# Whats the octane rating of diesel?
**URL:** https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447
**Category:** Factual Questions
**Created:** [August 31, 2005, 4:49pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447 "2005-08-31T16:49:56Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@justwannano](https://boards.straightdope.com/u/justwannano)
#### Post date: [August 31, 2005, 4:49pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/1 "2005-08-31T16:49:56Z")
How about after mixing gas/oil in a 2 stroke?
While we’re at it can a gasoline/ oil mix be used in a diesel?
The end of my questioning will be can I use used cooking oil fuel in my 2 stroke lawn mower?
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### Author:  [@bradministrator](https://boards.straightdope.com/u/bradministrator)
#### Post date: [August 31, 2005, 4:54pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/2 "2005-08-31T16:54:03Z")
Diesel is measured in cetane, not octane. I believe pump diesel in the US is 40 cetane.
I don’t think mixing gas + oil for a 2-stroke changes the octane of the gas.
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### Author:  [@N9IWP](https://boards.straightdope.com/u/N9IWP)
#### Post date: [August 31, 2005, 5:57pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/3 "2005-08-31T17:57:41Z")
Hijack: Can lawn mowers (in general) run OK on E85 (85% ethanol)?
Brian
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### Author:  [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)
#### Post date: [August 31, 2005, 7:38pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/4 "2005-08-31T19:38:50Z")
Petroleum is “cracked” (refined) into a variety of fractions with names you are probably familiar with. Each fraction has a different number of carbon atoms in it’s hydro-carbon chain:
1 C atom, methane
3 C atoms, propane
4 C atoms, butane
5 C atoms, pentane
6 C atoms, hexane
7 C atoms, heptane
8 C atoms, octane
Conventional automotive fuel (gasoline or petrol) is a mixture of octane and heptane, with the percentage (or _effective percentage_\*) of octane given as the rating. This is because octane is less likely to sponteneously ignite when compressed, and so is more desirable in high compression engines.
Diesel fuel requires less refining than that used to produce gasoline. A more qualified person can come along and correct this, but IIRC, diesel fuel has 14 C atom hydrocarbon chains.
With regard to using diesel fuel in a gasoline engine, unless the engine has been designe for it, it won’t work. For one thing, diesel engines operate at far higher compressions, and don’t use spark plugs. A quick Google will undoubtably turn up a lot more information about how the various engines work.
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### Author:  [@Cerowyn](https://boards.straightdope.com/u/Cerowyn)
#### Post date: [August 31, 2005, 7:43pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/5 "2005-08-31T19:43:15Z")
To add the missing bit from my post above:
- I say “effective percentage” because there are ways of using additives to make a given fuel behave as though it had a different percentage of octane. Lead (actually tetraethyl lead) was found circa WWII to make fuel behave as though it had a higher percentage of octane. This is how fuels can actually have an octane rating higher than 100 (“Super Test” is a name sometimes applied to such mixtures, I believe).
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### Author:  [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)
#### Post date: [August 31, 2005, 7:48pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/6 "2005-08-31T19:48:17Z")
[http://www.procarcare.com/images/shar/encyclopedia/88525G01.gif](http://www.procarcare.com/images/shar/encyclopedia/88525G01.gif)
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [August 31, 2005, 9:12pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/7 "2005-08-31T21:12:41Z")
Last time I saw a diesel pump, it was marked with a 42 octane sticker.
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### Author:  [@Kevbo](https://boards.straightdope.com/u/Kevbo)
#### Post date: [August 31, 2005, 10:16pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/8 "2005-08-31T22:16:00Z")
The cetane rating for diesel is a measure of how quickly it spontainiously ignites at high temperature and pressure. A too low cetane rating may not work at all, or may be noisier.
This is nearly the opposite of an octane rating, which is a measure of how well a fuel resists detonation. But not an exact opposite or inverse. It is measuring a different property.
In both cases, this is a factor in how well they work in an engine designed for that particular fuel. But you can’t convert from one to the other. If Dave is 6’3" and Bill weighs 173#, then how much does Dave weigh? Human height and weight do, in general, have some corilation, but not really enough to be useful in any particular case.
In general, the octane rating for diesel fuel would be very low. The noise typically associated with a diesel IS a detonation of the fuel that was injected prior to combustion occuring. The burn rate for the remaining fuel is determined by the injection rate.
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### Author:  [@yerba\_buena](https://boards.straightdope.com/u/yerba_buena)
#### Post date: [August 31, 2005, 11:23pm UTC](https://boards.straightdope.com/t/whats-the-octane-rating-of-diesel/319447/9 "2005-08-31T23:23:20Z")
[http://www.straightdope.com/mailbag/mdieselvsgas.html](http://www.straightdope.com/mailbag/mdieselvsgas.html)
> [@](#):
>
> SDSTAFF Una replies:
>
> What happens when you use gasoline in a diesel engine? Either something expensive or something very expensive. Since gasoline is designed to be resistant to self-ignition, gasoline in a diesel engine either won’t ignite or will ignite at the wrong time.
>
> …
>
> Trying to use diesel fuel in a gasoline engine also has unpleasant consequences, but generally not as dire in terms of damage to the fuel system. Depending on the proportion of diesel fuel relative to gas in the tank, a gasoline engine will either run poorly or stop altogether, necessitating another $500 trip to the repair shop for draining and flushing.