how does a steam locomotive increase speed?

[gregc]

a steam locomotive is traveling 20 mph on and approaching level grade

• throttle is open full
• cutoff is reduced to the minimum
• brakes are off
• boiler pressure is being maintained near maximum
• coal is being added to maintain the fire and boiler pressure

what needs to happen to increase and maintain speed at 30 mph?

[wjstix]

Take it to the backshop for repairs? If the throttle is fully open, I believe the cylinders are getting all the steam they can get. Kinda like if you're driving your car with the accelorator to the floor and you're only going 20 MPH. When starting the engine, the engineer might put the throttle fully open to get it started, but then back off as the engine gained speed.

[gregc]

• pop-off valves are not opening

[PM Railfan]

First, you answered your own question - How to make it go faster: open the throttle and brakes off. (ofcourse you have steam or it wouldnt move to begin with)

However, you posted throttle wide open - I'd say just sit back and watch her go at this point. You cant open the throttle any more than WOT (wide open throttle).

When shes 'over the shoulder' (meaning throttle wide open) and "lever's in the corner" (meaning full forward - no cutoff), you cant get more from a loco than that. If you cant move the train with all that..... get a helper loco youve exceeded the tonnage rating for this one loco.

With the 6 points you listed, all you need do is spot your pocket waltham and count posts as they go by ( how engineers used to gauge their speed).

 

A#1 North!

PMR

 

(PS: you just dont WOT a steam loco. They must be coaxed to speed. Do so, you will most certainly spin the drivers, yank a coupler [which is all kinds of bad news], pound the rails, even blow a few pipe or piston seals. One must remember, even the sportier versions of steam locos must be finessed to speed. They arent race cars. And the only traction control steamers had was sitting in the cab. Dont forget shock control - [slack action] people and products dont like being slammed around. Ive heard steamers break cadence into din of exhaust noise even at a good clip. impressive, but detrimental. PRR T-1s are known for slipping, so much so I think it was part of their design. Very touchy throttles requiring almost a master of the art. You cant just throw it over your shoulder if you want to gain speed. In all actuality, your rear end does more driving than your brain does, its a seat of the pants job - experience! The best job on the planet BAR NONE!)

 

 

 

[allegedlynerdy]

To echo what is already said - it seems the only way to go faster there would be to go downhill! There may be some situational environmental conditions that could effect stuff - if it is cold out you might need to open the cylinder cocks to let out condensed water in the pistons, if it is cold you may also check for wheel slip - you may need some sanding action. This is why it tended to be important for engineers and firemen to be used to their locomotives, there's a lot of little things that they'd know or not know from experience about the situation to know why they aren't hitting 30.

[gregc]

wjstix

If the throttle is fully open, I believe the cylinders are getting all the steam they can get

so you think the boiler is always producing the same amount of steam, as much as needed when running at top speed or up the steepest grade, and the throttle just limits the amount of steam needed when running at slower speed?

what happens to the excess steam that isn't needed at lower speeds?

PM Railfan

First, you answered your own question - How to make it go faster: open the throttle and brakes off.

those are the conditions when the loco is running at 20 mph.

allegedlynerdy

it seems the only way to go faster there would be to go downhill!

trains needs to be able to run at different speeds.   this could be a passenger train capable of running a 80 mph that is moving thru a yard

286

[allegedlynerdy]

gregc

 

 

wjstix

If the throttle is fully open, I believe the cylinders are getting all the steam they can get

 

 

so you think the boiler is always producing the same amount of steam, as much as needed when running at top speed or up the steepest grade, and the throttle just limits the amount of steam needed when running at slower speed?

what happens to the excess steam that isn't needed at lower speeds?

 

 

PM Railfan

First, you answered your own question - How to make it go faster: open the throttle and brakes off.

 

 

those are the conditions when the loco is running at 20 mph.

 

 

allegedlynerdy

it seems the only way to go faster there would be to go downhill!

 

 

trains needs to be able to run at different speeds.   this could be a passenger train capable of running a 80 mph that is moving thru a yard

286

 

If the boiler is at max pressure, and the throttle is fully open, that means all of the steam the boiler is capable of producing (maximum pressure) is being used (let out through the throttle). If the boiler was under temperature, the pressure would be going down with open throttle, as there would be more steam exiting than being produced. If there was more steam being produced than there was being used, it would be gaining pressure. In this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph. 

