- Harry
HarryHotspur wrote:Thanks guys. Very interesting and informative. But this sentence confuses me, "Electrical transmissions can be modulated by changing field excitation on the generator and thus are less finicky about rpm range."Are you using the term "electrical transmissions" in the sense of how the electricity is transmitted (i.e. volts, watts. amps. etc.) or is there such thing as an electrical transmission similar to a manual or automatic car transmission? Sorry to be dense.
Thanks guys. Very interesting and informative.
But this sentence confuses me, "Electrical transmissions can be modulated by changing field excitation on the generator and thus are less finicky about rpm range."
Are you using the term "electrical transmissions" in the sense of how the electricity is transmitted (i.e. volts, watts. amps. etc.) or is there such thing as an electrical transmission similar to a manual or automatic car transmission? Sorry to be dense.
Not a dumb question, no apology needed.
The term "transmission" refers to the means of getting power from the source to the point where it does work -- the wheel. It also means a gearbox, and that's the way many people think of it. In a diesel-electric locomotive, the transmission is the combination of main generator, cabling, traction motor, and reduction gear from traction motor to the axle. In a diesel-hydraulic locomotive, the transmission is the torque converter, drive shafts, and gearing to the axle. Small gas- or diesel-mechanical locomotives like the Plymouth had an all mechanical transmission, using a clutch, gearbox and drive train to transmit power to the axles, and as you can imagine there was a lot of clutch slip and wheel slip to get one started when coupled to a couple of loaded cars.
RWM
There were also a number of smaller diesel hydraulics, similar to a 44 Tonner in size and appearance, and don't forget the RDCs.
Wayne
The larger a diesel engine becomes, the more compressed its rpm range can be and the slower its maximum rpm can be -- otherwise the force reversals are too large and its difficult to engineer the engine to keep it from coming apart. Conversely mechanical transmissions have size restrictions on them; at large sizes they are not very feasible. Ships with diesel engines are in effect using the fluid environment around the propellor to avoid having a transmission (though they might have a single-speed gear reduction) but land vehicles do not have that capability.
Mechanical transmissions work best for engines with a very large rpm range, not so good for an engine with a narrow rpm range -- there are too many gear changes required to keep the engine within its optimal torque curve. Electrical transmissions can be modulated by changing field excitation on the generator and thus are less finicky about rpm range.
Cost of manufacture for a mechanical transmission is fairly linear with the size of the engine. Cost of an electrical transmission is less linear with size of engine and has a high threshold cost even at a small size. Thus for small engines the mechanical transmission is inexpensive but as engine size goes up the mechanical transmission soon costs more than the equivalent electrical transmission.
Locomotives require their greatest tractive effort upon starting, which happens to be where the electric transmission exerts its greatest force.
Electric transmissions are finding increasing interest in areas where mechanical transmission has long dominated. Large mining trucks have since the 1960s been offered as both electric-transmission and mechanical transmission. Caterpillar with roughly 60% market share is all mechanical drive; Komatsu, Liebherr, and Terex with the other 40% of the market offer both electric and mechanical but are all-electric in the large sizes; Caterpillar is now testing an electric-transmission 400-ton class truck. LeTourneau makes a very large electric transmission wheel-loader, container top lifter, and log handler, but the competitors are all mechanical transmission. Caterpillar just showed a prototype of a diesel-electric D7 at Conexpo in Las Vegas.
Diesel-hydraulic locomotives are very common in Europe and Japan. Diesel-mechanical and gas-mechanical locomotives of very small size made were manufactured in significant quantities in the U.S. from the 1940s through the 1960s; their replacement today is the TrackMobile which is diesel-hydraulic.
I realize that the great majority of locomotives have been diesel-electrics vs diesels. On the other hand, the reverse seem true for cars and trucks. Anyone know why this is so? It occured to me that the relatively small size of cars might be a factor, but I don't see how that would apply to large trucks.
Just wondering.