Steam & Excursion > Steam vs' diesel tractive effort

I know this dead horse has been beaten enough, but output horsepower and tractive effort is different between steam and diesel. Diesel's have all their TE and start and steam has it a certain MPH. Does this explain how an alleghany steam locomotive could pull a 15,000 ton coal train with one engine and at would probably take more than one AC type diesel to pull it safely?

There are a lot more factors than just TE in your question. Grades on a line determined how much a steam engine could haul vs a totally gradeless line. The fact that a 2-6-6-6 has four cylinders helps it especially if they are not in time with each other (front engine vs rear engine).The power stroke of a cylinder is limited to two during the revolution of the driver for a full turn. And, during that turn, at the beginning it gets a strong push but, that push diminshes during the half turn of the driver. On most engines the cylinder got steam on 80 to 85% of the stroke, on others just 60%. The excessive weight of the 2-6-6-6 was another factor in how much it could pull vs two 2-6-0's. The speed required by the railroad is another factor.

Essentially, if a steam engine could START the train moving it could accelerate it to a speed in excess of what a diesel would do. A diesel could start the same train and not attain the speed of the steam engine. In addition, the diesel could start an even heavier train than the steam engine and keep crawling at a very low speed due to the CONTINUOUS tractive effort of the traction motors.

Another answer to the question: Cylinder horsepower in a steam engine boiler and cylinders is not equal to drawbar horsepower at speed. Generally as speed goes up so does the drawbar horsepower up to a certain speed and after that it declines. On diesels, say you have 2000 hp prime mover. There are horsepower losses in the generator, any auxiliaries (air pumps etc.), and traction motors that reduce the horsepower at the drawbar to less than 2000 hp.

I don't know if I have answered to your satisfaction or not but, I hope so.

The most important basic fact to remember when comparing steam locomotives to diesel electric locomotives is:

1) The steam locomotive is essentially a constant torque, variable horse power machine.

2) The diesel electric locomotive is just the opposite, i.e. variable torque but constant horse power (the slower it goes the higher the torque or TE).

Yes that helps a lot, lets say you have a GE ES44AC the HP rating is 4000 to 4400 roughly how much of that is getting down to the rails, forgetting about TE?

Pretty much all of that HP, above a few miles per hour. I'm not qualified to offer opinions on GE power, but I can tell you that I have been on three BN SD70MAC units with an 18,017 ton unit coal train (126 cars) at 5.4 MPH with each unit at full 4000 HP on Jan 7, 1998. As a general rule, AC traction diesel electric locomotive units can sometimes APPEAR to defy the laws of physics, under extreme slow speed conditions.

On a test performed on a Southern GP-38 and a GP-18 mu'd together with a rated horsepower of 3800 they discovered with the diesels wide open that they produced between 3200 to 3300 drawbar horsepower. Maybe that can give you some idea.

Getting back to steam what is amazing to me is, for example a lima berkshire had around 68,000 TE and was able to move pretty fast with the tonnage they put behind them. Their drivers being pretty much the normal size for a freight locomotive.



Edited 1 time(s). Last edit at 01/31/11 21:10 by daylightfan.

Also you have to remember because of the inverse horsepower curve, a steam locomotive can pull a train it cannot start by itself,but once she get's rolling,her horsepower also increases to a certain point. For example: let's take an medium sized 2-8-2,like SP 786,and pit her on level track against a GP9. The Mike has just a bit more horsepower,but less tractive effort than a GP9. With the same size train,I'll run off and leave a GP9 because the Mike has more horsepower.If you give the Geep maximum rated tonnage, and give the Mike a shove out of the yard, the results will be the same.. the Mike wins because of her ability to produce horsepower at speed. The Geep is locked in at her 1750 horsepower forever. Now if you cheat and pit her against two GP9's, the Mike loses because the combined tractive effort and slightly greater horsepower gives them the edge in a slugging contest. This is one of the reasons some railroads couldn't get diesels fast enough.

Like with anything new, I bet engineers didn't miss steam when they switched to diesel.

daylightfan Wrote:
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> Like with anything new, I bet engineers didn't
> miss steam when they switched to diesel.

