You're making it tough to comment inline by posting this as a graphic...

To get the ball rolling: While it's useful to show stoich calculations with the elements separate (and presumably perfectly carbureted, etc.) the actual process of practical combustion more accurately resembles the premise of organic chemistry: first you have to break bonds in order to form new ones. In hydrocarbon fuels, you have to provide energy to separate the C-H and C-C bonds before you can do practical oxidation, and there is both energy and time involved in this (on a practical level). Here is a reference that is a bit simplistic but may be useful to people in this community who have had a lamentable lack of interest in this aspect of chemistry up to now:

https://www.wou.edu/las/physci/GS361/Energy_From_Fossil_Fuels.htm

There is also a natural tendency for 'atomic hydrogen' to recombine to diatomic gaseous hydrogen (and the energy represented in this bonding is surprisingly large - it's of interest in the aerospace propulsion field) rather than react directly with an oxygen molecule (which was also fundamentally diatomic as sourced in primary or secondary air)

Note that the 'hydrogen' part of most fuel combustion is not exactly invisible; it's responsible for the blue flame of natural-gas combustion. That does not change the valuable point about 'invisible products of combustion' (although I would re-introduce the Besler tube as an interesting way around some of the problems, and those tubes were demonstrated to show demonstrable improvement in practical heat uptake from gas to water in the convection section. (Can someone who has the original table showing this please provide an online copy? I have lost mine in computer changes over the years...) Something that might be useful here would be the emission spectrum of the various gases and species here, perhaps paired with the absorption characteristics of the various surfaces the emitted radiation affects...

I have tried, unsuccessfully, to figure out how the 'boiling-water' penalty applies to combustion, or how any characteristic of water at 70 degrees is supposed to be relevant to formed 'water of combustion'. In my opinion you'd have been better off looking at how the formed water 'absorbs' some of the released bond energy of combustion -- this being cognate to the 'latent heat of vaporization' at combustion temperatures, and it will not be given up in radiant, convective, or conductive heat transfer until the point of phase change, even lower than 212 F in the below-atmospheric-pressure environment of induced-draft combustion. Can you rephrase the argument a little so I understand why the boiling-water penalty is important in context?

This issue was taken up, in a different context, when the EPA tested the Donlee TurboFire XL boiler in the '80s. This used steam injection to reduce NOx generation, and "conventional wisdom" said this represented an irrecoverable loss of combustion energy. However, when an adequately long gas path is available (as was the case in the 'return' Donlee boiler, and at least theoretically for some forms of multiple-return or package boilers), and 'bottoming' recovery to a practical Rankine cycle is provided, you can get higher efficiency as well as lower-NOx benefits out of staam injection. And it follows, as for the Franco-Crosti ideas earlier in steam history, that much of the enthalpy "trapped" in the combustion water can be recovered to the Rankine cycle too...

While we are on the subject of inadequate draft for combustion: the first place I heard about this was semi-apocryphal, and had to do with the (somewhat amateur-hour, imnsho) experimentation with front ends that UP conducted in the latter half of the '30s. Supposedly at speeds over 80mph (probably with one of the 800 classes) the most efficient cutoff needed to hold high speed did not produce enough draft to keep steam production up even at the reduced mass-flow requirement. I never saw data to corroborate this, and I suspect Mr. Austin of all people knows the exact detail of this story ... and perhaps other examples of similar significance. I would like to hear further details, in detail.

Nice article! Some remarks:
In textbooks about combustion I found this formula and used it in my thesis:
Air(lbs or kg)=11.6C+34.7(H-O/8)+4.34S
The above data for carbon and hydrogen is now extended to coal which contains oxygen and sulphur.
I have some trouble with the concept of "the boiling water penalty". Firstly because this concept is not applied to the carbon dioxide but only to hydrogen oxide. Secondly because I question the starting temperature. Theoretically this is only valid for the primary ignition.
After that the heat contained in the combustion products is sufficient to heat the oil/gas molecules to their combustion levels where they fall apart in carbon and hydrogen atoms and react with the available oxygen. My point being that the combustion product is gaseous carbon diode and water vapor right away.
Kind regards
Jos Koopmans

This gets way too deep for me PDQ, but I agree with the parts I can understand...

Presumably diesel (everybody's favorite subject) has more hydrogen to carbon than the good stuff. What I notice is that I have to heat the good stuff up until it's about the consistency of diesel before I can fire with it. The big difference is that it takes a lot more firing adjustments (fuel/atomizer/blower) to keep the train happy with diesel than it does with the good stuff --- is this because of the lower carbon content in diesel ??? (Please dumb down your answers a little if you would.)

