Power-to-Weight Ratio

5AT 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 output of the 5AT, which is very high for an 80 tonne steam locomotive, is the key to its high-speed capability.  Its very high power/engine-weight ratio is evidenced in Table 1 below where its cylinder and drawbar power outputs per tonne of engine weight are compared to the best produced by earlier British designs.

The 5AT’s drawbar power/weight ratio is less impressive simply because of the weight of its very large tender, whose mass and rolling resistance have to be taken into consideration in estimating drawbar power/weight ratios and drawbar tractive force respectively.  Notwithstanding, Table 1 (below) demonstrates that the 5AT’s drawbar power/weight ratio still remains impressive when compared to earlier locomotive designs.

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

  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

In his 2003 article for Steam Railway magazine, Wardale included a bar-chart comparing the power-weight ratios of the 5AT with a range of train sizes with those of several well-known types of “modern traction”.  The chart is reproduced below:


Note: As explained in The α Coefficient page, compound expansion locomotives generally offer higher power-to-weight ratios than simple expansion machines.