Tractive Effort, Horsepower and Slugs

[martin.knoepfel]

another question as to slugs.

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.

[Anonymous]

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.

[dharmon]

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:

1. 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?

2. 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?

Dan