Flat-4 engine

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A FlatHead-4 cylinder engine is an engine that is horizontally-opposed with with four cylinders arranged horizontally in two banks of two cylinders on each side of a central crankcase. The pistons are usually mounted on the crankshaft such that opposing pistons move back and forth in opposite directions engines are better balanced than other engine types in 4 cylinder configurations. The more common straight-4 configuration suffers from an engine balance problem, causing up-and-down vibrations, which grows worse with increased engine size and power. In general, straight-4s above 2.0 L usually have balance shafts and ones over 3.0 L are not seen in passenger cars. However, the flat-4 does have a less serious secondary imbalance that causes it to rotate back and forth around a vertical axis. This is because the cylinders cannot be directly opposed, but must be offset somewhat so the piston connecting rods can be on separate crank pins, which results in the forces being slightly off-centre. It does not usually require balance shafts, but unlike flat-6 and inline-6 engines, the flat-4 is not a fully balanced configuration.