Putting more coal/wood/oil on the boiler may give an increase in speed, as going over pressure may give an increase in throughput, but that would very much be a matter of judgement for an experienced engineer, I wouldn't want to increase pressure on the boiler past maximum if I wasn't certain of the result.

[gregc]

allegedlynerdy

In this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph. 

so are you suggesting that speed depends on boiler pressure?

302

[gregc]

here's test data showing that boiler pressure is roughly the same (near 200 psi) for various speeds: ~9, ~20, ~800, ~40 mp

313

 

[allegedlynerdy]

gregc

here's test data showing that boiler pressure is roughly the same (near 200 psi) for various speeds: ~9, ~20, ~800, ~40 mp

313

 

 

While this is interesting, I would note my qualifier relating to the boiler being at maximum pressure - unless we know the "red line" for this locomotive was 200 psi, this would not fall under that criteria. Most consolidation boilers ran around 215 psi from what I've read.

It's like with your car - if you aren't redlining, you can rev up more to get to a higher speed then settle into a similar RPM at that new speed, but if you are already redlining and aren't increasing speed, you aren't going to get any faster.

[allegedlynerdy]

gregc

 

allegedlynerdy

In this particular circumstance, it seems that the maximum speed this particular locomotive can go given its current load is 20 mph. 

 

 

so are you suggesting that speed depends on boiler pressure?

302

 

 

 

To be more accurate, changing speed depends on boiler pressure. To go faster, you'll have an increased boiler pressure while accelerating, but it'll settle back down when you return to status quo; the issue in your scenario is that you are already at maximum pressure.

[gregc]

allegedlynerdy

the issue in your scenario is that you are already at maximum pressure.

gregc

boiler pressure is being maintained near maximum

i said "near" for a reason.

what would cause an increase in boiler pressure (other than closing the throttle)?

allegedlynerdy

but it'll settle back down when you return to status quo

does status quo mean orignal speed or new (higher) speed?

allegedlynerdy

It's like with your car - if you aren't redlining, you can rev up more to get to a higher speed then settle into a similar RPM at that new speed

i don't believe the car analogies work here because motor rpm is directly related to speed.

the table shows that locomotive speed is not directly related to speed boiler pressure

362

[wrench567]

  Watch the video that Ed posted earlier on valve gear. There's actually 2 valves inline in the steam passages. One is the throttle/regulator. The next in line is the cylinder valve gear and valve. The throttle admits a given volume of steam to be used by the cylinders. The valve gear regulates the steam to the cylinders.

 To start a train. The engineer first moves the quadrant to either forward or reverse, then opens the throttle. Like a manual shift car. Select first then gas. As your going down the track and you want to go faster, move the quadrant a little further. Once you are at desired speed, move the throttle valve to almost closed. Your still admitting steam but not the volume to go faster. Closing the throttle valve is like taking your foot off the gas while in gear but no braking pressure from the engine. Putting the quadrant in the center with the throttle closed is essentially like neutral. Opening the throttle with the quadrant in center can blow steam into the smoke box because of leaking cylinder valves. Not a good idea.

   Pop valves are safety valves to keep the boiler pressure up to a certain level. A good fireman will try to keep them from popping off. That means he is wasting steam. Before a large use of steam he will add more heat and maybe shut down the injectors. When boiler pressure is almost to the valve setting he will open the injectors and add water instead of making steam.

    Hopefully this explains it for you.

     Pete 

[Overmod]

Sheesh, there is some wack stuff in this thread.

Standard operating procedure for 'best thermodynamic efficiency' has long been 'get the throttle fully open as fast as you can (to avoid throttling losses from the boiler/superheater to the steam chests) and then 'drive on the reverse'.

Under the stated initial conditions, developed power is just balancing running resistance at 20mph.  To 'go faster' you will lengthen the cutoff to increase the mass flow per stroke.  This will draw additional steam from the boiler, which will cause a (relatively slight) drop in boiler water level.  (And if the pops were getting ready to lift, they won't then...)

Water level in the glass will drop and eventually require the injector or feedwater-heater hot-water pump to be actuated.  Even heated feedwater is many degrees colder than the saturation temperature of the boiler water, so nominal pressure will tend to drop.  On the other hand the additional mass flow of steam means additional mass flow in the exhaust... hence more draft, hence a brighter fire.  (This is part of what the English call 'automatic action')

At some point, probably before balancing at 30mph, you will resume the 'firing early and often' that you should have been attending to at 20mph.  A good fireman will estimate the additional fuel the engine will need at the new higher speed on the relevant part of the railroad... and fire and trim accordingly.

