I've read that some locomotives don't work well together in consists due differing loading times.. i.e. SD40s were not well matched with newer GEs for example. What factors determine loading time?
My MIT SB-EE thesis discussed this issue in 1953. A short answer: When moving the "throttle" (controller) from idle to a run condition, assuming this NOT one of the constant-rotating-speed prime-mover commuter locomotives which are special cases (head-end-hotel power from prime mover. snappy acceleration, and often lots of noise), the incrased power from increased fuel and air injection both starts to move the train and increases the rotating speed of the prime mover. First, how should the instantaneous powere be divided between the two functions? Historically, switchers favored instantaneous power to the rall, passenger units favored getting the prime-mover up to speed and then ramping-up power to the rail. Beginning with the GP-9 (after Boston & Maine GP-7s 1567 and 1568), and I beleve continued, almost all EMD production divided the power at the instant 50% - 60%, power to the traction moters and power for accelerating the prime moiver, because a careful mathmatical analysis says this gives the fastest overall acceleration and minimizes a jerk-sensation at the same time. I don't know if this analysis was applied to any other manfacturer's locomotives. They may use different ratios.
Second: The heavier the rotating components and pistons and valves (for prime-movers of a particular power rating), the more energy it takes to accelerate the rotation. Thus, in general, a four-cycle engine will load more slowly than a two-cycle engine.
I was waiting for Mr. Klepper to answer because he has 'distinctive competence' in this specific area.
Remember that Diesel engines don't have throttles; their governors only set and maintain rotational speed and adjust fuel flow through the injectors to do this (see the operation of the Woodward governor, and in particular EMD's way of implementing MU engine speeds on that governor).
No Diesel engine 'likes' accelerating into a load, and when excess fuel is required you get Alco-style smoke shows. Many GEs were neurotic about this, and are set for very slow engine acceleration... this limits how much electrical power the generator/alternator can develop (and as Mr. Klepper noted, send to the traction motors).
Ideally the practice would be to get the engine up to programmed speed 'in selected notch' and then start energizing the generator/alternator field. (The governor will then act very quickly to maintain that speed) But here, too, the excitation may not be immediate, both in order to preserve electrical components and not 'lug' the engine unexpectedly.
Meanwhile, there are more modern consequences of engine and electrical control that can make a locomotive either drop power or accelerate electrically in a manner that is not smooth (see for example the explanation of 'earthquake mode') and this can give just the effect noted in the video, where the slack between engines visibly runs in and out (but you may not be able to recognize if the second engine is 'bumping' the first or the first is lagging back against the second...
UlrichI've read that some locomotives don't work well together in consists due differing loading times.
For you to say that the units "don't work well together" is really a falsehood. Yes, some load quicker than others. However, in a mixed consist, the difference in loading is not anything drastic as your statement may suggest. For example, if you are coasting along with a GE lead and an EMD right behind, when you open the throttle you may feel a slight bump. That is all. No big deal. Other than that, the units work perfectly well together pulling their load.
The big difference, and it is a really big difference, is in the performance of like consists over a line that has many speed changes because of curvatures and many gradient changes due to hills and dips. Here you find that the slow loading of the GE hinders accelleration out of curves and the ability to stretch the train out when coming out of dips. Here, EMD's and especially the SD40-2's were the weapon of choice!
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I've had some 4 axle GP38-2s that were a pain when they were together. Usually one was really slow at loading, and the other one quicker. So the one would pull to the point of wheelslip before the other one would decide to get moving.
Really noticable when doing yard/industry work.
It's been fun. But it isn't much fun anymore. Signing off for now.
The opinions expressed here represent my own and not those of my employer, any other railroad, company, or person.t fun any
IF two identicle units behave very differently, then one or both is/are out of adjstment. The shop should be notified.
daveklepper IF two identicle units behave very differently, then one or both is/are out of adjstment. The shop should be notified.
I mean, we do, but this is the world of PSR. Sometimes you have to dance with the one that brought you.
Some EMD units had a switch that would select a faster response for yard work.
I preferred GP40-2's over the SD40-2's. Too many times when I advanced the throttle on an SD it decided it was time to make transition and it would drop load to do so. The GP40-2's did not have to make transition.
Never thought about the power needed to accelerate the engine itself. So you'd think a passenger engine that supplies HEP from the prime mover would load faster, and get the train from 0 to 15 mph quicker. But I've never noticed a difference between Caltrain's F40's, a few of which still have their constant-speed 645s.
