HORSEPOWER, SAE, DIN or kW ?

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This is an extract of the article, with small photos. You will find the complete article with full-sized photos in my e-book View America: North East - Part 1

In the travel series View America, North East - Part 1 covers Michigan and Wisconsin. It is not a traditional travelogue, but a non-commercial and more or less objective chronicle of an in-depth exploration of these states. Each state is described with its own brief historical background and its main sights, tourist attractions and points of interest.

My book does not describe lodgings, restaurants or entertainment, except where these may interact with the narrative. It is illustrated with more than 100 full-sized photos.

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The origin of "Horsepower"

The horsepower of a car is generally specified as SAE HP in the U.S., whereas in Europe it is usually specified in DIN horsepower. More recently it is also specified in kW (kilowatts). The definition of "horsepower" is the amount of work that is performed in a given time. But what is the origin of the name horsepower?

It is obvious that it has something to do with a horse, but what? Well, the term comes from the inventor of the steam engine, James Watt (1736-1819). His name became immortal because it was linked to the unit of power, and every time you need a new bulb, you reverently pronounce his name...

In order to sell his steam engines, Watt had to provide at least an idea of their power. Before his machines, everything was compared to horsepower, because the horse was the only industrial power available.

Around 1770, Watt "calculated" that the horse pulled with a power of 180 pounds, although no one knows how he arrived at this number. He probably estimated it, comparing its strength to human power. After more than 200 years, we still use this "standard value".

One horsepower stands for the power to lift 550 pounds one foot for one second, or 33,000 pounds per foot per minute. Converted to the metrical system, one horsepower stands for the power to lift one hundred and fifty kilograms thirty meters high in one minute. So 1 HP = mgh / t = 150 x 9.81 x 30/60 = 736 watts. The horsepower was also defined as the power required to slowly lift a load of 75 kilograms (at a speed of 1 meter per second, or 3.6 kilometers per hour). A dropping acceleration of 9.81 m/s² provides the following formula: 1 HP = 75 kg x 1 m/s x 9.81 m/s² = 736 watts.

SAE HP, DIN HP, and kW

There are two differences between DIN and SAE horsepower. The first difference concerns the basic units, with which the data are measured. A second difference concerns the way in which the power is measured. Cunning U.S. automakers came up with all sorts of tricks to suit the new standard to their own needs. In the 1960's the manufacturers fought a real publicity war among themselves with their famous Muscle cars, that needed at all cost to be advertized with ostensibly more power than the competition's car. Therefore they ran the tests on their engines without fixed accessories such as an alternator, a water pump, or even a frame...

"horsepower" 1 "horsepower" 2

In the 1970's the SAE (Society of Automotive Engineers) imposed standard tests on American manufacturers, which improved comparative values. However, they are still allowed to test their engines in different ways. The SAE J1349 Engine Power Test Code specifies a net power, measured by a dynamometer. The basis of the current test is about thirty years old, and the last "adaptation" dates from 1995, so that the manufacturers still provide higher values than the actual power and torque. In 2004 finally a uniform SAE standard arrived.

In Europe, the DIN (Deutsche Industrie Norm) 70020 and DIN 6270 were accepted, and they impose a measurement of the engine's power with ALL fixed accessories, such as inlet and outlet, fan, water, fuel and injection pump, and no alternator.

Other power values

Apart from the SAE, DIN and kW values, there is also the brake horsepower, calculated as the braking power needed for the movement to stop, and the fiscal horsepower, which is calculated on the basis of the cylinder content, mixed with some fancy and very political formulas...

fiscal horsepower tractor pulling with 4500 hp...

Today, and more and more, the engine's torque is also specified, which is the turning power to the drive wheels. A higher horsepower usually indicates a higher top speed, but a higher torque indicates a stronger acceleration power and a greater ability to pull heavy loads.

The formula between these two values is HP = torque x 2 x pi x rpm, which explains the reason why most engines deliver their maximum power at higher revolutions.

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