(Incidentally, the more correct way to start, if you are concerned with slipping/spinning -- as in a PRR T1 with a heavy train negotiating a bunch of station switches with low joints and worn frogs -- is to open the throttle only partway and 'drive on it'.  A slip then produces wiredrawing loss that more or less promptly exhausts the steam in the chests, in the same sense that a slipping LP engine on a Mallet quickly exhausts the receiver.  And the slip stops, without the overshoot and stall that would likely result if you tried arresting a momentarily overpowered slip by slamming the throttle closed and trying to horse it out again.)

[Overmod]

There is an interesting quirk about reciprocating steam locomotives, which explains why big boilers and fireboxes/chambers are a key determinant of high speed capability.  The 'balancing point' between steam generation and steam demand with the engine(s) in full gear, even with radical fixed cutoff like the 50% in a PRR I1s, would limit speed to a comparatively low number.  So you shorten cutoff to increase how effectively the steam mass in the cylinder produces piston thrust -- you let it expand and do work, rather than blow a cylinder still full of pressurized steam wastefully to exhaust.

But eventually you'll reach a point where the decreasing steam mass going into the cylinder in a given admission no longer produces acceleration.  To go faster, you need to start lengthening cutoff again, which of course requires more mass flow out of your boiler.

Now, plenty of 'fast' engines run into a wall of pain at this point because their steam tracts or valves are incapable of handling the increased necessary mass flow in the shorter and shorter interval that admission occurs per stroke.  The ATSF 3460 class, with a ginormous boiler and 84" drivers, could easily run 100mph on the relatively flat east end.  But it could barely reach 105mph under the best conditions... valve limited.  The C&NW E-4b, another monster Hudson, couldn't even crack 100mph with the AAR test train in 1938.

Engines not so limited turn out to have an interesting statistic: the English A4 Pacific, the German BR 05, and the N&W J all made their fastest road speed with cutoff in the lower 40s

[PM Railfan]

Greg) "...so are you suggesting that speed depends on boiler pressure?"

Yes, that and volume. Pressure AND volume. 

Pressure = The force you can throw at the work load. (if you dont have psi, you cant turn the wheel over once)

Volume = The amount of time you can continue that work load. (if you dont have volume, you cant turn the wheel over a second, third, of fourth time)

 

 

".... the table shows that locomotive speed is not directly related to speed"

wait.... what? That doesnt make sense. In any lan'guage'.

 

 

I wanna go back to your very first post. You said,

"you are doing '20', throttle is 'open full', cut-off at 'minimum', brakes 'off', bpsi near 'max', fire and psi are 'maintained', .... "

"what needs to increase and maintain at 30?"

 

First off, if thats your top speed at WOT then (full open throttle and boiler is ok you said), your either on an 0-8-0 with 25" drivers (LOL) or the Best Friend of Charelston. Running light!

Second, if thats WOT, then thats all shes got. WOT is WOT! You cant really change things on the fly with a built loco in operation. Oh sure, you could tie down the safetys, drop off on the cut-off a bit and flog the cylinders, maybe gain a few miles that way..... until she blows or flies apart! 

10mph increase to 30mph is 1/3 the rate of speed for this (????) loco. Thats a big jump for a loco. You might up the driver size and the boiler psi. that might do it. though for a bit as youd run out of steam (no volume). And you cant make those changes at 20mph. SO,

For a 1/3 gain in speed, Id do what the Railroads did.....

get a bigger, more modern loco.

 

 

Now,

if you mean your loco (????) is already capable of 30mph., but only doing 20mph at WOT and you wanna do the 30 its capable of, well.... then either your doing something wrong, your train is too heavy (too much tonnage for this loco), or you have something dragging (like brakes or a hot box). 

Again a loco at WOT doesnt have any more notches left to go thru. 

 

I think a bit of clarification would be required here.

PMR 

 

[gregc]

PM Railfan

AND volume. 

yes and what determines the amount (lbs.) of steam produced?

PM Railfan

if you mean your loco (????) is already capable of 30mph., but only doing 20mph at WOT and you wanna do the 30 its capable of, well.... then either your doing something wrong,

i'm suggesting it's capable of 80 mph

aren't steam locomitives designed to run at different speeds, efficiently?

(what determines, not controls, the horsepower produced by a car)?

