Diesel/electric locomotives use a diesel motor to drive a generator, the output of which drives electric motors at the axles of the locomotive. The generator/wires/electric motor part of the set-up is, therefore, just a means of transmitting power from the engine to the wheels*. What makes this set-up so efficient in a locomotive, but impractical for a car? It is impractical for a car, isn’t it? Or was that how the (never adequately explained to me) Owens Magnetic got the power from the engine to the wheels?
I don’t think it’s about efficiency, more about control. With electric motors you don’t need all the gearing malarcky when running at different speeds, and you run the diesel engine at an optimum speed when you need power. I expect you need a certain size before the losses aren’t too high. I worked on a ship that was driven this way, a car would be too small.
Waiting for a mechanical engineer too shoot me down in flames. . .
Porsche designed several petrol- and diesel-electric drive panzers during the war. None acheived widespread use, but ‘test batches’ of their versions of the PzKpfw V and PzKpfw V Ausf.B did see limited action. Also, some variants of the unproduced E- and P-series ‘super’ tanks used electric drives, some using locomotive diesels! Seeing as these were 30 ton and up vehicles, dunno how well they would scale down to a ton or two.
FWIW, the Wehrmacht seemed to like these diesel/petrol-electric drives, but copper shortages prevented their adoption. Since we copied just about everything else from them, at some point or other, except for these drives, their must be some other reason they are not practical. Too complex, perhaps?
Energy transfer between one form and another always involves some inefficiencies and therefore loss of some of the energy.
The corollory is that chemical --> mechanical energy conversion is likely to be more efficient than chemical --> electrical --> mechanical.
In the case of a loco, economies of scale (in a thermodynamic sense rather than an economic sense) make it viable.
Added to this is the nature of diesel engines. small ones are not as efficient as large ones. That is why diesel has been used for years in ships and trains, but is a relatively recent thing with cars. And I think it will be a while before you see a model aeroplane with a diesel engine.
So, anything is possible, but a very clever design is required before the idea is actually practical.
I have always thought it was because electric motors were so great at producing torque, which locomotives need more than horsepower. Plus what the others have said about control and economy of scale.
Well, I just found this HowStuffWorks page which explains it a lot better than I ever could. The short answer is because it eliminates the huge transmission that would be required.
We might be getting them soon. I don’t see it any more complex then a electric/gas hybrid car.
On such a small scale and becasue the demand on a car engine is much more variable and sudden then a train, I would think a sizable battery bank would be needed.
Also, you have to realize that diesel technology is much better than it has been in the past. Modern diesel engines use precise injectors with an extremely high pressure in order to disperse the fuel correctly to get to most efficient burn. One such car that you can get in America today is the VW TDI Jetta, Beetle, or Golf. They each get around 49 MPG highway with a manual transmission.
I’ve flown model aircraft off and on since the 50’s, and several of them have been powered by diesel engines. Most of these engines were imports from Eastern Europe. Tricky to start, but ran well once adjusted.
Today you can buy the parts required to convert glow engines to diesel operation.
The largest and easiest to understand reason for diesel electric transmissions in a locomotive is that the forces necessary to start a train would leave a mechanical transmission all over the roadbed. An added advantage is dynamic braking, where the traction motors are used to generate electricity, and the resulting mechanical loading used to control a train’s speed in hilly terrain.
A German company, Krause-Maffei, sold some diesel-hydraulic locomotives to the Southern Pacific and Rio Grande railroads in the early sixties. They were the equals of the EMD and GE locomotives of the day, but were maintenance nightmares, and wound up scrapped before the end of the decade.
Small industrial locomtives are made with mechanical transmissions, but are not meant for moving more than 2 or 3 cars at a time.
Diesel electric transmissions can also be found in large off-road mining equipment, such as the humonguous dump truck made by Terex, or the huge strip mining shovels
Older locomotives with DC traction motors have a process known as ‘transition’, which is analogous to shifting gears. When a locomotive starts from rest, all the motive power is fed to the traction motors in series, to increase the resistive load, and prevent burn out. At some point (I don’t know what it exactly), the generated back EMF generated by the spinning coils puts a limit on the efficiency of the traction motor, and the electronics shift a bank of relays to reconnect the motors into a series-parallel configuration. At top speed, the traction motors are all parallel. This was a 4 step process, and was independent of direction of travel.
Modern DC locomotives have 2 transistion steps, and AC locomotives don’t have any.
Electric trains can draw their power from an electric rail. Electric cars have to carry their power source around with them. Batteries tend to be large, heavy, and inefficient, with limited range. You have to recharge them every 100 miles or so, and charging takes hours.
