Here’s a basic question I have pondered for a long time and never seen a good explanation for: How is a diesel-electric’s horsepower rating determined? Is it a measure of the output of the diesel itself or a measure of the output of the traction motors or some combination thereof? It would seem to me you would want a measure of the output of the traction motors since they are what actually moves the locomotive and the train. Or is this what “Tractive effort” measures?
Thanks for your help!
Dad Howe
In North American practice diesel electric locomotive horsepower ratings reflect the maximum output of the traction motors. Thus an SD90MAC-H really does produce 6,000 HP at the rail.
Keep in mind,though,that General Electric has been known to round the HP number up in the model numbers of it’s locomotives. For instance,the U23B is actually a 2250 HP unit. GE also built some upgraded B36-7 units which were rated at 3,750 HP.
It is my impression that the rating is that of the diesel engine not the traction motors. A diesel engine will not deliver its full output to the traction motors because part of it has to be used to run auxiliaries such as traction motor blowers, exhaust fans, and the like.
Rudy,If you look up the engine specs on North American Diesel locomotives,you will find that the engine output is always higher than the specified horsepower of the unit. In most of the rest of the world,a locomotives output is calculated in gross engine power as you stated,but that is not the way it’s done in the US.If you check out the TRAINS magazine article about the SD90MAC in the EMD anniversary issue,this is explained in detail.
Thanks for the info! Most helpful. I missed that Trains issue- I’ll have to look for it. Helps me understand the various ratings and comparisons of various locomotives a little better. -Dad Howe
Diesel-Electric locomotives in North America are rated by the horsepower available into the TRACTION ALTERNATOR, not the traction motors. The diesel engine itself does indeed put out more horsepower-the extra is used to run things such as auxillary generators, cooling fans, air compressor etc. There is some power lost from the traction alternator to the rail. I have fiqures for the GE AC6000, but the EMD SD90 would be similar. The diesel engine in the AC6000 puts out around 6300 net HP. Out of that, at least 6000 HP is available for traction. The conversion from mechanical energy to AC electrical energy (main alternator)to DC (rectified AC to create a stable waveform) back to AC (inverters) and finally to the AC traction motors is around 93% efficient. So out of the 6000 Hp into the alternator, about 5600 HP come out at the rail. This complicated process is actually more efficient that the AC alternator/DC traction motor locomotives like the SD40-2, which are about 88% efficient. So for the SD40-2, which has 3000 HP for traction, about 2650 Hp is reaching the rail. Older locomotives like the EMD F7 where about 82% efficient (1500 traction HP, 1200 rail HP). For comparison, steam locomotives often loose up to 35% of their power from the boiler to the rail, so rail HP was only around 65% of boiler HP. Diesel-Electrics are able to produce their maximum power over a very large speed range; steam locomotives can produce their maximum power at only one specific speed.
THERE IS A SEPARATE GENERATOR ON TOP OF THE MAIN GENERATOR FOR SOME OF THOSE PLUS THE FRONT PART OF THE MAIN GENERATOR IS AN ALTERNATOR.
A FORMER ENGINE MAN