The Fell Locomotive - No 10100

This page supplements the information in the Fell Locomotive section of the Diesel Rail Traction page and is in two parts.

The first section provides a technical description of the Fell Locomotive and is the text of a brochure produced in the early 1950s by the designers of this prototype locomotive, Fell Developments Limited of 20 Bishopsgate, London EC2.

The second section consists of notes by John Cove, who joined Paxman in 1948 and worked for the Company while the Fell locomotive was being developed and throughout the time it was in service. We are indebted to him for these insights gained from personal involvement with the Fell project. John later became a Director of Davey Paxman & Co before progressing to senior positions in other parts of the Group.



The direct mechanically driven locomotive with which we are concerned is the subject of inventions by Lt. Col. L. F. R. Fell. At an early stage it was realised that the successful development of these inventions must involve considerable resources and this realisation led to the formation of a company called Fell Developments Ltd.

In 1948 this Company concluded a contract with the Railway Executive for the construction of a prototype locomotive. This locomotive (British Railways No. 10100) has been jointly developed by Mr. H. G. Ivatt, M.I.Mech.E., Chief Mechanical Engineer of the London Midland Region, and Fell Developments Ltd. and has been built at the L. M. R. Works at Derby.


Fell Developments have made arrangements whereby Messrs. Ricardo & Co. Engineers (1927) Limited shall be responsible for the licencing and exploitation of the inventions. This Company will also provide consulting services to the licensees. All technical and business enquiries should therefore be addressed to that Company
21, Suffolk Street, Pall Mall, London, S.W.l.
or to
Bridge Works, Shoreham-by-Sea, Sussex.


The Fell is a multi-engined, mechanical transmission, Diesel locomotive designed for main line traction.

The nature of the mechanical transmission employed is such as to facilitate the grouping together of the power outputs of a number of propelling engines.

The use of a number of main propelling engines having the required torque characteristics has certain advantages as compared with the use of one main engine:-

  1. Increased dependability as the locomotive can proceed at reduced speed even if all but one of the main propelling engines are out of action.

  2. The individual engines can be of a higher speed type and therefore lighter and more compact.

  3. As a consequence of "2," the engines themselves and their component parts are, for a given power output, lighter and therefore more easily and more rapidly handled for replacement or repairs.

As the transmission employed is purely mechanical without the use of either electrical or hydraulic torque conversion, it is an essential feature of this system of locomotive construction that the main propelling engines shall, for the purpose of starting the train and for ascending steep grades, be capable of developing very high torques at low rotational speeds. To achieve this result the engines are relatively highly supercharged at their lowest speeds and progressively less highly supercharged as their speed rises. It is therefore necessary that the engines employed shall have a good response to supercharging in order to economise air, and be of a type in which high supercharging at low speeds does not give rise to unduly high cylinder pressures.

In order that the required supercharge involving the supply of large volumes of air may be available when the main engines themselves are running at low speeds, the air blowers of the displacement type are separately driven by auxiliary engines - preferably Diesels which are themselves supercharged off the same air supply in order to reduce their size and weight.

These auxiliary engines are provided with variable speed governors which are in turn influenced by the supercharge pressure so that a fall in this pressure automatically causes the auxiliary engines to speed up in an endeavour to maintain the air pressure. The capacity of the blowers, however, and the maximum speed to which these engines are permitted to run are such that very early in the speed range of the main propelling engines the supercharge pressure commences to fall, and continues to fall progressively as the speed of the main engines increases, so that at the highest speed of the main engines the supercharge has practically reached zero. The governors on the main engines limit the fuel supplied in relation to the available air charge and the engine speed. The decrease in supercharge pressure with speed, taken in conjunction with the limitation of fuel by the governor, results in the main engines developing approximately constant horse power over their speed range.

For the purpose of connecting and disconnecting individual engines and the common mechanical transmission, each engine is provided with a hydraulic coupling of the scoop-controlled variable filling type.