I'll bet you're just about right! Some engineers today, mostly tourist roads, say they love the ole steamers but put 'em in the cab all day long on a hot summer day or cold winter's day (especially switching) and 99% of them will pick the diesel every time for obvious reasons. I've run both and that's my take on it. Also, if you're a mainline engineer you'll find a diesel rides like a passenger coach compared to a steam locomotive. And if you're running a coal burner glasses are a must unless you enjoy cinders in your eyes. I will say there's nothing quite like running a good steaming locomotive at 90+ mph, I had my turn with one of N&W's Class J's (#604).

Still steam draws the most fans though, no matter who you are. When I was watching 4449 on Blu-ray going through Chicago it was awesome to see that many people coming out to see her go buy. Makes all the hard work well worth it.

When we used to flat switch cars in the Army, the 2-8-0s could kick a car or a cut from a stop, better than any diesel.


Nudge

Remember those Lima ads in the late '40's: "When locomotives of 6,000, 8,000 even 10,000 horsepower are needed...STEAM is the only answer. We will CONTINUE to build such locomotives!!!"

"Big Jawn, Big Jawn, big bad Jawn".

This is a classic subject for discussion and there are many view points. The technical responses posted so far are all correct.
So let me add just some thoughts that will be of interest and may de-mystify the old arguments and confusion....

The first point has to do with the fact that DC traction motors, when spinning, become generators of electrical energy -- the technical term is "Back EMF".
Back EMF is short for Back Electro-Motive Force. Its is the bane of all DC motors, for at high armature RPMs, the voltage produced in the armature reaches the applied voltage---- ONLY ITS OF OPPOSITE POLARITY! The result is that Main Generator voltage appears to be high, but its not capable of applying beneficial magnetic attraction to propel the train. The Back EMF is an unavoidable result of a copper winding spinning inside a dense magnetic field. Thomas Edison's DC electricity works at low armature speeds, but at high rotational speeds, the Back EMF prevents any increase usable tractive effort.

Another troubling characteristic of DC motors and their commutators is that the sparking of the brushes rubbing on the copper commutator bars ionizes the air in the traction motor housing -- add very high traction voltage and BAM! You get destructive flash overs --both in the traction motors and in the main generator housing.
These lightning-like flashes are extremely destructive to the operating guts of the DC machines.

Another factor is reflected in the various gear ratios available for traction motor pinion gear/axle gear ratios. For passenger speed locos, 90-100 mph gear rations required limiting the maximum armature speeds, thus a lower ratio made passenger diesels struggle with freight trains.
AC traction is fascinating ... more later.

The Back EMF factor limited the early generations of locos, preceding the advent of AC traction, so that to get additional speed from DC machines many slaved units were required at speeds above 20-25mph. Ergo, the popularity of MU'd engine consists.

With electricity its easy to convert the Watts of electricity into Horse Power (James Watt's advertising language to sell his stationary steam power plants as he built his business). For SALES purposes, diesel manufacturers advertised their locos in 'catalogs' according to crankshaft horsepower. I think the formula is something like 747 Watts equals 1 HP. Couple the main generator's output to a known resistance, add a volt meter and an Ammeter, (rev up the engine), multiply the two and Voila' You get the Wattage output --divide the Watts by 747 and the result is Crankshaft power in Watts' "horsepower" equivalent. (see also Joules.)
The diesel locos also has tremendous fuel economies that hastened its quick takeover of the RRs rosters.

The old adage was true: A diesel will start with any train it can't run with, and a steamer will run with any train it can't START!
(There are stories of NKP 0-8-0s that were used to shove west bound Berk-powered freights out of Conneaut. [Ask Doyle]
Sometimes when over the crest west of Conneaut, the Berks would run so fast that the 0-8-0 couldn't go fast enough to get slack in the couplers to let go --- some were dragged to Cleveland before they could separate!).

Now, the original question was: what is the best way to compare the HP produced by the two types of propulsion. Its not an easy question because the weak first generation diesels were essentially slow speed, drag service locos. Their maximum power was developed at about 12 mph. A train that encountered a small hill would loose speed all the way to 12 mph, then it would drag the train at that speed until the traction motors overheated.

