Steam with Diesel helper

[yankee flyer]

 
Good morning.

I'm trying to understand the need for a diesel behind steam locos to, as some have said to "maintain track speed". Does the actual power or torque of a steam loco fall off as speed is increased? I assume the capacity of boiler output would be one factor. How about the time for steam expansion in the driver piston? Do they pull like an electric motor with max torque at stall speed?

Happy railroading.

Lee

[selector]

There are several reasons for the diesel, and almost always they are mandated by insurance and/or the host railway's policies/dicta if the steamer is using rails provided by another commercial carrier.

Often the diesel is just there to provide electrical power to the consist (air conditioning, lighting), sometimes it helps with dynamic braking on hills, and sometimes it actually assists on severe grades.  Of course, if the steamer should experience a failure and have to cool the boiler, the diesel would presumably be capable of handling the stand-in function to get the train off the tracks at some point further along.

One thing is certain: the diesel isn't there either for the show or to ruin a good photo op.  

-Crandell

[4merroad4man]

If you are speaking in terms of 1940's and 50's railroading, the lead locomotive was often a helper engine.  So if the diesel was against the train, with a steam locomotive on the point, the steamer was the helper. 

Steam locomotive assignments were frequently downgraded as more and more diesels were delivered.  One final assignment for many steam locomotives on many roads were as helper engines in heavy grade territories.  The helper assignments kept the aging and maintenance hungry locomotives near the shops that serviced them.  Adding them to trains allowed the railroad to maximize train speed on heavy grades when there simply were not enough diesels on the roster to do the job adequately. 

In passenger service, the steam locomotives were used as helpers to maintain track speed and assist in accelerating heavy trains. 

While diesels had a much better factor of adhesion, making them superior at low speeds, frequent over-tonnage conditions required the use of helpers.  Steam locomotives just happened to be the choice of so many roads at the end of steam.

If you refer to modern day excursions, a diesel will be used to provide HEP or "hotel power" for the trains' lights, etc. if so equipped.  Since many modern day railroad managers simply do not trust machinery that is well in advance of 70 years old in some instances, they add a diesel or two in case the steam locomotive breaks down, thus allowing operating personnel to use the diesels to keep the train moving and keep the railroad fluid.  Diesel are also present, in many cases to accelerate the train and provide dynamic braking, since a steam locomotive's greatest consumption of water and fuel is in accelerating and stretch braking.  Conserving water and fuel allows the train to make better time with fewer stops.

[henry6]

Are you referring to modern day railfan and publicity excursions or back in the day when diesels were coming on replacing steam?   Back in the day, it was because the steam may have been the helper and would be pulled off at some point.  And it, of course, was dependent upon which was the road engine and which was the helper.  But for today's purposes it is so that the steam locomotive can be photographed unelipsed by the diesel!

[tomikawaTT]

Back when, the Louisville and Nashville purchased 2-8-4 road locos for a newly-upgraded line, and provided 4-unit covered wagons for helpers on the worst of the adverse grades.  Sometimes the diesel really is a helper.

In present-day excursion service, the diesel is there to provide HEP to passenger cars that are no longer fitted with steam-powered accessories, to provide the tractive effort so that a smallish steamer can move a large-ish train over heavily graded rails and as insurance in case the steamer curls up and dies.

The Clinchfield used to run a rather small steam loco, with a pair of cabless covered wagons painted to resemble baggage cars, on excursion trains well beyond that loco's tonnage rating.  A homemade control stand in the cab allowed one engineer to run both forms of power.

Chuck

[yankee flyer]

 Thanks for all the replys

yankee flyer

. Does the actual power or torque of a steam loco fall off as speed is increased? I assume the capacity of boiler output would be one factor. How about the time for steam expansion in the driver piston? Do they pull like an electric motor with max torque at stall speed?

My secondary question was, where is the steam engines power band? Is their max torque at start up ( like an electric  motor) and then taper off as they build speed, Or does it peak some where in the middle like a gas engine?
My Uncle was a steam and diesel engineer but I was too young to ask him good questions before he died.

