Since each cylinder bank is essentially a straight-6, this configuration has perfect primary and secondary balance no matter which V angle is used and therefore needs no balance shafts. A V12 with two banks of six cylinders angled at 60° or 180° from each other has even firing with power pulses delivered twice as often per revolution as a straight-6. This allows for great refinement in a luxury car. In a racing car, the rotating parts can be made much lighter and thus more responsive, since there is no need to use counterweights on the crankshaft as is needed in a 90° V8 and less need for the inertial mass in a flywheel to smooth out the power delivery. In a large displacement, heavy-duty engine, a V12 can run slower than smaller engines, prolonging engine life.
V12 engines were first seen in aircraft. By the end of World War I, V12s were popular in the newest and largest fighters and bombers and were produced by companies such as Renault and Sunbeam. Many Zeppelins had 12-cylinder engines from German manufacturers Maybach and Daimler. Various US companies produced the Liberty L-12; the Curtiss NC Flying boats, including the NC-4, the first aircraft to make a transatlantic flight, had four V12 engines each.
V12 engines reached their apogee during World War II. Fighters and bombers used V12 engines such as the British Rolls-Royce Merlin and Griffon, the Soviet Klimov VK-107, the American Allison V-1710 or the German Daimler-Benz DB 600 series and Junkers-Jumo. These engines generated about1,000 hp (750 kW) at the beginning of the war and above 1,500 hp (1,100 kW) at their ultimate evolution stage. The German DB 605D engine reached 2,000 hp (1,500 kW) with water injection. In contrast to most Allied V12s, the engines built in Germany by Daimler-Benz, Junkers-Jumo, and Argus (As 410 and As 411) were primarily inverted, which had the advantages of lower centers of gravity and improved visibility for single-engined designs. Only the pre-war origin BMW VI V12 of Germany was an “upright” engine. The United States had the experimental Continental IV-1430 inverted V12 engine under development, with a higher power-to-weight ratio than any of the initial versions of the German WW II inverted V12s, but was never developed to production status, with only 23 examples of the Continental inverted V12 ever being built.
The Rolls-Royce Merlin V12 powered the Hawker Hurricane and Supermarine Spitfire fighters that played a vital role in Britain’s victory in the Battle of Britain. The long, narrow configuration of the V12 contributed to good aerodynamics, while its exceptional smoothness allowed its use with relatively light and fragile airframes. The Merlin was also used in the Avro Lancaster and de Havilland Mosquito bombers. In the United States the Packard Motor company was licensed by Rolls-Royce to produce the Merlin as the Packard V-1650 for use in the North American P-51 Mustang. It was also incorporated into some models of the Curtiss P-40, specifically the P-40F and P-40L. Packard Merlins powered Canadian-built Hurricane, Lancaster and Mosquito aircraft, as well as the UK-built Spitfire Mark XVI, which was otherwise the same as the Mark IX with its British-built Merlin.
The Allison V-1710 was the only indigenous US-developed V-12 liquid-cooled engine to see service during World War II. A sturdy and trustworthy design, it unfortunately lacked an advanced mechanical supercharger until 1943. Although versions with a turbosupercharger did give excellent performance at high altitude in the Lockheed P-38 Lightning, the turbosupercharger and its ductwork were too bulky to fit into typical single-engine fighters. While a superb performer at low altitudes, without adequate supercharging, the Allison’s high-altitude performance was lacking.
After World War II, V12 engines became generally obsolete in aircraft due to the introduction of turbojet and turboprop engines, which had more power for their weight, and fewer complications in large aircraft.