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[quote user="silicon212"] You are correct in that 'loading' specifically relates to load placed upon the main generator, i.e. power available to the traction motors after the throttle is advanced.  The term 'loading' also is used generically to refer to the time between the throttle is advanced to the time when the locomotive begins motion in the desired direction.The two stroke engine will advance RPM faster than the 4-stroke engine.  I suspect there are two factors at work here - there is a power stroke on every engine revolution for a two stroke, meaning that it can increase RPM faster (more power per RPM) than a 4-stroke can (the 4-stroke has one power stroke for every two revolutions).  The bigger factor is that since the 4-stroke can breathe on its own, air isn't forced into it for scavenging, so the turbocharger (and they're ALL turbocharged) spins free and depends entirely upon exhaust gas velocity for operation.  When a 4-stroke diesel is given the instruction to increase RPM, fuel is increased to the injectors to a degree where the air/fuel ratio is correct with turbocharging, but when the boost isn't there, the engine will run extremely rich (hence billowing black smoke - you've seen it on GE locomotives as well as Chevy/Ford/Dodge diesel trucks under hard acceleration - until the boost builds and the air/fuel ratio stabilizes.  The newer GE locomotives (-9, AC4400 and the ES series) have software that prevents the engine from increasing too fast (to control the smoke) and if the engine can't spin fast, the generator won't load fast.On the other hand, the EMD two strokers, since their turbo is engine driven to a certain speed (the turbo actually begins freewheeling around notch 5-6), don't generally have a smoke problem since there is no turbo lag (since the turbo up to notch 5 or 6 is driven directly from the engine), air picks up immediately and the engine can obtain proper RPM quick.Although I don't believe EMD's electrical system waits until an engine notches out before the generator loads - I've seen GP60s begin motion almost immediately upon throttle movement, and before the engine notches out. [/quote]Thanks for that clarification silicon 212.  I can see that the load-up cycle could be considered to begin the moment the throttle is advanced, and therefore the engine rev-up time would be part of that cycle.  I can also see why a 2-cycle engine would rev up faster than a 4-cycle engine.  And I think you are likely correct that the actual electrical load-up may, at least with some locomotives, begin the moment the throttle is advanced.  I had speculated that the load regulator does not begin loading until the engine reaches the RPM that matches the advanced throttle setting, but I don't see any reason why that delay would be essential.  I would guess that the load regulator bases its decision on the rate and curve of load-up on several variables, including a manual setting on locomotives where such a setting is available.
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