Let me…ahem…play devil’s advocate for a moment. First, I am only as knowledgeable as the next guy about gas and diesel engines. I make no claims to training or experience that places me in a position to catch any of the responders in such a way as to make a cheap point. So, please just take the question at face value as an academic exercise.
My understanding is that a diesel is different from a gas engine by virtue mainly of its ability to generate a lot of power at low rpm’s, and this is because of various characteristics such as compression and efficiencies that accrue thereby. Diesels rely more on torque than do gas engines, application for application…or am I wrong? That is, a diesel is not meant to generate tons of hp, whereas a higher revving gas engine does just that.
Another question: as the load demand on the traction motors is either realized or imposed, will the diesel prime movers not have to rely more heavily on their torque characteristics (vice a gas engine’s hp from revs) to keep the alternator/generator at speed to supply the power?
My questions, as naive as they may appear to be, could be at the root of the fracas a few posts back. If there is a fundamental misapprehension by some or many of the readers (including me) of this thread, perhaps some of you could fill us in a bit?
It’s continuous and effectively maximum torque. The motor has a higher figure called “Breakdown Torque” but this can only be achieved instantaniously and is followed by the motor stalling and torque dropping to zero.
All SD70MACs were built with 85/16 gearing and 42" wheels. SD90MACs also used this motor and gearing, some of them had 44" wheels.
At least on CSXT, the nominal continuous speed of an SD70ACe is 8.8 MPH. It can be relied upon, under normal rail conditions, to produce 157,000 lbs of TE at or below that speed; and rail conditions permitting, it can produce up to 191,000 lbs of TE within that speed range.
Torque is a measure of work. Horsepower is a measure of how fast the work is done. A tractor and a motorcycle may both have 100 hp. The tractor runs slower, does more work. The motorcycle does less work, but does it faster.
The hp-torque curves of any engine should cross at 5252 rpm (assuming the engine can run that fast) . I’m sure there are formulae for figuring torque at a given hp and rpm level.
Work is “energy usefully expended”. Linearly, it’s force X distance. Rotationally, it’s torque X revolutions (both with the proper fudge factor to get the English units to work out)
If you apply a force to an object and it doesn’t move (or a twist and there is no rotation), then no work is being done.
Power is a measure of how fast you can do work. Force X distance / time. Since distance/time = speed, Power = force X speed. The rotational equivalent is torque X revolutions/time or torque X RPM (with appropriate English fudge factors)
The 85:16 ratio is for the SD70MAC. The SD70ACe is 83:16. I don’t know what either of their torque limitations is (although the SD70ACe does have a higher limitation than the SD70MAC has) – to me it’s whatever the software thinks that it is.
I am just gonna sum things up this way… Horsepower is a basic rating and from what I understand has just as much to do with speed. A 5000 hp locomotive can pull a 5000 ton train, so can a switcher, just a lot slower. Lets also look at it this way, a muscle car can have 600hp, and a small switch engine also has same rating. Do you think the car is gonna pull the same kind of weight as the switcher? Doubtful. It explains that there are other forces than horsepower at work. An intermodal train will be given the same power as a large drag freight which could be a lot heavier, because it needs to move fast. The Contemporary Diesel Spotter’s guide explains a lot of this also at the end of the book.
That’s because torque is the ONLY force a reciprocating engine (gas or diesel) produces. I’ll repeat that for the “experts” on this forum: The diesel engine on a locomotive is designed to produce “X” amount of torque to turn the traction alternator. “Horsepower” ratings are derived from the torque output at a given RPM; it is never the other way around.
Both types of engines rely on torque and ONLY torque to produce useful work. Torque is the only force they produce. The difference between a (typical) gas and diesel is their torque curves occur at different RPM’s.
Torque is twisting force. Horsepower is work. The characteristics of a diesel usually cause it to operate at considerably slower speeds than a gasoline engine. Hence it must produce more torque at a slower speed to achieve like power.
