Same reason for both... the turbocharged 12 cyl 645... They wanted low-horsepower models, but needed them to be turbocharged for a number of reasons... the biggest is that turbocharged diesels work better and are more efficient at high altitudes than roots-blown diesels (i.e, GP38)
I'm not sure, but if my memory serves me correctly, I thought Santa Fe's SD39's were originally bought specifically for the Raton Pass area. The steep 3%+ grades there, and slow running speeds, lent themselves well to the SD39's mix of HP/tractive effort/fuel consumption discussed above. My 2 cents worth. Just my guess.
Slugs are widely used... in yard service, as this is where you see the greatest benefit of a slug, and not the drawbacks (more later)... Some roads still use road slugs (there is really no difference between a road slug and a yard slug, operationally) like CSX and it's GP40-2 slug sets... NS used GP40 slug sets in road service for years, these sets are now in yard service. Very basically, adding a slug with the same number of axles and the same weight as its mother increases max tractive effort twofold, while reducing the minimum speed max TE occurs at by half... If a SD39 is capable of 83,000lbs TE at 8mph, adding a six axle slug to it (that weighs the same) gives it a max TE of 166,000lbs at 4mph Take it a step further, like the N&W/NS SD40 slug sets using two slugs.... The SD40 capable of 83,000lbs TE at 11mph goes to 166,000lbs at 5.5 with one slug... With two, it's 249,000lbs TE at 2.75mph... small wonder they're used in yards! With any slug, there comes a point, speedwise, where the locomotive can no longer supply enough power to make the slug effective, from that point, the slug becomes nothing but dead weight for the locomotive to pull... the lower hp the mother has, the lower this speed limit becomes... As an aside, note that as long as they weigh the same, a SD38, SD39, SD40 and a SD45 will ALL have the same TE, just the lower hp SD38/39 has a lower speed max TE occurs at... one reason why low-horsepower six-axle units are sometimes used in yard service, or as helpers.
Well, thanks, I didn't know any of that. One Nitpicky correction Roots-blower refers to a type of Turbocharger. (actually, based on what I'm finding on the internet, it's actually a supercharger, but as EMD calls it a Turbo, that's what I'll call it, I assume that's what it really is) GP-38s and early EMDs are normally asperated. No fancy term for them.
Also, I would assume that tractive effort is also determined by electric motor type both current type and general specs. What was the EMD standard D77 or some such?
A Supercharger is spun through a direct drive on the engine such as a belt or gear train. Whereas a Turbocharger is spun by a gas turbine placed inside the exhaust pipe. A Supercharger "robs" useable engine power from the output shaft. Whereas a turbocharger only adds back pressure to the exhaust gas stream which is overcome by the turbocharger adding pressure to the intake air stream. A Roots supercharger was a specific type of older supercharger used on small and medium sized diesel engines, especially trucks.
Well, from what I was reading, a Roots Supercharger was named after the man (roots) that invented the airmover that is the basis for it. I would assume the same mechanism is in use on EMD's Turbos. In either case, my point is that the name EMD uses to describe their Turbo on the 2 cycle 567,645 and 710 is roots blown.
OK, I stand completely and totally corrected. Per wikipedia, Rootblown is the normally asperated version. Hmmm, I'm going to have to go over some old documentation I have and correct it. My apologies to everyone I misled.
To an extent, TE is determined by the motor and it's rating... but the unit weight and truck design also determine max TE... it's all intertwined... Since the SD38/39/40 and 45 all use the D77(B) traction motor, what I stated is still valid. You bring up a point I wanted to mention, but didn't becuase my thread was getting long... Traction motor design is one reason why you don't see newer units mated with slugs. Newer DC motors are capable of handling more power at lower speeds than earlier DC motors, and while that doesn't prevent the usage of a slug, it makes them less effective...
So do those slugs pictured have the same traction motors as the second gen engines they're mated to? or are they just more similar in performance? It still sems to me that a Green goat style Hybrid drawing power from a mother would increase the value of the "slug" in road situations.
