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Currently, many small stationary (and some not so stationary) spark ignited engines are sold with the capability of being converted ("on the fly" even) from gasoline operation to operation on propane as a fuel. This is useful because propane is highly storable (it doesn't go "bad" the way gas or diesel does, and is usefully dense without being cryogenically cold when liquified, as well), burns cleanly compared to fuels made of bigger molecules, doesn't mess up the environment much if it leaks, and is already widely and conveniently distributed in many parts of the world.

However, like gasoline, there's no easy way to produce the propane molecule itself when all you have is a pile of big, fat biomolecules. As a result, the biofuels folks went hunting for a replacement molecule, and they discovered one with mostly comparable properties: dimethyl ether. It doesn't go bad readily in storage as it is not vulnerable to peroxide formation unlike its larger kin, it burns even more cleanly than propane does thanks to not having any carbon-carbon bonds whatsoever, it has comparable liquification/vaporization properties to propane, and it is largely compatible with propane fuel systems with only a few changes required to the materials used.

However, while I can find documentation of its use as a propane co-fuel or substitute in heating, lighting, and cooking applications, as well as for its use in compression ignition engines, I cannot find any documentation of it being applied as a fuel in spark ignited engines. This is unfortunate, considering that most propane-burning small engines are spark ignited, and that the notion of a storable, relatively carbon-neutral emergency fuel source that is largely or entirely compatible with existing portable generators would ease many people's concerns about whole-home electrification.

Is there someone out there who's tried it before, or is there some obvious reason why DME can't be burned in a spark ignited engine that is set up for propane?

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  • I haven't heard anything about it (no, that doesn't mean much, lol), but would think any ICE engine setup for burning propane could easily be converted over. All that would need to happen is to change the orifice size, something like you'd need to do between propane and natural gas. It would be of different size than NG, but the process is would be the same. Just an assumption on my part, though. Mar 31, 2023 at 1:47

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This paper* (pdf) explains that while DME has a high cetane number, making it usable in a diesel engine, the octane number of DME is too low to be useful as a fuel in spark ignition engines on its own. It knocks badly.

It can be used in a fuel blend for spark ignition, and the blend must be adjusted for starting vs running, but then your long term storage advantage goes right out the window because you need a second fuel (perhaps gasoline) in a separate tank and a significantly altered fuel system.

If a diesel engine is altered to roughly double the fuel injection amount for DME due to its lower calorific content, DME works well, and diesel engines for generators, as you know, are a thing.

*“Dimethyl Ether as the Next Generation Fuel to Control Nitrogen Oxides and Particulate Matter Emissions from Internal Combustion Engines: A Review” by Yanuandri Putrasari and Ocktaeck Lim, ACS Omega 2022, 7, 32−37

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  • Ah, interesting. I wouldn't have figured that it'd be an octane problem since most other oxygenates (ethanol, the heavier fuel ethers) have very good octane ratings Mar 31, 2023 at 3:17

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