The common mechanical transmission referred to involves the use of differential gears as the means of grouping together the output of a number of propelling engines.

Fell Locomotive Gearbox

In the case, for example, of four propelling engines, see Figure 1, the transmission uses three differential gears so arranged that two, A1 and A2, act as primary differentials and the third, B, as a secondary differential. Each engine, E1, E2, E3 and E4, is connected through its hydraulic coupling, H1, H2, H3 and H4, to one of the four sun wheels of the primary differentials. The planet carriers of the two primary differentials are connected by gears GG one to each of the sun wheels S5 and S6 of the secondary differential and the planet carrier of this last differential delivers the combined power of all the engines to the road wheels through a train of gears, D, and a final reduction drive. The gears of the reversing train are in constant mesh and by means of a novel arrangement the direction of locomotive travel can be altered.

By employing differentials in this manner a progressively changing speed ratio is provided without the use of change speed gears involving a series of stepped ratios. Furthermore, inherent coupling slip is minimised.

This feature will be apparent on considering the events which take place when starting up the train and progressively bringing all four engines into action to propel the train.

Assume all four engines to be running idle with their hydraulic couplings empty. Then on opening the regulator and at the same time allowing one of the couplings, say H1, to fill, the associated engine, E1, will commence to drive the sun wheel, S1, to which the output side of that coupling is connected. It must now be pointed out that each of the four primary sun wheels, S1, S2, S3 and S4, has associated with it a uni-directional device, U, which prevents any backward rotation of that sun wheel. It will be appreciated that as a result of driving one of the sun wheels, say S1, in a forward direction and of preventing backward rotation of the other sun wheel, S2, of that primary differential by means of the uni-directional device U2, the planet carrier of that differential will drive the sun wheel, S5, of the secondary differential, B, to which it is connected through the gears GG, and so transmit the driving torque of the connected engine, E1, to the road wheels via the gear, D. This engine, E1, will thus be coupled to the final drive and road wheels with a superimposed gear reduction ratio of four to one, namely, two to one due to its primary differential A1 and again two to one due to the secondary differential B. When the train has reached a suitable speed, the coupling H2 of the second engine, E2, associated with the other sun wheel, S2, of the primary differential A1 is allowed to fill and this engine's output is increased by its governor until the torque which it is developing exceeds the backward torque on its sun wheel, S2, which is equal to that of E1 the first engine engaged. As soon as this condition occurs, the sun wheel, S2, of this second engine moves in a forward direction and its rotational speed is added to the rotational speed of the sun wheel S1 which is already running. This results in an increased speed of the planet carrier of A1 and so of the road wheels.

In a similar manner the remaining engines, E3 and E4, are in turn brought into action, and it will be seen that the speed ratio between the engines and the road wheels, when the connected engines have attained the same rotational speed, will be four to one for the first engine E1, two to one for the second engine E2, one and one-third to one for the third engine E3 and one to one for the fourth engine E4.

The uni-directional devices, U, may conveniently be of the "Legge type" in which a castellated sliding member is caused to move axially along its shaft by inclined splines. Any tendency of the shaft to rotate in a backward direction causes the sliding member to move axially to engage a stationary castellated member, thus locking the shaft against backward rotation.

It will be appreciated that with a device of this nature on each sun wheel shaft any attempt to push the locomotive in a backward direction will result in all the devices U becoming operative. It follows that if, during buffing operations, the buffer springs of the train become compressed when the engine backs up to the train, it might be impossible to operate the reversing lever owing to load on the gear teeth due to the expansion of the buffer springs. To meet this condition one of the devices U (for example U2) is arranged so that its stationary member is connected to the gear box structure through a suitable clutch, C, which may be released to free the transmission. In the prototype locomotive this clutch is held "in" by a vacuum cylinder so that the transmission is free with no vacuum.

