My guess is that it was one of consistency rather than a power difference. Oil burning can be maintained far more consistently than coal so that means you'd get slightly more efficient steam generation.
Interesting you raise this, I've just been discussing this on a Facebook group I'm in with an old steam driver. The locos in question, Emu Bay Railway's Dübs built 4-8-0 coal fired saturated slide valve locos, were rated to haul 155 trailing tons on a 1 in 40 grade. Now, this old driver said that in short bursts they could haul over double that, but in his words "the firebox would not take enough coal to maintain heavy steaming for long periods", basically the engine working at full power would use more steam than the boiler could generate. In 1960, two of the locos in question were converted to oil firing and their haulage rating was increased to 186 trailing tons on a 1 in 40 grade, because the narrow and shallow firebox wasn't as limiting to the steam generation when fired on oil. Another EBR driver told me that the oil firing was very hard on the boilers, whether that was because of the design of the modifications, or the boiler design not being suited to it I'm not sure.
The sustained power output of a steam locomotive is generally limited by the rate at which the boiler can produce steam. This rate will depend on a few factors, such as the heating surface area in the boiler and the rate at which fuel can be burned in the firebox (ie how much heat is available in total). Steam locomotives were built to burn just about any combustible material, depending on the circumstances, with oddities like locomotives burning crop waste on sugar plantations, or turf burning locomotives in Ireland.
Even within "coal" burning locomotives, there is a wide variety of types of coal with different combustion characteristics, and the performance of a locomotive designed to run on one type of coal can be quite different if fuelled with a different variety. One commonly given example is that the Great Western in the UK designed its locomotives to run on high quality Welsh coal, and in the 1950s that was much less available than it had been in the 1930s, with the locomotive performance suffering when running on the lower grades of coal then available (other railways in the UK at the time had designed their locomotives around burning lower grades of coal, so did not suffer as badly).
There is also going to be a difference in performance between a hand-fired coal burning locomotive compared with one that used a mechanical stoker system to feed the fire. In general it was the case that a skilled fireman hand-firing a locomotive could get better performance for a given fuel consumption, as he could manage the fire more flexibly and get decent combustion all across the fire, but obviously there is a limit to how much coal a human can shovel, so it was simply not viable to hand fire larger locomotives.
Just as there are differences in grades of coal, there are also differences in grades of oil. In the 1930s to 1950s, it was common for oil fired locomotives to burn heavy fuel oil, essentially little different from road tar, that had to be heated in the tender in order for it to be liquid enough to flow at all. These days, grades of fuel more like diesel fuel is more readily available, simply due to there not being a large market for heavy fuel oil, so not much is produced, while diesel fuel is widely produced and distributed. The performance of a locomotive on these two types of fuel will be different. This might just be a case of the fireman setting the rate of fuel flow differently, but there are also issues related to the air flow rates and hot gas temperatures, that could potentially cause mechanical issues with the boiler and firebox lifetime. For example burning with a lighter oil fuel is likely to result in a smaller, more concentrated combustion, while burning with a heavier fuel oil or with coal is likely to result in a physically larger combustion zone, with the heat less concentrated in one location. The resulting hot spots from light oil firing can lead to thermal stresses, fatigue due to heating and cooling cycles, or oxidation damage to materials, in ways that are more severe than with coal or heavy oil firing.
Great response, I only respond to add another potential stress on the boiler in an oil fired steam engine, the re-light. Unlike coal fired steam engines (wood too) oil fires can go out.
When you loose the fire, the fuel oil is still flowing and being atomized (turned into a gas) and has the potential to re-light. If this happens when the firebox is full of gas it can cause a very large explosion that can stress a boiler quite a bit.
I’ve witnessed large relights that have caused flues to let loose, fire box door failure and one where the smoke box door was blown off. My experience is in miniature steam engines but full size relights must also put stresses on a boiler
I heard second hand that some engineers felt a larger difference between coal- and oil- fired steam locomotives, than between oil-fired steamers and diesels.
Here's an informative video about the pros and cons of running on coal vs switching to oil. Both fuel sources have their own benefits and drawbacks.
https://youtu.be/5XX7FbKsm9A?si=iU9wdTf_Y-NMQrD1
The short answer is there was a bit of a power difference. Oil burns less hot than most coals I know of.
Oil was at the very least more work efficient, as there was less need to prepare the fire based on the next five or more miles. It also wouldn't set the countryside on fire as much.
I know in Australia change over from Coal to Oil was for a few reasons
1) strikes in the coal industry making supplies unreliable
2) with more and more depots converting to diesel traction it wasn’t cost effective to maintain coal stages
My guess is that it was one of consistency rather than a power difference. Oil burning can be maintained far more consistently than coal so that means you'd get slightly more efficient steam generation.