STEAM!
Modern steam locos with large grate areas staggered at low driver RPMs (low speeds) until about 18-23 MPH -- then the cylinder events per revolution were rapid enough to accelerate heavy trains to high speeds. The faster the loco went, the hotter the fire, the hotter the fire the hotter the steam, the hotter the steam, the less water was needed and soon the locos were sailing along with the freight cars merrily in tow -- watch movies of the NKP's fast freights, they're fascinating.

The problem: High speed freights soon arrived at their destination yards with no receiving tracks open! Then they would wait for an open track. Steamers don't 'idle' well! Only a few RRs were well run enough to always have open yard tracks for incoming freights.

Diesels, however, idle very well, even un-manned -- therefore, they became popular with the Operating departments. Eventually, the Mechanical departments saw that they needed only a couple of union 'crafts' to keep the diesels running -- this realiztion led to the extermely fast transition to diesel locos (helped by the fact that the steam fleet was worn out by extreme heavy traffic of WW2). Another factor was the ability of Mechanical departments to decimate the ranks of workers that formerly kept the steamers going ... that was the real battle that the diesels won!

So High speed horse power is what moves trains FAST, low speed horsepower is what drags them out of the yards!

AC traction...
Today's AC power locos use 3 phase, induction powered, traction motors to convey the crankshaft HP right to the wheels! The AC frequency is computer generated to be a few cycles ahead of the rotating armature copper bars --- this produces a very slowly rotating magnetic field around the entire armature. As the wheel/armature drive picks up rotational speed, the synthetic frequency generator increase its field rotational speed to keep the armature bars just slightly behind the rotating field. The result is a DC-like, low speed, magnetic-attraction- field even though the track speed may be very high.

The result is that the diesel crankshaft is virtually driving the wheels like a "mechanical" transmission. This is the desired result, over the ages, of all locomotive designers. The challenge, going forward, will be to provide enough fuel to meet the huge HP AC locos coming down the pike -- diesels with tenders!
These monsters will have large diameter drivers -- 58 inches to 72 inch diameters and huge traction motors. Maybe even 2-C+C-2 diesels with two traction motors per axle, or even 2-D+D-2 monsters!

Watt became a student of electricity after he mastered steam, just as George Westinghouse mastered electricity after he graduated from the air brake (he partnered with Nicola Tesla and led their AC domination of the commercial power field).
Edison's DC world met the same fate as the current existing DC diesels will meet -- save for the battery powered Green Goats, etc.

Wes Camp

I read an article where the 765 did this with a GP 38. They had the diesel in behind the 765 for help on a grade otherwise the diesel just went for a ride. On the return trip they ran the diesel up to run 8 and just gave 765 enough steam to keep her lubricated. The speed went up to 35 MPH and that is as fast as the diesel could go. This went on for several miles when the road foreman got concerned about the train being late. The engineer decided to end this experiment, with the road foreman's concerns, and brought the 765's throttle out and shut off the GP 38. With 765 now pulling the train, the speed increased to the track speed of 50 MPH. The 765 and the GP 38 diesel are a close match as far as tractive effort but the 765 could pull the train faster. I think if they wanted they could have took that train to 80MPH.
I read this about 5 years ago on a web site that I linked to from the Fort Wayne web site. I apologize for any major errors, I can't find the web site again. There were two other interesting articles there. One when they were on the CNW, the other when they were on Conrail Buffalo to Corning.

Enginecrew Wrote:
-------------------------------------------------------
> daylightfan Wrote:
> --------------------------------------------------
> -----
> > Like with anything new, I bet engineers didn't
> > miss steam when they switched to diesel.
>
> I'll bet you're just about right! Some engineers
> today, mostly tourist roads, say they love the ole
> steamers but put 'em in the cab all day long on a
> hot summer day or cold winter's day (especially
> switching) and 99% of them will pick the diesel
> every time for obvious reasons. I've run both and
> that's my take on it. Also, if you're a mainline
> engineer you'll find a diesel rides like a
> passenger coach compared to a steam locomotive.
> And if you're running a coal burner glasses are a
> must unless you enjoy cinders in your eyes. I will
> say there's nothing quite like running a good
> steaming locomotive at 90+ mph, I had my turn with
> one of N&W's Class J's (#604).


