This Canadian columnist is all upset about some car manufacturers using metric horsepower instead of Imperial horsepower to rate their engines.
http://www.canada.com/montreal/montrealgazette/news/driving/story.html?id=c369d48c-18d9-4435-bf9a-4f9926b6d530
snippet only:
<<< . . . . Thanks to Watt's work, we now know that one horsepower is defined as the ability to produce 33,000 pound-feet of work every minute. In the case of the pit pony, this meant lifting 330 pounds of coal 100 feet in one minute, 33 pounds of coal 1,000 feet in a minute or 1,000 pounds for 33 feet - any combination of pounds and feet that combine to produce the number 33,000 in one minute gives you one horsepower.

To arrive at this number, vehicle manufacturers use a dynamometer - or rolling road - to measure power production. However, herein lies the rub. Dynamometers actually measure torque (twisting power) in pound-feet, so manufacturers convert this number to horsepower by multiplying it by engine speed (rpm) and then dividing it by 5,252. It should be simple.

However, some confusion creeps in because not all automakers use the same format to arrive at an engine's optimal output. Under the metric system, one horsepower is defined as 4,500 kilogram-metre per minute. This is the equivalent of 32,549 pound-feet per minute, meaning each metric horse is only 0.986 the size of an imperial one. It's a confusing situation that will change for the better if all auto manufacturers follow General Motors' lead. . . . >>>

Of course, "proper SI" would be to rate output in kilowatts, but in a metric country, metric horsepower would presumably be more correct (less wrong?) than Imperial horsepower.

<<
Of course, "proper SI" would be to rate output in kilowatts, but in a metric country, metric horsepower would presumably be more correct (less wrong?) than Imperial horsepower.
>>

Since one horse-power (metric, imperial or customary) is about three-quarters of a kW, motoro-car manufacturers perfer horsepower to kW - it makes for goog advertising because slighty higher numbers can be quoted. For example, the VOlswagen Golf 70PS has a 55kW engine. (PS = pferdesterke; pferd = horse, sterke=power).

But in the SI, it is a deprecated unit, in part because its definition uses another deprecated metric unit, the kilogram-force. I certainly hope the EU doesn't allow such practices, why it is as bad as using Imperial. :)

No!

(thats the answer to the thread question, can't be more honest than that)

"""one horsepower is defined as 4,500 kilogram-metre per minute.

But in the SI, it is a deprecated unit, in part because its definition uses another deprecated metric unit, the kilogram-force."""


the original definition of a "metric horsepower" was the power required to raise a 75 kg mass, one metre in one second.

Thus, the metric horsepower was by:

75 kg x 9.806 65 m/s^2 = 735.498 75 N

735.498 75 N x 1 m/s = 735.498 75 J/s = 735.498 75 W or 735.5 W (rounded)


The definition above comes from multiplying the 75 x 60 to get 4500 kg. The kilogram here is still a mass unit and not force unit. Thus if one raises a 4500 kg mass a height of one metre in one minute, one is doing the same work as lifting a 75 kg mass in one second.


""" I certainly hope the EU doesn't allow such practices, why it is as bad as using Imperial. :)"""


The pferdestärke is not a legal unit in Germany.

See:

http://www.ptb.de/de/publikationen/download/einheiten.pdf page 7 of 9

Also note when viewing the pages that those items highlighted in blue are not legal units in Germany. You will they include old, non-SI units as well as imperial.

Measuring work by "raising a mass" inherently involves the assumption of a standard gravity to treat the effort as force. The reason the kilogram-force is deprecated is because the SI has a proper force unit, the newton, and wishes to avoid the FPS confusion of pound-mass and pound-force, which comes directly from the concept of "lifting" pounds against earth's gravity, versus unit (horizontal) acceleration using chosen units of length and time. (It did take me a minute, if you'll pardon the pun, to figure out the 4500 kgf*m/min as I was aware of the 75 kgf*m/s definition.)

<<The pferdestärke is not a legal unit in Germany.