 

Lee

 

[timz]

yankee flyer

where is the steam engines power band?

A steam locomotive (like a diesel-electric or an electric-electric) exerts its greatest pulling force at zero miles/hour, or close to it. Like any other locomotive, its horsepower at the wheelrims is zero at zero speed and climbs to a maximum at 20 or 30 or 40 mph-- but pulling force always decreases as speed increases.

But diesel locomotives with mechanical transmissions used to be common in England (and elsewhere?) and maybe they're an exception to that.

[henry6]

I believe you are referring to what is called tractive effort.

[De Luxe]

Here you have a video of my HO scale SP Alco PA-1 A-A Unit, being helped by a SP GS-4 4-8-4 steam loco on the 13 heavyweight car Pacific Limited Chicago-Oakland. 

Video

By the way: In my video this doubleheading action is just for fun. The PA-1 can handle the 13 without any problems.

[blue streak 1]

The positioning of Diesels and steamers gets to be complicated.

1. What is the labor operationg agreement on that particular RR and maybe even that division? Does the diesel helper require a fireman at all times? If no fireman required when helping behind a steam loco is one reequired if leading or if deadheading back to helper station? Can another crewman say front brakeman substitute for the fireman when helping the steamer? Does he get fireman's pay?

2. .Also no steamers of that era had diesel controls so an engineer was required to run the diesel consist.

3. On a passenger run did the diesel consist have steam generator(s) installed?. Did diesel if no generator have pass through steam lines and the extra signal air line?

4. So all those factors have to be taken in account when placing a diesel in front of a steamer.

5. If diesel or steamer helper runs a whole crew district what crewmen are required?

6. Get the idea? 

[wjstix]

yankee flyer

 Thanks for all the replys

yankee flyer

. Does the actual power or torque of a steam loco fall off as speed is increased? I assume the capacity of boiler output would be one factor. How about the time for steam expansion in the driver piston? Do they pull like an electric motor with max torque at stall speed?

My secondary question was, where is the steam engines power band? Is their max torque at start up ( like an electric  motor) and then taper off as they build speed, Or does it peak some where in the middle like a gas engine?
My Uncle was a steam and diesel engineer but I was too young to ask him good questions before he died.

 

Lee

 

My understanding was that a steam engine's power curve (or whatever the correct term is) is quite different from a diesel. A diesel has more power available at start up, which is why a diesel can often make a faster start with a train than a steam engine, which often takes a long time to get up to speed. I've read this can cause a situation where a diesel seemingly starts a train easily, but is unable to get it up to the desired speed, whereas a steam engine may just barely be able to start a heavy train at a slow crawl, yet is able to continue to speed up until it's going very fast.

[henry6]

I think if you will compare the tractive effort statistics per unit and horsepower, you will get your answer.

[selector]

Steamers exert their maximum drawbar pull at the instant the drivers start to turn from a standstill.  As the speed rises into the thirties and on up to the practical maximum speed of the type of engine (freight, fast freight, passenger, limited speed passenger), the drawbar pull falls, but the horsepower rises.  This depends on the engine, its driver diameter, its working pressure at the piston face, the surface area of the pistons, the porting shape and area at the side of the valves, it's adhesion on the rails, and so on.

Steamers share their weight distribution on the rails with truck wheels in the cases of modern road engines...as opposed to switchers of the 0-6/8-0 type.  Diesels place all their weight on all their axles, whether only four or six.  So diesels can start and accelerate a heavy train more effectively and efficiently than can a steamer due to their substantially greater adhesion on the rails.  As stated above, it is just the case that they can't always remain at an effective/efficient speed unless more units are added.  Conversely, steamers in some cases had the need of assistance in order to start heavy trains, but once they got them above 25-35 mph, they were usually able to sustain a good clip if the grades were kept under well under 1%.  Once you approach 1% and beyond, your power needs to keep the train moving double, then triple, and so on.

