David Wardale's Article from Steam Railway Magazine

A NEW STEAM LOCOMOTIVE DESIGN FOR MAIN LINE SERVICE

This is the unabridged text of the second of two articles about the 5AT project, written by David Wardale. A slightly abridged version appears in Steam Railway magazine's July 19th 2002 edition No. 273. The articles are reproduced with the kind permission of Steam Railway Magazine.

PART 2: THE CLASS 5AT 4-6-0 [Turn back to Part 1]

The Class 5AT (Advanced Technology) 4-6-0 proposal is for a new high-performance steam locomotive for the haulage of high-speed main line charter trains. It would be a 'state of the art' design giving the kind of all-round performance which may be mandated for main line service in the future and which significantly exceeds that of former steam, yet without sacrificing the steam locomotive's rugged simplicity, nor its aesthetic appeal. It would feature the following general characteristics, so as to have the highest level of acceptability to all sectors of the railway industry involved in charter train operation.

  1. High power:weight ratio, the key to high-speed capability.
  2. High thermal efficiency.
  3. High maximum operating speed, as required for slotting in with other traffic.
  4. High level of inbuilt safety, incorporating many operational and safety features mandated by Railtrack, Railway Safety and HM Railway Inspectorate.
  5. High reliability, giving high availability for service. (Reliability is one issue of immediate concern to Railtrack and the charter train operators. Heritage steam is not reliable - in fact by a more scientific criterion than that normally used, relating operational reliability to maintenance input, it never was.)
  6. Low fuel and water consumptions.
  7. Long operating range between supplies replenishment, a consequence of the low fuel and water consumptions and the use of a high-capacity tender.
  8. Low overall operating cost.
  9. High route availability.
  10. High level of convenience for operating crews.
  11. Aesthetically attractive for creating the true steam 'ambience' and a striking 'image'.
  12. Environmentally friendly operation, e.g. minimal pollution.

An engineering specification has been drawn up for the Class 5AT as part of the project's business plan, but here we will confine ourselves to points of general interest to the enthusiast. When considering the following it should be remembered that for an ideal design the overall concept and the details must both be right, but that compromises in both are inevitable in practice.

Firstly some of the design's proposed vital statistics and predicted performance data are given (most figures are converted from SI metric to Imperial units for those who may be unfamiliar with the former). The performance figures reflect what is attainable from present 'state of the art' design, which owes much to the work of Chapelon and Porta.

  1. Engine weight in working order = 80 ~ 90 metric tons (depending on the leading bogie load, which would only be determined at the detail design stage). The maximum axle load would be 20 metric tons (same as the BR Class 5) and the adhesive weight 60 metric tons.
  2. The tender weight with full supplies = 80 metric tons, giving an overall engine and tender weight with full supplies of 160 - 170 metric tons. Assuming oil fuel, the tender would carry 7 metric tons of oil and 10 200 gallons of water.
  3. The approximate length over buffers of engine and tender = 72 feet 6 inches, with an approximate overall engine and tender wheelbase of 62 feet.
  4. The working boiler pressure = 305 lb/in2, coupled wheel diameter 6 feet 2 inches (same as the BR Class 5), and the two cylinders 17.7 inches diameter x 31.5 inches stroke. This would give a nominal starting wheel rim tractive effort of 32 830 lb. and a starting factor of adhesion of 4.03.
  5. Walschaerts valve gear with piston valves would be used, probably two valves per cylinder each of about 7 inches diameter, tentatively with 2.6 inches steam lap and 0.4 inches exhaust lap.
  6. The rated maximum steam supply (cylinder plus auxiliary steam) from the boiler would be approximately 37 500 lb. per hour and the maximum steam temperature = 842°F (450°C). Exhaust steam feedwater and combustion air preheaters would be fitted.
  7. The maximum rated drawbar power on level tangent track would be some 2 535 hp at 71 mph (113 km/h) when carrying the high capacity tender. The maximum indicated cylinder power is predicted to be 3 460 hp at 106 mph (170 km/h), equal to 43.3 hp per ton of engine weight for an 80 ton engine.
  8. The overall thermal efficiency of the locomotive, when oil fired, referred to the cylinder output corresponding to the maximum rated drawbar power = 14.1% (this would not be the locomotive's maximum figure). The corresponding indicated specific steam consumption, based on steam to the cylinders only, = 11.2 lb. per hp-h.
  9. The maximum continuous operating speed = 112.5 mph (180 km/h). The locomotive would be designed for some 10% overspeed, i.e. 125 mph. Note that this would be the design speed and does not imply permission to operate at such a speed.
  10. The operating range at constant maximum drawbar power, i.e. 2 535 hp at 71 mph, would be some 350 miles based on fuel supply and 230 miles based on water supply. 2535 drawbar hp at 71 mph equates to the haulage of a 1 075 ton 29 coach train, and with the trailing loads more likely to be found in service (say 300 - 500 tons) the full-power ranges would be significantly greater than the above figures, and greater still under average service conditions (i.e. at an average power which is less than the maximum).

