British Rail Class 52: Encyclopedia II - British Rail Class 52 - The Diesel-hydraulic experiment

British Rail Class 52 - The Diesel-hydraulic experiment

When switching to diesel traction, as part of the Modernisation Plan of the 1950s, BR designed and commissioned designs for a large number of locomotive types. At this time (and arguably right up until Sectorisation in the 1980s), BR's regions had a high degree of autonomy, which extended as far as classes of locomotives ordered, and even the design criteria for those locomotives. Whilst almost all other diesel locomotives were diesel-electric, the Western Region employed a policy of utilising diesel-hydraulic traction, commissioning three classes of main line locomotives—the Westerns, and the lesser-powered Hymeks and Warships.

The theoretical advantage of diesel-hydraulic was simple—it resulted in a lighter locomotive than equivalent diesel-electric transmission. This resulted in better power/weight ratio, and decreased track wear. Unfortunately, it had two key disadvantages:

• The technology was proven in continental Europe, particularly Germany, but was new to the UK. It was considered politically unacceptable at the time for the UK government to order trains from foreign companies let alone German companies so soon after the second world war.
• The most robust hydraulic transmissions were only capable of handling engines with power output of around 1500 hp (1120 kW); to build a more powerful locomotive would involve two diesel engines and two transmissions.

Thus the Westerns—high-powered locomotives for top link Western Region services—needed two diesel engines to achieve the required power output. Prototypes sited the engines behind the driving cab, but drivers found this too noisy; moving the engines centrally meant making the locomotive heavier, removing some of the design's advantage. In production use the dual engine arrangement turned out to have some advantages, in particular the Westerns were able to continue operating with a single engine running in situations where more conventional single engine designs would require rescue by another locomotive. This valuable property was intentionally duplicated in the later high speed trains and was one reason for them having two power cars.

Whilst the Class was successful, it was also non-standard; the early 1970s saw the decision taken to retire all the diesel-hydraulic types. Class 47s and 50s began to take over on the Western Region, whilst the arrival of the High Speed Train was the final nail in the coffin for the Westerns. The final class member was withdrawn in February 1977. Unlike the similar, but lower-powered Warship class locomotives, most Westerns did receive air brake equipment in addition to their vacuum exhausters, thus significantly extending their working lives. However, as for the Warships, it proved impossible to equip them with electric train heating (ETH, or head-end power in US terminology). The Western Region faced particularly stiff competition for its prime inter-city services in the mid to late 1970s from the M4 motorway and it was generally felt within BR that significant speed up and comfort increases on the prime Paddington-Bristol route were necessary. The lack of ETH meant the Westerns could not power the newly-introduced air conditioned BR Mark 2f coaches - a shortcoming that the Class 50s, which were built with ETH, did not share. A more serious problem was the design fault mismatch between the Maybach MD655 engines and the Voith L630rV three speed hydraulic transmissions. The top gear ratio in the transmission was too high for the torque characteristics of the engine: the result was that a Western struggled to reach its claimed 90mph top speed in the absence of down grades or tail winds. These factors all conspired against the Westerns continuing in top-line service and their replacement by High Speed Trains provided the speed and comfort increases the Western Region sought.

Adapted from the Wikipedia article "The Diesel-hydraulic experiment", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki