Engineers began anticipating the engines that would be required for what became the aircraft of World War II as early as 1930, with some preceding that. Many of the engines which would see service in World War II started production in 1935.
Engine design was driven by the push for altitude capability, where thinner air placed great emphasis on the supercharger. This push for power via increased supercharger pressure ratios and engine compression ratios also required improvements in fuel quality.
While other approaches were tried (liquid-cooled radials, and air-cooled inlines) they were not major factors.
For many fighters and bombers, engines of at least 1,000 horsepower were specified in the procurements of the mid-1930s. By 1938 and 1939, 2,000 hp engines were specified for fighters and bombers. Engine design had to become more sophisticated as greater power was sought at higher engine rotating speeds. This focused attention on small design details because of the resultant higher stresses and temperatures.
Consequently, the engines used early in World War II often had up to two stages of supercharging, with the pilot able to select from two different supercharger impeller speeds. The effect of this high supercharged power rating was lower engine life, with time between overhauls ranging from 50 to 500 hours.
In the main, all power requirements were met with radial air-cooled engines and inline liquid-cooled engines. While other approaches were tried (liquid-cooled radials, and air-cooled inlines) they were not major factors. The bomber typically needed efficiency, reliability, and system survivability, and was usually equipped with air-cooled radials. The fighter needed higher power per unit of weight per unit of frontal area, and initially received the supercharged liquid-cooled inline engine such as the Rolls-Royce, Allison, or Daimler Benz. Later, when cooling and cowling aerodynamics were improved, air-cooled radials were often used, as in the Republic P-47 and Focke Wulf Fw 190. In many cases, the same basic engine was used in both fighters and bombers. Trainers and utility aircraft needed low power and high reliability, and received both air-cooled radial and inline engines.
The ten engines produced in the greatest quantity were the following:
- Pratt & Whitney R-1830 – 173,618;
- Rolls-Royce/Packard Merlin/V-1650 – 165,000;
- Shvetsov M-11 – 130,000;
- Klimov M/VK-105 – 101,000;
- Mikulin AM-35/-42 – 100,000+;
- Daimler Benz 601 – 100,000+;
- Shvetsov Ash-82 – 71,000;
- Allison V-1710 – 69,233;
- Junkers 211 – 68,000;
- Bavarian Motor Works 801 – 61,000.
(The M-11 only powered trainers and utility aircraft. )
Engines : The Heart of Aircraft Performance
It is noteworthy that with very few exceptions, almost all improvements in aircraft speed and load-carrying capability during World War II came from increased engine power and not from aerodynamic, structure, or system improvements in airframes.
In turn, engine power increases came mainly from improvements in fuel and superchargers. This was made possible by improvements in stress analysis, aerodynamics, and metallurgy in the rest of the engine.
The structure, size and streamlining used in most of the production aircraft were all demonstrated before September 1939. Two examples of engine benefits are the Boeing B-17 bomber and the Messerschmitt Bf 109 fighter.
The B-17B had a maximum gross weight of 46,650 lbs. It weighed 27,650 lbs empty. With payload of 4,000 lbs and 15,000 lb of fuel, it had a range of 1,250 miles at cruise speed of 231 mph, using four R-1820 engines that produced 800 horsepower at 25,000 feet altitude. The B-17G had a maximum gross weight of 65,500 lb. It weighed 36,135 lb empty. With payload of 6,000 lbs. and 21,780 lbs of fuel, it had a range of 2,000 miles at a cruise speed of 182 mph, using the same R-1820 engines that were now rated at 1,200 horsepower at 25,000 feet altitude.