The extensive use which is being made of thousands of Stuka (Junkers Ju-87) dive bombers by the German Air Force in the European war focuses attention on the power plants with which these warplanes are equipped. That they are all equipped with the same type of engine goes without saying inasmuch as standardization is one of the essentials of military equipment in Germany. Junkers Jumo 211 engines are also used in high-speed Junkers Ju-88 bombers and they have been installed in Heinkel bombers and fighters to an appreciable extent. Primarily, one associates the Jumo 211 with the Stuka and it must be admitted that the two make a very effective combination.
The basic requirements insisted upon when the Jumo 211 was designed were high power output at ground level and altitude, low weight, compactness, economical fuel consumption and maximum reliability. That all of these requirements have been combined in one engine is quite an achievement and is due to the wide experience which Junkers have had with water-cooled power plants. High power output is obtained by means of a two-speed supercharger, low weight is made possible by refinement of design, compactness is obtained by building the engine with its cylinders in an inverted vee, economical fuel consumption is obtained by means of direct fuel injection, and reliability is ensured by exercising the most rigid inspection control during the process of manufacture.
When the Jumo 211 was first produced in quantity in 1938, it was rated at 1,025 hp at 5,600 ft and 975 hp at 13,800 ft, with 1,000 hp available for takeoff. Since that time its performance has been considerably improved so that now it has a rating of 1,200 hp at 6,500 ft and 1,000 hp at 16,400 ft and develops 1,200 hp for takeoff. The weight of the engine is 1,290 lb and its specific weight is 1.07 lb/hp. These power outputs are obtained with 87-octane gasoline.
Apart from its inverted cylinder construction, the Jumo 211 follows conventional practice. The crankcase and cylinder blocks are a one-piece casting of aluminum alloy with the blocks set at an angle of 60°. The crankshaft is of twelve-throw construction with counterbalances and is supported in seven main bearings. On the front end of the crankshaft there is a spline-mounted spur wheel which drives a spur gear wheel on the propeller shaft and provides a reduction ratio of 1.55:1. At the rear end of the crankshaft there is a tailshaft from which drives are taken for the supercharger and the electric generator mounted on top of the crankcase. A spur gear wheel is also provided at the rear of the crankshaft from which a gear train extends down to the two camshafts, the oil pumps, the centrifugal water pump and various small accessories.
The valve mechanism is operated by means of an overhead camshaft along each cylinder head and two inlet valves and one exhaust valve are provided for each cylinder. The pistons are fitted with three pressure rings above the piston pin and one oil scraper ring below it. The connecting rods are machined from I-section steel forgings and have big-end bearings of unusually large area. The propeller shaft is hollow so that a shell gun can be fired through it and it is made with a large flange with serrated teeth and holes for eight bolts for attaching the propeller in accordance with German standards. The propeller reduction gear has an efficiency of from 98-99%.
Instead of the down-draft carburetor usually found on high-performance aircraft engines, the Jumo 211 is equipped with a twelve-unit high-pressure injection pump which injects the gasoline direct into each cylinder where it forms the correct mixture with the air forced into the cylinder by the supercharger during the suction stroke. Fuel injection is used on many of the military aircraft engines in Germany and has the advantage that there is no danger of engine stoppage after a power dive or during aerobatic maneuvers. It also makes the engine less fuel-sensitive and permits a slightly lower grade of fuel to be used without impairing the performance of the engine.
The two-speed supercharger is mounted so that the axis of its impeller is at right angles to the crankshaft of the engine instead of parallel with it as in most installations. The impeller is of the high-speed closed type and sucks in the air through curved vanes around its hub and discharges it at high velocity through openings around its periphery. The altitude control is in the form of a capsule or metal sylphon controlled by atmospheric pressure which transmits its expansion and contraction to a small hydraulic servo-unit which opens and closes the throttle valve in the air intake of the supercharger. At high altitudes the capsule expands due to the reduced atmospheric pressure and this causes the servo-unit to increase the opening of the air throttle valve. The fuel consumption of the engine is approximately 0.45 lb/hp/hr when it is cruising at an output of 900 hp.
The lubrication system functions on the dry sump principle with one pressure feed and two scavenger pumps of the rotary gear type. The oil pressure ranges from 60 to 90 lb/sq in. Water is used as the coolant and is circulated by a centrifugal pump with a circular header tank around the front part of the engine. The use of water as a coolant instead of ethylene glycol which is used in most liquid-cooled engines enables the Jumo 211 to be serviced quickly wherever it may be without requiring supplies of a special coolant. Drives are provided for an electric generator, an air compressor or vacuum pump, a shaft-driven and an electric tachometer and two machine-gun synchronizers.
When the writer saw the Jumo 211 in mass production in the Junkers factories in Germany last year it was particularly noticeable that rigid inspection control was maintained during all stages of manufacture. Even the crankcase castings were tested with Rontgen rays to ensure that the material was satisfactory and the roentgenographs of each casting were filed away for future reference. Much of the inspection work on small precision parts was carried out in air-conditioned rooms with up-to-date electrical and optical apparatus. Complete assemblies such as pumps and superchargers were tested and calibrated on special testing machines before assembly on the engines. Interchangeability of parts was excellent and the workmanship of the engines was of a very high order.
The use of inverted vee-type engines such as the Junkers Jumo 211 and the Mercedes-Benz DB601 for high-performance warplanes in Germany is in sharp contrast to the use of upright vee-type engines such as those used in the United States and the Rolls-Royce Merlin in Great Britain. The Germans claim their engines provide better visibility for single-engined planes and are easier to build and service in the field. It might be well worth while for us to investigate the possibilities of the inverted vee-type of high-powered aircraft engine before committing ourselves to the mass production of vast quantities of high-performance liquid-cooled engines of a type which has been discarded in Germany.
This article was originally published in the September, 1940, issue of Aviation magazine, vol 39, no 9, pp 52-53, 132.
The original article includes a photo of the engine and a diagram of the cooling system.
Photo is not credited.