See:

http://www.ptb.de/de/publikationen/download/einheiten.pdf page 7 of 9

Also note when viewing the pages that those items highlighted in blue are not legal units in Germany. You will they include old, non-SI units as well as imperial.>>

That wasn't too helpful, all being in German. :)

Actually, that table would be better divided into three tables:
*Derived units of the SI
*Units outside the SI but accepted for use with SI
*Forbidden or unacceptable units

<<
The reason the kilogram-force is deprecated is because the SI has a proper force unit, the newton, and wishes to avoid the FPS confusion of pound-mass and pound-force, which comes directly from the concept of "lifting" pounds against earth's gravity, versus unit (horizontal) acceleration using chosen units of length and time.
>>

Well, the newton was established in order to avoid all the "confusion" that results from introducing gravity into the definition of force. But well after the newton was established, people persist in using the kilogram-force. What does this tell you?

It tells me that sometimes having force and mass numerically equal is convenient for some people. The SI system is too rigid, so people don't follow it.

Actually, I've never seen the kilogram-force used much. Germany didn't adopt it until 1958, and the SI was formalized in 1960 introducing the newton.
http://www.sizes.com/units/kilopond.htm
<<kilopond

A unit of force, mid 20th century, the Central European and Scandanavian equivalent of the kilogram-force, the force exerted by a mass of 1 kilogram at the standardized value of the acceleration due to earth's gravity, 9.80665 meters per second per second; in other words, roughly the weight of a kilogram, 9.80665 newtons. Symbol, kp. Thus instead of a meter–kilogram-force–second system, they had a meter-kilopond-second system. The French equivalent is the kilopoid.

The kilogram had been used in the 19th and early 20th centuries both as a unit of mass and as a unit of weight, which is a force. Distinguishing between the two in favor of mass created the need for a unit of force. In 1934, for example, the Swedish Royal Mint ended the uncertainty in Sweden by declaring the kilogram to be a unit of mass.1

The word “kilopond” was used internally by the Physikalisch-Technische Reichanstalt, the official German standards laboratory, as early as 1939. In 1945 Sweden adopted the kilopond as its unit of force. The Verein Deutscher Ingenieure (Society of German Engineers, VDI), however, objected in 1950 to the term's adoption in Germany on the grounds that units ought not to be adopted on a national basis and that “pond” was too close to “pound.” The decision by Technical Committee 12 of the International Standards Organization to recommend adoption of both the kilopond and kilogram-force as synonyms (1955, 1957) removed the first of these objections, and in August 1958 the VDI recommended provisionally that “kilopond” be used for the unit of force>>

http://www.physicsdaily.com/physics/Kilopond
<<Kilogram-force
(Redirected from Kilopond)
The deprecated unit kilogram-force (kgf) or kilopond (kp) is the force exerted by one kilogram of mass in standard Earth gravity (defined as exactly 9.806 65 m/s²). One kilogram-force is equal to exactly 9.806 65 newtons.

The kilogram-force has never been a part of the International System of Units (SI), which was introduced in 1960. The SI unit of force is the newton.

Grams-force and kilograms force were never well-defined units until the CGPM adopted a standard acceleration of gravity of 980.665 cm/s² for this purpose in 1901, though they had been used in low-precision measurements of force before that time.>>

"""Well, the newton was established in order to avoid all the "confusion" that results from introducing gravity into the definition of force. But well after the newton was established, people persist in using the kilogram-force. What does this tell you?

It tells me that sometimes having force and mass numerically equal is convenient for some people. The SI system is too rigid, so people don't follow it. """



It really says they are ignorant and don't understand the concepts of physics or mechanics and shouldn't be engineers.

"""Well, the newton was established in order to avoid all the "confusion" that results from introducing gravity into the definition of force. But well after the newton was established, people persist in using the kilogram-force. What does this tell you?

It tells me that sometimes having force and mass numerically equal is convenient for some people. The SI system is too rigid, so people don't follow it. """


It tells me that there is a degree of inertia out there, cause IMHO mainly by people who do not have an immediate benefit from making the change.

Apparently, you CAN take the horse out of the horseless carriage, but not the horsepower.

A camel is a horse designed by commitee

Did someone say that some units are "forbidden"

Cool! I'm going to be naughty now!


"Inch"

Come and get me!!!!







