Hp numbers are obvious...but torque? - Trains Magazine

In locomotive applications, the diesel engine power output is usually rated at 900 - 1000 rpm, and is the maximum continuous rating. For automotive and marine applications the same engine would usually have a higher rating, on the assumption that these applications don't run the engine at maximum output for long periods of time.

The important figures for a locomotive are the 'tractive effort' figures -

for an SD70ACe :

157,000 lbs continuous tractive effort
191,000 lbs starting tractive effort

for an SD70M-2 :

113,100 lbs. continuous tractive effort
163,000 lbs. starting tractive effort

(from )

The tractive effort is basically the pulling force the loco can exert at the drawbar. The continuous tractive effort varies with speed, so the continuous figures above will be at specific speeds. Horsepower mainly becomes important at higher speeds.

Note that both the locos above use the same diesel engine - the difference is in the transmissions (AC for the SD70ACe versus DC for the SD70M-2). The electric transmission system performs the torque/speed multiplication function that the gearbox in a car provides. The gears between the motor and axle are fixed ratio, and match the optimum speed range of the motor to that of the axle (so a low-speed freight loco will have lower gearing - higher ratio - than a high-speed passenger loco).

Very simply, in a DC-drive loco, the diesel engine drives an alternator, the current from this is converted to DC, and this current then flows through the coils of the motors. The current creates rotating, opposing magnetic fields between the rotating (rotor/armature) and stationary (stator) parts of the motor which produce the force to turn the wheels. The principle is the same with AC-drive, but complex electronics generate a variable strength and rotational speed magnetic field in the stator coils of the motor which drag around an iron rotor with copper bars embedded in it.

It's quite possible to run the diesel engine at a fixed speed e.g. 900 rpm, and just vary the power output - this is common practice in passenger locos with head-end power (HEP) e.g. Amtrak F40's etc - it makes it easy to provide a constant voltage/frequency HEP supply.

Tony