5AT
 
5at Train

The 5AT Group - Steaming Ahead with Advanced Technology

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Power-to-Weight Ratio


The power-to-weight ratio of any powered vehicle, whether car, aeroplane or locomotive, is not a fixed quantum, but varies throughout its speed range.  At zero speed, the power output (by definition) is zero. As speed rises, power output will rise to a peak value, and will often decline as speed increases further.

When quantifying a locomotive's power output, it is necessary to define where the output is measured.  In the case of diesel locomotives, their nominal power rating usually refers to the power delivered at the crankshaft, taking no account of the further electrical and mechanical losses that occur in the rest of their drive mechanism.

The power of a steam locomotive is quoted either as "indicated power" or "drawbar power".

The power-to-weight ratio is important for a number of reasons.  In the case of motor cars, the power-to-weight ratio governs the acceleration that can be achieved.  In the case of locomotives, the benefits of a low power-to-weight ratio are more subtle because the weight of the train behind the tender vary enormously.

Essentially the power-to-weight ratio of a steam locomotive provides a clear indication of the efficiency of the design, efficiency in this case meaning "power output per tonne of steel" and/or "power output per dollar of cost".

Wardale makes the following observation about power-to-weight ratio of the Red Devil on page 277 of his book:

"For high speed operation, a high power-to-weight ratio is essential, which implies the need for a smaller boiler, requiring highly efficient draughting and high combustion rates, requiring a highly efficient combustion system.  Failure to realise this means that at high speed most of a locomotive's power is absorbed in pulling the locomotive itself.  In fact any locomotive has a 'zero drawbar power and thermal efficiency' at which all its power is used to pull itself along, this speed being largely a function of the inbuilt power-to-weight ratio.  That this ratio was not high enough in steam locomotives was the basic reason why steam was perceived as being unsuitable for the accelerated services which many administrations, especially in Europe, saw as essential if rail transport was to remain competitive.  It was, however, an inherent characteristic of most First Generation Steam locomotives, not steam traction per se."

 


 

 

On page 273 and page 494 of his book “Red Devil and Other Tales from the Age of Steam”, Wardale compares the power-weight-ratio of his Red Devil and the 5AT, with several well-known locomotive classes.

Table 1 – Power-to-Weight Ratio Comparisons

Locomotive Type

Engine Weight

Max cyl power

Cyl power/wt ratio

Max equiv drawbar power

db power to engine weight ratio

Engine + Tender Weight

db power to total weight ratio

Tonne

kW

kW/tonne

kW

kW/tonne

tonne

kW/tonne

5AT

80

25751

32.2

18821

23.5

160

11.8

SAR Class 26

123

3750

30.5

3000

24.4

236

12.7

BR Class 8 Duchess2

108

2495

23.1

18496

17.14

164

11.33

BR Class 7 Britannia3

94

 

 

15046

16.05

143

10.53

German Class 45 2-10-2

17.57

SNCF Class 240P 4-8-0

23.37

New York Central Niagara Class 4-8-4

18.87

Rio Turbio Railway Santa Fe 2-10-2

20.67

Porta's experimental 4-8-0 Compound

23.27

  1. The estimated power outputs for the 5AT are “continuous rated” (not transitory maxima).
  2. Built 1937. Reckoned to be the most powerful class of steam locos to run in the UK.
  3. One of the last main line express passenger class of steam locos in the UK. Built 1951.
  4. Transitory maximum figure - refer “Red Devil and Other Tales from the Age of Steamp273
  5. Refer “Red Devil and Other Tales from the Age of Steampage 494
  6. Values deduced from loco and tender weights, and Power/Weight ratios given above.
  7. Values taken from “Red Devil and Other Tales from the Age of Steampage 273

Information on the 5AT's power-to-weight ratio can be found on the 5AT Power-to-Weight Ratio page of this website.