I believe "load up" means that a particular locomotive unit in a consist starts applying tractive effort in response to a throttle command. Sometimes it is in reference to a "unit that won't load up" meaning that you have multiple locomotive units in a consist, but one of them is not contributing its share of tractive effort to moving the train owing to some defect in the unit itself or in the MU connections.
An automobile produces near instantaneous tractive effort at the wheels in response to "tipping in" the throttle pedal. I was told that about a 20 ms lag is all most drivers will tolerate. I used to think that this means most drivers are way impatient, but what it means is that if you have 40 ms or more lag in a drive-by-wire gas pedal or other system controlling power in a car, you will experience a noticable hesitation that a lot of people find scary when pulling out into traffic.
Obviously, railroad personnel tolerate a much longer time for a big Diesel prime mover to develop power in response to operating a locomotive throttle, but apparently there are differences, with the 4-cycle GE locomotives reputed to being particularly slow.
I believe the 2-cycle EMD locomotives "load up" more quickly on account of the apparatus required for "air scavenging" on a 2-cycle Diesel. The non-turbo EMDs have forced-air scavenging with a Roots blower while the turbo EMDs have a shaft-driven turbocharger that only runs free of the prime mover in Notch 7 or Notch 8. It seems that EMD Diesels can get air into the engine faster to get things moving. The 4-cycle GE units, however, have free-wheeling turbochargers in all notches that require time to "spool up" -- they have considerable turbo lag. The control system has to hold off on feeding in fuel so the Diesel doesn't smoke badly.
I also understand that many of the AlCOs were turbo 4-cycles only they weren't holding back on the fuel, and that an AlCO "loading up" would belch smoke and sparks in simulation of a steam locomotive.
I drove a 4-cycle turbo Diesel rental car in Europe once. I can tell you that car was noticably slow to "load up."
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
This refers to the dash-2 lines and before:
The speed at which a locomotive can load is governed by the (surprise) load regulator. It can be set to a variety of positions depending on the needs of the service it is assigned to.
On switch engines, the load regulator is set to "maximum field"; this results in extremely quick loading. The Metra suburban F7s and E8s were also set at max field.
The CNW geeps as well as the suburban power all had switches on the control stand so one could use either quick or slow start as needed.
Road power is set for "minimum field", thus assuring more gentle starts.
Quick-starting is much more demanding of the traction motors.
I am not sure if modern locomotives have the old-style load regulators, or if it is (likely) computer controlled.
Just to clarify, if any clarification is needed, and if I am not mistaken: Notching up the throttle, causing the fuel injectors to deliver more fuel, and thus causing the engine to increase RPM is not "loading up."
In the power mode, when the throttle is notched-up from the idle position, the following stages occur:
1) The fuel injectors' rate of delivery increases, thus causing the engine RPM to begin to increase. During this time of engine speed increase, there is no loading on the engine. It is simply spinning the generator or alternator but they are not producing power for propulsion.
2) The engine completes its RPM increase, and the RPM then matches what is prescribed for that throttle setting.
3) Once the engine RPM has stabilized at the higher level, matching the higher throttle setting, the load regulator gradually increases the electrical power taken from the generator or alternator, and begins to apply this power to the traction motors. This begins the process of placing a physical load on the engine by making the generator or alternator harder to spin.
4) As the load regulator gradually loads the engine, the engine governor causes a corresponding increase in fuel delivery beyond the fuel delivery increase that brought the engine RPM up in the first place, before the load regulator began applying the load. The intent of the governor is to maintain the engine RPM to precisely match what is prescribed by the throttle setting even though the engine load is being increased by the load regulator, which would tend to slow the engine RPM if more fuel were not added.
Zardoz mentioned, the ability to change the rates of loading on some locomotives for different types of service. And I have often heard that certain makes of locomotives load fast or slow. However, I cannot see any fundamental reason why a locomotive with a 2-cycle engine would load faster than one with a 4-cycle engine unless a 4-cycle engine fundamentally takes longer to reach an increased RPM for an increased throttle setting. In that case the loading would simply begin later, so the overall cycle from throttle advance to reaching full loading would take longer. Maybe somebody could shed some more light on how loading might be affected by 2-cycle versus 4-cycle engines.