Most of the pre-war and later production steam engines produced upwards of 5500 dbhp above about 45 mph if they were of the Mountain, Northern, and specialized freight and passenger engines such as the Allegheny and T1 Duplex.  Diesels could not match that hp, so the early ones needed multiple units to make up the difference if they were to pull comparable consists at comparable speeds over the same routes as their steam counterparts.  It wasn't until the late 60's (I seem to recall reading..?) that a single diesel locomotive approached 5500 dbhp (someone help me out..).

-Crandell

[yankee flyer]

Crandell   

Excellent Just what I was looking for. I now have a better understanding.

One of my friends sent me an article about an pure electric car that has been blowing all comers away at the drag strip. The electrics acceleration is so great at the start that no one can catch it.  IIRC it's a 1972 Datsun with batteries in the back seat area.

Have a good day.  

Lee

[AgentKid]

selector

It wasn't until the late 60's (I seem to recall reading..?) that a single diesel locomotive approached 5500 dbhp (someone help me out..).

You are correct. I read several years ago where someone, I think it was an Engineering School, had taken the old circular and line graph's taken from the CPR Dynomometer car, that measured TE, and used modern computer technology to convert that to Drawbar Horsepower. The outcome? CPR Selkirk 2-10-4's had a dbhp of approximately 6000.

I also remember my father attending some function, or it was some kind of announcement in the mid seventies, that in the SD40-2, after several years in service, the CPR had finally found a diesel that could perform equivalently to a 5900. It had taken since 1949 when the last Selkirk was built. My father sure enjoyed that because he had worked on the Laggan sub as an operator at almost all the stations open then between 1947 and 1953. It really made his day.

AgentKid

 

[rrboomer]

On the Soo Line it was found that if a multi unit diesel helper was in the lead it could pull the wooden pilot beam off the front of a trailing steam engine.

[Deggesty]

rrboomer

On the Soo Line it was found that if a multi unit diesel helper was in the lead it could pull the wooden pilot beam off the front of a trailing steam engine.

That could be embarrassing. "And now, we start our steam excursion...oh, no!"

Johnny

[rrboomer]

Would be an unlikely issue on an excursion.  This would be on a full, or slightly over, tonnage freight train. Territory this happened was between Valley City and Enderlin, ND.

[dredmann]

You've gotten some good information, but to clarify a bit: there are two distinct properties that are of interest, tractive effort and power output.

 

Tractive effort is an approximation of how much force the locomotive can exert on the drawbar or coupler at the back of the tender (or for a Diesel, the back of the locomotive). It basically affects how heavy a train the locomotive can start in motion. With steam locomotives, tractive effort is at its maximum at a stop (or nearly so), stays relatively constant up to 5 ot 10 mph, then falls off after that. Depending on the details, a steam locomotive's tractive effort at 50 mph might be, say, 25% of what it is at 5 mph. This can be a problem even with a train already moving, e.g., when attempting to pull the train up the hill.

 

Power, on the other hand, is more related to moving trains at relatively high speeds. Power is the product of pulling force (exerted tractive effort, which will often be less than the maximum tractive effort at that speed) and speed (or arguably more technically, velocity). Power is 0 at a stop. Typically it increases to a maximum, in a steam locomotive often somewhere in the 20-to-40 mph range. After that it slopes off again.

 

As a practical matter, with steam locomotives, tractive effort was usually the limiting factor, as usually the locomotive could pull the train at the desired speed if it had sufficient tractive effort to get it rolling. On the other hand, with Diesel-electric locomotives, power is usually more important, because in most cases the Diesel can get the train rolling, but without sufficient power, it can only pull the train slowly.

 

There is some good stuff on these subjects in Prof. Lamb's book Perfecting the American Steam Locomotive. If you are interested in these issues and not afraid to wade into somewhat more technical details, I recommend it.

 

Dave

 

[Kootenay Central]

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[BerkshireSteam]

yankee flyer

Crandell   

Excellent Just what I was looking for. I now have a better understanding.

