A U engine, also known as a double or twin bank engine, is composed of two straight engines that work together and share some parts with one another, though they do each have a separate crankshaft. The two crankshafts are connected by a set of chains or gears to ensure they spin together. U engines are rarely used since the V-engine, found in many of today’s automobiles, can share parts without all the excess weight the U engine suffers from.
Like most car engines, the U engine is a piston engine which relies on internal combustion to work. The pistons power a crankshaft which turns the wheels of the car. In this way, the small explosions created within the engine turn the car's wheels and allow it to move forward. The U engine contains two engines which each power a separate crankshaft. Since two crankshafts turning freely of each other would cause problems and often prevent the car from moving, the crankshafts are attached to each other so they can work together to power the car.
Other variations of the U engine exist as well. A square four engine resembles two U engines used together. This equals a total of four straight engines all working together to share certain parts. The idea was much the same as for U engines. In theory, if the square four engine shared parts it could save on parts, act more efficiently, and produce more power to the vehicle.
Still, few engines of this type are used. Other alternatives exist and provide more efficiency while weighing significantly less. The modern day car does not need the power of four straight engines to reach its maximum speed. Neither U engines nor square four engines really took off and both were quickly forgotten by many in favor of the V-engine.
The very first U engine was designed by Ettore Bugatti around 1916. He patented his work and gave license to companies in both America and France. Roughly 40 to 55 engines total were produced before production stopped. Other attempts, both for gasoline and diesel U engines, were made over the years but the U engine simply proved impractical compared to other available engine designs.