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.
silicon212 wrote:On the other hand, the EMD two strokers, since their turbo is engine driven to a certain speed
bigbird_1 wrote:So would the Roots type blower be classified as a supercharger/turbocharger combination, as it has direct mechanical as well as exhaust gas drive?
I can see your point, however, I think you question pertains to the EMD turbocharger with the overrun clutch and not the roots blower, which is purely mechanical all the time. The EMD turbochargers with the overrun clutch are mechanically driven some of the time, and exhaust driven the rest of the time, so they are may be considered superchargers when running mechanically. However, when they run mechanically, it is a low RPM where the added air charge is less necessary and less effective. I suppose there may be some effect of the boosted charge at the low RPM, but a big reason for this mechanical spinning is to give them a head start on getting up to speed when the engine accelerates. Fundamentally though, they are turbochargers during the time where they do most of their work.
silicon212 wrote: 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.
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.
Bucyrus wrote: bigbird_1 wrote:So would the Roots type blower be classified as a supercharger/turbocharger combination, as it has direct mechanical as well as exhaust gas drive?I can see your point, however, I think you question pertains to the EMD turbocharger with the overrun clutch and not the roots blower, which is purely mechanical all the time. The EMD turbochargers with the overrun clutch are mechanically driven some of the time, and exhaust driven the rest of the time, so they are may be considered superchargers when running mechanically. However, when they run mechanically, it is a low RPM where the added air charge is less necessary and less effective. I suppose there may be some effect of the boosted charge at the low RPM, but a big reason for this mechanical spinning is to give them a head start on getting up to speed when the engine accelerates. Fundamentally though, they are turbochargers during the time where they do most of their work.
The only thing I would add is that gear vs turbine isn't really and either/or thing. It's a both.
Two cycles require scavanging air. It takes energy to provide scavenging air. The exhaust gas stream is a good place to get energy to run a blower to do this job because it's otherwise wasted.
The trouble is, the amount of energy you can extract using an exhaust gas turbine isn't a good match for the energy needed to power the blower. If you design it to give a good supercharge boost at the high end, you don't have the bare minimum scavenging air charge at the low end.
EMD's solution was to augment the turbine with a direct gear drive (and overrunning clutch). So, even at idle, some of the power to turn the blower is coming from the turbine, but most is coming from the gear train. This load sharing arrangment exists all the way up to roughly notch 6 where the turbine finally has enough oomph to do the whole job itself.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
Randy Stahl wrote:The load regulator is only a tool the governer uses to maintain engine RPM at the correct RPM for each throttle notch. Electrically the LR is in series with the notch reference signal from the TR (throttle response) panel.
Randy, a clarification, please; I would appreciate your input..
I remember the F7s and E8s load regulators: if we had a problem with loading, it was one of the first things we checked for trouble. We could actually see the position of the armature (?) thru the glass window. In "quick-start" the arm would be in a certain position (I cannot remember if it was the 4:00 or the 8:00 position), in "slow-start" the arm would be in the opposite position. The position of the load regulator seemed to have no effect on motor rpm's, only how quickly the amperage would build.
FYI: in quick-start, the unit would be loading 1500+ amps within 1-2 seconds after opening the throttle to the notch 1. We'd have to keep the jammer partially applied to reduce the initial G-force on the passengers.
The governor is fundamentally a flyball governor. It adjusts fuel to maintain engine speed (RPM). On an E8 (or any EMD up through the Dash2s), on top of this, a locomotive's governor will adjust the load regulator to maintain a set fuel rack balance point. It will move the load regulator to increase or decreas load until the fuel rack balances at the right point.
On an E8, which has battery field excitation, the circuit for the field for the main gen is the battery, the throttle switch resistors - the higher the notch, the less the resistance, the load regulator and the main gen field. In notch 8 with the LR in max position, you get the full battery volatage applied to the main gen field.
A Roots blower on a two stroke diesel is considered 'normal aspiration' since it is required for the engine to run. The Roots blower is displacement-matched to the engine, so the air moved by it is what's required by the engine and nothing more.
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