Why do diesel locomotives utilize electric motors rather than running drive rods directly from the engine (like an automobile)?
I recommend that you read a book like “Modern Locomotives” by Hollingsworth and Cook, where you’ll get the full skinny on how and why motive power technology progressed from steam to diesel power and from mechanical, thru hydraulic to electric traction.
There are still diesel/mechanical DMUs (Class 101s) operating in British passenger service. They’ve even got automatic gearboxes and all – but they are rapidly being phased out and replaced by diesel/electric units.
Bottom line is that diesel/electric is presently the economical, most efficient choice wherever there are no O/H or 3rd Rail power supplies, for freight and passenger trains.
…I would imagine providing a mechanical clutch to connect the prime mover to the drive wheels would be a little touchy…and a maintenance issue. Of course then there would be the requirement of a transmission too…The alternator and traction motors take care of that requirement.
There have been reasonably successful diesel/mechanical drive arrangements. Michael mentioned the DMU’s in Britain, and there are a number of others in Europe. The Krauss-Maffei diesels which ran years ago on the Southern Pacific and D&RGW were good, well-regarded units, too – and, of course, the famous (?) Budd RDC cars – a number of which are still operating here and there – had two diesels each driving one axle on each truck through a torque converter transmission (think: Hydramatic on steroids!). However, that being said, the diesel-electric arrangement offers some real advantages in terms of control and layout flexibility and power management, since the electrics handle all of the power transmission. Especially now, with the new emissions requirements, it is possible to run the prime mover in its most efficient power range, and handle all the speed variations with the electrics. Dynamic brakes are easier to manage with the electrics, too – although hydraulic retarders are possible, if you can figure out a way to cool the transmission fluid (not so simple). Maintenance is an issue, too – changing out a traction motor, while not exactly a do-it-yourself proposition, is a good deal easier than changing out a big automatic transmission, never mind Cardan (universal) joints and the like.
The biggest disadvantage is weight (generators or alternators are heavy muggers) – but the weight is needed somewhere, anyway, for tractive effort, so why worry?
The modern hybrid cars (Toyota Prius, Honda Insight and Civic) use basically the same arrangement, in miniature – an engine and generator and traction motors (although there the generator and traction motor are on the same shaft) and almost all really big earthmoving equipment is diesel electric (traction motors in the wheels or track drives).
I would note that many ships use or used diesel (or in some cases steam turbine or gas turbine) /electric drives, again for control flexibility and layout flexibility. Even – o
The transmission would be huge. The train would take forever to get up to max speed.
…And I will say in agreement the hydraulic retarder would have to really be man size to do the job as the dynamic’s do…and as for the cooling of the oil…That would be a man size job too. Many years ago I was involved in testing such a retarder in the mountains in Pennsylvania on a semi truck test vehicle and we had a high capacity heat exchanger on it to do the cooling and it had it’s work cut out for it. One thing about it we had the full benifit of the truck radiator on the down grade…Engine wasn’t using much of it’s capacity on the downgrade. The retarder was quiet effective too. But it sure did generate the heat…!
The mention of a mechanical transmission makes me think of having a gearshift in the loco… You’d have to double clutch… Some trucks have upwards of 18 forward gears, I think. How many would it take in a loco to get from 0 to 60 with a hundred car train. And what happens if you miss a downshift on a grade?
Random thoughts…
Way too random…[:-^]
The D&RGW sold their units to the SP. The SP made two separate purchaces of Kraus-Maffei (which differed in appearance) and also tried diesel hydraulics made by ALCO. All the diesel hydraulics had a relitavely short operating life because the SP decided it was better just to have one system of power transmission to maintain.
In the US diesel-hydraulics are still used for some small switchers. Many small industrial switchers are diesel-hydraulics. There are also gas/diesel-mechanical industrial switchers.
The answer to your question is well-described above, but perhaps I can add more detail. The key fact that makes a transmission essential is this: at zero rpm, the diesel (or gasoline) engine develops zero power. Moreover, a diesel (or gasoline) engine only produces anything close to its maximum power inside of a very narrow rpm range. For instance, with an EMD engine rated at 1500 hp at a maximum rpm of 800, you don’t start getting anything close to 1500 hp until you get above 700 rpm.
