Being an automotive technician, i see a lot of numbers of horsepower and torque in ft.lbs.
Most people go by horsepower numbers alone, not having a clue where or when that power is reached etc (think dyno graphs and hp and torque curves).
Anyway, I never really knew how a civic dx with 100 ft lbs of torque was able to drive around until i understood torque multiplication with transmissions and differentials BUT i still dont have a clue how it works on a locomotive.
An SD70 or C40-8 have claimed HP numbers of 4000 hp but when in the RPM range does that arrive, and what are the torque measurements? I know theres other forms of measuring them, but can someone explain it to me? They’ve got to be huge, especially when broken down into foot pounds.
Also, is there even a form of torque multiplication since there are no transmissions, but I am aware there are traction motors, with gears inside also, so can someone also explain this to me?
I’ve always always wondered about this but always tend to forget to ask when i’m lurking these forums.
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 -
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 sp
Are you talking the diesel engine or the whole locomotive?
The locomotive is powered by diesel engine that operates at 8 fixed power points, each with a certain, fixed engine speed and load (which determine the torque). (HP = torque x rotational speed)
The diesel engine is connected to the traction motors through an “continuously variable electrical transmission” where the output is constant HP, but the volts and amps can vary (HP= V x A/746) according to how fast the locomotive going (high amps, low volts is like 1st gear in your car. high voltage, low amperage, is like 5th gear).
The traction motors, gearing and wheels turn this into HP at the coupler, where HP = TE x speed/308 (the 308 includes transmission ineffiency)
Can someone explain a couple of terms in more detail: “Power Points” and “Continuously variable electrical transmission” and"just vary the power output" Thanks Wayne
Wayne – Don was referring to the 8 throttle settings which most (not all – some GEs have 16) which most diesels have (GG-1 electric motors had 48, and no one ever used them all…) when he mentioned ‘power points’. Unlike a car’s accelerator, which you can vary continuously, with a railroad engine you set the throttle in one of the notches (power points) and let her go. If she gets going a bit fast, notch back a bit. But keep in mind that a train is so heavy that it changes speed very very slowly – even at full power (‘notch 8’). Both he and I referred to the overall drive as a ‘continuously variable electrical transmission’ – by which we meant that the rpm of the drive wheels is not related to the rpm of the engine in any way at all – in fact, the only connection between them is a bunch of wires. It acts a little like a continuously variable mechanical transmission, but without any mechanical bits involved. ‘Just vary the power output’ is a little hard to visualize, perhaps, but again, think in terms of your car: suppose that you are driving along at 60 in overdrive, or you have a manual transmission. Now, if you start up a hill, you have to press harder on the gas pedal to maintain the constant speed, even though the engine rpm doesn’t change. Now in a diesel engine, the power demanded from the engine is determined by the throttle setting, but the engine governor (in some cases) determines the engine speed. The throttle will cause more power to be drawn from the generator for the traction motors.
The current EMD 710 produces 4300HP and 25,100 lbs. ft. of torque at 900 rpm.
In both EMD and GE locomotive engines, peak HP and peak torque are reached at full RPM.
Automotive engines are usually rated in SAE NET HP, which is the MAXIMUM HP available using the vehicles stock exhaust system, full oil pan, and routine accessories running.
Locomotive engines are rated in NOMINAL HP, which is the MINIMUM HP available (at full throttle) for traction with ALL accessories running. So in other words, EMD says the 710 will have a minimum 4300 HP available for locomotive traction under all standard operating conditions. That is a big difference than SAE net HP.
Electrical transmission is continiously variable because of the interaction of voltage and amperage on the motors. Basically, in a DC motor the amps controls the torque (higher amp draw=more torque), voltage controls the rotational speed. By continiously varying each, you end up with a infinite amount of “gear” ratios. AC induction motors vary the voltage, amps, and frequency.
Edit: calpoly48, if you want a direct automotive comparison, the current version of the EMD 710 would be rated in the neighborhood of 4700 HP and 27,500 lbs. ft. torque @ 900 RPM using current SAE guidlines.
“The traction motors, gearing and wheels turn this into HP at the coupler, where HP = TE x speed/308 (the 308 includes transmission ineffiency)”
The 308 constant is for DC generator/ DC traction motor locomotives, i.e “first generation” diesel-electrics.
Since then locomotive electrical systems have been optimized.
Rule of thumb:
Use 330 for AC alternator/ DC traction motor “second generation” locomotives
Use 341 for the latest AC/DC locomotives
Use 356 for the latest AC/AC locomotives
As you can see, it takes less HP output for a modern locomotive to produce a given amount of pull than an older locomotive. This is one of the main reasons railroad are quickly replacing older “second generation” locomotives (i.e. SD40-2, GP40-2 etc) with the high tech, computer controlled uber locomotives most railfans seem to hate. Not only are they more powerful in terms of engine HP, but the are also vastly more efficient in getting that power to the rail.