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[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 ratio -- the engine crankshaft is turning at the same speed as the drive shaft coming out the back of the transmission. You use the clutch to change gear ratios. (This is not the same as a gear reduction. Those are fixed, single-ratio gear trains between an engine and an output shaft, and recognize the fact that usually you want the engine or motor to turn a lot faster than whatever it's driving. For instance, all EMD and GE locomotives have a single-ratio gear train between each traction motor and the axle it drives, as do most ships between the turbine or diesel and the propeller shaft. For many years, the common ratio on EMD locomotives geared for a maximum speed of 70 mph was 62:15 -- a 62-tooth gear on the axle and a 15-tooth pinion gear on the end of the traction motor.) The hydraulic transmission is a variation that uses an oil-driven torque convertor to do the same thing as the clutch: temporarily convert some or all of the rotational energy of the engine into heat until the input and output speeds match. The hydraulic transmission may include more than one gear ratio. The K-M diesel-hydraulic locomotives described above had only one gear ratio, so as they accelerated a train they converted a lot of diesel fuel into heat and wasted it. Hydraulic transmissions have become common in earthmoving machinery and medium-duty trucks (garbage, city bus) because they do lots of starts and stops, which burn up clutches quickly. But they're not popular in heavy-duty trucks (which don't do a lot of starts and stops) because they waste fuel. The electical transmission is the common choice for big power applications. Unlike gear-based transmissions, which have a fixed number of ratios, this is a continuously variable transmission. Moreover, the electric motor develops maximum horsepower at zero speed. No clutch is needed: the electric motor can be directly coupled to the wheel. Now you can run the diesel engine at any speed you want totally independently of the wheel speed. Why not use an electrical transmission in an auto? Because you don't need it, so long as fuel is cheap and your car is light weight. Electrical transmissions are very expensive, and other than in locomotives the only common transportation application is in large off-road mining trucks and some ocean vessels. LeTourneau also makes large front-end loaders and log-stackers that are diesel-electric. You also find applications that use a lot of electric motors, for example oil rigs, have gone all-electric, with a diesel engine driving a generator that powers everything. In light railroad applications the hydraulic transmission is cost-effective. That's why it was used in RDCs and small switching locomotives, and has reappeared in the DMU that's now testing in the U.S. Big clutches are problematic to design and easily damaged unless you have a highly skilled operator. The only application of big clutches I know of are mine hoists, which are electrically driven. I should add that a fourth variation has appeared in recent years: pure hydraulic. This consists of an engine driving a big oil pump, and oil motors driving rotational applications. This is the system used on excavators, which got that way because they already required the oil pump to drive their hydraulic actuating rams, sit in one place most of the time and when they move, don't move far. It also shows up on container/trailer loaders (as does diesel-electric). [/quote] AAAHHH!!! That's right!!! [:D][#ditto]
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