Tunnel crankcase

This requires a crankcase with a large tunnel instead of cross web supports for narrower main bearings, hence the name.

They were favoured by some makers more than others, notably Saurer in Switzerland[2] and Maybach-Motorenbau GmbH (now MTU) Friedrichshafen.

They are described as both 'tunnel crankcases' and 'roller bearing cranks';[2] the two aspects are related and it is unclear as to which gave rise to the other.

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.

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.

The British Beardmore Tornado was based on medium-speed diesel engine practice and was both heavy, underpowered and unreliable.

[6][7][8] Thousands of these engines were produced, although surviving examples are now extremely rare, particularly in working order.

Following German practice, these high-speed engines were used in conjunction with a hydraulic transmission, in order to produce a powerful, but lightweight locomotive, compared to the competing diesel-electric designs.

[16] A later development was to enlarge the main bearings sufficiently to be larger than the entire crank web.

Each bearing now only needed to be slid through the length of its own width, being small enough to pass easily through any preceding housings.

For this reason, the tunnel crankshaft is most appropriate to engines running for long periods at constant speed, such as generator sets, railway locomotives and boats.

[17] Some multi-cylinder two-stroke engines also use crankcase compression, almost all of them marine outboard motors.