611

[gregc]

Overmod

get the throttle fully open as fast as you can

as that table suggests, the engine is typically operated near max boiler.   allowing full boiler pressure into the cylinders while the engine moving slowly and cylinder valve is opened for long periods of time is likely to cause slip.

while that table also shows the throttle open full, this is while running.   my understanding  is the throttle is just "cracked" to get the locomotive started to limit the amount of steam into the cylinders and the cylinder pressure.   throttles are designed to have more finer control near the beginning of there range 

the throttle is opened fully once the locomotive is running.

Overmod

To 'go faster' you will lengthen the cutoff to increase the mass flow per stroke.  This will draw additional steam from the boiler, which will cause a (relatively slight) drop in boiler water level.

as the locomotive picks up speed, more steam is required to maintain the steam density/pressure in the cylinders (twice as much steam is needed at twice the speed).

while increasing cutoff may allow more steam into the cylinders and increase speed momentarily as more steam flows from the boiler, boiler pressure will drop ... until a new equilbrium is reached, more likely at a lower speed

my understanding is cutoff is reduced to improved the efficiency of the steam of the engine

• more energy is extracted from the steam as it expands

• 5% cutoff uses 1/20 the amount of steam but mean effective pressure is ~50% of initial cylinder pressure

• less steam is drawn from the boiler
• less coal i needed
• the fireman works less

at the least, reduced cutoff allows greater boiler pressure to be maintain if not a greater amount of steam to enter the cylinder resulting in higher density/pressure

630

[PM Railfan]

Greg) What determines the psi produced in a boiler? Holy cow any number of a million things, at any different time, under any number of a million circumstances. 

thats rather an unfair question as there are so many ways to make steam psi. in a steam loco. 

 

 

Ok, your loco is capable of 80mph. that tells me you have to have atleast 70" drivers. Having that size driver, and the fact it can do 80 tells me its a modern steam loco (90% chance). Being a modern loco tells me its prolly Superpower design capable of producing all the steam it can use, and a good chance of being superheated aswell. 

Knowing this, plus your givens about throttle, boiler status, cutoff, and speed of 20mph, its safe to say your prolly spinning the drivers right now.

After you correct that and open the throttle again - more gently this time, your train should pick up speed.

sit back and count the posts you pass lineside against your watch your on your way to 30mph. Thats if you are doing 20 and JUST opened the throttle to full. Shouldnt take long if your train is of proper tonnage and your loco is up to snuff after you stop the wheel slippage. 

However, if you pulled the throttle open about 10 miles back and your still doing 20...... 

 

Your definately doing it wrong. Hit alt-Q and reset your game. 

 

 

What determines hp in a car? Again, that is such a can of worms question it cant really be answered here. suffice it to say everything from cylinder size to octane of the gas your using, to the intelligence of the engine builder, is in the formula to figure that one out. 

And just so you know, since the 70's, the "controls" ARE part of the horsepower making formula. So you cant discount that.

 

 

PMR

 

[gregc]

PM Railfan

PM Railfan wrote the following post 8 minutes ago: Greg) What determines the psi produced in a boiler?

gregc

what determines the amount (lbs.) of steam produced?

not pressure (PSI, lb/sq.in), mass (lbs)

PM Railfan

Knowing this, plus your givens about throttle, boiler status, cutoff, and speed of 20mph, its safe to say your prolly spinning the drivers right now.

so a steam engine can only run at one speed ???

PM Railfan

What determines hp in a car?

does a car only generate a constant amount of hp? only runs at one speed?

681

[PM Railfan]

Wait LOL, I totally missed that one.....

 

"What 'determines' hp in a car?"

 

I feel so stupid! What determines hp.... lol.... its the same in railroading as in cars.....

 

A DYNAMOMETER!

 

For trains its a dynamometer car. For cars its just Dynomometer, or 'Dyno' for short. It can be in ground or in room. 

dang cant believe i didnt catch that! Ya got me!

 

PMR

 

[PM Railfan]

Lbs of mass? Now why in the world would you wanna know that? No one I know figures that into the weight of a loco. 

At any rate, the mass would be the area of steam in the cavity between water level and roof of boiler. This would be forever changing due to changing water level itself and the rocking of the locomotive (though not much as the flues would act as baffles). 

Take the cubic area of the boiler, minus the volume of water, now you have the void. Multiply that amount of cubic area times the density of steam and you have your answer.

 

PM Railfan

Knowing this, plus your givens about throttle, boiler status, cutoff, and speed of 20mph, its safe to say your prolly spinning the drivers right now.

 

so a steam engine can only run at one speed ???

 

Now, you know very well they can run from one mile an hour to their repected top speed.