Yeah I noticed the date after I posted (stuff on the net hangs around forever dunnit). The basic problem with cars using electric motors is that they all seem to need honking great batteries, with no driving batteries and just a small diesel engine you get the power of - a small diesel engine. Google on diesel locomotives and you’ll find motors with 6000 horsepower, if you could put that in a car it’d be quite nippy.
Didn’t follow the Ford Hybrid link above, but the hybrid Escape is imminent. Yeah, it uses Toyota technology and is late to the market, but at least it’s a useable size compared to the other hybrids!
Diesel-electric won’t really work in a car right now, nor gasoline-electric, where by these I mean a purely electrical drive. That is, without a huge containment system, like batteries (heavy and expensive), a huge capacitor (the size of the car?), or a flywheel (I like this idea).
The problem with a purely electrical drive is demand horsepower, meaning accelleration. You’d need a big, big generator (and consequently a motor) to give instant power needed for the type of accelleration we expect. Or, as I said, another energy storage system for that reserve power.
Hybrids do both, though. Their motors run an electric drive system, and directly drive a transmission when power is needed.
Consider that something the size of a Escape may only use 25hp to maintain a constant 70mph, but use 200hp to get to that speed in 10 seconds. So you see that for cruising, electric drive is enough.
Interesting thread. Hope this isn’t too much of a hijack, but given the relatively higher fuel efficiency of diesel vs. gas, why have all the manufacturers that produce hybrid vehicles so far selected gasoline, rather than diesel, for the drive/battery-charging engine?
You need both, actually. Starting torque on a steam locomotive is very low and they would sometimes need help in starting a heavy train (a booster truck that would cut out at about 15 mph, for example, or a good hard shove from another locomotive in the rear). During the transition days when both steam and diesel-electrics were being used, the expression was that steam could run more than it could start while diesel could start more than it could run – the motors could be temporarily overloaded to get moving, but would overheat if the load was too big for sustained pulling.
True, but this might not have been so much because they had to do it this way, but because the method was available, so why not use it? Steam locomotives were the sole form of motive power on the tracks for nearly a century prior to the arrival of the diesels, and railwaymen muddled through somehow.
I think it’s not so much the low start up torque generally as the fact that a steam locomotive trying to accelerate at very low speeds delivers its power in rather cumbersome pulses, and this leads to serious wheel slip problems. In many cases, helper locomotives (either steam or diesel) were required to move heavy freight trains, but even a powerful mainline steam locomotive pulling a relatively light passenger train experienced problems. Even starting “light engine” (no cars) in rain or snow could be a challenge. The main cause of wheel slip in this case was the regulation of power (often these big locos would be applying too much), and this required an experienced hand on the throttle, and liberal application of sand to the metals. Steam locomotives also have a thing called a “reverser”. This is the big wheel-looking thing you’ll see inside the cab. As the name implies, it is used to change the direction of travel, but it is also used in a way somewhat analogous to a variable gearing system. Pulling away from a standing start, the engineer would set the reverser to maximum, usually expressed as a percentage. Max tends to be around 75, depending on the loco. This means steam is admitted to the cylinders for 75% of the stroke. It provides massive power, but also uses up steam at an alarming rate. As the engineer slowly opens the throttle and picks up speed, she gradually winds back the reverser to a cruising speed setting of maybe 25 or so. Just like going up through the gears in a car.
As a diehard railfan and steam lover, I have to be honest with myself, and say that, despite her beauty, steam is a total bitch of a form of motive power. Very inefficient. Cool though. The dads with their sons standing by the side of the tracks may have mourned the loss of steam, but the engineers sure as heck didn’t.
The main reason is emissions. With the U.S. having high sulfur content in the diesel fuel, and diesels also having higher NOx levels than gas cars, diesels are almost locked out of the California and other “Green” states because of their tighter emissions laws. As a matter of fact, current diesel technology means that when the Feds tighten the emissions laws(I can’t remember what year that is), automotive diesels will be basically outlawed in this country. Barring an adoption of low-sulfur diesel fuel, and some monumental change in the marketplace, I doubt that anyone will bother to sell diesel cars in the U.S. because the investment to get a diesel to pass these future emissions laws will be too high. Which is a real shame, since the modern diesel is much much better than the horrible diesels inflicted on us in the early eighties.
Mercedes-Benz has at least toyed with the idea of a diesel hybrid drivetrain. This being a Mercedes, no one is shooting for impressive gas mileage (30 MPG), but for a 300+ HP half-SUV that’s pretty good.