# 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 The estimated power outputs for the 5AT are “continuous rated” (not transitory maxima). Built 1937. Reckoned to be the most powerful class of steam locos to run in the UK. One of the last main line express passenger class of steam locos in the UK. Built 1951. Transitory maximum figure – refer “Red Devil and Other Tales from the Age of Steam” p273 Refer “Red Devil and Other Tales from the Age of Steam” page 494 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.