Yes... the slugs here have the same D77 motor as the mother... in other slugs the motors are simliar in generation, i.e, the NS RP-F6Y slugs built from former FM Train Masters use the GE 752 motor As for the hybrid road slug... The slug would have to be able to provide enough power at high speeds (i.e, over 15-20mph) to propel itself and still provide TE, something a hybrid really can't do unless it has a large prime mover... The mother itself would have to have an excess of power not used at low speeds (this is where those newer traction motors come into play)... if you put DC traction motors on an AC6000CW, that'd be a good mother
Hmm, didn't they try the Green Goat out in help service on Cajon and it blew something out? Why would the prime mover be a limiting factor? It would be a matter of getting enough Kw out of the battery banks. Which of course is based on the size of the banks. The Diesel in a hybrid is primarily used to charge the batteries. My assumption was that a GP-40 used as the slug mother to a hybrid slug would be providing less TE to it's 4 motors, but would be supporting the 2000HP equivelent tractive effort of the Hybrid. Again, assuming you have a battery bank that won't blow out at that load. I guess what I'm saying is that they should treat the engines as a power grid rather then as units. Essentially every single Diesel electric engine is a powergrid alread. It has a supply and a drain. In my mind, it seems that there could be an advantage to treating a set of engines as a grid instead of each engine being seperate. So for instance, (using completely fake numbers) you have 2 GP-40s that put out 150Kw nominally, an SD60 at 200Kw and a Green Goat at 100Kw. To me, it would make sense to divide that 600Kw between the 18 traction motors in the most efficent manner rather then having to divide it in a more limited fashion. Honestly, I think this is the next evolutionary step in locomotive technology, because they have the ability to detect where tractive effort can best be applied on a per engine basis If you could then control that on a consist wide basis, you'd be able to more efficently use your equipment. And hybrids and technolgoy like GE's regenerative braking would be more useful as you are dealing with a grid. That's just the electrical engineer in me talking though.
Yo, one thing you must consider is that a national power grid is supplying loads that are all independent of each other. Each load will draw as much power as it needs without affecting any other load. However, MU'd locomotives are all mechanically locked at the couplers and the wheel/rail interfaces. Therefore, each traction motor must have the same design characteristics and power demand as all the other motors. If there are motors with different power characteristics, some motors could backfeed your hypothetical grid while other motors are at maximum demand, which could cause the grid to unbalance and possibly burn up.
Why is the prime mover an issue? You need to charge the battery bank at a rate higher than will be consumed,. i.e, feeding four traction motors at 50mph... Hybrids work well as switchers... speeds are low, and movement is not constant (i.e, the batteries have time to charge between shunts... but they can not be used consistantly, eventually they have to have time to recharge... GE's forthcoming hybrid operates much differently... it uses energy stored from dynamic braking to charge the batteries, this stored energy can then be used in conjunction with the regular prime mover...
Hytec, We already have Microprocessor controlled limited slip and indivdual control. The only difference is in scale. And considering computers have been networking for 30 plus years, getting a couple units to coordinate tractive effort and power use should be easy. Don't get me wrong, it's a challenging issue, but it's something that modern locomotives and cars essentially do already. As for the issue of battery charge, you're basing your assumptions on current battery technology and the specific design of the green goat (RPT is working on a road switcher) This may not be the most logical way to look at it. A Stack train is using far less of it's available power when shooting across the mojave compared to when it is surmounting Cajon at a low speed. When it's shooting across the mojave the battery banks are going to be asked for a constant, but relativly low current and will have maximum life while the "mother" units will also be running at a very constant efficient rate possibly even keeping up some of the charge on the batteries. When going up Cajon, the battery system may or may not be of use, but the big diesels could potentially be getting more tractive effort out of the total package. And of course, coming down the other side would recharge the batteries pretty effeciently. Really what I'm talking about is GE's technology applied across additional axles.
Yo, what I didn't bring up discussing your grid concept was the inductive kick-back from traction motors. Inductive kick-back can occur when the motor control voltages abruptly change, as in moving the selector from Run-8 to Run-1. This is especially critical with DC motors. The best example I can think of is when a DC relay is denergized. The inductive kick-back, or induced current flow from the collapsing magnetic field around the relay coil back into the power source can be more than 100X the current that the relay initially needs to become energized. That is why a shunting diode is placed in parallel with the relay coil, with an opposite polarity to the source current. The diode must be able to short-circuit the inductive kick-back current directly back through the relay coil. This prevents the kick-back current from reflecting back to the source and blowing the socks off the power supply. The reason that a shunting diode can not be used on a locomotive traction motor is that the locomotive must be capable of running in either direction. That is to say, the traction motor drive current must be fed by voltage of either polarity, depending on the locomotive's direction. Therefore, all traction motors being fed by a locomotive power "grid" must be nearly identical so that a single circuit can be designed to prevent power supply destruction from inductive kick-back. When you consider that traction motors on modern locomotives can draw up to 2000 Amps, an inductive kick-back of more than 200,000 Amps can get a little testy. Trust me, inductive kick-back has bitten the best design engineers at least once. [ July 28, 2005, 07:55 PM: Message edited by: Hytec ]
To me though that would be a function of the load half of such a system rather then the source half. Essentially you have a source and you have a load and you can design the eltrical isolations of each load to match the characteristics of that load. At the same time the source is really irrelevent. Kind of how my lightbulbs don't care if it's coal, uranium or water that are the fuel that turns the turbines. I understand that there's some engineering work to be done here, but lets face it, there hasn't been much of a technological innovation in Locomotives in 15ish years (AC) beyond the Hybrids.
There is something inherently wrong with this statement... Was physics required for your electrical engineering degree?