It is a valuable feature of this mechanical transmission that the changing speed ratios are passed through without shock by finger operation of triggers, permitting the hydraulic couplings H, to fill in turn with the regulator held in the maximum power position until all four engines are on load and running at the same speed. After this condition has been reached the regulator is manipulated as required to give the desired train speed with all four engines in operation.

It is interesting to note that the maximum tractive effort can be developed when only a single engine is propelling the train, and that the bringing into action of the remaining engines permits the train's speed to be increased. Consequently any route which is within the capacity of the locomotive when all the engines are available could be completed (at reduced speed) even if only one engine remained effective.



The prototype Fell locomotive built at the L.M.R. Locomotive Works, Derby, employs four main propelling engines each of nominally 500 H.P. and two auxiliary engines driving the blowers etc., each developing 150 H.P.

The general features of this prototype 4-8-4 locomotive are shown in Figure 2.

Fell Locomotive Layout

It will be seen that the four main engines are arranged in pairs at the ends of the locomotive. These engines are Vee type 12 cylinder R.P.H. series made by Davey Paxman & Company Ltd., Colchester. They are of 7" bore and 7¾" stroke and operate over a speed range of 500 to 1500 r.p.m.

The main engines are supercharged by two Holmes Connersville Roots type blowers which are directly driven by auxiliary engines. These latter engines are A.E.C. 6-cylinder marine type A210D units 120 m/m bore and 140 m/m stroke employing Ricardo Comet III combustion chambers and are themselves supercharged from the same air system and to the same inlet pressure as the main engines. These engines operate over the speed range 1300 r.p.m. to 1800 r.p.m.

In Figure 3 are graphs showing how the supercharge pressure, the brake mean pressure and the brake horse power of one main engine vary over the speed range.

In order to limit the power output at any given engine speed to the predetermined value for that speed, a device is provided which is in effect an adjustable limit stop on the fuel pump rack, and which is under the control of a suitable centrifugal governor. To this end each main engine is provided with a special centrifugal governor which, through hydraulic servo mechanism, controls the angular position of a cam. This, in turn, limits the power output to the characteristics shewn by Fig. 3. Movement of the driver's regulator controls the power output of the engines within these limits by operating a pendulum lever in each governor.

The pin on which the pendulum lever swings can be displaced by a vacuum actuated diaphragm in opposition to a spring in such a manner that when it is displaced in one direction by the pull of the diaphragm, movements of the driving regulator can give the engine any quantity of fuel up to the maximum permitted by the cam. Release of vacuum allows the spring to move the pendulum lever suspension in the opposite direction so that the engine receives only idling fuel irrespective of any movements of the regulator.

Each main engine crankshaft is provided with a Wellman Bibby coupling carrying on its output member a shaft which conveys the torque to a Layrub coupling mounted on the input side of a Vulcan-Sinclair hydraulic coupling. This coupling is of the scoop-controlled variable filling type size 36 and its output side is connected by a shaft with one of the four input shafts of the gear-box. This latter shaft is also fitted with Layrub universal joints at both ends.

All controls of the main engines and of the transmission employ vacuum as the actuating means, with the single exception that the coupling together of the regulator movements between the ends of the locomotive is effected mechanically by a longitudinal rotatable shaft.

Supercharge System

The auxiliary engines are provided with variable speed governors, and a control device influenced by the supercharge air pressure is directly connected to the governor speed lever so that when the air pressure falls the engine is speeded up and vice versa. Spring-loaded blow-off valves are provided on the air trunking so as to limit the supercharge to the desired value of 10 lbs./ square inch. In order to avoid unnecessary blower work when the locomotive is at rest with auxiliary engines and, consequently, the blowers running, these blow-off valves are so arranged that during such periods when supercharge is not required the air has a free escape to atmosphere. When supercharge is required, vacuum is applied to a diaphragm which causes the blow-off valves to operate as spring-loaded valves.

On the air trunking there are also provided inwardly opening automatic valves for the purpose of enabling the two main engines associated with that trunking to run as atmospherically aspirated engines in the event of any failure of the pressure air supply.

