The importance of horespower in locomotives?

Trains Magazine General Discussion
1

I am “beginning” to understand some of the principles of horsepower in diesle locomotives. My understanding is that locomotives, especially when carrying heavy loads, have to excelorate to a certain speed before they may apply all of their horsepower–mostly due to wheel slippage.

This concept was really brought home to me when reading about steam locomotives with high drivers and how railroads would buy high-drivered steam locomotives with high horse power and use them in manners that rarely, if ever, allowed them to use all of that horsepower. As most avid readers of Trains probably know, such railroads would have been better off with cheaper low-drivered locomotives–or at least using them in fast freight service.

Altough to a far lesser extent–my understanding is–this also applies to diesles–i.e. they may not apply all of their horsepower when traveling at low speeds, carrying heavy load, and/or going up grades?

My question is, is there “any” use for the entirety of 4400hp in a locomotive when the locomotive is only intended to be used in heavy-haul, low speed, grade service?

Does A.C. power change this equation at all?

Rio Grande’s SD-45s come to mind. Were they able to use all that horsepower, or was it an unnecessary expense that would have been better spent in buying more SD-40-2s?

Please enlighten me.

Thanks,

Gabe Hawkins

2

This has been discussed at length in past forums. Look through the post regarding HP vs Tractive effort and steam vs diesil HP vs tractive effort
Randy

3

Brief recap:

Most diesels (note sp, all) can’t put full hp to the rail (in the form of drawbar-pull-at-a-particular-speed) below a given speed, which Randy knows precisely for each of his classes of locomotive. This is an adhesion and traction-motor cooling problem. AC diesels can achieve full hp at a lower speed, but not (contrary to the gist of some locomotive advertising) all the way down to locked-rotor stall. Corona-discharge numbers (e.g., off the point on top of my head) – 11mph for DC, about 5mph for AC, don’t know differential for 6-inverter GEs vs. 2-inverter EMDs.

Conversely, at high speed the diesel engine starts to run out of shaft horsepower to overcome train resistance and losses. This is independent of the motor/wheel gear ratio, the back EMF at fixed field in DC traction motors, and any rotational speed limits on DC motors due to problems with brush & commutator integrity or birdsnesting, etc.

The principal reason for extremely high horsepower in a single locomotive is for power at speed, against train resistance. Many services today can operate at sustained speed just above transition, where imho the full propulsion rating of the diesel engine can be used in the transmission – again, Randy can give you more detail. But it’s a waste (in a variety of operational ways) to use a 6000hp diesel for this kind of service – or a 4000+hp B-B diesel.

I think the difference in hp between Rio Grande’s SD45s and SD40s may not be the determinant. First, did the extra hp translate into added train capacity, or meaningfully faster times? Did the fuel consumption of the bigger engine work out more effectively in the service the railroad used them? Perhaps most importantly, was the availability and maintenance expense of the 45s proportionally better or worse… on a unit-for-unit basis adjusted for ton-miles produced… than alternatives?

4

The Alaska Railroad commonly uses its SD70MACs on the passenger train to Fairbanks.Since AC traction is best suited to heavy drag freights,it seems a waste to use them in this service,when two GP40-2s would seem to be more effecient.The SD70 MACs are best used on their coal trains.

5

Note that there were a fair number of railroads that stuck with WWI era steam designs until dieselization, including the Southern and when you come to think of it, also the PRR. The “J” Texas types were bought off the shelf and not a Pennsy design. If they hd their druthers they might have bought a modernized 2-10-0, but war restrictions forced them to use an existing design. The T-1 was, of course, a failure and went to scrap long before the last K-4 (or E-6 or H-9 for that matter!). I also found it interesting that the J-2 Boston and Albany Hudsons were snapped up by other parts of the Central, when the B&A became the first dieslized segment of the NYC system. They had lower drivers than the J-1’s and J-3’s. Some were re-equipped with the larger tenders of the other J’s. Again, it was a matter of being able to use the horsepower at lower speeds.

6

There are two basic measures of a locomotive’s performance. One is the maximum tractive effort that is available on an everyday, all weather basis. This will tell you how many tons you can take up a grade without stalling. So, for any given route, you can calculate the drag rating (you need 20 lbs of TE per ton per percent of grade). Horsepower is irrelevant here.

The other measures is horsepower. This determines your running time. More HP/ton, the faster you go.

Consider one SD70MAC versus two SD40s. Roughly the same TE rating (about 140,000#). But the two SD40s total 6000 HP versus 4000 for the SD70.

Now, we have a 3500 ton train to move up a 2% grade. At 20#/ton/%grade, you need 140,000# TE, so either consist can do the job. The difference would be that the SD70MAC will only make about 8 mph versus 12 for the SD40s.

On any railroad route from A to B there is a ruling grade where you must satisfy the TE demand and a schedule where you must satisfy the HP demand. Doing so, you wind up with a certain ratio of HP and TE you need in a locomotive. Buying too much TE or HP tends to cost you more, altough the TE generally comes cheaper, in the long run, than the HP because there is no fuel penalty for having too much TE on a train.

