Horsepower: Where do we measure it?

[Anonymous]

Someone rang me the question I have in mind for long long time. I wonder where do we measure the horsepower output, at the engine, at the alternator (or generator), at the TM, or anywhere else? Thanks!

Karn[:)]

[Randy Stahl]

For our purposes WE measure horsepower in kilowatts from the main alternator/ generator. Volts times amps = watts, divide by 700 . This is actual horse power NOT rail horsepower, too many veriables to calculate drawbar or rail HP , all theory anyhow.
Randy

[adrianspeeder]

I dont like horsepower #s only. Horsepower is only a mathamatical conclusion anyway. Torque is what i like. How much tprque do locos put out?

Adrianspeeder

[Overmod]

Adrianspeeder, torque is not particularly meaningful as an index of *electrical* performance, as the developed torque curve of the engine at each of the 'governed' speeds in Run throttle positions will be affected by generator excitation and some other factors. You can derive the torque fairly easily from hp by 'correcting' time back out of the measurements for a given set of output conditions.

"Horsepower" in Mark's sense is really an advertising number, similar in a way to locomotive ihp calculated from theoretical parameters. Many steam fans will make an analogue to adrianspeeder's comment by noting that wheelrim or drawbar TE, not "horsepower", is a better indication of a locomotive's performance at many speeds, not just at or near starting. (I disagree, but for complicated reasons almost certainly boring to anyone reading this!)

As with automobiles, or for that matter with stereos, there are different ways to measure power, some of which constitute thinly-veiled advertising or 'one-upsmanship'. DIN horsepower is IIRC the same measurement we used in SAE before 1971 or so; naturally, the current SAE measurement which includes 'necessary' ancillary power draws is more meaningful for an engine installed in a working vehicle. But a still more meaningful number is the 'dyno' output, which is directly equivalent to wheelrim torque or hp measurement, and represents the actual engine power that's making it out to 'where the rubber hits the road'.

'Dynamometer' testing on railroads measures something even a bit more 'realistic' -- the actual power through the drawbar used to pull trains, with input from time base, rotating machinery and speed sensors, control position, etc. as needed to determine a horsepower figure and compare/contrast it with more 'theoretical' numbers or measures. THIS is the 'best' meaningful number (imho) that describes locomotive power output -- more meaningful still in that it's directly related to the economic purpose for which the locomotive was built, and not to some index of 'thermodynamic efficiency' that only peripherally addresses how well the locomotive actually works in service.

The measurement of 'torque' that is meaningful for diesel-electrics is the aggregate torque, at a given actual factor of adhesion, measured at the wheelrims of the traction wheels. Note that this number might be somewhat less than theoretical achievable torque based on adhesion characteristics, and some 'correction' could be made based on loading characteristics for the motors, losses in the connections, instantaneous/hourly/continuous TM ratings, etc.

[Randy Stahl]

We all know that a deisel locomotive is an electrical device, not much different than a powerplant. How much horsepower is a powerplant turbine generator and how do we measure it? You load it , check volts and amps and calculate HP by KW or MW out put.
A diesel electric locomotive IS a power plant and has been used as a stationary powerplant in the past. Locomotive horsepower is KILOWATTS period. Kilowatts available for traction.
Tractive effort is a variable that changes even when the fuel tank starts to empty out. I presented the AAR formulas in the thread Steam loco TE vs DE hp. The formula's for steam horsepower is also there.
Randy

[adrianspeeder]

Yeah ok, but what would the torque be if we pulled the generator and hooked up a dyno?

Adrianspeeder

[Anonymous]

Torque is a static measurement independent of time, a snapshot of an amount of rotating force at a given moment.

Torque is stated in foot/pounds, because a twisting force is being represented as a linear or straight line force. For example: 1 ft/lb = 1 lb of force turning a rod or wheel one foot from the center of rotation.

Horsepower is an amount of torque for a period of time. For example, 1 horsepower is 550 ft/lb for one minute, either 550 lb for one foot, or one pound for 550 feet for one minute of time, its all the same. That distance can be in a straight line, or the circumference of a circle for example: 550 rotations of a one foot circumference circle in one minute.

When an engine is tested on a dynanometer, both torque and horsepower are measured because either one by itself, does not show the whole picture of what is happening.

In an internal combustion engine torque is a resault of cylinder pressure, surface area of the piston, and mechanical advantage for the angle of the con rod and length of crank radius.

Horsepower tells us how quickly is energy being released.

So, comparing the two tells us how much energy is being released and how much is getting converted to work.

To consider one more important than the other is not correct. When a diesel makes more torque than horsepower, it not because it is making poor horsepower, it is because it is making good torque !! In other words, more of the given energy is being converted to work - higher efficiency. Does this make a top fuel dragster with 3000 hp and 1000 ft/lb torque innefficient? no, because a large amount of energy is being released very quickly. The 1000 ft/lb is in a smaller fraction of time.

[jchnhtfd]

Randy's got it (why am I not surprised?) (and we'll just leave things like power factors out of it, ok?!). And folks, you simply can't compare torque and horsepower. Torque is a variety of force: twist instead of push. A force (or torque) times a distance gives you work (believe it or not, from the standpoint of physics and engineering, no matter how hard you push, if you aren't moving, you aren't working!). A force or a torque times a distance (work) divided by how long it took to get there is horsepower...

Just as a homespun example... I have a 12,000 pound stock trailer. I also have a 230 hp Chevy truck with a hitch. I also have a 30 hp Ferguson tractor with a hitch. They can both pull that stock trailer up the hill near my place, no problem. The only difference is, the Chevy can do it at a speed of 45 mph, and the Fergie tops out at about 5... !