One of my friends sent me an article about an pure electric car that has been blowing all comers away at the drag strip. The electrics acceleration is so great at the start that no one can catch it.  IIRC it's a 1972 Datsun with batteries in the back seat area.

Have a good day.  

Lee

That went fast because of torque. As once said, something to the effect that if electric power was so great then every body in drag racing would be running geared-down drills. And not to through mud in your face just going to compare since this is a train forum, but there is also a diesel powered dragster out there that runs the 1/4 in the high 6's/low7's. They are currently working on getting it faster . As long as money is no option, you can quite easily go out and buy an older Dodge ram with the Cummings 5.9L turbo diesel and have it ramped up to around 800-900 hp and still be very street able. Torque? Go put a diaper on first. Torue in the particular truck I am thinking of is 1400 lb-ft. Just to compare, the typical 12-13L turbo-diesel in semi's makes around that much torque, mind you I am talking about a 5.9L. There was also a Ford that was juiced up to 1400hp, but it also had every possible power increaser made for Fords. NOS, twin turbos, methanol injection, power programer, semi-truck sized exhaust, aftermarket pistons and guts, cam, on lets see oh yea fuel injectors and fuel pump. Also on a side note your typical car/truck fuel pressure can go from 45psi (older chrysler) to 65psi with fuel injection. Typical direct injection modern diesel engines can see upwards of 23 000psi. Makes me wonder how much water pressure those water jet machines are at that can cut through 2 inches of solid steel...with water !

No what was this post for....oh yeah. With the steam engines, a valve controls the volume/pressure of steam that is allowed to go into the cylinders. Wouldn't the volume and pressure of the steam puffed into the cylinder determine how much, "tractive" power the engine was making? Because if I have that right then it should work that the more/high pressure steam put into the cylinder the higher the power.

[selector]

MILW-RODR

...With the steam engines, a valve controls the volume/pressure of steam that is allowed to go into the cylinders...

Not quite.  The valve uncovers and covers ports at the side of the sleeve, or cylinder, in which the valve slides back and forth.  When it uncovers an inlet ported to the dry-pipe, incoming steam pressure is governed by the pressure in the dry-pipe, not by what the valve lets in or out. The volume of gas is indeed controlled by the cut-off, or timing (the duration that the valve allows the port to be uncovered).  Pressure is essentially instantly to the max and then it begins to expand agains the only movable part presented to it...the piston face.  By then, ideally, the valve has closed the port again so that the volume admitted can do its expansive effort.

MILW-RODR

...Wouldn't the volume and pressure of the steam puffed into the cylinder determine how much, "tractive" power the engine was making?...

Volume and pressure are directly related.  So, the pressure on the piston face imparts momentum to the piston rod, to the crosshead, to the main rod, and around the main crank.  The distance from the center of the mainrod crank pin to the axle of rotation of the driver is what imparts the torque on that axle via the force tangential on the crank pin and parallel to the axis of movement of the driver.

What makes for a torquier steam engine (a drag freight tank engine, say, versus a high-drivered 4-4-0) is the diameter of the drivers as a ratio of the amount of rotational force the main crank can generate, which is itself a function of the distance away from the axle.  If you have a large throw on the main crank and tires on the drivers just outboard of the circle in which the crank turns, you will have a very powerful set of driving wheels.  As you increase the diameter of the drivers, thus making the crank throw a smaller circumference during a complet driver rotation, you reduce torque and traction effort....but only at the tire circumference!   At the crank, the torque is the same as it was in the first place.  All that changed was the larger circumference of the working surface.  The effect of that is to reduce tractive effort...which is why two similar steamers, say a 65" diameter set of drivers on one and 80" drivers on the other, will have different capabilities.  The former can pull more from a start, but have to do with a lower maximum sustained speed, while the 80' drivered engine will only be able to start a lighter trailing load, but deliver that load much faster over the same distance. (Assuming the road is flat, or very nearly so.)

-Crandell