Consider the simplest possible case of a diesel engine coupled directly to a wheel. You have to get the engine turning before you can move. The simplest way to do this is introduce a clutch: two friction plates a small distance apart rotating around the same axis, one attached to the engine, the other to the wheel (or output shaft). You rev up the engine independently, then move the two clutch plates toward each other until they contact. The clutch slips, temporarily converting some of the engine’s rotational energy into heat, allowing the wheel to come up to the same speed as the engine without stalling the engine. Then the clutch locks up and you can vary speed by adding or subtracting fuel to the engine. But if you want to stop, you must disengage the clutch, unless you plan to stall the engine. If you slip the clutch too much, it overheats and burns up: you can’t use the clutch for more than a few seconds at a time,
At a wheel speed of 10 mph, your direct-coupled engine is at idle. You want to run your engine at maximum rpm all the time to get maximum power and maximum efficiency for your fuel dollar. Since you can’t continuously slip the clutch, you introduce a set of gears with varying ratios between the clutch and the wheel: high ratios (little gear on engine, big gear on wheel) for slow wheel speeds, low ratios (same size gear on engine and wheel) for high wheel speeds. In fact, in your typical car, “Drive” is a one-to-one ratio – the engine crankshaft is turning at the same speed as the drive sh
…As part of the conversation lets add…Hydrastatic transmissions such as used in Lawn tractors…Which I believe is similar as Mark’s last illustration of pure hydraulic’s. Infinite ratios and forward and backward operation are accomplished from the unit. No Mechanical clutch and no gear sets in the transmission. But the size of the engine in the lawn tractor and the locomotive are somewhat different.
Automatic Transmissions are making some headway in trucks now after years of a very slow start. Our projects were testing them over 45 years ago.
Diesel and the electric traction motor with the strong dynamic brake feature seems to be the best choice for the present.
I forgot about lawn tractors! I hate mowing lawns. That’s probably why.
Caterpillar track loaders have a hydrostatic drive, too. But not their track dozers. I’ve read lengthy arguments as to which type of drive on a dozer is best. And you think EMD vs. GE gets heated …
[quote]
QUOTE: Originally posted by Mark W. Hemphill
The answer to your question is well-described above, but perhaps I can add more detail. The key fact that makes a transmission essential is this: at zero rpm, the diesel (or gasoline) engine develops zero power. Moreover, a diesel (or gasoline) engine only produces anything close to its maximum power inside of a very narrow rpm range. For instance, with an EMD engine rated at 1500 hp at a maximum rpm of 800, you don’t start getting anything close to 1500 hp until you get above 700 rpm.
Consider the simplest possible case of a diesel engine coupled directly to a wheel. You have to get the engine turning before you can move. The simplest way to do this is introduce a clutch: two friction plates a small distance apart rotating around the same axis, one attached to the engine, the other to the wheel (or output shaft). You rev up the engine independently, then move the two clutch plates toward each other until they contact. The clutch slips, temporarily converting some of the engine’s rotational energy into heat, allowing the wheel to come up to the same speed as the engine without stalling the engine. Then the clutch locks up and you can vary speed by adding or subtracting fuel to the engine. But if you want to stop, you must disengage the clutch, unless you plan to stall the engine. If you slip the clutch too much, it overheats and burns up: you can’t use the clutch for more than a few seconds at a time,
At a wheel speed of 10 mph, your direct-coupled engine is at idle. You want to run your engine at maximum rpm all the time to get maximum power and maximum efficiency for your fuel dollar. Since you can’t continuously slip the clutch, you introduce a set of gears with varying ratios between the clutch and the wheel: high ratios (little gear on engine, big gear on wheel) for slow wheel speeds, low ratios (same size gear on engine and wheel) for high wheel speeds. In fact, in your typical car, “Drive” is a one-to-one rati
There is a very good presentation on how a diesel electric locomotive works [and why electric motors] at
http://travel.howstuffworks.com/diesel-locomotive.htm
rrbrewer