Is there any unit to measure cleanliness of a fuel?

Sort of like calorific value which measures the fuel efficiency, is there any unit which signifies the cleanliness of a fuel.

I have heard of "methane no", but I am not sure whether it is really a unit to measure cleanliness or whether it can be used for fuels other than hydrocarbons?

  • $\begingroup$ What are you talking about? $\endgroup$ – Mithoron May 13 '16 at 18:34
  • $\begingroup$ If we consider the degree to which a fuel produces carbon dioxide on burning as the opposite of "clean", then hydrogen would be a perfectly clean fuel (producing only water on combustion); methane with a 4:1 ratio of hydrogen to carbon atoms would be less clean; and long chain hydrocarbons with almost down to a 2:1 H:C ratio would be less clean still. $\endgroup$ – iad22agp May 13 '16 at 20:35
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    $\begingroup$ The amount of sulfur present in a fossil fuel such as coal or oil is another "clean fuel" factor: a fuel that does not contain sulfur does not produce sulfur dioxide on combustion (leading to acid rain). $\endgroup$ – iad22agp May 13 '16 at 20:38
  • $\begingroup$ Why not purity (of hydrocarbons?) by w/w assay? $\endgroup$ – Beerhunter May 14 '16 at 22:54
  • $\begingroup$ Please clarify what you mean by cleanliness of fuel. I can think of at least two ways of defining it. $\endgroup$ – Jan Jun 14 '16 at 18:47

The trouble with the idea of a "clean" fuel is that the fuel is often not the dominant factor in determining whether burning is clean in an engine.

Usually the stuff we don't want in a combustion reaction in an engine consists of oxides of nitrogen, carbon monoxide, particulates, sulphur oxides and other similar things that contribute to harmful things getting into the atmosphere. Some of these are caused by the content of the fuel, like sulphur oxides, but these are mostly very low in actual modern fuels. Raw coal contains a lot of nitrogen and sulphur which makes it dirty however you burn it. But raw coal is almost never used any more because of that problem.

Most vehicle fuels will now have very low levels of things like sulphur. The problem is that just because the fuel doesn't have nasty components doesn't mean the exhaust won't. The trouble is most fuels are burned in air and the reactions in real engines are hard to control so combustion is perfect. So a range of undesirable compounds can still be formed. Some occur because combustion is incomplete (carbon monoxide and particulate carbon, for example). Others are formed because other components of air react to form undesirables (e.g. nitrogen oxides). The nasties are as much a product of the specific environment in the engine as they are of the content of the fuel itself.

What this implies is that the cleanliness of a fuel can not be defined without some consideration of the engine conditions it is burned in. So there will never be a perfect scale of fuel "cleanliness." Obviously if the fuel contains something like sulphur it will be dirtier than a sulphur-free fuel but even a pure hydrocarbon has ways to produce nasty output.


I do not know of an established way to do this, but it would certainly be a good indication to measure and publish the following things:

What is the ratio of hydrogen to carbon? The more hydrogen, the more water is produced. Usually this means more energy density (compare oil to methane). Despite the scare in mass media this is almost irrelevant regarding both health and environmental concerns however (water vapour and carbon dioxide are very comparable in regards to greenhouse effects). This can used to guess the next point:

How much 1) soot 2) carbon monoxide is produced in a particular engine (or oven or whatever). Something like nitrocellulose burns rather clean even if there is not much oxigen present.

Are there other elements present(for example nitrogen, sulfur etc) Those certainly burn much "dirtier", often producing irritating gases (nitrous gases etc.) This is probably the greatest concern regarding health.

As a side note: One should also consider whether there are volatile things which are unhealthy, benzene rings come to mind.

  • $\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$ – bon May 14 '16 at 16:55
  • $\begingroup$ Why does this not answer the question? I hardly think it is possible to have a one dimensional quantity for "cleanliness" as there are several aspects. To my knowledge those are represented rather well in this answer. The units themselves should be rather obvious for all points. $\endgroup$ – caconyrn May 14 '16 at 17:02
  • $\begingroup$ I felt that it was rather speculative when I was reviewing it. Other people may feel differently. $\endgroup$ – bon May 14 '16 at 17:20

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