Paxman Hi-Dyne engine

Diesel-mechanical locomotives

Diesel locomotives appeared in the 1930s, after the availability of reliable, compact diesel engines. The first were low-speed shunters with mechanical transmissions. These were followed by more powerful high-speed express locomotives with diesel-electric transmissions. These electric transmissions and their traction motors were expensive and complicated though, often requiring entirely new skills for their maintenance staff.{{Cite book

One of the main needs for a transmission is to match the speed of the engine to the speed of the locomotive, so that the engine can work in its useful operating speed range. All locomotives need to deliver high torque from zero rail speed for startup. The overall range of gear ratios required from the transmission thus depends on the maximum speed of the train. The narrower the power band of the engine, the more precise control of gearbox ratio is required, either by using a continuously variable transmission or a discrete ratio transmission with more ratios.

Rather than building increasingly complicated transmissions, Paxman chose instead to develop a more flexible engine.

Hi-Dyne principle

The output power of an engine is the product of its torque and speed. The torque varies with speed, increasing to a peak value and falling away both above and below this. The range for which the torque is a useful proportion of the maximum is described as the 'power band'. Diesel engines generally have a broader band than petrol engines and they also lose less fuel-efficiency at part throttle settings.

The Hi-Dyne principle was to produce an engine where the torque curve was the inverse of the usual: a maximum at low revolutions and decreasing gradually with increasing speed, so that the overall power (the product of torque and power) would remain constant, whatever the speed. Such an engine cannot be achieved by normally-aspirated engine design alone.

Experiments in supercharging diesel engines from the 1930s onwards, initially by Sulzer{{Cite book

By applying a turbocharger, it is possible to increase the torque (and thus power) at low speeds, so as to match the naturally-aspirated maximum power at the torque peak. If the turbocharger and its inlet manifold are carefully sized to provide the necessary boost at low pressure, but for the volume delivered to stay relatively constant above this, an inverse torque characteristic with rpm can be achieved.

Engines

Fell locomotive

Main article: British Rail 10100

The same concept of a constant-power diesel engine by variable supercharging had earlier{{efn-lr|The broad design for the Fell locomotive was established by the time of the 1947 agreement to build it. It first ran on the main line in 1951.{{Cite book

Hi-Dyne

The Hi-Dyne used a turbocharger characteristic curve that was adequate to provide full boost at low rpm, but was choked above this as engine speed increased, so that the boost reduced and thus also the output torque.{{Cite report

Engine fuel supply through the injection pump was controlled by a governor. Rather than the usual arrangement where a control input from the driver's lever sets an engine speed that is maintained by a governor, in the Hi-Dyne engine the control input selected an output power level, which was maintained by the governor.

Applications

In 1954 the first prototype Hi-Dyne was installed in Enterprise, a 48-ton Hudswell Clarke industrial locomotive.{{Cite web

One of the more lasting, although obscure, legacies of this locomotive was due to its name. This locomotive first appeared in 1954, during the construction of DP1, the prototype Class 55 Deltic. DP1 already had the internal project name Enterprise and it had been intended to name the locomotive similarly on its delivery. To avoid confusion with the Hudswell Clarke, and association of this prestige project with such a small and minor locomotive, DP1 instead acquired the name DELTIC, after its Deltic prime mover.{{cite book

Paxman subsequently supplied sixteen Hi-Dyne engines, based on the 6 cylinder RPHXL (contracts 55096-103 and 55721-8) to Hudswell Clarke for locomotives for Sierra Leone Government Railways.

Similar engines

Paxman was not alone in trying to develop diesel engines which gave high torque at low speed. Italian inventors Enrico Hocke and Fausto Zarlatti patented a "Diesel type locomotive with direct transmission and with automatically supercharged motor when decreasing the velocity" in 1938, patent US2115525.{{Cite patent

References

References

1. General Arrangement drawing, British Railways