What I am saying is you just opened the throttle to WOT on a slow moving train with a powerfull locomotive. Thats like stomping the gas from a slow roll.... you think that VW Bug aint gonna pop a wheely and tear outta there? 

 

As for the car part - same answer. On a dyno-scope, you will see a graph - RPM vs 100's. The 100's is the range for HP and Torque (both are usually measured at same time - SOP). 

As RPMs grow, so does HP and Torque until @ 5252 RPM they meet. At some point the HP and Torque tend to drop off. Who knows where in the grapgh  but always after the 5252 point. 

And you know as well as the rest of us, cars run at varying speeds. lol.

 

[gregc]

PM Railfan

A DYNAMOMETER!

not measure.   By "determine" i mean the thing (purposely vague) that when changed results in a change in HP and i don't mean what "controls" it.

the thing that needs to change to accelerate or run at 50 vs 20 mph

[gregc]

PM Railfan

Lbs of mass? Now why in the world would you wanna know that?

gregc

what determines the amount (lbs.) of steam produced?

not the amount "in" the boiler.

doesn't more steam result in more power ???

[PM Railfan]

When running you cant change the physical parameters of an engine. But yo can alter its fuel, air, timing, all of which can either enhance or deter hp/torque.

In the old days we called them carberatuers. Now, its fuel injection, electronic spark control, and a computer controls it all through the reading of a myriad of sensors. this is why you cant discount the control side. It is half of the part that make HP now.

And, it even adjusts speed! My 2012 tacoma is not fly by wire aymore. Meaning the cable that connected my foot to the "speed control" is gone. Its now done by sensor. 

Since it comes down to what happens in the cylinder, all you need to do is alter fuel, air, compression (turbo), timing. thats it.

(But what type of fuel? what temp is the air? what boost you running? How much advance in the timing you have? Alot of stuff huh!)

 

[gregc]

PM Railfan

But you can alter its fuel

and on a steam engine?

 

[dehusman]

gregc

the thing that needs to change to accelerate or run at 50 vs 20 mph

More steam to the cylinders.  Not necessarily more pressure in the boiler, more steam.

The nuance your question misses is that that there are multiple things that can affect that, the pressue, the timing of when the steam is admitted, how long before it's exhausted, the physical ability of the pipes and valves to admit the steam, etc. 

[gregc]

dehusman

More steam to the cylinders.

gregc

PM Railfan

Lbs of mass? Now why in the world would you wanna know that?

gregc

what determines the amount (lbs.) of steam produced?

not the amount "in" the boiler.

dehusman

The nuance your question misses is that that there are multiple things that can affect that, the pressue, the timing of when the steam is admitted, how long before it's exhausted

there are of course a myriad of environmental and design factors that affect performance.  I believe i addressed the run-time timing aspect by specifying cutoff.  The first chapter of Firing the Steam Locomotive is Cooperation, so it's not all mechanical/thermal/...

but no one has mentioned what i believe is fundamental, even on the earliest of steam locomotives

[wjstix]

gregc

so you think the boiler is always producing the same amount of steam, as much as needed when running at top speed or up the steepest grade, and the throttle just limits the amount of steam needed when running at slower speed?

The fire heats the water in the boiler creating steam, and the steam goes up into the steam dome. The throttle controls how much of the steam in the steam dome goes to the cylinders. If the throttle is closed, the cylinders are getting no steam, so the engine doesn't move (or if it is, it drifts to a stop).

In your question, you state the throttle is fully open, so 100% of the available steam in the steam dome is already going to the cylinders. You can't go above 100%, so 20 MPH is apparently as fast as the engine can go - at least pulling the train it has. 

Unless the engine in your question is a geared logging locomotive with a very low top speed, or unless the engine is pulling a very heavy train that the engine can just barely pull, it's unlikely (impossible?) an engine going 20 MPH would be at full throttle. A 4-6-2 with a six car passenger train would probably, once it got up to 20 MPH, be maintaining that speed at let's say 30% throttle. To increase to 30 MPH, maybe you'd increase the throttle to say 40-45%. If you increased to 100% full throttle, on level track, you'd be going 70-80 MPH.

[gregc]

wjstix

The fire heats the water in the boiler creating steam,

how much steam (lbs)? (not a specific number). 

what would it take to produce half/twice as much steam?

wjstix

so 20 MPH is apparently as fast as the engine can g

what does it take to run a loco at different (higher) speeds

wjstix

you state the throttle is fully open, so 100% of the available steam

this table shows that the speed of the locomotive is determined by something other than the throttle