On a real train, if you have several diesel engines pulling the train, how is the power on each one of the engines regulated. Are they all set to the same throttle setting by some control signal that goes from one engine to the other, and if so, and one engine was stronger than say the other two, wouldn't that cause a problem? Just curious.
Multiple engine question on a real train
- Thread starter Robert2d6
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Back in 1897, a guy named Frank Sprague invented "MU", Multiple Unit control, for one of the Chicago elevated railroad companies. When the "diesel" locomotive came out it was natural to use that - because surprise surprise, a diesel locomotive is not diesel powered, it's electric just like the elevated, interurban, and subway trains. The difference is that it carries its own electrical power generating station with it, that's the only thing the diesel engine is for. Connected to a generator, the diesel engine has no direct connection to the wheels, instead it just generates electricity which is used by the electric traction motors in the bogeys. If one loco has a 1200 horsepower diesel and the other has a 2000 horsepower diesel it doesn't matter, because the electric current is the same and the electric motors are what are actually being multiply controlled. The diesel throttle is controlled by a governor that responds to the power demand from the electric motors.
(confused) what?
Couldn't be more clear, except for a few oddballs like the "diesel hydraulic" locos, what we call "diesel" locomotives are more correctly diesel-electric - the diesel doesn't actually power the wheels, it just drives a generator to make electricity, the actual movement is from electric motors.
jjanmarine3
Active member
To cut a long story short - Locomotives are all preset to certain reference values within specified limits. For example : Many diesel locomotives have electronically controlled modules which are plug in , these are adjustable and one can measure these values at certain points on these modules while the locomotives are 'live' and do any adjustments needed. This is part of the maintenance scedule, and there is also a 'load box ', which is a simulator that places a load on the locomotive ( external resistors ) and then tests are performed to check these values are within reference limits in the manuals.
Overpower, underpower and similar faults are also reported by drivers on their trip reports and the locos are then pulled in for non-sheduled maintenance and checked.
The drivers know the locos like we know our cars, and they quickly determine if there are power and other relevant differences between locos.
It can cause problems like jerking and smothering, which all causes lack of proper train control.
There are many mecahnical and electrical faults that can occur like turbo's that are delivering too much or too little air pressure and modules that deliver too high or too low reference voltages for power control .
Certain of these electrical reference signals are relayed in MU working through the MU jumper cables to each locomotive which then responds as desired providing the settings are all within limits. Interestingly too these trainlined signals are 'crossed' at each cab or tail end to respond correctly in reverse or forward gears.There is also a system where the driver can shutdown selected locomotives in the consist remotely for fuel saving etc.The leading locomotive can be shutdown too ( nice , quiet ride ) and a trailing locomotive left live.
Thanks for the very in depth explanations. I taught electronics for many years, so this all makes sense now.
A related question.
On the Canadian Pacific as depicted on Cayden's route this shows 6 diesels 2 at the front 2 in the mddle and 2 at the back. Are all 6 locos linked or are they manned and they use radios to communicate. I'm sure I saw a program on the Horseshoe curve where the lead loco driver was requesting more power from the back as at the front they were going downhill but the back was still going uphill.
Ken
On the Canadian Pacific as depicted on Cayden's route this shows 6 diesels 2 at the front 2 in the mddle and 2 at the back. Are all 6 locos linked or are they manned and they use radios to communicate. I'm sure I saw a program on the Horseshoe curve where the lead loco driver was requesting more power from the back as at the front they were going downhill but the back was still going uphill.
Ken
It depends. These days they have a gadget called Distributed Power, so you can remotely control a loco like it was an RC car.
http://en.wikipedia.org/wiki/Distributed_power
Operationally that's used only for long heavy trains since it takes time and labor to hook all that up. For helper service (what the Brits call a "Banker") to get a train over a short grade, the usual practice is for a different engineer to couple on behind and push, then uncouple and run back to base when he's no longer needed. Usually these are MUs, 3 locos at the head controlled by one engineer, the helper is usually a 2 or 3 loco MU controlled by its own engineer, with radio communication between the two engineers. Not as complicated as it used to be, before radios and diesels they communicated with whistle signals.
But the one you saw with 2 in front, 2 in the middle, and 2 at the back was probably distributed power controlled by a single engineer in the lead loco.
http://en.wikipedia.org/wiki/Distributed_power
Operationally that's used only for long heavy trains since it takes time and labor to hook all that up. For helper service (what the Brits call a "Banker") to get a train over a short grade, the usual practice is for a different engineer to couple on behind and push, then uncouple and run back to base when he's no longer needed. Usually these are MUs, 3 locos at the head controlled by one engineer, the helper is usually a 2 or 3 loco MU controlled by its own engineer, with radio communication between the two engineers. Not as complicated as it used to be, before radios and diesels they communicated with whistle signals.
