In the 1930s, BMW took out a license to build the Pratt & Whitney Hornet engines. By the mid-30s they had introduced an improved version, the BMW 132. The BMW 132 was widely used, most notably on the Junkers Ju 52, which it powered for much of that design's lifetime.
In 1935 the RLM funded prototypes of two much larger radial designs, one from Bramo, the Bramo 329, and another from BMW, the BMW 139. BMW bought Bramo soon after the projects started; unsurprisingly BMW folded the Bramo engineers into the BMW project, cancelling the Bramo design. The resulting proposal was essentially a two-row version of the BMW 132, the 1,550 PS (1,529 hp, 1,140 kW) BMW 139.
The BMW 139 was originally intended to be used in roles similar to those of the other German radials, namely bombers and transport aircraft, but mid-way through the program Kurt Tank suggested it for use in the Focke-Wulf Fw 190 fighter project. Radial engines were rare in European designs as they were considered to have too large an area for good streamlining and would not be suitable for high speed aircraft. However, radials were often used in American and Japanese fighters at the end of the 1930s, and improvements in the cowlings for radial engines were reducing the concerns about drag. Tank felt that attention to detail could result in a streamlined radial that would not suffer undue drag, and would be competitive with inlines.
The main concern was providing cooling air over the cylinder heads, which generally required a very large opening at the front of the aircraft. Tank's solution for the BMW 139 was to use an engine-driven fan behind an oversized prop-spinner, blowing air past the engine cylinders, with some of it being drawn through S-shaped ducts over a radiator for oil cooling. However this system proved almost impossible to operate properly with the BMW 139; early prototypes of the Fw 190 demonstrated terrible cooling problems. Although the problems appeared to be fixable, since the engine was already fairly dated in terms of design, in 1938 BMW proposed an entirely new engine designed specifically for fan-cooling that could be brought to production quickly.
The BMW 801 would eventually be fitted with a magnesium alloy cooling fan that rotated at 1.72 times the crankshaft speed (3.17 times the propeller speed). This provided effective cooling although at the cost of about 70 PS (69 hp, 51.5 kW) required to drive the fan when the aircraft was at low speed. Above 170 miles per hour (270 km/h), the fan absorbed little power directly.
The cowling used for the BMW 801 was generally identical for almost every aircraft it was fitted onto, as it was designed and supplied by the BMW firm itself. The oil cooling provision existed within the curved forward section of the cowl, just aft of a 360º perimeter armored metal ring forming the cowl's extreme forward component, and had airflow directed to flow from the inside of the cowl, over the oil-cooling core block and drawn out into the cooling fan's slipstream within the cowl and over the engine cylinders. Integrating the oil cooling provision within the cowling's sheet metal made it more possible to "unitize" a powerplant installation, with minimal exterior fluid and mechanical mounting and control linkage connections - the ideas behind it were inspired by the Kraftei standardized format engine mounting, possibly first done on the liquid-cooled Junkers Jumo 211 inverted V12 liquid-cooled engine. The Jumo 211 most often used an integrated annular radiator for both the engine coolant and motor oil just behind the prop for a unitized installation, as used for the many examples of the Junkers Ju 88 built, an airframe that just as easily could take a pair of BMW 801 radial engines with minimal design changes.
Differences between the BMW 139 and the new design were fairly minor and limited primarily to details except for the use of fourteen larger cylinders instead of eighteen smaller ones. The engine's cylinders were in two rows of seven, the bore and stroke were both 156 millimetres (6.1 in), giving a total capacity of 41.8 litres (2,550 cu in). The unit (including mounts) weighed from 1,010 to 1,250 kg and was about 1.29 m (51 in) across, depending on the model.