I never got a chance to clock a NJ Transit PL42 (or whatever those 710-powered units are called) but I had the impression their loading was quicker than anything else around now. Wonder if the Chargers can do well too, when they're allowed to. Do engineers not trust them to accelerate smoothly but quickly?
The ERIE railroad used to put PAs in front of E8s because the PAs were loading faster and bumping the E8s. The DDX40s were always bumping other locomotives. Gary
timzSo you'd think a passenger engine that supplies HEP from the prime mover would load faster, and get the train from 0 to 15 mph quicker.
I covered this earlier. Some diesels ran at constant RPM to supply head-end power without all the electronics needed to derive it from a diesel'generator with varying RPM. So full power was available much quicker. But fuel economy? Forget it! Noise.
The verry old locomotive term throttle comes from the fact that it actually thottles the steam flowing from the boiler to the cylinders. Obviously, electric and diesel-electric and diesel-mechanical locomotives have no such device. But some operating people still refer to the controller, or at least did during my rfegular riding the ebgine on the B&M in 1952-1953.
The power handle has "throttle" written right on it, and both the manufacturers and our operating manual refer to it with this name. So of course we are going to continue to call it the "throttle".
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zugmann I've had some 4 axle GP38-2s that were a pain when they were together. Usually one was really slow at loading, and the other one quicker. So the one would pull to the point of wheelslip before the other one would decide to get moving. Really noticable when doing yard/industry work.
This happens in pacesetter too, sometimes due to a defective unit but it's really noticeable when you have an older GE (Dash-8, Dash-9) paired with a faster loading unit like a GEVO or SD70. The quick loading unit will end up starting the train by itself and then when the slower one catches up you'll get a big surge, then they'll fight each other for a while before things settle out.
And that's assuming the different types of units actually work together in the first place. In a smart world uncompatible units wouldn't get put on the same train, but again, gotta run what they give you.
As with the age-old fun of what to call the diesel engine installed in a locomotive engine, we need to be careful about the sense of 'throttle'.
Perhaps obviously, on a steam locomotive the valve that regulates steam flow is a throttling valve. But when I say 'diesel engines don't have throttles' I'm referring to something technical about the prime movers themselves.
On many older non-GDI gasoline (or spark-ignition) internal combustion engines, speed and power control is accomplished by controlling the flow of fuel and air mixture into the engine -- on engines with carburetors, this involves throttling airflow past the jets with the butterfly. A diesel engine, and technically port- or direct fuel-injected gasoline engines, use active fuel metering control to regulate speed. There is no restriction of airflow on these engines, therefore no 'throttling losses'. (We might have fun with semantics by considering whether the fuel control is a 'throttling' valve... but the initial sense of the word is cutting off air going through the throat, not liquid flow.)
The use of 'throttle' as the control to increase power on a locomotive is long-established semantic usage. It is fun to see terms like 'combined power handle' used to try and implement military-industrial-style precise imprecision in language, but I suspect most actual engineers just call it the throttle whether or not it does fancier things sometimes when moved.
No matter it's technical term, we still call it a throttle.
Just like a parking brake is called a handbrake, even if it's a push button.
On many engines the bell switch turns on a electronic speaker that goes ding-ding. We still refer to it as a bell. Although I guess the bell wording could be used as a verb, instead of a noun?
mvlandsw Some EMD units had a switch that would select a faster response for yard work.
Some of our geeps downgraded to yard/local service still have this capability. I just had a consist with one set up for yard switching. When moving the engine consist by itself around the terminal, it would give a little kick in the butt when first starting. (That was one of those nights. It took 10 hours to go 10 miles. Actually, only took about 20 minutes to go the 10 miles. Trains tied down on one main track and then a autorack got a right end draw bar just west of town shut down everything for about 5 hours.)
Jeff
jeffhergert mvlandsw Some EMD units had a switch that would select a faster response for yard work. Some of our geeps downgraded to yard/local service still have this capability. I just had a consist with one set up for yard switching. When moving the engine consist by itself around the terminal, it would give a little kick in the butt when first starting. (That was one of those nights. It took 10 hours to go 10 miles. Actually, only took about 20 minutes to go the 10 miles. Trains tied down on one main track and then a autorack got a right end draw bar just west of town shut down everything for about 5 hours.) Jeff
Wrong end and it would likely have been the night right there.
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