But the one you saw with 2 in front, 2 in the middle, and 2 at the back was probably distributed power controlled by a single engineer in the lead loco.
I try to think of it as tug of war, the 300lb. guy in front of you is probably pulling more than you are, but when you start pulling you just add that extra energy. I still get confused and my head hurts a little when I think about it too much, the physics and such. I always wondered the same thing when you would see a ES44, Dash 9 and a GP38-2 all pulling at the same time but you know they each put out different tractive effort.
Doesn't have anything to do with the topic, this is yet another kid whose parents let him go on the internet unsupervised.
Back on topic;
"I try to think of it as tug of war, the 300lb. guy in front of you is probably pulling more than you are, but when you start pulling you just add that extra energy. I still get confused and my head hurts a little when I think about it too much, the physics and such. I always wondered the same thing when you would see a ES44, Dash 9 and a GP38-2 all pulling at the same time but you know they each put out different tractive effort."
Not really. Think of it this way, keeping in mind there's no direct connection between the diesel and the wheels - when you move the throttle lever, it's connected to an electronic control circuit. That circuit sends different amounts of voltage to the traction motors depending on which notch you have the lever in. In a 4 axle loco you have four different motors, and in (for example) notch 4 the control circuit is sending 300 volts simultaneously to all four motors. So essentially when you're running a single loco, you're already doing a form of MU control - you're controlling four electric motors with one lever. Want to control more motors, hook more motors into the control circuit, which will feed the same voltage to all the motors.
The only difference with multiple units is you're driving a GP38 with a Dash 9 and SD40 MU, your control circuit is sending the exact same voltage to 16 electric motors (4 in the GP38, 6 each in the Dash 9 and SD40) making all 16 motors run at the same speed. Not all 16 motors are drawing power from the same source, the 4 in your geep are drawing 300 volts from the diesel-generator in the geep, the six motors in the Dash 9 are drawing from that loco's generator, the six in the SD-40 are drawing from the SD40 generator. But the voltage is regulated by the control circuit so if one motor is getting 450 volts, all 16 are getting 450 volts. The reason the bigger locos are bigger in the first place is that there will be more electrical power available to the motors, but when they're MUed together the control circuit is not gonna let the motors in the Dash 9 run faster than the ones in the GP38.
There will be some minor inequalities like one wheelset slipping when it hits an oily spot, but the reason electric motors are used in the first place is because they tolerate that sort of thing easily. Nothing compared to the problems you would have trying to get 3 different direct drive diesel engines working in unison.
Back on topic;
"I try to think of it as tug of war, the 300lb. guy in front of you is probably pulling more than you are, but when you start pulling you just add that extra energy. I still get confused and my head hurts a little when I think about it too much, the physics and such. I always wondered the same thing when you would see a ES44, Dash 9 and a GP38-2 all pulling at the same time but you know they each put out different tractive effort."
Not really. Think of it this way, keeping in mind there's no direct connection between the diesel and the wheels - when you move the throttle lever, it's connected to an electronic control circuit. That circuit sends different amounts of voltage to the traction motors depending on which notch you have the lever in. In a 4 axle loco you have four different motors, and in (for example) notch 4 the control circuit is sending 300 volts simultaneously to all four motors. So essentially when you're running a single loco, you're already doing a form of MU control - you're controlling four electric motors with one lever. Want to control more motors, hook more motors into the control circuit, which will feed the same voltage to all the motors.
The only difference with multiple units is you're driving a GP38 with a Dash 9 and SD40 MU, your control circuit is sending the exact same voltage to 16 electric motors (4 in the GP38, 6 each in the Dash 9 and SD40) making all 16 motors run at the same speed. Not all 16 motors are drawing power from the same source, the 4 in your geep are drawing 300 volts from the diesel-generator in the geep, the six motors in the Dash 9 are drawing from that loco's generator, the six in the SD-40 are drawing from the SD40 generator. But the voltage is regulated by the control circuit so if one motor is getting 450 volts, all 16 are getting 450 volts. The reason the bigger locos are bigger in the first place is that there will be more electrical power available to the motors, but when they're MUed together the control circuit is not gonna let the motors in the Dash 9 run faster than the ones in the GP38.
There will be some minor inequalities like one wheelset slipping when it hits an oily spot, but the reason electric motors are used in the first place is because they tolerate that sort of thing easily. Nothing compared to the problems you would have trying to get 3 different direct drive diesel engines working in unison.
im gonna pretend i didnt see that, not offended just that i get alot of that