Traction engines are mighty machines capable of pulling heavy loads or powering threshers and sawmills, yet they are often rated at a very low horsepower, maybe just four or five, from an engine that is weighed in tonnes and gulps down coal.

This would appear to make them hugely inefficient and wasteful of resources, however that figure is somewhat misleading as it will refer to nominal rather than brake horse power, which is the more modern term, although that in turn is now being displaced by kilowatts.

Nominal Horse Power (NHP) is a calculated figure which is based on the piston diameter and connecting rod speed.

However, the actual definition was never set out or recognised in engineering circles, despite it being adopted by the Royal Agricultural Society of England (RASE) in the 1840s as a measure of an engine’s ability to do work.

Made by Allchin of Northampton, Lena has a nominal horsepower of 8

It must be remembered that the engine was very much a new fangled thing nearly 200 years ago and the manufacturers were, just like today, keen to simplify the method by which they could be compared by potential customers.

Watt – engineer and marketing genius

It was James Watt who apparently came up with the calculation to enable this comparison while his other measurement of power, brake horsepower, was, at first, used to calculate royalties payable by mine owners who were switching from horses to steam to keep the mines dry.

A comparable situation was in place for much of the late 20th century where the cubic capacity (cc) of an engine was taken as the main indicator of a car’s power and was proudly displayed on the boot for a while.

Engine size on Ford Cortina — Ford was one car company that differentiated its models by engine size, in this case a 2,000cc Cortina estate

The Austin 1300 had a bigger displacement than the Austin 1100 for instance, and would therefore go faster, actual power output was rarely mentioned and cynics would suggest that this was because power output on mass produced internal combustion engines was rarely the same from one unit to the next.

Meanwhile, motorbikes are still sold by engine size, although nobody is suggesting that modern production methods are at all sloppy; it’s that the bigger the engine, the bigger and heavier the bike, important factors in the buying decision.

Comparison by nominal horse power

Thus it was that the NPH was more a comparison between the displacement of steam engines rather than useful power output, yet it was a convention that stuck until the 1920s when reciprocating steam engines started fading from the scene altogether.

The question arises as to whether there is a way of calculating brake horse power (BHP) from nominal horse power.

The answer is not really, there are too many variables, not least of which is steam pressure.

Traction engine cylinder — Nothing new under the sun. Cylinder, piston, flywheel and even injection pump can be seen on this Ransomes, Sims and Jefferies engine from Ipswich in Suffolk

However, there is a rule of thumb which suggests that one NP is equivalent to between six and seven BHP, so for a 8hp traction engine such as Lena, the actual BHP will be around 50hp to 55hp.

This goes a long way to explain the capabilities of traction engines, and why they were so easily replaced by the internal combustion engine.

The Fordson Major is in the same power bracket, is a lot lighter and cheaper, takes a minute to start rather than hours and is less prone to set light to the straw when threshing.

The traction engine was an incredible piece of machinery with a character and presence that may be sorely missed, yet they were machines of their time, a stage in mechanisation that passed long ago.

Thankfully, there are still a number preserved and running to be appreciated and enjoyed by all.

Watt – engineer and marketing genius

Comparison by nominal horse power

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