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Horizontal engines have cylinders that travel horizontally with the ground, like the Volkswagen Beetle’s air-cooled four-cylinder engine, which Porsche and General Motors also used. A flat-plane crankshaft allows for high-performance operation, but overheating can be a problem. Aircraft and motorcycles also use horizontal engines, but external oil coolers are recommended for severe service.
A horizontal engine has cylinders that travel horizontally with the ground instead of in a V-shaped configuration like a V-6 or V-8 engine. Other engines, such as the typical inline-four and six-cylinder models, run in a straight vertical line with the pistons running 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 at 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 Corvair model line of the 1960s.
A key to high-performance horizontal engine operation lies in the use of a flat-plane crankshaft. The flat-flat crankshaft allows the four-cylinder engine to perform as if it were a big two-cylinder engine with two pistons firing opposite each other. While the idle is a bit rough, the power output is smooth. The problem with many horizontal air-cooled engine designs is that the rear cylinders are blocked by the front cylinders, creating an overheating problem for the rear cylinders.
Cars are not the only vehicles that use the horizontal engine. Many aircraft, both airplanes and helicopters, use the engine design. The low profile of the horizontal power plant allows for greater workability around the engine. The engine’s flat profile also helps it form a smooth line within the wing structure. This helps both aerodynamics and aesthetics. Many air boats, including those operating in the Florida Everglades in the United States, rely on horizontal air-cooled engines for power.
Oil temperatures can often get very high in a horizontal layout, and any engine that is scheduled for severe service would benefit from an external oil cooler. The flat horizontal engine design creates a slow cycle of engine oil from the cylinders to the crankcase. This creates an overheat condition for the oil and accounts for the early end of the bearings and related components within the engine. By positioning an oil cooler away from the engine and mounting it in the predominant area of the windshield, the oil is easily cooled, extending the life of the oil, the engine, and its components.
Another type of vehicle to use 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, with motorcycles like the Honda Gold Wing series and the Valkyrie line relying on the horizontal design.
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