Okay, I think I’m good on the general horsepower vs. tractive effort concept (starting power vs speed) but now I have a couple of questions:
I was on the GATX leasing site. The GP38s are listed as2000 HP, weigh 250K average, and have a TE of 58K/lbs @ 10 MPH. The GP40s show as 3000 HP units, weighing 257K average, but have a lower TE at 55.4K/lbs @ 10 mph. Both show to have the same traction motors. I was under the impression that the greater weight would give a greater TE, but in this case it seems it doesn’t. Similiar but less. Why would this be?
In Gabe’s thread about reduced horsepower in rebuilds, one of the members mentioned that a shortline may not need the additional 1000 HP of a GP40 when a GP38 with essentially the same TE would suffice. Makes sense to me. Now that being said…St Lawrence and Atlantic uses GP40s as mothers for their RM1 RoadMATES. Many of the pictures I have seen show two mother / slug units in consist (4 units total) in the lead. My understanding is that about 25 mph after the train gets rolling, the slugs get shut down…extra tractive effort to start the train, then HP from the 2 GP40 mothers take over once its going…so I guess the question would be why GP40s instead of GP38s as mothers? Is the extra 1000 HP what is needed to pull the train once in motion or is it also needed to generate the extra juice for the the slug or both?
I will proceed to pronounce my ignorance in all it’s entirety on the problem of Tractive Effort.
I understand T/E as a value of work that is placed on the rails to pull against a mass (train) by the wheels. In steam engines smaller diameter wheels were able to maintain high T/E values against larger wheels which could not really put as much on the track.
The WEIGHT on the wheels determines what work the wheels can place on the rails. This is limited by availible traction. The number of wheels that can be on the rails doing the work creates the T/E
Finally… Both the GP38 and GP40 are 4 axle units. The Gross Weights on them are very similar. The extra 7,000 pounds of weight will not significantly add to the T/E
If I was on grades and hills I go with the GP38 If I was on the level I’ll choose the GP40 as I may be able to do the same work of 0 to 10 mph in “less” time perhaps haul a bigger train.
I think the GP40 will be too slippery. I would want a engine that can get down and pull, not carry around excess horsepower that would be wasted due to lack of gross weight etc.
I would want 6 axle units with more gross weight to create a T/E of perhaps 70,000 pounds or more. The addition of the extra set of axles may save me from having to buy a second desiel and give me a better load on the bridges etc.
A slug is a desiel with no prime mover, just the traction motors with perhaps more weight added, it would need a mother unit to feed it.
wouldn’t it make sense to equip them with an extra-large fuel-tank, to avoid refuelling-stops? the larger tank can occupy the space under the empty hood. of course, you need a flexible pipe-connection between mother and slug.
Do not confuse work and force. Work is the energy required to move an object using a force over a distance. Tractive effort is a force. Actually, the minimum speeds are different, 10.7 mph for all types of GP38s and 11.3 mph for all types of the GP40s. I do not know for sure why the TE is lower for the GP40s than for the GP38s, however, I can take a few guesses. One reason my be the if the locomotive is pulling its maximum load (force), the small difference in speed causes the wheels to slip. I do not know if there are locomotive wheels made from different material, if there are, it might be that since the GP38 is use more for low speed, high force service and the GP40 primarily used for higher speed, lower force service that the GP40 has wheels made from a material with a lower coefficient of friction. Maybe they were inconsistant in their test and have the coefficient of static friction for the GP38 and the coefficient of kenetic friction for the GP40.
Probably the reason why slugs do not have extra large fuel tanks is that diesel fuel has a lower density than steel or concrete.
Different gear ratios would explain the difference. The higher tractive effort results from a gear ratio that has the wheels turning slower for given speed of rotation of the motors.
The flexible coupling between units is only one reason not to do this, of course when the fuel is emptyed from the slug fuel tank , there goes your adhesion. Don,t forget that you will also need another fuel pump.
Randy
What I was getting at was… Does the extra 1000 HP of the GP40 make it more advantageous because that HP is going to support the slug’s tractive effort (ie powering it) without affecting it’s own tractive effort or does that extra 1000 HP over the GP38 let it move the train at say 40 mph vice 20 mph once it gets moving and the slugs shut down? Why would they choose the GP40 as a mother for the RM1s vice a GP38?
With a GP-38 it may not get up to 20 mph very fast if at all. Remember that you must divide the current output from the main generator by 8 instead of 4. The AR-10 is a constant kilowatt machine. After 20 mph the slug really isn’t needed anymore . Time to go into parellell and really get going.
The GP -38 is not a low speed locomotive at all. On the Milw we used them for short fast trains.
As far as I know, for a while now railroad wheels are all made from the same thing, steel.
Randy
Do they always use the same steel, or have different compositions of steel been used? Do you think that the different tractive efforts is a result of wheel slip since the locomotives are pulling all the weight they can and the GP40 has a higher minimum continuous speed? I calculated coefficients of friction of 0.232 for the GP38 and 0.216 for the GP40, which is only a 3.672 percent difference.
The difference in TE is due to the difference in MCS, which is due to higher amperage for at any given throttle position for GP40 than GP38. Both are likely limited by adhesion, which is why the values are so close together. It is possible that GP 38 is adhesion limited and GP 40 is limited by motor current. You would have to look at both power curves side by side to figure it out and I havent got them handy.
Slugs are inherently low-speed motive power, and are often used with a high-horsepower road unit to get the extra tractive effort at low speeds out of the extra horsepower that otherwise could not be used due to adhesion limits. A GP40/slug set would be roughly equivalent to 2 GP7’s. Notice that most road slugs are paired with GP40’s (CSX and PAL) and SCL’s MATE’s were paired with U36B’s.
CSX road slugs and perhaps others are equipped with fuel tanks to supply extra fuel to the mother unit.
That’s what I was trying to confirm. I suspected that the reason the GP40s were used as mothers over GP38s because the higher horsepower was available to power the slug initially to get the train moving, then to have sufficient horsepower to maintain speed once on step, which a lower HP unit may not be able to do.
It seems that one of the hybrid locomotives could be used as sort of a self powered slug in a sense, to help get a train moving or at low speeds, and shut down at higher speed. Just a thought.
I dont understand why the slugs CANNOT run at speed with the mother units at 50 mph on a train??
In fact why not equipt rolling stock with traction motors and run eletrical feeds in the entire train?! Let the mother units feed the train and perhaps we can get really good numbers in performance versus money costs.