The air delivered by the blowers passes directly into water-cooled after-coolers on its way to the engines. The water for these coolers is pumped through roof radiators and forms a circuit independent from the engine cooling system.

Cooling System

The jacket water of all the engines is cooled by gilled tube Serck radiators located at the extreme ends of the locomotive. One pair of main engines and one auxiliary engine are coupled to one radiator.

In order to hasten the warming up of the main engines in cold conditions the water circuit is arranged so that the auxiliary engine associated with a pair of main engines discharges its hot jacket water into the jackets of this pair. A part of each main radiator is devoted to cooling the main engine lubricating oil which is circulated from the engine sumps to the radiators by low pressure pumps located in the sumps of the main engines. Each auxiliary engine has a water-cooled lubricating oil cooler mounted on the engine. When pulling a train the rear radiator is less favourably placed as regards cooling air than the forward radiator. The water circuits have therefore been arranged so that the auxiliary engine associated with the pair of main engines at one end of the locomotive draws its water supply from the bottom of the radiator at the other end of the locomotive. The jackets of the auxiliary engines are connected to an equalising pipe at roof level. It will be seen that with this arrangement there is a continuous but limited interchange of water between the rear and forward radiators which serves to equalise the temperatures of these radiators.

Auxiliary Equipment

The fans for cooling the main radiators are rotated by shafting driven by the auxiliary engines. Power is taken from the end of one of the rotor spindles of each blower and thence by two pairs of bevels to shafting running fore and aft down the centre line of the locomotive. The shafts each drive a Westinghouse type 3V.72 exhauster and a pair of centrifugal pumps which draw from the bottom of one of the radiators and deliver to the main engines between which they are located. Means are provided to prime the main engine lubricating, systems and ensure servo oil for the functioning of the governors immediately on starting up.

All the engines are started electrically on a 24-volt circuit from batteries charged by 1300 watts generators on the auxiliary engines.

The gear-box is lubricated by a system of feeds and jets supplied with oil drawn from the sump of the box by two Rolex pumps. Direct radiation from the surfaces of the gear-box is relied on for cooling the lubricating oil.

Adequate capacity for train heating is provided in two oil fired boilers installed on either side of the transmission gearbox. These are associated with four feed water heaters accommodated in the main engine exhaust system.

Driving Controls

The controls in the driving cabs comprise a row of four small levers, a regulator and a forward-reverse lever. The four small levers actuate vacuum valves to enable vacuum to be applied to or released from diaphragm cylinders each controlling the scoop of one of the hydraulic couplings. The vacuum valve also controls the application of vacuum to the diaphragm on the governor of that particular engine to permit the regulator movement to give more than idling fuel.

Assuming the two auxiliary engines and all four main engines to be idling and that it is desired to start the locomotive into motion, the driver: -

  1. Moves the regulator through the early part of its travel: in addition to setting the governor to permit load to be taken, this puts vacuum on to a diaphragm which loads the clutch on the unidirectional coupling, and also puts vacuum on to the blow-off valves and on to the governors of the auxiliary engines.

  2. Pulls over one of the four levers: this by vacuum puts in the scoop of the hydraulic coupling of its associated engine and also puts vacuum on the diaphragm of the governor of the same main engine to permit the governor to give the fuel represented by the regulator setting.

  3. Engages other engines in a similar manner to obtain predetermined locomotive speeds. The following speed limits comprise the drivers instructions in the case of the prototype locomotive and apply when either ascending or descending in the speed range.
    First engine0-6 MPH.
    Two engines6-17 MPH.
    Three engines     17-24 MPH.
    Four engines24-78 MPH.

Miscellaneous Features

It may be of interest to draw attention to the completely universal nature of the transmission between the main engines and the gear-box. The presence in these lines of shafting of the Bibby and Layrub couplings in the shaft connecting engine to the hydraulic coupling and the Layrub couplings at each end of the shaft connecting the hydraulic coupling and the gear-box, render these shafts completely immune from the effects of any distortion of the main frames when travelling.