Well...I'd have to say that I enjoy running both,for different reasons. I've handled 5,000 ton freights with steam and 15,000 ton freights with diesels,and the diference really between the two is...to get the most out of a steam locomotive,you have to be an ENGINEER. To get the most out of a diesel,you open the throttle.That may be a bit simplified,but that is the way I see it. I've run steam an entire year on work trains in the summer,tourist trains, switching,and whatever else needed to be done, in all kinds of weather...and I would do it again in a heartbeat.

Aka the difference between the head end CREW and the current head end crew, nothing against a good
diesel engineer but the fireman in steam days was just as important as the engineer. I've single man hostled
steam and it's a pain, a good fireman earned his keep.

A follow up to the technical discussions....

While HICO was restoring the NKP #759 to operation in 1968, a retired NKP engineer/RFE (Wesley Brown?) toured our efforts at the Conneaut roundhouse and brought out his 1940's note book made during the EMD F7 sales demonstrations on the NKP (1947-48?). Brown showed me that their tests established that a Berk could outperform a 3 unit combo of (blue & silver) F7s; however, a four unit F7 consist outperformed a single Berk -- both types of motive power handling similar heavy-duty trains.

We were told that the report prepared for management ---calculating the anticipated growth of NKP freight traffic-- recommended that the 'demands of the service' could be met with an order for either 11 S2-type Berks or 44 F7 locos (A & B units mixed). A steamer at that time cost the price of a single-unit F7 diesel.
The Diesel order could have cost 4 times the expense of the steamers, not including the costs of constructing entirely new service facilities at ALL the major cities, etc. The NKP managers made a wise decision at that time -- as 1960 approached a different strategy was needed.

As I recall a single F7 was rated at 1500 HP, three units would be 4500 HP. This means that a well drafted and well fired Berk, at full-cry,
was capable of more than 4500 'diesel equivalent' tractive HP -- as good a comparison as we have to work with (and this was confirmed by the entire series of side-by-side tests performed on trackage all over the NKP system -- EMD lost the order!).

The NKP subsequently ordered 10 LIMA Berks, in 1947, and they held off the more expensive diesels for 10 long years... the 779 is the last of the greats.
The NKP Operating department knew how to RUN a railroad -- lots of High Speed freights (about 80 cars), single track Mains, lots of LOOONG passing sidings and open tracks in the receiving yard at the destinations (Imagine a "flying meet" at better than 65-per --for each train-- at a passing siding fitted with "high speed" switches).

Doyle's dad was a retired NKP Dispatcher at Conneaut --and he told us many stories of the every-day high speed meets that he would set up and watch on the CTC board as occupancy lights marched towards each other, one train entering the high speed sidings, passed each other and proceed of the opposite ends of the sidings and off CTC boards --all at FULL SPEED! Such total teamwork by the entire work force is hard to maintain --year-after-year.

In the rest of the industry, the slow speed, drag freight, DC diesels allowed the yard masters to build huge trains in the class yards, order the crews for late in the day, double the cars from 6 to 7 departure yard tracks together, grab 6 & 7 diesel MU consists and watch them struggle --- at 12 mph out of the yards--- with more train than they could run with!

Together with the 'Wreck of The Penn-Central', the demise of the freight traffic (and slow speed running) ALMOST put the RRs out of business, save for the efforts of Rep. Harley O. Staggers (D-WV, deceased) and his successful drive to get the 4-R Act made into law in the early 1970s (which largely deregulated the RRs [end of the ICC] and provided for more favorable RR tax treatments which aided the RRs competitive efforts).

The 759 was the last 'overhauled' NKP steamer in 1958. And today she still is the favorite of many fans.
The 765 is also a wonderful Queen --its so good to hear about her continued successes and the Can-Do boys of the Ft. Wayne preservation efforts!
Now, lets string together 50 Passneger cars and listen to her BARK as shes reaches for 70-per!!

Roll-On, Beautiful Berks!

Wes Camp.

I don't know about putting together 50 cars to run at 70, but we may be able to gather together about 20 cars some day soon and run them at 70. Keep your eyes on our web site.

I'm looking forward to hearing the rapid-fire machine gun of an NKP Berk running 70 mph at about 22% cutoff.