How to keep a constant speed

[faraway]

How does the engineer keep a defined speed constant with a diesel engine if there are only 8 notches available?
If e.g. notch 4 with 5 car is good for 30 mph and he picks up one empty car is notch 4 not enough power anymore but notch 5 would be possible too much to proceed with 30 mph. Is he constantly switching up and down to maintain a constant speed? A longer train might be less sensitive but how about a very short train and a powerful engine?

[e-paw]

Hmmm. :? That's a good question. I would have to guess that the biggest problem in that scenario would be the start up from a dead stop. Once the train is moving I don't think one more empty would cause all that much more drag to effect train speed. The wheel sleep system, voltage regulator, and act of the loco making transition should help to even out the train speed under load. I think that the change in track elevation on hills probably gives the engineer more work to do as far as throttle goes, but I'm sure his left hand is kept busy because of several factors on any run. But remember, I'm an electrician not am engineer I really only have a rail fans experience on such things.. 8-)

[steinjr]

Al Krug's excellent page on running locomotives:
http://www.alkrug.vcn.com/rrfacts/hp_te.htm

Smile,
Stein

[Charlie B]

Reinhart, On all the locomotives I ran I never paid any attention to the "notch" I was in. You feel the throttle on the train the same as you would a car, if you need more speed, more throttle. Sometimes you are coasting, because when you close the throttle the traction motors disengage. The new locomotives are computer controled (never run one). Maybe someone with experience on one of them will post.

[faraway]

Reinhart, On all the locomotives I ran I never paid any attention to the "notch" I was in. You feel the throttle on the train the same as you would a car, if you need more speed, more throttle. ...

Ok, but you have only 8 steps while the car is infinitely variable. I have no vision how frequent you have to go up and down if the speed to go is between two steps. If the engine is at it's limit that might be a rather slow process but if you run with a GP60 and only two cars you will be too fast quickly.

[Charlie B]

The notches are only there to keep the throttle and the governor in sync with the electrical load. There are ports in the governor that control the fuel input needed to maintain the prime mover RPM and the main generator functions with the switch gear to make transition. You do the exact thing when using the throttle on a car only you don't have the notches. Just forget the notches are there in a locomotive. When driving an auto when you need more speed you have to increase the pressure on the accelerator, when a locomotive is in a certain notch it will keep the engine at a steady rpm, similar to cruise control. the governor is what controls the fuel, but when the load becomes too heavy to maintain a speed in that notch you have to open the throttle more, to let the governor know the prime mover needs more fuel. When you run out of notches, as on a grade, the wheels start to slip and/or the engine stalls.
There is also transition made on locomotives which, to explain, is like shifting gears in a car, but this is done with the electrical switch gear. The Old Alco started in Series, at 6 MPH it made the switch to series parallel, and at 28 would go to parallel shunt. This was done by changing the electrical connections to the traction motors..Maybe EPAW can explain it fully
( I could run the passenger train at ten miles an hour in notch 3, but if I forced transition to parallel I could drop the throttle back to notch one and maintain the speed)
Charlie

[Brakie]

I watch former steam locomotive engineers open the throttle to notch five while switching and once he gets moving he would close the throttle and let the engine "drift" and if more power was needed he would use notch two or three till he reach the speed he needed then he would close the throttle and "drift"..

I was always amazed at the way these guys could manipulate the throttle and brake.

[e-paw]

Well I'll see if I can explain it Charlie. Unlike our toy trains that use permanent magnets to produce a magnetic field in witch our DC motors turn in. Locomotives, as do most large industrial DC motors, use a electro magnet called a field winding in there traction motors. Transition is the act of switching the field winding and armature winding ( the part of the motor that turns) in different combinations of series & parallel wiring to get either more torque or allow for more speed.I was able to find this drawing that helps to show what I am talking about.

<!-- m --><a class="postlink" href="http://members.localnet.com/~docsteve/railroad/motors.gif">http://members.localnet.com/~docsteve/r ... motors.gif</a><!-- m -->
and this one too.
<!-- m --><a class="postlink" href="http://members.localnet.com/~docsteve/railroad/transitn.gif">http://members.localnet.com/~docsteve/r ... ansitn.gif</a><!-- m -->

Different manufacturers had different steps to transition in what gets wired how, but the all are after the same results. In straight series wiring you get more torque as you work your way up to full parallel you get more speed.

[faraway]

Thanks for spending the time to explain the internals. But I am afraid that is not the subject I am asking for. May be we get closer if I ask the other way around.

Scenario:
You run a train at level terrain (in any notch:-) with constant 28mph since at least 5 minutes. You want to accelerate to 30mph because that is the max permitted speed.
What do you do?
1. Go up one notch
2. ... ? what happens ? I assume the train will accelerate to a speed higher than 30mph? But you shall run 30mph!

ps. To do the same with your car you would press the accelerator pedal very very gently to get 2mph more speed. But you would not press the pedal 1/8 more down just for 2mph.

[Charlie B]

You click the power on and off, sometimes all the way off. You can also use a little train brake, not good for freight as it wastes fuel, and from what I hear is not allowed anymore, but on a passenger train it stretches the slack and gives a smooth ride.
You may not realize it but your foot on the throttle in a car is never steady, you are always increasing and decreasing the pressure.
Charlie

[faraway]

You click the power on and off, sometimes all the way off....

Thank you. That is what I expected.