Diesel Locomotive Tractive Effort Calculation

Does anyone know where I can find a formula for calculating the tractive effort of a diesel locomotive? I found this on wickipedia but I do not know if it is correct:

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The reference quoted is: Hay, William (1978). Railroad Engineering. Wiley, New York. p. 100.

Your advice please.

 

If v is the velocity in mph, P is the power of the engine in horsepower, E is the overall efficiciency, and t is the TE in pounds force, then that is correct as far as it goes, but seriously simplistic.

The overall efficiency for that formula depends both on the transmission (electric, hydraulic or mechanical) and on the amount of power needed to move the locomotive itself. Those would normally be assessed separately, as they depend on different factors.

Also the formula would imply infinite TE at zero speed, which is clearly nonsense. For a diesel-electric, the starting TE (which more or less corresponds to the TE as normally calculated for a steam loco) depends on how much current the motors can handle. There is likely to be a time limit on how long they can handle that and hence on how long the maximum TE can be sustained. And of course, as with any locomotive, the TE actually available may be limited by adhesion.

Others may have additional caveats to mention.

 

When inputting the available horsepower, it is necessary to deduct any power used for auxiliaries. Apart from electrical carriage heating, power is also drained by various compressors/pumps etc.
As an example during the period when BRCW Type 3 (Cromptons) (class 33) were used on Waterloo - Exeter services the load limit varied according to train-heat season.
Summer load was 9 Mk1, Winter was 8 Mk1. The trains would not keep up with the timings if loaded to 9 in the winter.
As @MellishR has said the maximum starting tractive effort depends on what the motors can handle as well as the adhesion factor, prolonged use at that rate can overheat causing damage. The class 33 as mentioned above had I believe a 'minimum continuous power speed' of 18mph. Above that speed full power could be maintained almost for ever, but if speed dropped below that figure then the driver would be watching the ammeter and easing back as necessary.