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V12 engine
A V12 engine is a V engine with 12 cylinders mounted on the crankcase in two banks of six cylinders each, usually but not always at a 60° angle to each other, with all 12 pistons driving a common crankshaft. Since each cylinder bank is essentially a straight-six which is by itself in both primary and secondary balance, a V12 is automatically in primary and secondary balance no matter which V angle is used, and therefore it needs no balance shafts. A four-stroke 12 cylinder engine has an even firing order if cylinders fire every 60° of crankshaft rotation, so a V12 with cylinder banks at a multiples of 60° (60°, 120°, or 180°) will have even firing intervals without using split crankpins. By using split crankpins or ignoring minor vibrations, any V angle is possible. The 180° configuration is usually referred to as a "flat-twelve engine" or a "boxer" although it is in reality a 180° V since the pistons can and normally do use shared crankpins.
== Applications ==
These engines deliver power pulses more often than engines with six or eight cylinders, and the power pulses have triple overlap (at any time three cylinders are on different stages of the same power stroke) which eliminates gaps between power pulses and allows for greater refinement and smoothness in a luxury car engine, at the expense of much greater cost. In a racing car engine, the rotating parts of a V12 can be made much lighter than a V8 with a crossplane crankshaft because there is no need to use heavy counterweights on the crankshaft and less need for the inertial mass in a flywheel to smooth out the power delivery. Exhaust system tuning is also much more difficult on a crossplane V8 than a V12, so racing cars with V8 engines often use a complicated "bundle of snakes" exhaust system, or a flat-plane crankshaft which causes severe engine vibration and noise. This is not important in a race car if all-out performance is the only goal. Since cost and fuel economy are usually important even in luxury and racing cars, the V12 has been largely phased out in favor of engines with fewer cylinders.
In a large displacement, high-power engine, a 60° V12 fits into a longer and narrower space than a V8 and most other V configurations, which is a problem in modern cars, but less so in heavy trucks, and seldom a problem in large stationary engines. The V12 is common in locomotive and battle tank engines, where high power is required, but the width of the engine is constrained by tight railway clearances or street widths, while the length of the vehicle is more flexible. It is often used in marine engines where great power is required, and the hull width is limited, but a longer vessel allows faster hull speed. In twin-propeller boats, two V12 engines can be narrow enough to sit side-by-side, while three V12 engines are sometimes used in high-speed three-propeller configurations. Large, fast cruise ships can have six or more V12 engines. In historic piston-engine fighter and bomber aircraft, the long, narrow V12 configuration used in high-performance aircraft made them more streamlined than other engines, particularly the short, wide radial engine. During World War II the power of fighter engines was stepped up to extreme levels using multi-speed superchargers and ultra-high octane gasoline, so the extreme smoothness of the V12 prevented the powerful engines from tearing apart the light airframes of fighters (often made out of wood and canvas rather than aluminum). After World War II, the compact, more powerful, and vibration-free turboprop and turbojet engines replaced the V12 in aircraft applications.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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