Diesel vs steam: power at speed

[fredswain]

There are 2 things that need to be known to determine which can pull what amount. Those 2 things are TE (tractive effort) and Limit of Adhesion. Neither on their own tells us anything useful as you can have all the tractive effort in the world but no adhesion and you will go nowhere. You can also have all the adhesion in the world but without tractive effort you don't have the necessary force required to pull the train. Therefore you need to know both. This is easy.

Weight on drive wheels / Tractive effort = Limit of Adhesion

To figure out TE, we need to look at steam and diesel/electric separately. To figure out TE of a steam engine we need to know:

Boiler working pressure, piston bore, piston stroke, drive wheel diameter, and then we need a constant that allows for losses incurred in the real world.

To figure out TE of a diesel/electric engine, we need to know:

The torque of the traction motors, the gearing, and the diameter of the wheels. The power of the generator is not a factor.

Notice that I haven't listed weight on driver's as being important in figuring TE. That's because it isn't. TE is Tractive EFFORT. Effort doesn't mean traction. It just means available force. We need to use weight on drivers in conjunction with TE to determine traction or limit of adhesion. Even then we are trying to determine a theoretical maximum limit of adhesion. Real world can be very different and hence the reason for technology to control adhesion more effectively.

You quoted me as saying "hp per hp, the steam engine still passes the d/e in tractive effort quite early in speed." This statement is in fact true. However if you don't think this is so it is because you are forgetting limit of adhesion. Remember adhesion is traction. Effort isn't. Just because tractive effort on one is higher doesn't mean it can necessarily pull more. You need to also know adhesion. Keep in mind my above statement could in fact also be made false. You need to change all of the parameters, most importantly wheel diameter. Play with some numbers, you'll see it. Generally speaking though, with the way in which each engine is typically designed with their typical wheel size differences with steam engines getting larger wheels, that statement does generally hold true but it can be made to go the other way. We just haven't designed an engine that way yet.

Most of the argument that went back and forth only concentrated on TE and there was nothing pertaining to adhesion. You need to know both for there to be any comparison. Without adhesion, the argument didn't tell us anything useful from either side.

I wasn't going to get into the steam being more efficient overall argument because I do have an opinion on it but don't want to get into it. Needless to say, it's not coming back so whether or not we feel it could or not, it won't. Can and will are also 2 totally different things. 

The ultimate engine would in fact be steam. It would be a closed loop nuclear heated steam engine much like submarines and aircraft carriers. The steam generated would spin a steam turbine which would turn a generator which would then power the traction motors in the wheels so really it would be a nuclear hybrid of current engine and steam engine technology with a twist.

[timz]

The "can steam make a comeback" thread has included the inevitable confusion about the supposed advantage the conventional steam locomotive has over a diesel locomotive at, say, 50-60 mph and up.
 
"...ton for ton a steam locomotive can and does pull more tons at higher speeds than a diesel locomotive can."
 
"The H.P. of a diesel is less effective at faster speeds because more electricity is needed to keep the traction motors spinning at the higher speeds."
 
"hp per hp, the steam engine still passes the d/e in tractive effort quite early in speed."
 
"The tractive effort of a steam locomotive increases as the speed increases..."
 
"This is exactly the opposite of what is clearly shown by the published and generally accepted TE/HP curves for the respective motive power types." [The "this" that he was responding to was "the Diesel should always pull at least as much tonnage as a steam engine over the entire practical speed range for a given maximum dbhp."]

"At 19 mph, a reciprocating Steam engine that is putting out 5600 hp will generate 50% more TE than a Diesel-electric locomotive(s) generating 5600 hp at that speed."
 
In the last one he probably didn't say what he intended to say, and in the next-to-last he must have misread the other guy's claim, but it all adds to the confusion. This spinoff thread is an attempt to straighten out this one aspect of the argument-- no discussion here about fuel costs or capital costs or air pollution or mining practices.
 
Start with the diesel locomotive. The diesel itself (the prime mover) is happy to produce full horsepower at any locomotive speed, and clearly our aim in designing the locomotive is to arrange a transmission that can transmit that full power to the wheels over a range of speeds, the wider the better. As with any other locomotive, the rail-horsepower-vs-speed curve for a diesel starts at zero at zero mph, and it can only climb so fast as speed increases. But eventually it levels off and, hopefully, remains about constant up to the locomotive's top speed.
 
As it happens I have EMD's graph for the 57:20-geared SDP40F, which (they say) reaches 2500 rail horsepower at 17 mph and 2600 at 25 mph; it's supposed to maintain that to 100 mph. Peak is 2700 rail horsepower from 50 to 95 mph. No reason to think a 62:15 SD40-2 wouldn't do the same, with all the speeds multiplied by 0.69. Some fraction of the road diesels out there couldn't do as well-- it seems high-hp-per-axle diesels like the GP50 (and GP60?) and DDA40X only maintain full rail power up to maybe 90% of their maximum speeds, due to voltage limits, and some? many? all? early C-Cs never got out of series-parallel, so for all we know they did likewise. But plain-vanilla road B-Bs could probably maintain full power pretty well; I doubt anyone can find a power-vs-speed curve for an F3/F7 or RS3 that shows otherwise.
 
The diesel's drawbar horsepower (that is, whatever remains of the rail horsepower after subtracting enough power to move the locomotive itself) will presumably peak somewhere around 20-30 mph, so it isn't 100% true that "for a given maximum dbhp" steam and diesel pull the same-- a 5000-maximum-dbhp diesel will be slightly below 5000 at 45 mph (or whatever speed the steamer reaches its 5000 dbhp peak). As speed increases further the steam advantage may increase slightly for a few mph -- its rail horsepower may still be increasing, while the diesel's is about constant. Eventually the steamer's rail hp will start decreasing, but no way to find out how soon and how much except by trying it-- and in this country that kind of data has always been hard to find.
 
Which brings us to the N&W 2-6+6-4. The claim first appeared in 1936: N&W said an A had pulled 7500 tons (car count not given) at 64 mph on "comparatively level track". Nobody knows how much diesel horsepower we would need to match that, but hard to imagine it would be less than 15000 (on the level)-- probably 12000+ at the drawbar.

N&W originally claimed 6300 dbhp from the A at 45 mph; their graph stopped at 60 mph, but it suggested maybe 5600 dbhp at 64 mph. (Reportedly they later revised those downward a bit.) So people are naturally mystified: if the A can do 64 mph with less than 6000 dbhp, why can't the diesels?

Say the A really did make 64 mph on the level exerting 5600 dbhp-- i.e. 33000 lb of drawbar pull at that speed. That's 4.4 pounds per trailing ton, the same force we'd get by placing the train (without the engine) on a 0.22% downgrade and releasing the brakes. Like I said, nobody knows for sure, but-- anybody think such a train would accelerate to 64 mph?