Heavy american freight trains can be fast

[ttjph]

Well, it's all a matter of power:weight ratio.

If a Class 70 can put 90% of its 3700 hp to the rail, that's 3330 hp or 2500 kW. A moderate load would be 1200 tonnes including the loco.

At 70 mph or 31 m/s, the consist would have 0.5mv^2 = 0.5 × 1200 × 31 × 31 = 576600 kJ, so ignoring rolling resistance, aero drag, tractive effort limitations, etc., the shortest possible time to reach 70 mph is 576600 / 2500 = 231 seconds or about four minutes.

Looked at another way, it has around 3 hp/tonne - an average family car these days probably has 70-100 hp/tonne (and a Deltic plus 8 has something like 8 hp/tonne).

What would you call "quickly"?

[Kariban]

10% too much power - usually about 82% efficiency for older DE, I guess maybe a couple more these days, but that would( if you do it another way ) need an average TE of 155kN, without drag and so on. That would be remarkable

[Kromaatikse]

At low speeds, *no* locomotive can put all of it's horsepower down, because adhesion and tractive effort limits won't allow it. It will therefore be limited by tractive effort and possibly adhesion. Modern freight locos tend to have mechanisms to extend adhesion well beyond the limits of older types, but there is still a limit. Locos with AC traction motors tend to have very high tractive effort, when sufficient adhesion permits its use.

Only at higher speeds does horsepower come into play, preventing the loco from putting maximum tractive effort down. A Deltic - with an unusually high power-to-tractive-effort ratio - will enter this regime at 32.5 mph. The Class 70 has AC motors, so horsepower will become the limiting factor earlier (and at higher effort).

A quick calculation suggests that the 70 can maintain at least 155kN up to about 35 mph, and for most of the time up to that point, *much* more than that value. Another quick calculation suggests that 155kN is about 12% adhesion, which is maybe two-fifths or a quarter of the plausible limit on dry rail. At 75mph, the tractive effort falls to about 75kN, a good deal of which will presumably be consumed by various drag factors. So overall, it looks as though maintaining an average of 155kN over the entire acceleration sequence is not totally implausible, but not easily proven.

[ttjph]

10% too much power - usually about 82% efficiency for older DE, I guess maybe a couple more these days, but that would( if you do it another way ) need an average TE of 155kN, without drag and so on. That would be remarkable

Yes, I know - I was just providing a very optimistic, simplified estimate of the absolute maximum you could expect from a reasonably representative modern freight consist.

Also, I was quoted some efficiency figures recently by someone in the business (apparently he was involved in the electrical design of the Class 60) and they were rather higher than the 90%×90% I'd always assumed; so given that the Class 70 is an extremely modern AC-AC loco with a focus on efficiency, I went for a higher figure to give a 'best case'.

Starting tractive effort is listed on Wikipedia as 534 kN, giving full power around 10-11 mph! I'm starting to like the 70 - high-speed engine, high power-to-weight, high adhesion factor and thus starting TE; it's rather like a modern-day Western (only ugly)...

Interesting article if you haven't already seen it:
http://www.therailengineer.com/Featured ... r/view/157
"An example simulation revealed that, from a standing start, such a locomotive hauling 1,200 tonnes up the 1:75 gradient at Shap would be a mile ahead of a Class 66 after ten minutes."

[Kariban]

AC traction motor torque "curves" have all been flat lines at the start, for the ones I've seen - For 90s-onwards EMUs thats been a flat line to quite high speeds. Understandable given how they work, though.

Mechanically it's fantastic... just why can't rollingstock companies employ designers anymore!

[Torque55]

I tend to post a little to quickly I suppose, my apologies for that but I honestly thought that the train in my first post of this thread was fast.
Considering all the comments and various explanations it seems that the trains could go even faster at certain circumstances.
Thanks again all for all your inputs and thoughts and constructive replies. Also I got the various links and other video's from various posters in this thread, very nice ! thanks !

[Kariban]

One other thing it made me wonder was what drawbar pull screw-link couplings are rated for these days. The first round of diesels could give more than the 40-44 thousand lbs a 3 link can handle, which I guess is why we ended up with so many mixed-traffic types.