Tunnel crankshaft

A tunnel crankcase, tunnel crankshaft or disc-webbed crankshaft engine is a diesel engine where the crankshaft is designed so that the main bearings (the bearings that support the crankshaft within the crankcase) are enlarged in diameter, such that they are now larger than the crank webs (the radial arms that link the big end bearings to the main bearings). They thus form the largest diameter of any part of the crankshaft. Rather than a conventional crankcase that has webs across it to support the narrow bearings of a conventional crankcase, the crankcase now has a large tunnel through it, hence the name.

Tunnel crankcases appeared in the 1930s with the first high-speed diesel engines. They were favoured by some makers more than others, notably Saurer in Switzerland and Maybach-Motorenbau GmbH (now MTU) Friedrichshafen. They are described as both 'tunnel crankcases' and 'roller bearing cranks'; the two aspects are related and it is unclear as to which gave rise to the other.

With the development of the high-speed diesel engine around 1930, powerful diesel engines became available in the sizes previously used by lower-powered petrol engines. In particular, their high BMEP and high torque led to high forces on the crankshaft bearings. These forces were greater than could be sustained by the small whitemetal bearings used for petrol engines. Although aircraft and sports car engines in the 1920s developed to have considerable power in a small space, these were high maintenance machines with regular servicing. The new diesels were intended for long commercial service where maintenance was a key cost to be reduced.

The need for an improved bearing technology led to the adoption of roller bearings, rather than whitemetal. Although they might be considered esoteric today, ball and roller crankshaft bearings were already in use in the 1920s for such mundane engines as the Austin 7.

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