When you have two or more locomotives on a train, do all locomotives work at equal capacity, or do one or more work at maximum capacity while the others provide whatever else is required to move the train? The reason I ask, I’ve read that a diesel engine is most efficient at maximum output…thus it would make sense to have as many locomotives in the consist working at maxmium output for as long as possible …as opposed to having all locomotives working at half capacity (for instance)…
Ulrich
With standard MU cables each unit will operate in the same throttle notch as selected by the engineer. If lead and trail have Dynamic Brakes, the same is true with the dynamica.
With RCU the engineer can have head and rear NOT in same position.
Mac
As I recall from previous discussions along this line, each locomotive will “help out” to the limit of it’s ability. Obviously a 3000 HP locomotive isn’t going to be the same as a 4400 HP loco, but both will be pulling/pushing.
Thus a 1500 HP yard switcher could work with a 4400 HP road switcher (assuming they can be MU’d). It might not be pretty, but it will work.
Ideally, I’d imagine management would prefer to have “matched” locomotives where possible, but it’s not critical.
Of course, you have to deal with stuff like speed restrictions on locomotives, too. That 4400 HP loco might be geared for (and capable of) 80 MPH. Not so much the yard switcher…
There have been devices offered by various manufacturers to optimize power in consists in just the manner you describe (one unit operating at maximum throttle positions, others reduced as necessary), but they don’t seem to have caught on, for whatever reason.
I know that UP would instruct engineers to shut down unnecessary units, back in the days when horsepower-per-ton was the standard for power assignments. They now go by tons per powered axle, and I don’t know if this has modified any shutdown procedures.
Ulrich,
I think you may have read that a diesel engine (as opposed to a locomotive) is most efficient running at high rpm.
That is for the most part true, they do perform best at high revs, but a locomotive is a diesel electric machine, using the prime mover or diesel to spin an alternator, create current and power the traction motors.
Because the locomotive is in essence an electric machine that happens to bring its power supply along for the ride, it is very, very efficient.
But the higher your rev the diesel the more fuel it consumes, worse under load, but still quite efficient all things considered.
The connection between the alternator and diesel is a direct link, the faster the diesel revs, the faster the alternator spins and the more amps it creates.
An example you may be familiar with is the charging system in your automobile, which uses a alternator to power and run a direct current 12 volt system.
The alternator has a rectifier circuit that in essence converts the alternating current from the alternator to direct current, (A/C to D/C) and charges your starter battery, while also supplying the electric needs of the auto.
These alternators create 15 volts A/C at varying amperes, up to 200 amps depending on the car make and model.
The faster you rev your car engine, the more amps the alternator creates, at 13.5 to 15 volts.
Because your car also has a voltage regulator, the excess or un- needed voltage is sent to ground, your 12 volt battery stays charged, and the ignition system, stereo and such don’t fry from the excess amps.
If you turn everything on in your car, the regulator will allow all the amps the alternator is producing to be used…if all you have on is the engine, the regulator will simply allow the voltage at low end amps to flow to the battery and charge it.
In a diesel electric locomotive, somewhat the same system functions.
If you notice a diesel locomotive pulling a heavy train, you
(because they are all very nearly frauds!)
A locomotive’s incremental power generating efficiency is very nearly the same from any one notch to the next. Even if you exploit the very small differences that do exist, it doesn’t add up to a meaningful hill of beans.
Reducing HP/ton and shutting down engines when not being used is where it’s at!
So maybe someday we’ll see high-horsepower gensets? Like an articulated Centennial with three or four twelve-cylinder motors?
As for the initial question, each locomotive will provide the power it creates at that notch setting at that speed (mph). If you mix a 4000 and 2000 hp loco and set them at notch 5 you would get about 2000 hp from the big and 1000 hp from the smaller locomotive. (I am guessing at the notch setting since notch and HP are not proportional).
To save fuel, you would need to shutdown the 2000 hp loco and run the 4000 hp loco at a higher notch to get the needed 3000 hp for the speed you are trying to maintain.
It would be interesting to throw maintenance wear and tear on this equation. That is, maybe you save fuel by running a single locomotive at a high notch but does that appreciably shorten the life of the diesel and the electrical components?
Great input and explanations everyone…thanks for taking the time…
Shutting down locomotives in a consist that are not needed for a particular division doesn’t do much as far as fuel efficiency but does save wear and tear on the pistons and the blowers and other mechanical parts of the diesel engine itself as well as alternator bearings and slip rings. That is the reason, and not fuel savings, that some railroads still use this practice. There isn’t any difference in wear on the traction motors, which continue to rotate with the wheels even though unpowered.
See Al Krug’s “Amperage to Tractive Effort table for an SD40-2” for a partial technical explanation, at:
http://www.alkrug.vcn.com/rrfacts/amps_te.htm
See also his “Locomotive Fuel Use” chart - in Gals./ Hr. - for several common deisel locomotive models at:
http://www.alkrug.vcn.com/rrfacts/fueluse.htm
Also, the more detailed chart for “SD40 Fuel Use & Horsepower by Throttle Position” at:
http://www.alkrug.vcn.com/rrfacts/fuelSD40.htm
- Paul North.
Throttle notch setting is directly proportional to Kilowatt output / horsepower. One 4000 hp and one 2000 hp locomotive equals 6000 hp in notch 8 without any regulation for tractive effort.
Randy