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A horizontal engine has cylinders which travel horizontally with the ground as opposed to in a V configuration as in a V-6 or V-8 engine. Other engines, such as the typical in-line four- and six-cylinder models, operate in a vertical straight line with the pistons running straight up and down as the engine rotates. Perhaps the most popular horizontal engine was used in the Volkswagen Beetle. This horizontal engine consists of four cylinders, which are air cooled and mounted in the rear of the vehicle. Porsche borrowed the horizontal engine design for many of its models, and General Motors used the horizontal engine design in the 1960s Corvair model line.
A key to the operation of the high-performance horizontal engine lies in its use of a flat crankshaft. The flat crankshaft allows the four-cylinder engine to operate as if it were a large twin-cylinder engine with two pistons firing opposite each other. While the idle is a bit rough, the power output is smooth. The problem in many air-cooled horizontal engine designs is that the rear cylinders are blocked by the front cylinders, creating an over-heating problem for the rear cylinders.
Automobiles are not the only vehicles to use the horizontal engine. Many aircraft, both airplanes as well as helicopters, utilize the engine design. The low profile of the horizontal power plant allows for more viability around the engine. The flat profile of the engine also helps it to form a smooth line within the wing structure. This aids in aerodynamics as well as aesthetics. Many air boats, including those operating in the Florida Everglades in the United States, rely on the horizontal air-cooled engines for power.
Oil temperatures can often climb high in a horizontal design, and any engine which is slated for severe duty would benefit from an external oil cooler. The flat design of the horizontal engine creates slow cycling of the engine oil from the cylinders to the crankcase. This creates an overheating condition for the oil and spells out an early end to bearings and related components within the engine. By placing an oil cooler far away from the engine and mounting it in the predominant wind screen area, the oil is easily cooled, extending the life of the oil, the engine and its components.
Another type of vehicle to utilize the flat engine is the BMW motorcycle. The BMW motorcycle has been using the horizontal power plant successfully for decades. Other manufacturers have followed suit, and motorcycles such as the Honda Gold Wing series as well as the Valkyrie line both depend on the horizontal design.