The final drive to the driving wheels of the locomotive is by a form of Pennsylvania Drive in which tubular quill shafts surround the two centre axles - the ends of the quill shafts carrying spiders, of which the arms are provided with rubber pads making contact with the spokes of the driving wheels. Torsional flexibility and relative vertical displacement between axle and frames is thus provided.

Principle Dimensions and Ratings

Length over buffers50ft.
Weight in working order116 tons
Total installed power for traction     2060 BHP
Total power for supercharging
    and aux. drives
300 BHP
Tractive effort at 12 M.P.H.25,000 lb.
Maximum speed78 M.P.H.

The Fell Locomotive - British Railways No 10100
some notes by John Cove, formerly a Director of Davey Paxman & Co.

This locomotive employed for the first time in its horsepower range an entirely mechanical drive. The gearbox was the brainchild of Colonel Fell and design of the locomotive was coordinated for his company Fell Developments Ltd via a committee that met at regular intervals as the project took shape. Paxmans were represented on this committee and I believe that Geoff Bone (Managing Director of Paxman, 1954-64) mainly attended meetings. However, I also sat on this committee but only briefly in the later stages when the performance of the locomotive was being evaluated.

The engines were mechanically supercharged rather than turbocharged but otherwise operated on a somewhat similar principal to the Hi-Dyne engines in that they were very heavily supercharged for high starting torque but this was reduced to normal proportions as the loco picked up speed.

The heart of the locomotive was the gearbox and this was the essential invention of Colonel Fell. The principle on which this gearbox operated is described in the previous article on this page.

The locomotive was extensively tested and did prove the viability of an entirely mechanical drive for a locomotive of this power. However, in retrospect it is my opinion that a mistake was made in deciding that everything on board should be mechanically driven as this led to a complex and troublesome arrangement of long shafts driving auxiliaries such as the radiator fans. The positioning of the radiators at the ends of the locomotive also caused problems. Air supply to and from the trailing radiator which was partially blanked off by the attached carriage was marginal at best, and cross connections to compensate for this, coupled with the need to include cooling for four main and two auxiliary engines resulted in a very complex system that gave a lot of problems with air venting and overheating.

Finally there was the problem of the AEC auxiliary engines and the Rootes blowers they drove to supercharge the main engines. In the station, with maximum boost required for starting the train from rest the noise from these was hideous, especially as it was added to the noise caused by the radiator fans and six engines also running. At that time a number of competitors were running various other prototypes on British Rail and when the Fell locomotive passed them their service engineers always made a point of ostentatiously blocking their ears!

Despite the fact that the viability of its mechanical drive was proven the Fell locomotive did not succeed in producing further orders. An unfortunate accident caused severe damage to the main gearbox at a crucial time and this probably influenced BR's decision not to proceed with the Mark 2 version which Fell Developments Ltd proposed.

The failure had relatively small beginnings. The gearbox suffered the failure of a bolt in its upper part. I cannot recall whether this was attributable to faulty design, materials or workmanship but the main point was that the broken bits passed all the way down through the gear train and rendered most of the gear wheels unfit for further service. The locomotive was returned to BR's workshop where it was built and stripped for investigation but that was effectively the end of the project.

The Mark 2 locomotive was thus never proceeded with and I do not know how it would have turned out.

My personal opinion has always been that the prototype locomotive tested was somewhat before its time and a Mark 2 locomotive could have been greatly simplified and improved. About the time the prototype was doing its test running large torque converters were becoming available in sizes to suit the Paxman engines. If the engines could have been fitted with these they could have had normal turbocharging and the AEC engines and Rootes blowers could have been eliminated. Then there would have been room in the centre of the locomotive for side mounted radiators and this would have eliminated the cooling problems. Such a locomotive would have been much quieter too. But it was not to be.

John Cove
April 2005

Page updated: 04 MAY 2005