What is the correlation between the prime mover rated horsepower and the actual elctrical power to the traction motors at the railhead.
In an automobile it is direct from the engine/transmission to the final drive.
For example 250hp engine rating is approximately 250hp at the rear drive wheels
Many folks figure 10% loss due to shaft driven items like the air compressor/etc. A 3000 hp SD40-2 has a V16 engine that puts out about 3300 hp - 3000 hp available for moving trains.
Jim
Like he said, the SD40-2 is supposed to produce around 3300 total hp, 250-300 of which is absorbed by the traction motor blower and radiator fans etc. So 3000 hp into the main “generator”, which is 90+ percent efficient, so maybe 2800? hp into the traction motors, which are 90+ percent efficient, so maybe 2600? hp at the traction motor pinions and a bit less at the wheel rims. Overall efficiency is slightly less at, say, 10 mph than it is at 40 mph.
When the carmaker says the engine is 250 hp, it doesn’t mean at the tire tread, does it?
As far as I know, as generally was pointed out, a prime mover is rated slightly higher than the given HP for a locomotive, though they are generally considered to be 2000, 3000 etc. The actual horsepower is whats delivered to the generator, and then it seems to vary what actually gets to the rails. There have been several discussions here pertaining to that. Suffice it to say that a 3000hp SD40 isn’t producing all that to move the train. I guess that is where tractive effort comes in, though I must admit I get confused after that.
But is that true for newer locomotives? I recall reading that the SD90MAC-Hs prime mover put out (alternator output) approx. 6300 HP…maybe the newer designs don’t require the same percentage of net power to run the auxiliaries?
No, it does not.
In the U.S., automotive engine horsepower is measured by SAE Net standards. Power is measured at the crankshaft of the engine and includes all losses from belts, oil pump, fuel pump, cooling system, alternator, exhaust system, and pollution control. It does not include losses from the transmission. A typical 4 or 5 speed automatic transmission is around 75% efficient. So a car rated at 250 SAE Net HP would be producing around 185-190 HP at the wheels.
On a locomotive, traction HP is the power at the crankshaft available to the main alternator after all engine losses. In that way, it is similar to SAE Net HP as used on automobiles. Currently, DC motored locomotives are around 88% efficient in transmitting the traction HP rating to the wheels, AC motored locomotive are around 93% efficient. A 4400 traction HP DC motored locomotive produces around 3,900 HP at the wheels, a 4400 traction HP AC locomotive produces around 4,100 HP at the wheels.
On a SD40, about 400 HP is lost through the alternator and traction motors, so around 2600 HP is produced at the wheels. The only force any locomotive produces is tractive effort–it is the pulling force of the locomotive. The HP being produced by the locomotive is tractive effort X the speed of the locomotive.
…and some of that TE is used to move the locomotive, so the drawbar HP will be somewhat less than the HP at the wheel/rail interface.
Another difference is in the duty cycle. A railroad-size diesel engine reaches its rated horsepower at about 1100 RPM, which is roughly the top speed of the engine, and the engine spends a fair amount of its running time at that speed. Automotive-size engines tend to attain their advertised horsepower at speeds of about 4000-4500 RPM, a speed which is a small part of its duty cycle.
I’m standing very far away if you wind an EMD that high. Top RPM is mostly 900, 950 in the 50 series (until they broke).
In one if the Vernon Smith articles, he stated that a nominal 2000 hp GP38 delivered about 1630 hp at the rail.
I was thinking of four-cycle diesels like the 251 and FDL, which run a bit faster than the EMD 2-cycle engines. The Maybachs that were used in the KM diesel-hydraulics topped at 1585 RPM, but they did have a smaller bore and stroke.
So does that mean tractive effort = speed/hp? SAE is a bit leanient in their figures, a reason why everyone goes by them. You really think the new Corvette ZR1 ONLY makes 630hp? Try again. A car mag did an article long back that covered a build up of a ZR1 engine, it pumped out 639hp after break in. Give it a few thousand miles and a couple oil changes it will gain about 20 more. A bit off track but back in 1967 Chevrolet debuted a new model called the Camaro, of which a Z28 package could be made. Chevy rated the little 302 at 290hp for the goverment, in reality it make around twice that. The LS6 big block 454 used in Chevelles/Corvettes SS models was rated at 450, it made at least 100 more. The all aluminum 427 in the Corvette? Rated at 435, actual was closer to 600. Yes I’m a car guy, and my furute little girl will be named Chevelle.
TE=HP/speed
Did the car mag correct back to standard conditions for air temperature and pressure specified by SAE? The current crop of locomotives are all net traction HP regulated so they will make the same HP on a hot day in the Rockies as they do on a cool day in Seattle. EMD locomotives Dash 2 and earlier were fuel rack regulated so that they would make more HP with cool fuel than they would with hot fuel. (fuel expands with heat so less BTU’s being delivered for the same volume of fuel being injected)
Sooooo…all I would need to know, is… the particular trains rpm per throttle notxh, the hp per those particular rpm’s, aaand the particular attainable speed per throttle notch. It’s too bad my mind works in such mysterious ways, then I might actually be able to drop this lol.