<<<<<<""" I certainly hope the EU doesn't allow such practices, why it is as bad as using Imperial. :)"""


The pferdestärke is not a legal unit in Germany.
>>>>>>>

ErrorNB: Germany is not the EU.
Nice insight into your mind though.

<<
It tells me that there is a degree of inertia out there, cause IMHO mainly by people who do not have an immediate benefit from making the change.
>>

This is a case of what the people working think is better vs. what those in charge at the top think is better. Like with education. Who do you think knows more about education, a classroom teacher or the superintendent of the school board? Similarly, who knows more about engineering, the rank and file engineers or the directors of BIPM?

<<This is a case of what the people working think is better vs. what those in charge at the top think is better. Like with education. Who do you think knows more about education, a classroom teacher or the superintendent of the school board? Similarly, who knows more about engineering, the rank and file engineers or the directors of BIPM? >>

I believe there is an alternate explanation, the journalist who wrote the article is an idiot.

The automotive industry is metric. The engine's output would have been measured in kilowatts, and marketing would have decided whether to divide by 0.7355 kW or 0.7457 kW, according to the 'flavor" of horsepower they wanted (or thought was required) in the brochure.

Anyway, that is only a 1.4% difference, there are many more creative ways of deetermining advertised horsepower. There is a new SAE procedure which requires audit or observation of the testing for claimed horsepower by a third party. That should stop some of the "stretched" horsepowers of the past.

John, I was referring to force units in general, not in the context of cars or horsepower. But those are good observations.

The May 2005 issue of SAE's "Automotive Engineering International" has brief article on the new procedure. If you wish to advertise your horsepower as SAE, the test must be certified by a third party observer per requirements of SAE J2723. The test is still conducted per SAE J1349. This includes specification of temperature, air pressure, and humidity (or requires adjustment to the standard consitions).

The DIN measurement standard uses higher standard pressure and lower temperature. With increased air density, an engine will normally produce more torque at a given RPM under the DIN standard conditions. That is probably more significant than the different sized horses used to convert power from kilowatts.

Are the horsepowers actually measured in the tests or is it the kilowatts and the horsepowers are just an afterthought conversion?

Actually, torque and rotational speed are measured. Horsepower, kilowatts are calculated from that data. Also temperature, dew point and pressure must be measured. Engine power is related to the density of dry air, which must be calculated from those parameters, and corrected to standard conditions. Engine power doesn't vary DIRECTLY with density however, the SAE pub. gives an equation. But given different standard conditions in SAE and DIN, you get different torques from the same engine, a bigger air/fuel charge means more power. SAE standard condition is 100 kPa of air, including 1 kPa water vapor (99 kPa dry air) and 25 degrees C. DIN (I think) is based on ICAO standard atmosphere at sea level, 101.325 kPa (dry?), 15 degrees C. (In spite of "make your car run on water" claims, neither water in the air, nor in the gas counts towards horsepower)


Typically fuel consumption is also measured. It is a good time to do an engine map and calculate brake specific fuel consumption vs torque and speed. In metric, power, in W, is just torque, in Nm, times rotational speed in r/s. However, speed is probably measured in rpm, requiring conversion to radians per second. If you datalog all values, computers can make nice graphs in whatever units.

The days of "gross horsepower" are over and the engine must operate an accessory belt with normal loads. For a car, that includes alternator and a/c compressor, but the only electrical load would be what the engine draws and the a/c clutch would be off. The engine would have to operate a representive cooling system, whether mechnical or electrical fan. The goal is shaft power out while running itself as installed in a vehicle, but no miscellaneous non-engine loads.

Apparently DIN standard condition is based on 20 °C. Using the SAE correction formula, at DIN conditions, and engine would put out 1.038X more power (in kilowatts) and be derated by 0.9634. The big horse/little horse issue adds another 1.4%.

At conditions near the two standards, I would expect the same engine would have an output rating in kilowatts about 3.8% higher by DIN method than SAE, and if horsepower was cited, about 5.2% based on DIN PS vs SAE HP. As you get further away from the reference conditions, the derating equations differ somewhat and the spread would increase for extreme conditions. Note that the difference in test conditions (pressure, temperature) is about 2.7X more important than "how many watts in a horse."