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
Locomotive Type	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.