The Fell Diesel Mechanical locomotive was a collaborative venture between  Lt. Col. L.F.R. Fell and H. G. Ivatt of the LMS who was also responsible for the LMS 1600 HP locomotives. The locomotive was an attempt to produce a high horsepower relatively lightweight locomotive.  It was fitted with 4 Paxman 12RPH diesel engines each producing  a peak of 500 HP and capable of operating between 500 and 1 500 RPM. The engines drove through a central gearbox and the locomotive was designed to start on one engine with the others being progressively  started as the speed increased. A further novel feature was that the engines, which were supercharged, had the highest degree of boost at low revolutions. The superchargers were driven by two separate 150 HP engine.s The locomotive had a poor reputation for reliability however this was perhaps as a result of the low level of technical expertise that was provided during the various trials with some of the basic errors sounding more like the output from an Ealing comedy than a professional test.  As an example serious damage was incurred when a gearbox was allowed to run dry an seize.  The locomotive was the subject of a number of technical articles in the contemporary publication "The Engineer" and was subjected to at least one dynometer test which forms the basis of the curves shown below. It will be seen that the power curve follows an unusual profile with the peak power only being produced over a small part of the range around 40- 50 mph.  This has often led to technical reports on the locomotive performance incorrectly suggesting that the locomotive was not producing its full power.  It will be seen that the power (and tractive effort) drops to zero at 72 mph although various reports indicate 74  and 78 mph with a top speed of 84 mph.  This is likely to be a misprint or an error given the diameter of the coupled wheels.  The locomotive was classified as 6P5F and later as a type 4.  One of the claims for the locomotive was that it provided a significantly higher transmission efficiency through the use of a mechanical gearbox rather than electric traction motors. This was correct however once the auxiliary engine had been included the 95% fell to 82.6% which was comparable with diesel electrics.The locomotive eventually caught fire in 1958 and was cut up in January 1960 at Derby Works having operated for less than 80 000 miles.  

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