Multiple engine question on a real train

[jjanmarine3]

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.

Traction power has to be compatible with each other for proper train control ( read the manuals and keep to company rules & procedures ) , otherwise one might end up with valve bounce and overrevving
Each locomotive in a MU consist actually operates independantly , with it's own mechanical protection devices and electrical protection circuits , alarms buzzers lights and bells. There are numerous measuring devices that monitor relevant values permanently under all load and track conditions. For example, when a wheels slips or skids, a 'policeman' will detect imbalance in amps between the motors and cut excitation to all the motors and apply automatic sanding untill balance is restored ( unless synchronised wheelslipping occurs which is why the rails end up blue with hollows under each wheel ) .Then there are things like compressor control ( maintain main air pressures ) , transition or field shunting ( speed control ) etc.
In general : The diesel engine may shut down or return to idle for safety and protection purposes for example if there is excessive pressure or heat in the crankcase or the engine revolutions went out of limits etc.
Off topic : Imagine should one apply all these cool features to modern steam engines how efficient they would be ? : Back to topic :hehe: