What was the benefit of Gresley's wide fireboxes?

[Matthew T]

The impression I got from reading a million and one coffee table books as a kid was that wider boxes could burn more caloric and poorer-grade fuels. I also heard that GWR/Western region locomotives couldn't easily burn bad coal, and they all seem to have Belpaire fireboxes.

So how does this work?

 

[randyrippley]

GWR burnt high grade Welsh steam coal
LNER burnt lower grade local coal, presumably from the northeast.

The lower grade coal would have lower calorific output, so you'd need to burn more for the same power delivery - hence a bigger firebox being required.
The GWR locos didn't need such a big firebox - in fact it would have been wasteful, besides being heavier and wasting space.
Probably no coincidence that the GWR didn't need to build pacifics - the smaller firebox could fit onto the frame of a 4-6-0

Whether they were Belpaire or round topped seems to be irrelevant and depended more on the circumstances of the railway and designer preferences. For instance it appears that the LNER simply found round tops cheaper to build, while the GWR found the Belpaire cheaper. But that was probably due to the tradeoff between tooling/engineering costs vs standardisation. The LNER had around 150 different boiler designs to build and support, the GWR a fraction of that. Round tops were cheaper to build on a small scale due to the easier engineering (no sharp angles or tight radius turns)

 

[RLBH]

The lower grade coal would have lower calorific output, so you'd need to burn more for the same power delivery - hence a bigger firebox being required.

Another consideration, as I understand it, is that a wide firebox over a trailing axle can have a bigger ashpan. For the LNER (and LMSR), this was needed to allow longer-distance trains to run without changing locomotive - and therefore removing a relatively lengthy stop. The GWR didn't have much demand for long runs by a single locomotive, so didn't need the large ashpan that the wide fireboxes permitted.

 

[randyrippley]

Another consideration, as I understand it, is that a wide firebox over a trailing axle can have a bigger ashpan. For the LNER (and LMSR), this was needed to allow longer-distance trains to run without changing locomotive - and therefore removing a relatively lengthy stop. The GWR didn't have much demand for long runs by a single locomotive, so didn't need the large ashpan that the wide fireboxes permitted.

I've often wondered why ash pans weren't emptied onto the track while in motion.......would it have caused firebox draughting problems or was it just too messy?

 

[Spamcan81]

I've often wondered why ash pans weren't emptied onto the track while in motion.......would it have caused firebox draughting problems or was it just too messy?

Dropping hot ash onto wooden sleepers wouldn't have a particularly good idea.

 

[randyrippley]

Dropping hot ash onto wooden sleepers wouldn't have a particularly good idea.

I guess thats true........but I was thinking you could sparge it with water first before opening the bottom of the ashpan

 

[delt1c]

I guess thats true........but I was thinking you could sparge it with water first before opening the bottom of the ashpan

Water was also a valuable resource on Steam loco,s

 

[Taunton]

The advantage of a wide as opposed to long firebox (to get the same grate area) is that the fireman doesn't have to throw the coal as far. This may sound a bit trivial but it's not, you can make do with less-skilled firemen.

The GWR ashpans were cleverly designed (originally by Churchward, and never changed) to be both in front of and behind the rear driving axle, which they straddled. The key issue with ashpans is to prevent them getting choked to the extent they restrict the supply of air to the fire. Not all other railways managed to achieve this with their 4-6-0s or similar types.

GWR burnt high grade Welsh steam coal
LNER burnt lower grade local coal, presumably from the northeast.

The usual case of "burned what they hauled". The GWR was fortunate that what is generally considered to be the best steam coal in the world was on their system. Apparently the worst was up in Scotland, notably poor stuff.

 

[Matthew T]

I guess thats true........but I was thinking you could sparge it with water first before opening the bottom of the ashpan

That's a really stupid idea. You don't pour water in the firebox for the same reason you don't boil an empty teakettle -- the metal can warp and destroy parts of the engine!

 

[randyrippley]

That's a really stupid idea. You don't pour water in the firebox for the same reason you don't boil an empty teakettle -- the metal can warp and destroy parts of the engine!

You wouldn't be putting it in the firebox though, it would be in the ashpan

 

[Matthew T]

You wouldn't be putting it in the firebox though, it would be in the ashpan

Wouldn't it still warp though?

 

[randyrippley]

Wouldn't it still warp though?

depends on how it was forged, bigger worry would be making the metal go crystalline and stress fracture, but I doubt it would get hot enough to do that. However as was said upthread, there's a premium on water on a loco

 

[70014IronDuke]

Another consideration, as I understand it, is that a wide firebox over a trailing axle can have a bigger ashpan. For the LNER (and LMSR), this was needed to allow longer-distance trains to run without changing locomotive - and therefore removing a relatively lengthy stop. The GWR didn't have much demand for long runs by a single locomotive, so didn't need the large ashpan that the wide fireboxes permitted.

I thought it was this, plus the fact that on long runs clinker would build up across the firebox bars, so reducing the burning capacity of the firebox - rather than randrippley's point regarding lower calorific coal as such. Yes, he's right that to get the same heat out, you have to burn more - simple physics/thermodynamics - but a smaller firebox would have been perfectly capable of doing enough at the start of a long trip.

And, to the OP, it wasn't just Gresley that needed the wide fireboxes, Stanier agreed, of course, as did Bulleid (though I'm less convinced the SR needed pacifics).

All of which points to the fact that the really heroic enginemanship in the 1935 ? ... let me google it ... 1936 non-stop LMS runs from Euston to Glasgow and Edinburgh was not 6201 Princess Elizabeth on the Glasgow run, but the Edinburgh portion worked by .... a 4-4-0 compound! (IIRC). I suspect the loads were quite light for the compound though.

 

[trebor79]

Wouldn't it still warp though?

No. The ashpan is relatively cold, with lots of fresh air and little actual combustion taking place within it.
Water mixed with ash will make acid though.

We used to wash out the ashpan at the end of each day on the heritage lone I volunteered at. Didn't cause any warping.