Why horsepower needs to be retired

The Mercedes EQC electric SUV (shown) produces a total output of 300 kilowatts. Referring to the vehicle as having 402 hp is pointless.

The time has come to ditch a beloved old anachronism of the Industrial Revolution: horsepower. With the mass-market introduction of electric vehicles, the term has outlived its usefulness and eventually it may even become a relic of a bygone era. Fundamentally, the unit is an expression of work performed over time, a quantity in physics that describes how much energy is needed to move a certain amount of mass a certain distance over a certain period.

Now that electric vehicles are becoming a fixed quantity in the auto industry, it's only a matter of when and not if the public becomes more familiar with watts and specifically kilowatts, the standard used by automotive engineers. Take the EQC, the Mercedes-Benz electric SUV that debuted in September. Its two electric motors produce a combined output of 300 kilowatts, but only after they are fed the 80 kilowatts of power the battery supplies per hour.

Once depleted, it can be replenished using a maximum of 110 kilowatts of direct current (DC) from a fast charging station. Referring instead to the vehicle's 402 hp is both inelegant and pointless. It makes more sense to pick one single unit of measurement. One could demand that battery capacity be expressed in horsepower-hours instead of kilowatt-hours -- that would at least be consistent.

But there are some serious complications arising from the industry's favored unit. First, the term horsepower is outright confusing since it means different things to different people. There's the metric kind and the imperial. The latter is just another name for brake horsepower.

This, however, measures an engine's gross output that reaches the crankshaft, differentiating itself from the net result after adjusting for internal driveline friction, called wheel horsepower. Metric horsepower is also known alternatively by the initials from the word in German and French, PS and CV, respectively.

Pferdestaerke (literally "horse strength" in German) and cheval-vapeur ("steam horse" in French) sound close enough that one can be forgiven for mistaking them as a direct translation of what British and American consumers know as horsepower, which would be great -- if it were true.

The result is often odd and seemingly arbitrary numbers. For example, Volkswagen Group's initial reason for developing the Bugatti Veyron first launched in 2005 was to build a car with 1,000 PS.

But that goal flies out the window when converted to 987 bhp in the imperial system. At the very least that is a notable difference. When it comes to cars that mere mortals can afford, the difference is just a rounding error.

Somewhat frustratingly this means that the gap between metric and imperial horsepower -- or PS and bhp -- is just enough to be measurable, but not enough to be meaningful.

The problem arose because Scottish inventor James Watt wanted to come up with a term to compare the effectiveness of his steam engine against the horses it was designed to replace. One unit of horsepower derived from his crude estimate that a horse can lift 330 pounds of coal out of a mine shaft at 100 feet per minute. Compare this antiquated definition with the simple elegance of a watt, which is the energy transferred when applying one newton of force over one meter of distance in one second.

Speak to powertrain engineers, and they refer to their engine power in kilowatts. This is the easiest benchmark based on the approved measurement used in the International System of Units, better known by its French abbreviation SI.

Ask them why horsepower still survives as a term today and what you often hear is as cynical as it is refreshingly honest: "It's a bigger number, so it sounds better." We could all, for example, buy lightbulbs measured in tenths of a horsepower if it was such a flexible unit of measurement. Fortunately, no one suggests we do.

Now that electric vehicles such as the EQC and many others are becoming a reality, why not make life easier by adopting one logical and consistent method for measuring output. That means it's time to give horsepower -- in all its various forms -- a merciful death. Kilowatts are here to stay.

This story is from Automotive News Europe's latest monthly magazine, which is also available to read on our iPhone and iPad apps.You can download the new issue as well as past issues by clicking here.
You can reach Christiaan Hetzner at christiaan.hetzner@gmail.com -- Follow Christiaan on Twitter: https://twitter.com/christiaanhtznr

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