically driven 2nd stage two-speed centrifugal compressor. The power of this remarkable engine is said to have reached 1500 hp at 3000 rpm without any intercooler, but only 12 units were built.
Figure 7: Jumo 208
Due in large part to the scavenge air that was pumped through the engine, the temperature of the exhaust gases of these 2-stroke diesels did not exceed 550°C (against 800 to 1000°C for spark ignition engines) and thus did not constitute an obstacle to the installation of an exhaust turbine made of the steels available at the time, particularly in Germany during the war. Various systems of coupling the turbine to the compressor and the engine were tested. The above described scheme was finally also adopted for the Jumo 205, which was then renamed Jumo 207. In 1941, the production version 207 B3 was fitted with a semicircular intercooler laid out around the lower crankcase. This engine produced 1000 hp at 3000 rpm; that is to say 60 hp per liter of swept volume. Its specific fuel consumption did not exceeded 230 g/kW/h at full power. Some specimens were equipped with a GM-1 emergency power boosting nitrous-oxide injection system which allowed the rated power to be sustained up to 33 000 ft, the absolute ceiling being somewhere between 46 000 ft and 50 000 ft. The ultimate version 207 D appeared in 1943 with a bore increased from 105 to 110 mm for a capacity of 18.25 liters and a takeoff power of 1200 hp, i.e. 66 hp/l!
Figure 8: Jumo 207B3
Prior to starting, the dry sump lubrication system was primed and pressurized by a hand pump. The start itself was usually obtained by injection of compressed air of between 40 to 80 bars directly into the cylinders. However some engines were equipped with an electric inertia starter-motor while others had powder cartridge equipment.
Not really suited to the swift load and engine speed variations met in aerial fights, because of their use of a centrifugal air compressor, the Junkers diesels were normally fitted to long range observation planes and seaplanes during the war: Bv 138, BvHa 139, Do 18, Do 26, and Ju 86 D were propelled by Jumo 205s; Bv 222 and Ju 86P&R by Jumo 207s, whereas some Jumo 208's were installed on Ju 86 R. In all, more than 900 engines were produced.
Dr. Ing. Gasterstädt, who was in charge for the development of the Junkers diesels, revealed in 1937 not long before his death, some interesting figures at a technical conference. These engines equipped only civilian aircraft at the time, and were not covered by the military secrecy which had rendered the German direct gasoline injection equipment so baffling. It was learned that a 205 C developing 660 hp at 2400 rpm, had a compression ratio of 17:1, a scavenge pressure of 0.42 bars, a compression pressure of 61 bars giving a temperature of 600 to 700°C and a firing pressure of 102 bars, yielding some 1400°C.