Abuse of power?
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There would be an uproar if fuel suppliers created a new litre, equal to 986cc instead of 1,000cc, but still called it a litre. So why do we accept that some manufacturers are ambiguous about the size of the horses in their engines’ horsepower ratings?
Words: David Wilcox The first unit of power – the rate of doing work – was established by Scottish engineer James Watt in 1783 to help promote his improved design of steam engines. Watt coined the term ‘horsepower,’ deining it as the power needed to lift 33,000lb by one foot in one minute, equivalent to lifting 550lb by one foot in one second (550ft-lb/s). Converted into metric units, this becomes the power to lift 76.04 kg by one metre in one second (76.04kg-m/s).
In the late 19th Century, around 100 years after Watt’s deinition, German engineers created another unit of power, the pferdestärke (horsepower). This is abbreviated to PS and is sometimes referred to as metric horsepower. It is deined as the power required to lift 75kg by one metre in one second: 75.00kg-m/s.
When gravitational force is factored in, converting kilograms into newtons, Watt’s hp = 0.7457kW whereas one PS = 0.7355kW. So we have two units of horsepower that are confusingly close: the original hp (sometimes called ‘imperial’ or ‘mechanical’ hp) and the metric version, PS.
The latter is 1.4% smaller than the former. When converting kW to hp multiply by 1.341, and when converting kW to PS multiply by 1.36.
It is arguable whether hp or PS ratings should prevail. There is a strong claim for hp on the basis that it is the original value. And in choosing the name ‘watt’ for the future unit of power in 1882, a direct link was forged from James Watt to the new unit. The watt was subsequently adopted by the Système International d’Unités (SI) as the SI-derived unit of power.
And the watt is based on metric units (kilograms and metres) anyway, so one could argue that there is no need for PS as a second metric unit of power – just use kW.
A safe bet
The kW is the safe bet. It is unambiguous. It is the basis for European technical legislation. But the kW is rare in the US, where speciications invariably cite only hp. The US deinition of hp, by the way, is always an imperial hp, equal to 0.7457kW.
It is unsurprising that European vehicle manufacturers apply the PS conversion factor – it is shrewd marketing. Smaller horses mean bigger numbers on speciication sheets. For example, a 300kW engine may be described as developing 402hp or 408PS. Who wouldn’t opt for the bigger igure? Instead of rounding down 402 to 400, the output can legitimately be rounded up to 410 on the badge. Whether truck manufacturers convert kW into hp or PS is relatively unimportant. The real issue is the widespread portrayal of PS as hp.
Vehicle manufacturers routinely convert kW into PS but then express the answer as hp. At best this is ambiguous: at worst it is misleading, with buyers being short-delivered on horsepower by 1.4%. This shortfall amounts to 10hp-11hp on the 730hp-750hp units produced by Volvo and Scania; it is 6hp on a 450hp engine. Granted, that is not much, but why should there be any discrepancy at all?
This practice of bigging-up power ratings by describing PS as hp is rife. For example, Scania’s speciications quote hp but the conversions from kW are actually PS. Daf does likewise. Volvo Trucks’ power curves have an hp scale, but the curves actually graph PS.
An interchangeable abbreviation?
CM asked these manufacturers why they do this. The gist of their responses is that while they recognise the difference between imperial and metric horsepower, they believe it is reasonable to treat the hp abbreviation as interchangeable with PS. They reason that, because Europe is metricated, the meaning of hp has evolved to refer to metric horsepower. For example, Volvo Trucks’ engine strategy and planning manager Mats Franzén took the trouble to quantify the difference between imperial and metric horsepower to six decimal places of a kW, but implies it is only in the US where hp retains its original value. Volvo Trucks’ view is that throughout Europe – including the UK – hp now means metric horsepower. “Horsepower = Pferdestärke = Hästkraft,” says Franzén succinctly, introducing the Swedish term.
Scania (Great Britain) technical manager Clive Burnett says: “The main reason we use metric horsepower is to give uniformity throughout Europe and I believe there would be even more confusion if we started showing different power ratings for vehicles that are destined to operate in the UK.” He adds: “The primary power igure we quote in all of our speciication sheets for engine technical descriptions is in kilowatts, as this is the only term that can be used for EU certiication.” Phil Moon, UK product marketing manager at Daf Trucks, also says that hp retains its original meaning in the US, but in Europe it is legitimate to use hp when referring to the metric value because it is metric horsepower that predominates throughout Europe.
“Although there is ambiguity between metric hp and imperial hp, it was considered preferable to use hp instead of PS as PS is not recognised or understood by a lot of people,” explains Moon.
Other manufacturers believe it is better to use PS to avoid any chance of ambiguity, making it clear that they are measuring power in the smaller of the two units. Iveco’s power igures quote PS alongside kW. MercedesBenz has hitherto followed that strategy too, although the new Actros speciications quote kW and hp – the hp is the smaller metric variety. Mercedes’ sister brand Fuso picks PS as the alternative to kW in Canter speciications. Cummins Engines also quotes PS and kW, removing the risk of confusion about European and US deinitions of hp.
Confused yet?
MAN quotes PS igures but omits kW values in its UK speciications. Hino does the opposite, quoting only kW. And Renault Trucks UK has recently headed off in a different direction altogether, switching to kW and CH power ratings in its online speciications. CH is the abbreviation of the French word for horses (chevaux): one CH equals one PS.
When CM asked Isuzu Truck (UK) why its speciications quote hp and kW, even though the PS conversion factor had been used, operations manager Bob Morris accepted the company had slipped up in the translation. “It’s our mistake. We put our hands up to it,” says Morris, adding that Isuzu is about to reprint its speciication sheets. “We will drop references to hp and change them to PS.”
So there you have it. We can blame marketing strategy, alternative views on the use of the hp abbreviation or details lost in translation, but for certainty and precision when comparing power outputs, count the kilowatts. Watt’s deinition of what a horse can do may be superseded in Europe, but his name endures. n