A straight-seven engine or inline-seven engine is a straight engine with seven cylinders. Wärtsilä, with their RTA96-C, and MAN Diesel produce crosshead two-stroke diesel engines in this configuration. Wärtsilä also produces regular trunk engines in this configuration. It is more common in marine applications because marine engines are usually based on a modular design, with individual heads per cylinder.
Only one straight-seven engine for land propulsion is known to be in production, the AGCO Sisu 7-cylinder diesel engine. This engine configuration was chosen because of size, parts commonality, and power range issues. A straight-8 would be too long for the farm machinery application the engine was intended for, whilst a V engine would require a higher investment compared to the expected low sales volume for this power range. The straight-7 configuration is a lower investment because Sisu has reused cylinder heads from their I3 and I4 diesel lineups. This is possible because the cylinder volume, pistons and con rods are identical across the Sisu model range.
Some seven-cylinder engines have been produced for marine use. One example is the Burmeister & Wain Model 722VU37 2-stroke submarine diesel built starting in 1937, with an output of 600 horsepower (450 kW). These were used in the Danish H-class submarines. Another such engine was the 2,500 horsepower (1,900 kW) Sulzer-designed 7QD42 submarine diesel, which was built by the Dutch between 1939 and 1940 for use in the O-class submarines.
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