The new design was given the name BMW 801 after BMW was given a new block of "109-800" engine numbers by the RLM to use after their merger with Bramo. The BMW 801 retained the BMW 139's older-style single-valve intake and exhaust, while most engines of the era had moved to four valves per cylinder, or in British use, sleeve valves. Several advances were worked into the design, including the use of sodium-cooled valves and a fuel injection system. The supercharger was rather basic in the early models, using a single-stage two-speed design directly geared to the engine (unlike the DB 601's hydraulically clutched version) which led to rather limited altitude performance, in keeping with its intended medium-altitude usage. One key advancement was the Kommandogerät (command-device), a mechanical-hydraulic unit that automatically adjusted engine fuel flow, propeller pitch, supercharger setting, mixture and ignition timing in response to a single throttle lever, dramatically simplifying engine control.
The first BMW 801A's ran in April 1939, only six months after starting work on the design, with production commencing in 1940. The 801B was to be identical to the 801A except turning the airscrew in the opposite direction (counterclockwise, as seen from behind the engine) using a different gearbox. The A and B models were intended to be used in pairs on twin-engine designs, cancelling out net torque and making the plane easier to handle. There's no confirmation the 801B ever left the prototype stage.The initial BMW 801 series had a lot of overheating problems especially with cylinders in the rear cylinder row and a number of efforts were improvised in an attempt to cure them. The BMW 801A/B/C/L engines delivered 1,560 PS (1,539 hp, 1,147 kW) for takeoff. Major applications of the 801A/L engines include multiple variants of the Junkers Ju 88 and Dornier Do 217.
The BMW 801C was developed for use in single- or multi-engined fighters and included a new hydraulic prop control and various changes intended to improve cooling, including cooling "gills" on the cowling behind the engine. The BMW 801L was an A model with the hydraulic prop control mechanism introduced with the 801C engine. The 801C was almost exclusively used used in early variants of the Focke-Wulf Fw 190A.
The 801C was replaced with the BMW 801 D-2 series engines in early 1942, which ran on C2/C3 100 octane fuel instead of the A/B/C/L's B4 87 octane, boosting takeoff power to 1,700 PS (1,677 hp, 1,250 kW). The D-2 models were tested with a system for injecting a 50-50 water-methanol mixture known as MW50 into the supercharger output to cool the charge, and thereby reduce backpressure. Some performance was gained but at the cost of engine service life and other boost systems, using fuel for charge cooling (known as C3-injection), were developed and used until 1944. The serious fuel shortage in 1944 forced re-evaluating the MW50 system which was then installed instead of the C3-injection. With MW50 boosting takeoff power increased to 2,000 PS (1,470 kW), the C3-injection was initially only permitted for low altitude use and raised take-off power to 1870 PS. Later C3-injection systems were permitted for low-to-medium altitude use and raised take-off power to more than 1900 PS. The BMW 801G-2 and H-2 models were D-2 engines modified for use in bomber roles with lower gear ratios for driving larger propellers, clockwise and counterclockwise respectively. Just like the 801B engine the 801H-2 engine did not leave the prototype stage
With the engine now being used in higher-altitude fighter roles, a number of attempts were made to address the limited performance of the original supercharger. The BMW 801E was a modification of the D-2 using supercharger gear ratios tuned to higher altitudes. Although takeoff power was unaffected, cruise power increased over 100 hp (75 kW) and "high power" modes for climb and combat were likewise improved by up to 150 hp (110 kW), with the possibility of turning up to a four meter diameter, three-blade propeller for multi-engined use, such as on the proposed Heinkel He 277 Amerika Bomber four engined strategic bomber design. The E model was also used as the basis for the BMW 801R, which included a much more complex and powerful two-stage four-speed supercharger. Continued improvements to the basic E model led to the BMW 801F, which dramatically improved performance across the board, with takeoff power increasing to 2,400 hp (1,790 kW). It was planned to use the F on all late-model Fw 190's, but the war ended before production started.