The rationale behind basing the design (excluding the tender) on the size and format of the British Railways Standard Class 5MT 4-6-0 design of 1951 now needs to be explained. The reasons are as follows.

  1. Given the level of power:weight ratio now possible in steam traction (i.e. in excess of 40 continuous indicated hp per ton of engine weight, compared to about 30 hp per ton for the very best of heritage steam) it is an appropriate size of locomotive for its intended duty - nothing larger would be required.
  2. The deep firebox of a 4-6-0 has, size for size, a higher evaporative capacity than a shallow firebox boiler, and is ideally suited to burning oil or coal (using the Gas Producer Combustion System).
  3. Basing the design on an existing one would significantly reduce the design complexity, time and cost.
  4. All overall dimensions constrained by the moving structure gauge being kept within those of the BR Class 5MT would facilitate route acceptance.
  5. The route availability would be high - it would be a 'go-anywhere' type.
  6. A modest size locomotive would allow a large tender without exceeding the permissible length for turning facilities, this in turn maximising the very important parameter of operating range between supplies replenishment.
  7. The relatively small taper boiler would give good forward visibility from the cab and good exhaust lifting, factors of paramount importance to safe operation at high speed.
  8. The 4-6-0 is the quintessential British locomotive type, and the Class 5 may be considered to be the quintessential 4-6-0. It is considered most appropriate to build on this tradition, and the present proposal would define the current limit of performance for this type of locomotive.

Some might question the use of a 4-6-0 for very high speed, but it should be noted that Chapelon's 125 mph proposals of the 1930's and 40's were likewise 4-6-0's. The important parameter of adhesive weight would be adequate in a 4-6-0 at high speed, indeed it is worth noting that 4-6-0's can have the same level of adhesive weight as much larger 4-6-2's and 4-6-4's - very few European locomotives of these two wheel arrangements exceeded the adhesive weight of the GWR 'King' Class 4-6-0's. However modern traction combines high-speed capability with rapid acceleration, the latter requiring high low-speed tractive effort and therefore more powered axles than the three of a 4-6-0. Unfortunately more coupled axles would mean either a longer engine wheelbase, increasing the design difficulties and correspondingly reducing the permissible size of the tender, or smaller coupled wheels, which would limit maximum speed. We therefore come up against one of steam traction's inherent limitations, i.e. the difficulty of designing a locomotive for both high tractive effort and high speed. For the present, high speed has been judged more important than rapid acceleration. Should the latter prove more desirable an 8-coupled design would be indicated, perhaps based on the LMS 8F but with a heavier axle load, a type which would also be more useful on steeply-graded lines. However with such a design we could not expect a continuous maximum speed of more than about 85 mph, which is not significantly more than that of heritage steam nor, it is thought, adequate for tomorrow's main line conditions.

Like the BR Class 5, the new locomotive would be a 2-cylinder simple. Realistic alternatives would be a 3-cylinder simple or 3-cylinder compound, the unattractiveness of a two-throw crank axle in a high-power locomotive ruling out four cylinders. A comprehensive comparison was made of 3-cylinder versus 2-cylinder simples, from which it was seen that the advantages of three cylinders lay mostly in the realm of mechanical factors affecting performance, and the disadvantages in extra complexity and associated costs. In the final balance three cylinders gave no net advantage over two, a conclusion which no doubt echoes that found by the majority of steam engineers in the past.

As the present writer is no expert on compound locomotives, people more knowledgeable in this field, including Porta, were consulted. Their arguments in support of compounding were carefully considered, but a 3-cylinder compound was finally rejected, primarily because of the following:

(i) insufficient space for adequate l.p. cylinder volume;

(ii) the need for resuperheat and/or a l.p. cylinder steam jacket, with attendant complexity;

(iii) doubts about the possibility of adequate internal streamlining downstream of the h.p. cylinders at high speed and high steam flow rate;

(iv) improvements in simple engine design, which have to a greater or lesser degree negated the various advantages of compounding for a high-speed locomotive;

(v) general complexity;

(vi) the extra design workload and manufacturing cost, bearing in mind that the project's intellectual and financial resources would probably be limited; and

(vii) lack of experience with compound locomotives of almost all those who might be involved with the project.

