Horsepower Calculation ?

The formula for determining horsepower for an automotive/internal combustion engine is as follows:

Torque x RPM

Is this formula accurate for diesel engines as well?

Yep. Horsepower is proportional to torque and proportional to RPM, and that’s it; doesn’t matter how the torque is made, whether it’s diesel, gasoline, electric, turboshaft, wind, or a dude pedaling a bicycle.

Note that your equation is valid only for torque expressed in foot-pounds; you need a different value in the denominator if you’re using torque in N*m or some other unit.

The trouble is getting accurate measurements of torque because it’s difficult to determine outside of actual load conditions. Many engines are rated based on fuel consumption and an independent calculation of engine efficiency.

Or "what the opposition claim they produce, plus 50 ".

OK, so please help me with my long-forgotten and unused math skills.

Engine RPM: 720
HP: 1,600

Torque @ 720 RPM?

horsepower = torque * RPM/5252

Rearranging, we get torque = horsepower*5252/RPM

Assuming your 1600-horsepower was measured at 720 RPM, then:

torque = 1600*5252/720

torque = 11,671 foot-pounds.

Marine diesel?

Thanks much.

Emergency diesel/generator (1 of 2), USS Midway. 9 Cylinder, horizontally opposed 18 piston diesel. manufactured by Fairbanks, most likely very late 1930’s. My understanding is that it is pretty close to identical to what was used in railroad locomotives of that era. BTW, despite being in service from 1945 thru 1992, it’s spotless. If interested, will send pics. The version for the locomotives may have had 10 cylinders with 20 pistons.

Pics please!

I’m a volunteer docent on the Midway. Outside chance I can get down there tomorrow. Next scheduled shift is early next week. Earlier or later will be back at ya.

Yeah, if you want it metric, you have to cut up a horse into 10 equal pieces.
-Proud to be a 'merican!*

Don’t ask me what you’ll need to do if you want it in Planck units…

When the steam engine took over from horses in the mines during the early 1800’s, the best sales pitch was to say how many horses they would replace.

James Watt, did some calculations and decided to say that a horse could pull 550 ft lbs per second. This would have had to be a pretty strong horse, and of course horses need break, which makes the average a lot less. The point was that Watts engines could easily achieve what he said they could, so he impressed the mine owners and sold more engines.

550 ft lbs per sec became one horse power. This bears little relationship to the horse power of a modern engine.

Of course the Newton is better "the amount of energy needed to accelerate 1 kilogram of mass at the rate of 1 metre per second squared". From this we have the better known Watt "One watt is the rate at which work is done when an object’s velocity is held constant at one meter per second against constant opposing force of one newton"

And you guys stick to BThUs and Horsepower - jeez

The formula for power is just power = torque times rotation rate, with no magic numbers you need to memorize. The cryptic magic number only comes in if you’re trying to convert from one crazy unit for torque and another crazy unit for rotation rate (itself derived from a crazy unit for angles and a crazy unit for time) into an unrelated crazy unit for power.

Yes interested

SanDiegoTim, Thank you for what you do. My son and I spend a whole day aboard the Midway last year and ran out of time.

Funny, I learned that torque is lb·ft (pounds on the end of a foot-long lever) and work is ft·lbs (one pound of force moving something one foot).

The better car mags (Road and Track, Hot Rod) always use lb·ft for torque, not ft·lbs.

Just noted an error in one of my earlier posts. The pistons/cylinders are not horizontally opposed. They are vertically opposed. Thought it best to clarify. Will have pics soon.

Cool! Toured the Midway a couple of years ago, very very interesting visit.

Also got to tour the Fairbanks-Morse engine plant in Beloit, Wisconsin about 20 years ago. Your emergency generator engine is small potatoes compared to some of the stuff they build there; power outputs go up to almost 24,000 horsepower. They brought us into one small test cell that was so loud we couldn’t hear our guide talking; the sound was like someone hitting an anvil with a sledgehammer a couple of times per second. When we left the cell, he explained to us that the test cell had a fuel injector test stand; what we had seen/heard was a diesel fuel injector that was delivering about one cubic inch of fuel per shot. :eek:

A similar distinction is seen in the metric system, where work is generally expressed in joules, while torque is expressed in newton-meters. But strictly speaking, the units for work and for torque are interchangeable, and there’s nothing actually wrong with referring to a torque in foot-pounds, or in meter-newtons, or in joules. Or even in Calories or BTUs or something crazy like that, though I can’t think of any sane reason to do that.