Seems a proclaimed advantage of the 2-6-6-6 and N&W A was the deep firebox set behind the drive wheels. From a dimensions standpoint, it does make sense they have more interior height and of course, the ash pan design is easier.

However probably the majority of large coal fired articulateds had the firebox at least partially "over" the rear most drivewheels resulting in a shallower firebox, and much shorter throat sheet.

Was there a steaming or efficiency advantage to the deeper firebox ?

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Kelly,

This may seem like a novice question, but can you go into more detail on the combustion chamber? Is it just more space in the “firebox” without grates below it? Sort of like a platform if you will, before the tube sheet? Is there anything special about it, or just more space to allow for the fire to burn?

Thank you for your time.

Click on this link for the T1 Open house and you will see the partly assembled boiler shell for the T1.

https://prrt1steamlocomotivetrust.org/open-house-2022/

The first round section in front of the firebox - the section that carries the belpaire top forward of the firebox - is the combustion chamber.

T1 boilers were considered free steaming. It's one of the reasons that they were unofficially clocked at 140 MPH by technicians from Franklin Valve Company as they were researching why the 100 MPH Type A valve assemblies were failing.

Brian

It's interesting as you see references to similarly sized 2-8-0s vs 2-8-2s and the 2-8-2 with the deeper fireboxes are typically considered to generate more power, etc. Then the superpower engines expanded on this.

But then the 4-6-6-4s must not have suffered from having the shallow fireboxes vs say a N&W 2-6-6-4. Would something like the 3985 really have been more efficient/ generate more steaming capacity if the firebox was deeper and sat entirely behind the drivers ? Of course this wouldn't have worked for many of the big articulated as the length would be too much.

"Of course this wouldn't have worked for many of the big articulated as the length would be too much."

Most everything about stream locomotive design is a compromise. The boiler and locomotive itself would have been too long if the Challenger or an eight coupled articulated had the entire firebox behind the drivers. There would have been too much overhang at the rear of cab on curves.

In the beginning of the 20th century, some 2-8-2's and 2-10-2's had the firebox above the rear drivers. The grate did not go the length of the firebox, and there was a short firebrick wall at the front of the grate called Gaines wall. There was a empty space between the Gaines wall and the front of the firebox and that extra firebox volume may have made up for the shallow firebox.

Looking at drawings of several articulated engines, some had extra space in front of the Gains wall, and some did not. For instance the Big Boy had the Gaines wall right up against the throat sheet, but the B&O 2-8-8-4 and the D&RGW Challenger had space in front of the Gaines wall.

Consider the boiler as consisting of different zones in which different forms of heat transfer are effective.

A deep firebox usually includes an arch of some kind, often fitted with circulator tubes from the water legs or syphons dividing the combustion space laterally. From the grate up around the nose of the arch and then forward between arch and crown is the combustion plume, white with shining vaporized carbon that is burning. The firebox is not 'water insulated' to contain the fire safely; in a good design, the water is channeled in circulation to increase heat uptake -- radiant heat uptake proportional to the fourth power of temperature. To increase the TOF of the luminous plume, additional radiant uptake is provided ahead of the firebox, in an annular shape only partly correctly called a 'combustion chamber' (which can also have its own circulators and syphons to increase area or circulation, but many roads that tried them there later removed them.

It is pretty well established that active bright combustion ceases as the reducing-atmosphere, subatmospheric-pressure plume divides into the tubes and flues. Carbon may be very hot, above ignition temperature in air, but in the reducing atmosphere it does not burn, and rapidly gives up visible radiant heat. In the tubes and flues, the effect is more conduction from the gas molecules to the surface of the tubes and elements, and a careful mix of turbulence and laminar flow can be induced for best 'convective' heat transfer to boiler water.

Note that, unlike the situation in an industrial boiler or HRSG, the visible tube/flue containing boiler shell is not populated with tubes all the way to the 'top' -- nor is there water carried very high above the top rows. This in part is to provide necessary steam space above the boiler water level... and that water does NOT look or act at all as you may expect it does; Porta likened it to boiling milk, with all the frothing you might see on a stove ... to keep as much water from carrying over as possible.

In the Challenger design, the firebox rides over the last couple of driver pairs on the rigid engine, and the trailing truck does not 'support' the firebox in the same way that most deep-box boilers do. The extra length at the front required to have a four-wheel lead truck 'and all that implies' is balanced by having a shorter back end, and different guiding from the trailing truck. It's considered (by Alco or other proponents) to be a higher-speed design from a deep'box engine with a two-wheel lead truck (often done to save length, as with the L&N Emmas, which would have been good 4-8-4s had L&N had a better way to turn them) but the deep-box engines could develop far better horsepower at speed -- compare an Allegheny to a Big Boy at any particular speed the engine can reach with a train.

A locomotive like a Big Boy suffers reasonably less than other articulateds with fireboxes built this way because its fuel is friable and fast-igniting: a significant portion of the fuel is not burned on grates but is levitated, similar to PC firing, and forms an extended flameheld plume the length of the firebox and chamber. This to me shares some characteristics with rear-fed oil firing...