As to the comment regarding not having enough draft at high speeds to keep up with steam demand, most of the time I keep the blower going when running. The exception is when steam demand is balls to the wall, then you don't need to use your blower, but you need every drop of steam. Otherwise, a little bit of blower is a hedge against getting a flashback if your fire goes out, but is also used to assist the draft when steam demand is low.

It's a balancing act --- fiddle with the knobs until the train is happy. And don't forget to pick up your check on the 10th and 25th.

Take Care & WORK SAFE

As this is my speciality, I would suggest another blast-cap with more orifices. This forces
the chimney to do its proper work: suck!
Kind regards
Jos Koopmans

What I was referring to is something radically different -- although I'm not going to say it isn't a railfan-morphed legend in the form I heard it (now probably more than 30 years ago... and I do not have either the original quote or the original source to provide.)

The discussion didn't involve mechanical damage to the running gear at all; it was purely that when the 'engine' was adjusted for minimum water rate at high running speed, the actual mass flow could be reduced below what the locomotive required for drafting to generate steam even at that lower rate. I read this before encountering the irritating lack of technical knowledge in the various Kratville books that take up the subject of UP front-end design and refinement, so I have no real idea whether this was a transient problem from one of the experiments, or was first experienced during the early high-speed testing on the UP 4-8-4s (which ISTR were the first real high-speed 4-8-4s actually tested at high sustained speeds)

It would seem that lengthening the cutoff somewhat, or using the blower judiciously (as has been reported for excursion service, perhaps in a different context) would be ways of avoiding the problem, but both those have a deleterious effect on something often even more important for modern steam than fuel consumption: water rate.



This is particularly interesting because of the detail design of the intermediate 'floating bushing' in the 800-class main, with all the little cross-drilled holes supposedly acting as lubricant reservoirs but, here, acting as sinks to crash the hydrodynamic film integrity for a short - but consistent part of the main-pin circumference. I suppose this was not helped by the reduced bearing surface of the bushing due to the holes. There is also an issue of consistent peak bending moment at the root and seat of the main pins due to the high peak compression force.

These are likely to be consequences of the old 'thermodynamic desideratum' of having the tract pressure just match the inlet pressure as the valve cracks open to steam (thereby limiting pressure loss due to 'dead space' considerations). To get that approach to work right, you need reasonably reliable modulated reversible compression relief...

I don't know if I can mention SACA here without incurring displeasure of the practical engineers -- but Jay Carter has looked carefully at the 'next step' beyond Okadee compression-limiting blowoffs, and his approach should work just as nicely for a DA engine in locomotive configuration as it does for single-acting.

Be interesting to see the extent to which Timken lightweight roller-bearing mains actually 'solved' the problem of main-pin tribology during high compression. Seems logical to me that they would, but also logical that main-pin peak stresses and perhaps breakage might be enhanced where unexpectedly high compression pressures (e.g. as might be developed on throttle closing at high speed in the absence of a good bypass valve arrangement...

See the points Mr. Austin made responding to Mr. Koopmans.

What every oilburning fireman knows
But doesn’t know why ...

Historically, boi1er fuel was categorized as #6 or Bunker C, which was sucked out of the refinery cracking tower one nozzle aboye the asphalt drain. The difference between all grades of fuel oil is the ratio of carbon to hydrogen. The lighter the fuel oil, the greater the ratio of hydrogen atoms to carbon atoms. Inversely, the more carbon per hydrogen atoms in the fuel, the more BTUs of energy per pound of fuel liberated in combustion. Any fireman who has fired a locomotive under the same load conditions with #6, #5, #2, waste lube oil. And/or vegetable oil knows that the lighter the oil, the harder it is make steam. It is not a simple linear relationship, however.

The Fundamentals:

Heat of Combustion.
Combusting one pound of hydrogen releases 62,000 BTUs of energy.
Combusting one pound of carbon releases 14,540 BTUs of energy.
These numbers make it look simple. Everything should burn hydrogen and we get 4 times as much energy, right? Nope.

Combustion Air Requirements
To burn one pound of hydrogen requires 34 pounds of air and creates 9 pounds of waler.
To burn one pound of carbon requires 11.5 pounds of air and creates 3.67 pounds of carbon dioxide gas.

The Boiling Water Penalty
When hydrogen combusts with oxygen the product is water. When 1 pound of hydrogen burns with 8 pounds of oxygen at 70 degrees F in air, the temperature of combustion will be about 2700F, but the combustion product is water. At 70F water is a liquid. At 2700F water is a gas. To boil water to a gas absorbs energy. To boil 9 pounds of water and superheat it to 2700F requires about 10,000 BTUs. This must be subtracted from the gross combustion energy of 62,000 BTUs. The net available energy of the burning of one pound of hydrogen is about 52,000 BTUs.