[adrianspeeder]

I know my truck only puts out 270hp, but can put out 525ft#s torque. I thought that was impressive, but now dodge diesels are posting 600 for torque, and yesterday chevy posted 605 for torque. Ford has yet to post '05 numbers for its diesel, but i bet it's going to be at least 606ft#s. Now its turned into a peeing contest between the big three, but it still is cool to see that much power in a pickup.

Adrianspeeder

[TH&B]

My guess is that your 30hp Furgy could probably pull a heavier load up to 5mph then your 200+hp truck can.

[Randy Stahl]

Especially were it on steel wheels!!!!!
Randy

[Anonymous]

So what your saying is that a locomotives type of " torque " is the maximum starting tractive effort. Lets say for example 145,000lbs.

[GP40-2]

The formula that relates HP to torque is:

HP = (torque) X (rpm)/ 5252

The GE FDL produces a maximum of 4700 HP at the crankshaft at 1050 RMP, therefore:

torque = (HP) X (5252)/RMP = 23,508 lbs*ft torque@1050 rpm

Of the 4700 hp, a minimum of 4400 hp is reserved for traction, with the remainder reserved for auxillary use.

That 23,508 lbs*ft is a tad more than your truck! [:D]

[GP40-2]

As you can see HP is nothing more than torque (a twisting force created by the downward motion of the engine's pistons that the crankshaft converted into a rotational motion) multiplied by speed (the engine RPM) divided by a derived mathamatical constant (5252).

This same formula can calculate the TE of a locomotive if you know the traction motors HP and the speed of wheel rotation. You will also need to know the adhesion of the locomotive, as this is the limiting factor for maximum TE.

This formula also explains why an engine of small displacement (such as an Indy car engine) and relatively low maximum torque (torque is related to engine displacement) can produce a crap load of HP. The engine may be small and low torque, but it spins at such high RPM (10,000+) that its HP output is very high (1000+ HP)

jruppert: Top Fuel engines produce 6000 HP at 8000 RPM!!! Of couse, this is only for a few seconds. At the end of a run, engines must be totally rebuilt. Many of the internal parts are melted together!! To find out the torque:

Torque = (6000)(5252)/8000 = 3,939 lbs*ft@8000 RPM

[ericsp]

I’d like to say some things about the formulas used in this thread.

Never pluralize the units when they are abbreviated, it could be mistaken for seconds.

The English units for torque are lb-ft, not lb/ft. Lb/ft is pounds per foot..

A moment (torque) equals force multiplied by the lever arm (distance). Moment and force are not interchangeable or comparable.

Other than this the posts appear to be fairly accurate.

Here is how the constant in the HP formula is derive. 1HP=550lb-ft/s. If we know the torque and the speed of the engine we can multiple to gets its power. However, if RPM is being used the units are wrong. We have to convert the revolutions per minute into radians/second (rad/s). There are 2pi radians in one revolution, therefore 1 RPM = 2pi rad/min. To convert to rad/s we multiple by 1min/60sec, 1RPM = (2/60)*pi rad/s which is approximately 0.1047 rad/s. An engine producing 1000 lb-ft of torque and turning at 600 RPM is producing 62831.85 lb-ft/s of power. To convert this to HP divide by 550 HP/(lb-ft/s), this gives 114.24 HP. The 5252 comes from (2*pi/60)/550 which is approximately 1/5252.

[Randy Stahl]

RKARN ,
If you want to find the horsepower of your locomotive connect the generator to a load box, put the engine in throttle 8 and calculate volts times amps divided by 700.
Randy

[ericsp]

QUOTE: Originally posted by Randy Stahl

RKARN ,
If you want to find the horsepower of your locomotive connect the generator to a load box, put the engine in throttle 8 and calculate volts times amps divided by 700.
Randy

Randy:

With all of the posts, you should have started with "To make a short story long..."

Also, why do you use 700? According to the conversion table on page 646 Fundamentals of Thermodynamics, 5th Edition by Sonntag, Borgnakke, and Van Wylen one horsepower is approximately 745.7 Watts. With the power of locomotives, couldn't this result in a large error?

[Anonymous]

Randy is right on, because a generator's resistance to rotation when its being loaded is no different than a dynanometer's resistance to turning. In fact, an electrical dyno (vs. a water brake, or friction brake type) is the exact same thing.

When a brand new gen-set is finished, it is evaluated with a load bank, which is what dynamic brakes are. Kw/hr, Hp, it's all the same.

Larger engines like locomotive engines are not dyno tested because it's not practical to build a dyno for such amounts of power, so other methods of determining performance have to be used, like comparing actual fuel consumption to a BSFC chart, or fuel consumption vs. power generated. Locomotive engines are also far from the biggest engines, some marine engines develope 40,000 Hp, engines of this size are typicaly rated in Kw.

Rating output at the flywheel, makes sense to me because while an engine's output can be pretty constant, the power that gets "to the ground" can vary a lot, and this is true for any vehicle or application. In the case of locomotives, it is convenient to measure electrical output even though we know the the engine as a x,000 Hp unit.

SD-40-2, right on with the formula.

[Randy Stahl]

700 is the industry standard, you are correct that one HP=745.7 watts, how ever we have to compensate for fuel temp, air density, ambient air temp,water inlet temp. 700 is simply the generator efficiency factor.
Randy

[jrw249]

Originally posted by Randy Stahl

RKARN ,
If you want to find the horsepower of your locomotive connect the generator to a load box, put the engine in throttle 8 and calculate volts times amps divided by 700.
Randy
Assuming that you could keep adding load current, how do you know when you reach full load on the engine? When the manufacturer's full load rpm starts to drop???

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