A Reading T-1 is an interesting subject in this discussion. The rebuild included retaining the 94.5 sq. ft. firebox of a 1923/1925 I-10-s-a very heavy 2-8-0 and installing it on a T-1 4-8-4.

On the 2-8-0, the firebox sat atop the 61 1/2 in. drivers and was intended for a mix of anthracite and bituminous.

On the 4-8-4, the firebox is behind the 70 in. drivers and is intended for bituminous only. The boiler includes a combustion chamber. There is a brick arch with thermic syphons.

As you might expect, the T-1 is a faster engine with the same train. That's why they did the project. To do this, the same firebox in a T-1 generates more steam.

In the articulated world, the 2-6-6-4 and 2-6-6-6 engines have deep fireboxes behind the drivers. The P&WV 2-6-6-4 also had a belpaire firebox. The SAL, B&O and N&W let their 2-6-6-4's make good time and they did it well.

A lot of firebox design depends on the coal you intend to use. The Western roads generally had low BTU coal with limited volatile. The Eastern roads had their choice of good bituminous if they wanted to pay for it. The Reading was fighting but slowly changing from anthracite (high carbon) to bituminous (high volatile).

Phil Mulligan

Seems that ultimately overall grate area prevails vs depth.

Looking at 2-8-0 evolution, the early ones had a deep narrow box, vs the later ones like had a wide box above the drivers.

Wonder if there would really be a difference in steam generating ability or horsepower between something like the A vs the 4-6-6-4 firebox style.

Other than speed, I don't think there would be any real advantage to the L&N 2-8-4 being a 4-8-4. And for the L&N speed wouldn't be a tangible advantage as I'm sure the 2-8-4s were very comfortable at 55-65. Their 2-8-4s didn't have any major design proportion differences to other 2-8-4s.

There are well-established counterexamples that indicate the opposite: that a good narrow firebox is effective in good steam raising. The Chapelon 4-8-0 rebuilds are reasonable examples.

A wide firebox gives you the ability to make the same mass of combustion gas with slower draft through the grates, less tendency for holes in the fire, and this kinda-sorta makes up for some of the radiant uptake in the longer legs. But compare any wide-grate shallow-firebox engine to one with the same grate area in a deep forebox and the result will be reasonably clear...

The benefits of deep, narrow boxes can be enhanced with proper circulation to allow 'thinner' waterwalls in the confined space. With some care, a very good "Lamont"-style firebox can be operated in that space (the firebox legs and backhead space being fabricated of tubes through which water at near boiler pressure is positive-circulated at about 6x steam demand, with separate steam separation in tall cyclones -- steam up, water down to the equivalent of a hotwell from which it and the downcomers from the convection section go back to the forced circulation.

Remember, the point is not to burn fuel up to the grate limit, it's to perform heat exchange from the combustiojn plume to generate steam and superheat it for use. Do not use fuel cleanly burned per hour as the only criterion of locomotive performance...

I was always told the firebox depth was based on the regional style of pizza. East coast railroads and track lines liked a deep firebox for a couple of reasons.

1. The Atlantic Ocean. Steam engines evolved from being ships and they became trains. Also, pizza was invented somewhere near the ocean and sailors liked pizza and they didn't have very much money because there's not a bank in the middle of the ocean so they couldn't print very much money. (This was largely due to the lack of the ability to log the ocean because there are no trees on the surface of the water, unless it's a shallow swamp.)

2. A thin pizza that we associate with New York would require a lot of heat. A heat similar to one found deep beneath the Earth's surface, one deeper than the deepest ocean, which is the Atlantic.

3. Chicago is next to an ocean for entry level sailors. (Once sailors mastered the entry level ocean, they could then advance to Loch Ness monster challenges or mermaid hunts before graduating to the mighty Atlantic and be true men.

These sailors were also not good cooks and put the ingredients into the pizza in the wrong order. They also used the wrong cooking methods because they didn't know that a deep cast iron pan could better be used for being melted down and cast into a brand new copper firebox or exhaust nozzle which would be used to make the steam trains of the day go very fast. This type of pizza was ultimately best cooked in a shallower oven with less heat, but took more time. One of the reasons Central time was adopted for Chicago was simply to get one extra hour of sunlight in the day for the longer cooking times of this style of pizza. Don't forget that the railroad was a major employer at this time, second only to pizza bakers. These families would discuss their days and tricks of the trade each evening at the dinner table, and this led to shallow fireboxes being used on the steam engines of the Midwest.

Thankfully after the Dust Bowl event which decimated the native tomato species that once fed all the native bison and lost tribes, diesel locomotives had evolved from steam engines and Pizza Hut started Book It, which taught many children that New York style pizza was the superior type.

Funny analogy, John. You could be president of the anti-bloviation society.

But you forgot the discussion of coal-fired pizza ovens, which some pizza people consider an important thing.

On the other hand, perhaps you didn't...

How does the Old Forge, PA style of pizza fit into this theory?

Old Forge would have to have a wide Wootten firebox with a shallow fire: it's right in the middle of the Northern Anthracite Field.

Phil Mulligan