Would an engine designed for 100% ethanol require a catalytic converter?

From what I've read ethanol creates little to no carbon monoxide or hydrocarbons, and very low amounts of NOx. Not sure how accurate that is, but ethanol is supposedly a cleaner burning fuel.

  • I'm no scientist, but the structure of ethanol is C2H6O. I imagine it's creating some sort of carbon by-product, as the carbon needs to go somewhere. Much less than say, gasoline (octane), which is C8H18.
    – raydowe
    Feb 3, 2017 at 14:59
  • @raydowe - The carbon by-product is called carbon dioxide, and if the ethanol is from biological sources the carbon dioxide has in the fuel production stage been captured from the atmosphere, so there are no net carbon dioxide emissions. Hydrogen by-product is called water, and plenty of it is also produced. Water is a greenhouse gas but there is an equilibrium for it, so it doesn't count. But all of this assumes perfect combustion, which does not take place. So, yes, there will be NOx, CO and HC. In fact, read my answer to see that some emissions (aldehydes) are actually worse for ethanol.
    – juhist
    Feb 3, 2017 at 15:19

1 Answer 1


Don't believe in all marketing crap you hear. There are cars designed to run on E85 (85% ethanol, 15% gasoline). If you believe that the ethanol would not produce practically any emissions, the emissions of E85 would be 15% (or perhaps a tiny bit more) of the emissions of pure gasoline. This is so low that the catalytic converter could probably be made much, much smaller or perhaps even eliminated completely.

Look at what kind of catalytic converters such cars actually have. Yes, they have catalytic converters equivalent to the ones in gasoline cars.

According to a book Vehicular Air Pollution I found from Google Books, ethanol fuel actually has some emissions worse than gasoline: ethanol emits more aldehydes. Also, if cold start enrichment is required for ethanol engines, then it does definitely emit VOCs, as all of the ethanol in a rich mixture cannot burn. The only thing that ethanol definitely does not emit is SO2.

The real reason for the existence of ethanol fuel is that it is somewhat easier to produce ethanol from agricultural products than it is to produce biogasoline. However, technology is quickly advancing and we may see biogasoline in the future. At least in Finland, I have read that there are plans to build biogasoline plants. The purpose of ethanol fuel is not to reduce harmful emissions (except carbon dioxide, obviously, if the ethanol is produced from biological sources and not from fossil fuels).

  • 1
    To be fair, E85 cars also run on regular gasoline, so they'd need the same equipment as regular cars. The real reason for ethanol fuel (IMO) is to give the U.S. farmers somewhere to sell all of their excess corn.
    – Spivonious
    Feb 3, 2017 at 15:20
  • That's true they still have to run on gasoline. Also these engines are basically just normal gasoline engine that accommodate ethanol with proper air/fuel mixture. Would an engine designed for pure ethanol (higher cylinder pressure etc.) not result in cleaner combustion?
    – Guest
    Feb 3, 2017 at 15:30
  • Ah, yes, my argument was not entirely valid. But not all of it is invalid: it is still true that there are more aldehyde emissions according to the book I found, so if you want to control those, a catalytic converter is required.
    – juhist
    Feb 3, 2017 at 15:31

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