In short, although a compound might give better thermal performance, more especially at lower speeds, its potential advantage when comparing current 'state of the art' simple and compound design was not considered sufficient to justify the extra design and manufacturing cost which a compound would entail. As implied by point (i) above, the very restricted British moving structure gauge is a factor working against compounding, and as one of the compound experts has put it, 'perhaps the French were wise to build compounds, and the British wise not to'.

A thoroughly modern 2-cylinder simple would show little or no inferiority to multi-cylinder engines in respect of thermal performance. Mechanically, however, it is a different story. One of my correspondents has called the 2-cylinder engine 'barbaric', and he is correct. The problem is one of balancing - ideal balancing of a 2-cylinder locomotive with cranks at 90 degrees is not possible, and the problem gets worse as speed increases. At its proposed maximum continuous operating speed of 112.5 mph the drivers of the Class 5AT would be revolving at 8.5 revolutions per second, and with the long piston stroke the mean piston speed would be 2 666 feet per minute ('Mallard's was some 2 300 at 126 mph). The key to solving the resultant balancing problem would be to keep the mass of the reciprocating parts to the absolute minimum, certainly no more than 550 lb. (250 kg) per cylinder, and it must be stressed that this would be critical to the acceptability of a 2-cylinder locomotive for the envisioned speeds. By further refinement of the best former practice in the design of lightweight reciprocating parts, plus the use of an engine-tender connection allowing the tender mass to contribute effectively in absorbing fore-and aft forces due to the unbalanced component of the reciprocating masses, it is believed that satisfactory balancing could be achieved whilst limiting the dynamic augment ('hammer blow') to an acceptable figure.

Another area critical to the success of the proposal would be the design of the combustion equipment. To achieve an evaporation of 37 500 lb. of steam per hour would require a heat release rate in a BR 5MT - size firebox of 3.2 x 105 Btu per hour per cubic foot of firebox volume, or 1.9 x 106 Btu per hour per square foot of grate area. Such a high level can be reached - it has been achieved before with oil firing, and would be possible burning coal using the Gas Producer Combustion System - but achieving it with high combustion efficiency would be a difficult problem. The preferred fuel would be gas oil/diesel fuel.

It has been mentioned that high reliability is an important factor. This is because (i) in-service failures which disrupt other trains will be increasingly less tolerated, (ii) the intensive servicing and maintenance which steam received in the past (and which is still required on today's heritage locomotives) will become too costly, and (iii) spare parts will be expensive as they tend to be special items manufactured in small quantities. In its truest sense reliability means that locomotives must give high reliability 'on the road' with the minimum of maintenance effort. Even at the present state of the art reliability and simplicity do go together, and the format of the Class 5AT - a 2-cylinder single expansion 4-6-0 - is about as simple as a main line locomotive can be. Reliability is also very much a question of good detail design, and every attention would be given to this point at the detail design stage. Naturally features of proven high reliability, such as roller bearings, would be incorporated to the maximum possible extent. The level of reliability would be such that it is expected that major overhauls would only be required at approximately 250 000 mile intervals, with intermediate overhauls (dictated by tyre wear) at some 125 000 mile intervals, and major servicing (e.g. boiler washouts) at a minimum of 12 500 miles.

The foregoing has described just some of the important parameters having a bearing on the Class 5AT 4-6-0 design. It is a proposal which may be considered to be in the nature of a pilot scheme, to demonstrate the extent to which high-performance steam traction can satisfy the various requirements involved in the running of steam charters on tomorrow's railways, with the aim of securing their long-term future.

Finally, the main benefits to charter train operators offered by the Class 5AT 4-6-0 would be as follows.

  1. Offering more power, higher maximum speed and longer range than heritage steam, it would allow higher charter train speeds and make it easier to arrange suitable paths for such trains.
  2. A higher average speed would mean that long-distance charters could be run in less time, making them more attractive to customers who do not necessarily wish to spend too long on a train and increasing the list of possible destinations.
  3. The possibility of very high speed behind steam traction should be a commercially exploitable factor.
  4. The long operating range would offer the operator greater flexibility in the choice of routes and minimise the logistical problems of providing supplies.
  5. The running and maintenance costs would be low due to the low fuel and water consumptions and high reliability respectively.
  6. The use of 'shadow' diesel locomotives following steam charters to cover for possible engine failure could be dispensed with, giving a corresponding cost saving.
  7. The new and striking appearance of the locomotive should generate interest and attract passengers.
  8. Last, but most important of all, the very possibility of being able to run steam charters in the future may depend on such a locomotive as here proposed becoming available.

Turn back to Part 1 of David Wardale's Article or close window.

Note: David Wardale's book describing his work and his achievements is titled "The Red Devil and Other Tales from the Age of Steam". Copies of the second edition of this book can be procured from:

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