The Combustion Process From the Oxygen Side:

If we are burning pure carbon in the firebox and suck in 100,000 pounds of air we release 3,793,000 BTU of heat energy.
If we are burning pure hydrogen in the firebox and suck in 100,000 pounds of air we release 1,529,000 BTU of heat energy.

What this shows is that for each pound of air consumed during combustion, we get 3793/1529 = 2.48 times more thermal energy out of each carbon-oxygen atomic bond created, than for each hydrogen-oxygen bond created.

Scientific Analysis Follows… Proceed at your own risk

Hydrogen vs. Oxygen
It is not fair to compare one pound of hydrogen to one pound of carbon. One hydrogen atom can only make one atomic bond. Each carbon atom can make 4 atomic bonds. The formation of an atomic bond gives off energy. The amount of energy released in the formation of a hydrogen-oxygen bond is not the same as the formation of a single carbon – oxygen bond. Also, to accurately understand combustion it is necessary to compare the reaction of the same number of atoms.
In chemistry this was defined by Avagadro as one MOLE = 6.02 x 10E23 atoms. The convenient way to measure the number of atoms is to base it on the atomic weight of each atom. The atomic weight of hydrogen is 1. The atomic weight of carbon is 12 and that of oxygen is 16. One pound. of hydrogen, 12 pounds of carbon and 16 pounds of oxygen all have the same number of atoms. Hydrogen can make one atomic bond, carbon 4 atomic bonds and oxygen only 2 atomic bonds.
The process of combustion is to lake atoms in their natural state, hydrogen(H2) and carbon(C) and burn/combust/oxidize them all with oxygen to create water(H2O) and carbon dioxide (CO2). When burning hydrogen and oxygen in free air, it must be taken into account that free air only contains 21 % oxygen by weight. The majority component of air is inert nitrogen.

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Ignoring steam locomotive mechanical efficiency and drafting efficiency, let's just say that for a certain quantity of steam used from the boiler and out the smoke stack creates a certain quantity of vacuum in the smokebox. Since the cross sectional gas area of the tube bundle is constant, the volume of air sucked through the firebox for combustion is a function of the amount of vacuum at the front tube sheet and the cross-sectional area location of air holes into the firebox. For a coal burner. the air intakes are the area of holes in the grates (plus overfire air if equipped). For an oilburner, there may or may not be dampers, but with the dampers open the area of air openings are constant. The point to understand is that for a given steam load(throttle opening plus cutoff) the volume of combustion air through the firebox is the same, based on the geometry of the firebox air openings and has nothing to do with the BTU content of the fuel whether oil, gas or coal.


Since the draft is constant for any particular load setting, adjusting the steam generation is controlled by the amount of fuel injected into The firebox for combustion. As the carbon/hydrogen ratio decreases. the amount of BTUs released in combustion decreases. resulting in less steam being generated. It is possible, that as the carbon/hydrogen ratio decreases. a point will be reached such that insufficient heat energy will be released to maintain the steam flow required to create the vacuum that maintains the 100,000 pounds of air flow limited by the geometry of the firebox and boiler. The only solution to this situation is to create rnore draft by using more steam to allow for burning more fuel oil. The penalty for this condition is loss of thermal efficiency. More heat energy is being thrown away for the same load due only to the lower carbon/hydrogen ratio of the fuel.

Also relevant to the carbon vs. hydrogen combustion equation is that the radiant heat energy given off by each reaction is of different wavelengths. A hydrogen flame gives off no photons in the visible spectrum. The hydrogen flame is invisible. In a carbon flame, a significant part of the energy is released in the visible wavelengths of orange and yellow. This allows for radiant heat adsorption by the boiler sheets. With hydrogen giving off little radiant heat, the heat energy is transmitted by conduction to adjacent combustion gas molecules. It is a well known rule of thumb in the power plant industry that when a power plant is converted from coal to natural gas(CH4) fuel firing. the design steam output is reduced at least 10% to 30%. To promote the extra hydrogen combustion. the draft fans force more air into the combustion furnace and the transfer of heat to boiler surfaces occurs by convection as the gas flows through the boiler. The only way to maintain the original steam generating capacity, due to the lack of radiant heat energy input and increase in combustion air required, is to add more convective heating surface area to the boiler.

The Math
Carbon
14,540 BTU = 1 lb carbon
12 lb carbon = 1 mole carbon
1 mole carbon 4 mole carbon-oxygen bonds (MCOB)
1 lb carbon = 11.5 lb combustion air

Hydrogen
52,000 BTU = 1 lb hydrogen
1 lb hydrogen = 1 mole hydrogen
1 mole hydrogen = 1 mole hydrogen - oxygen bonds (MHOB)
1 lb hydrogen = 34 lb combustion air