If the RR on the whole has gentle grades and demanding schedules, you’ll find a lot of high horsepower four axle locomtives. The NYC comes to mind here. If you have stiff grades and mostly bulk traffic, you’ll tend to see lower HP six axles. The L&N comes to mind. I’m sure others can come up with better examples.

Working “backward”, let’s ask on which kind of route would an AC4400 make a useful intermodal locomotive. Assuming you need 2 HP/ton for your intermodal trains, a pair of AC4400s could handle 4400 tons. With 280,000# TE, they could handle a grade up to 3.2%. There aren’t many intermodal routes with 3.2% ruling grades, so an AC4400 is not a good fit for this service. A pair of Dash9

7

oltmmand:
Two exceptions to your post come to mind:

Chessie System (C&O/B&O/WM) used heavy 4 axle locomotives (GP40/GP40-2) in heavy hual service -quite successfully- on some of the most severe mountain terrain in the country for decades. Above 13-14 mph a SD40 has no advantage in pull over a GP40, and they didn’t feel the added expense of the six axle SD was justified. SD’s were limited to mostly helper service.

On the other hand today, CSX will use its big AC4400s on everything from coal to high speed intermodal. AC’s are not just for slow speed! The AC drive is more efficient at speed than DC drive, so intermodal work is a natural fit for AC’s. The big turn off for ACs on other railroads for intermodal is for the reason you stated (i.e. the higher initial price of AC vs. DC locomotives)

When EMD and GE get the price levels more equal- and they will- you will start to see fewer and fewer new DCs being produced.

Remember, the high tractive effort of CSX’s AC4400s (rated at 180,000 lbs continious now) is not “wasted” at high speeds. The reason they can produce so much tractive effort at low speeds is due to their AC drive, and software upgrades. They produce the 180,000 lbs of pull because they can run at full power at such a low speed. At high speeds, they actually produce more drawbar HP than a C44-9 because of the inherent efficiency of the AC drive vs. DC drive.

More TE at low speed, more HP at high speed = a better all around locomotive IF you do not mind paying the current higher price than for a C44-9.

8

espeefoamer:
The two GP40-2 you used in your example would use a lot more fuel that 1 modern SD70MAC in this service, especially when 1 SD70MAC has more that enough HP to pull a lightweight passenger train.

9

Gabe said:

“My question is, is there “any” use for the entirety of 4400hp in a locomotive when the locomotive is only intended to be used in heavy-haul, low speed, grade service?”

Yes! It’s the 4400 hp that is producing the high pull at low speed. (Assuming you have the adhesion to put that power to the rail) CSX’s AC4400s can produce their full continious HP as low as 8 mph.

10

A couple of questions. On Diesels, there is all that gee-whiz computer-controlled high traction stuff – the ground radar, the wheel creep, and all that. How does all that high tech traction control work when you have wet rail or leaves on the track? Yeah, yeah, sand the rails, but there are still adverse traction conditions. In terms of adhesion numbers there are all kinds of answers, but one reason for rating locomotives for lower adhesion is to allow for Murphy’s law that the track conditions are never perfect and you don’t want a train stalled on a grade because someone was too optimistic.

On steam, what is the story with all of those idler axles – the 4-8-4. I suppose the trailing axles allowed for a bigger firebox – more steam and all of that. What did the 4-wheel leading truck do? Was a 2-8-2 relegated to slower speed than a 4-8-4?

I was browsing the Web at the Ultimate Steam Page and Porta was promoting a 2-10-0 design – more traction axles. None of this ACE-style recreation of a Pennsy T-1 duplex with interconnected driver pairs. It was this odd looking deal – this really skinny boiler mounted up high: that way the firebox could fit over driving axles. I guess Porta felt he could balance that large number of small-diameter drivers.

Oh, and what is the proper adhesion comparison between Diesels and steam? In the pre-electronic era, Diesels were not only afflicted with DC traction motor short-time ratings and minimum speeds. Each wheel had its own motor so if one wheel spun you were slipping. Steam (apart from Mallets and Pennsy duplexes) has all of the drivers coupled, so you would think the adhesion would be higher (averaged adhesion not worst-case of slipperiest wheel) – this was supposed to be the case with the Diesel hydraulics. But then it is said that the pulsed nature of the reciprocating engine worked against adhesion. What was the real story?