One horsepower equals 550 foot pounds per second. From that you should be able to calculate anything. [swg] (But, I’ll bet 4000 Budweiser Clydesdales could drag any loco anywhere ! If you could only find the right harness. )
Thanks for the illucidation. I still don’t know how the prime mover in a locomotive responds to an increased demand for work. If the operator notches up, take me through the “system” so that we understand what happens as a result of what. Does the diesel increase revolutions by nearly two times, or does it merely aspirate more, get more fuel, and how does that affect the torque output, which I now understand is the primary motive force once converted to tractive effort by the motors? (I had always understood that a diesel gets air metered, while fuel is constant. Probably outdated knowledge now that we have common rail and other improvements…although I don’t know if common rail has any application here…what does?)
Todays diesel engines are computer controlled as to both air and fuel (injectors) metering, but neither have ever been constant flow. Ideally you would want to have a nearly constant ratio of fuel to air, but rarely has this ever been accomplished throughout the range. When the operator asks for more power or speed, the diesel will produce more torque and/or a higher speed, which will be utilized by the generator due to it being also stepped up via the field currents at the same time. Torque will be effected by how much fuel is burned on every combustion stroke, and power will be determined by how fast the torque is produced (i.e. engine RPM). The traction control will monitor every wheel and control the electrical power going to each motor to limit slip.
A locomotive with battery field excitation (e.g. a GP9) works as follows.
The governor is the “brains”, whose job is to balance engine speed, fuel and generator excitation. The governor is basically a fly-ball governor that will adjust the fuel rack to maintain a set engine speed for each notch. If the load increases, the engine will start to slow down, the fly-balls move toward center, causing (through linkage) the fuel rack to shorten, giving more fuel to the engine. So, your GP9 in notch 8 will have the engine turing at 835 RPM regardless if the generator field switch is open or closed and whether the locomotive is moving at 10 mph or 70.
That’s half the governor’s job. The second half is that it will try to maintain a given rack (fuel) setting for each notch. That rack setting is set so that the engine will try to produce roughly, a constant, set HP for each notch. It does this by balancing the generator excitation against the fuel rack using the governor’s load regulator (a hydraulic vane motor powered rheostat). The load regulator is a big rheostat that’s in series with the battery and the generator field.<
And then, of course, we have steam locomotives where we can discuss horsepower in multiple ways. There is drawbar horsepower, cylinder horsepower, boiler horsepower, etc. Which is most useful depends on which question you are asking. The same is true for a diesel-electric or any other locomotive.
Ultimately, horsepower is a measure of the rate at which a machine can do work. It is computed by measuring a force times a distance per unit time. One horsepower is equal to 550 foot-pounds per second. It can be 550 pounds moving through a distance of 1 foot in one second, 1 pound moving through a distance of 550 feet in one second, or any equivalent combination. For a rotating machine it is measured by multiplying revolutions per minute times the torque the machine is producing. It is the same thing if you look at the units. It is always a force times a distance divided by a time.
Drawbar horsepower for any locomotive is the ultimate measure of the work it can do. And surprisingly, when the locomotive is pulling to start the train but the train is not yet moving, the work it is doing is zero. That’s right, zero. Remember, even a very large force, the starting tractive effort, multiplied by the distance of zero feet per second means the locomotive is doing no work. (But don’t tell that to the locomotive!)
Once the train begins to move, the locomotive is doing work. For a steam engine, the tractive effort is limited by two things. One is the adhesion of the drivers at the rails and the weight on the drivers. The other is the force generated by the steam pressure on the pistons averaged through a single driver rotation. A steam engine can produce its maximum tractive effort when it first starts a train and that tractive effort stays more or less constant when the engine is pulling slowly. During this time the horsepower may be quite low because the speed is low. But the horsepower the locomotive generates increases linearly as the train speed increases. (Not exactly true because once the train