I have been reading the new Classic Trains Magazine about EMD E-Units and it raises a question which I had not thought of before.
Unlike ALCO which used a single diesel engine in their PA/PBs – and presumably a single electrical generator – EMD utilized two engines each driving a seperate electrical generator. Did these two generators provide power to individual trucks – engine #1/electrical generator #1, for instance, providing power to the forward truck – or was the output of these electrical generators paralleled to provide power to both trucks?
Obviously the engineering departments in the two builders had their reasoning for why the did things their way but were there advantages – and disadvantages, of course – to the two systems?
The number one reason for dual prime movers in the E units was to get enough power. In the 1940’s EMD prime movers just didn’t have 2000 horsepower, so the designers put in two of them to get the needed power. Number two reason, should one prime mover break down on the road, the train could proceed on power from the other one.
The traction motors were series wound DC motors. For maximum tractive effort at slow speeds, the locomotive connected all traction motors in series with each other. Once a reasonable speed was attained, the traction motors were re arranged into a parallel hookup. After you have put in enough heavy duty electrical switchgear, to do this, arranging to disconnect the generator on a dead primer mover isn’t hard.
I don’t know for certain, but my guess is that the combined power from both diesels was fed in common to the trucks.
Otherwise, you’d have a nightmare of controls to keep both trucks operating in tandem, instead of fighting each other. And if one diesel goes off line, this would also provide the means for backup propulsion from all wheels, rather than just those tied into the second motor. Half the amperage, but at the same voltage.
Wish I could remember more from the electronics classes I took more than a decade ago, as I could give a fancier answer.
An E unit is two locomotives in one car body. Front engine for front truck , rear for rear. Why? because parelleling generators isn’t easy, if the voltage on one gen is lower than the other the higher voltage will try to turn the lesser one into a motor.
The thing with using separate generators for the trucks makes sense to me. I remember an experiment when I was in school with running two generators in parallel on one circuit and it was very tricky. In using one generator for one truck I can see the load demand on the generator being somewhat lower. It was only pulling two traction motors (A1A = 2 powered axles and 1 idle axle) instead of four.
As others have explained, the front engine powered the front two motors, the rear engine powered the rear two motors.
I have the EMD Operators manual on a PDF, it has all the wiring diagrams. I would be happy to send it to you. Drop me a PM if you are interested.
GM did not have an engine with that kind of HP, and while the 567 was a modular engine, built in various cylinder quantity configurations 8, 12, 16, building one above the 16 cylinder version in the GP7/F7 was not seen as practical - crank shaft flex/strenght, etc.
So to get 2000 HP, two seperate engines were used. And it proved to be a very effective design. Truth is many of the assignments for these locos only required about 1000 HP for most of the run, so they proved very reliable.
With any application of motive power, the engine that can do the job easily usually lasts longer, has fewer breakdowns and uses less fuel compared to one working at its limit.
Trick question - what gets better fuel economy a Toyota Prius or a BMW M3?
Having been the shop foreman in a BMW service department, it’s not really how much time they spend in the shop as it is how much of your wallet is emptied by the visit - leaving you no money to spend on gas. Just ask the guy at the Benz dealer what BMW stands for = Bring Money With you.
Great driving cars - just way past the point of deminishing return in terms of cost of ownership.
The EMD E8 on the other hand was the ulimate evolution of the passenger diesel locomotive.
Drifting off topic but, having owned 4 BMWs now over the years I find them to be every bit as reliable as any other car I’ve driven. ANd the secret to not payign to much for service - take it to an independent shop! There’s oen by me that specialiaze in only German cars. Cost of an oil change in my old 328 from the dealership: $135. Cost of oil change at the independent: $40. Same work, same synthetic oil, OEM filter.
A brand new one will cost you less - since you get 4 years scheduled maintence included. Although I prefer older ones, my present one is a 99 540 6-speed. Bought it used for less than my daily driver Kia cost, 111K miles on it now and still runs like new.
SO why not get 2 F units??? Same prime mover, same total HP ?? If one breaks down, remove it and replace with naother
The answer is union labor agreements. During the transition steam to diesel, the unions were fighting to keep jobs. WIth a steam locomotive, you need an engineer and fireman to run it. For double heading, there is no MU in steam and all locomotives had to have a crew of two.
With diesels, the engine crew of the lead locomotive can run all units in the consist via MU connections; the union did not like this. In the 1940s, they wanted and engineer and fireman in the cab of all A unit locomotives and a fireman in all B units. The railroads, naturally, objected to this. One of the reasons for a 2 diesel E unit was it could be run by a crew of 2. Other ways this was accomplished was to connect units with drawbars instead of couplers, number the whole thing with one number, and considder it one locomotive. so A-B set was numbered xxxA-xxxB, an A-B-B-A set was numbered xxxA-xxxB-xxxB-xxxA
The unions eventually gave in to progress and accepted that an MU diesel consist could be run by a single crew.
Yes in the early days of diesels there were lots of questions/disagreements about union rules, safety, crew size,etc. But none of that drove the chioce by GM to install two 12 cylinder prime movers in the E unit.
ALCO PA’s were turbo charged and made 2000 HP from a single 16 cylinder prime mover of 10, 688 cubic inches.
EMD’s 16 cylinder prime mover, used in the F3, F7, etc, only produced 1500 HP from 9,072 cubic inches - NO turbo charger.
The E unit has two, 12 cylinder, 1000 HP, 6,804 cubic inch prime movers - again NO turbo chargers.
And that is exactly what the Santa Fe eventually did. While an early adopter of the E units, the flexibility of multiple F’s soon won the day. During the labor issues the Santa Fe actually ordered a pile of FTB units and reconfigured them to be A-B-B-B sets so there was only one cab and so no issue.
F units also did not have the space for the water and steam boiler for larger passenger trains. Hence the FP7 for a smaller passenger loco, lengthened frame to fit a larger water tank for the boiler, but still basically an F7, single diesel, 4 axles. Since the Santa Fe had F units equipped for passenger service, the Reading wanted EMD to convert some fo theirs, but EMD said it would not be economical to do that, and pushed the FP7 instead, so the Reading ended up with some FP7s for name passenger trains until replaced with RDCs.