11

Every body summed it up pretty good. The bottom line is you want to get em moving, accelerate to track speed and keep em there. You need HP if you want to get em over the road. Remember that amps = torque… Volts =speed, the wheels are gonna slip when?
The biggest advantage deisels had over steam as far as physics is weight on drivers. Steam locomotives did not carry 100% of their weight on the drivers. Even prior to the electronic age diesel locomotives had a wheel slip system. they were simple yet effective through cable transductors and voltage dividing bridge circuits. The technology was trolley car tech.
Randy

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Although the title does not suggest HP or TE is discussed, nor were they the original topic, the following thread diverged into this topic.

http://www.trains.com/community/forum/topic.asp?page=2&TOPIC_ID=19048

13

Two items:

Fisrt, the “break even” speed between a GP40 and and SD40 is around 18 mph - don’t be fooled by the presence of performance control/power matching! Is suspect there were other issues at play - such as track forces/curvature and/or a desire for a “universal” locomotive. If all you want to do is drag tonnage up a hill,

14

M.W., Your comments about Chessie wanting a “universal” locomotive are correct. It was EMD’s own claim the the GP and SD 40-2 were equal locomotives above 13mph, however, if it was 18 as you state I don’t think it would of changed Chessie’s locomotive policies in the 1970’s when they amassed their large fleet of GP40-2s. Chessie’s over the road speed, even on branches and secondary lines was well above the break even point you stated. Even on the mountain grades, they tried to keep the speed over 20 mph, except on the heaviest drag trains. Mainline general frieght usually ran at 50-55 mph, while intermodal at 60-70mph. At those speeds, it made no sense to have a fleet of SD40-2, other than for helper service. Of course, with the current management of CSX seeming to like to run trains as slow and underpowered as possible (I have seen 20,000 ton trains with only 2 AC4400’s on the point!!) , the 4 axle locomotive really doesn’t fit the picture anymore. However, I firmly believe that as the price between AC and DC locomotives becomes more equal, you will see most, if not all railroads adopt the AC as the “univeral” locomotive.

Paul, steam locomotives, even with their interconnected axles had lower “real world” adhesion levels even than the first generation diesels. It has to do with the fact that the steam does not enter the pistons in a uniform manner, along with the geometry of the connecting rods. In a sense, it is like a jerking motion, which will cause the wheels to slip sooner. Power to an electric motor is applied in a very smooth and continious fashon, allowing for higher levels of adhesion. Add fancy electronics and wheel creep, and modern AC’s easily have 2 to 2.5 times the adhesion of the typical steam locomotive.

15

I was told once by a retired UP engineer that a SD40 could pull just as much over Cajon pass as an SD45.He also said that a 3000 HP GE could pull more than a 3000 HP EMD,when it’s working.

16

Would fuel consumption pay any part in 1 vs 2? Fuel vs Time?

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talbanese, something that matters is the specific fuel consumption of the diesel engine at part throttle. In just about every diesel engine, there’s a spot called the ‘torque peak’ which represents, essentially, the place where the engine is most effectively converting fuel to shaft horsepower. You can get the engine to make more power above that point, but only by burning proportionally more fuel.

So IF YOU DON’T INCLUDE COST OF CAPITAL

18

Overmod, Thanks! But what about two vs one and the time difference to climb the grade. If onr takes a few minutes longer, and in the example it was 4 minutes, is there a huge difference in fuel consumption to make up the time gap? I am asssume that 2 engine would burn more fuel? Or maybe not? Maybe one needs to burn more for the same HP output as 2?

Thanks!

19

talbanese, something that matters is the specific fuel consumption of the diesel engine at part throttle. In just about every diesel engine, there’s a spot called the ‘torque peak’ which represents, essentially, the place where the engine is most effectively converting fuel to shaft horsepower. You can get the engine to make more power above that point, but only by burning proportionally more fuel.

So IF YOU DON’T INCLUDE COST OF CAPITAL it can easily be more fuel-effective to use, say, three SD-40s instead of two SD-45s on a comparable train… and, perhaps needless to say, get greatly increased maintenance life out of the locomotives. If you’re running locomotives that have already been depreciated, it becomes almost definitive… now start looking at the differential cost of maintenance (including ‘surprise’ repairs or big-ticket majors).

I’ll save discussion of steam-locomotive adhesion for later.

20

M.W.;

If I remember correctly, the B&O West End was one of the few (only?) parts of the Chessie System where 6 axles were the power of choice for heavy drags. However, on general frieght and intermodal, where they could keep the speeds higher, the GP40-2 even ruled the infamous West End.

The GP40-2s definitely ruled the line to Pittsburgh over Sand Patch. That line had a high enough traffic that you needed to keep trains moving at a good clip. One slow or underpowered train could tie up several trains behind it. Usual power was 4 GP40-2s on the head end. Lighter trains, such as intermodal or autoracks, would take the east or west slope unassisted, while heavier trains would get a set of SD40-2 helpers (on the rear end only) for assistance over the summit. The line was good for 70 mph, and Chessie used it.

Concerning you comments about the performance control available on GP40-2s when MU’d to SDs: The one thing I never saw on Chessie was GP40-2s MU’d to SDs. GP40/GP40-2s always ran together, or were MU’d to other GPs.
Chessie loaded their GP40-2s to 280,000lbs for traction, and the engineers weren’t afraid to peg the amp meter on a hard climb. The way those locomotives would go into the red zone, leads me to believe that the performance control (HP reduction) feature was never activated. Its a testimony to EMD’s engineering of the 40 series that those things are still running on CSX!

In a sense, Chessie was a two fleet railroad too: SDs for dedicated helper service and dedicated drag service in severe terrain, and GP’s for every thing else.