A number of attempts were made to use turbochargers on the BMW 801 series as well. The first used a modified BMW 801D to create the BMW 801J, delivering 1,810 PS (1,785 hp, 1,331 kW) at takeoff and 1,500 hp (1,103 kW) at 40,000 ft (12,200 m), an altitude where the D was struggling to produce 630 hp (463 kW). The BMW 801E was likewise modified to create the BMW 801Q, delivering a superb 1,715 hp (1,261 kW) at 40,000 ft (12,200 m), power ratings no existing Allied radial engine of a similar displacement could touch. However none of these engines ever entered production due to high costs, and the various high-altitude designs based on them were forced to turn to other engines entirely, typically the Junkers Jumo 213.
Engines were typically delivered from BMW complete in their cowling, ready to be bolted to the front of the aircraft, since 1942 as Motoranlage (M) and 1944/1945 as Triebwerksanlage (T). The Motoranlage was the original form of the interchangeable Kraftei, or "power-egg", unitized powerplant installation concept used in many German wartime aircraft. It was most often used with twin and multi-engined designs, with some need for external add-ons, and the Triebwerksanlage was the Motoranlage plus some external mountings, such as exhaust pipes, as a completely interchangeable unit, still within the Kraftei definition. Both M and T formats were also used with various inline engines, like the Daimler-Benz DB 603 used for both the inline-engined versions of the Do 217 and the enormous Bv 238 flying boat, and the Junkers Jumo 213 powerplants used for later marks of the Ju 88 multirole aircraft.
The M and T unitized engine formats added secondary designator suffixes, which confuse the naming considerably, as they initially referred to these complete kits and their "bare" engine counterparts almost interchangeably. The A, B and L models were known (logically) as Motoranlage style MA, MB and ML engines in this form, but the common D-2 apparently initiated the "differing" second suffix letter usage when unitized into a Motoranlage powerplant unit, and was instead known as the MG. Deepening the confusion, the E model was delivered as the Triebwerksanlage style TG or TH, seemingly suggesting a relation to the G and H engines, but in fact those were delivered as the TL and TP. It is rather common to see the turbocharged versions referred to only with the T as Triebwerksanlage installations, notably the (most notoriously of all) TJ, and the TQ models, further confusing the issue.
Production versions of the engine generated between 1,560 and 2,000 PS (1,540-1,970 hp, or 1,150-1,470 kW). It was the most produced radial engine of Germany in World War II with more than 28,000 built.
BMW 801 A, C, L (B)
1,560 PS (1,539 hp, 1,147 kW)
BMW 801 D-2, Q-2, G-2, (H-2)
1,700 PS (1,677 hp, 1,250 kW)
BMW 801 E,S
2,000 PS (1,973 hp, 1,471 kW)
BMW 801 F
2,400 PS (2,367 hp, 1,765 kW), development halted by the end of the war
Blohm & Voss BV 141
Blohm & Voss BV 144
Dornier Do 217
Focke-Wulf Fw 190
Focke-Wulf Fw 191
Heinkel He 277
Junkers Ju 88
Junkers Ju 188
Junkers Ju 288
Junkers Ju 388
Junkers Ju 290
Junkers Ju 390
Messerschmitt Me 264
BMW 801 C
Type: 14-cylinder supercharged two-row air-cooled radial engine
Bore: 156 mm (6.15 in)
Stroke: 156 mm (6.15 in)
Displacement: 41.8 litres (2,560 in³)
Length: 2,006 mm (79 in)
Diameter: 1,290 mm (51 in)
Dry weight: 1,012 kg (2,226 lb)
Valvetrain: One intake and one sodium-cooled exhaust valve per cylinder
Supercharger: Gear-driven single-stage two-speed
Fuel system: Fuel injection
Cooling system: Air-cooled
Power output: 1,560 PS (1,539 hp, 1,147 kW) at 2,700 rpm for takeoff at sea level
Specific power: 27.44 kW/L (0.60 hp/in³)
Compression ratio: 6.5:1
Specific fuel consumption: 0.308 kg/(kW·h) (0.506 lb/(hp·h))
Power-to-weight ratio: 1.13 kW/kg (0.69 hp/lb)