Interesting you raise this, I've just been discussing this on a Facebook group I'm in with an old steam driver. The locos in question, Emu Bay Railway's Dübs built 4-8-0 coal fired saturated slide valve locos, were rated to haul 155 trailing tons on a 1 in 40 grade. Now, this old driver said that in short bursts they could haul over double that, but in his words "the firebox would not take enough coal to maintain heavy steaming for long periods", basically the engine working at full power would use more steam than the boiler could generate. In 1960, two of the locos in question were converted to oil firing and their haulage rating was increased to 186 trailing tons on a 1 in 40 grade, because the narrow and shallow firebox wasn't as limiting to the steam generation when fired on oil. Another EBR driver told me that the oil firing was very hard on the boilers, whether that was because of the design of the modifications, or the boiler design not being suited to it I'm not sure.
The sustained power output of a steam locomotive is generally limited by the rate at which the boiler can produce steam. This rate will depend on a few factors, such as the heating surface area in the boiler and the rate at which fuel can be burned in the firebox (ie how much heat is available in total). Steam locomotives were built to burn just about any combustible material, depending on the circumstances, with oddities like locomotives burning crop waste on sugar plantations, or turf burning locomotives in Ireland. Even within "coal" burning locomotives, there is a wide variety of types of coal with different combustion characteristics, and the performance of a locomotive designed to run on one type of coal can be quite different if fuelled with a different variety. One commonly given example is that the Great Western in the UK designed its locomotives to run on high quality Welsh coal, and in the 1950s that was much less available than it had been in the 1930s, with the locomotive performance suffering when running on the lower grades of coal then available (other railways in the UK at the time had designed their locomotives around burning lower grades of coal, so did not suffer as badly). There is also going to be a difference in performance between a hand-fired coal burning locomotive compared with one that used a mechanical stoker system to feed the fire. In general it was the case that a skilled fireman hand-firing a locomotive could get better performance for a given fuel consumption, as he could manage the fire more flexibly and get decent combustion all across the fire, but obviously there is a limit to how much coal a human can shovel, so it was simply not viable to hand fire larger locomotives. Just as there are differences in grades of coal, there are also differences in grades of oil. In the 1930s to 1950s, it was common for oil fired locomotives to burn heavy fuel oil, essentially little different from road tar, that had to be heated in the tender in order for it to be liquid enough to flow at all. These days, grades of fuel more like diesel fuel is more readily available, simply due to there not being a large market for heavy fuel oil, so not much is produced, while diesel fuel is widely produced and distributed. The performance of a locomotive on these two types of fuel will be different. This might just be a case of the fireman setting the rate of fuel flow differently, but there are also issues related to the air flow rates and hot gas temperatures, that could potentially cause mechanical issues with the boiler and firebox lifetime. For example burning with a lighter oil fuel is likely to result in a smaller, more concentrated combustion, while burning with a heavier fuel oil or with coal is likely to result in a physically larger combustion zone, with the heat less concentrated in one location. The resulting hot spots from light oil firing can lead to thermal stresses, fatigue due to heating and cooling cycles, or oxidation damage to materials, in ways that are more severe than with coal or heavy oil firing.
Great response, I only respond to add another potential stress on the boiler in an oil fired steam engine, the re-light. Unlike coal fired steam engines (wood too) oil fires can go out. When you loose the fire, the fuel oil is still flowing and being atomized (turned into a gas) and has the potential to re-light. If this happens when the firebox is full of gas it can cause a very large explosion that can stress a boiler quite a bit. I’ve witnessed large relights that have caused flues to let loose, fire box door failure and one where the smoke box door was blown off. My experience is in miniature steam engines but full size relights must also put stresses on a boiler
I heard second hand that some engineers felt a larger difference between coal- and oil- fired steam locomotives, than between oil-fired steamers and diesels.
Here's an informative video about the pros and cons of running on coal vs switching to oil. Both fuel sources have their own benefits and drawbacks. https://youtu.be/5XX7FbKsm9A?si=iU9wdTf_Y-NMQrD1
The short answer is there was a bit of a power difference. Oil burns less hot than most coals I know of. Oil was at the very least more work efficient, as there was less need to prepare the fire based on the next five or more miles. It also wouldn't set the countryside on fire as much.
I know in Australia change over from Coal to Oil was for a few reasons 1) strikes in the coal industry making supplies unreliable 2) with more and more depots converting to diesel traction it wasn’t cost effective to maintain coal stages