Reply to post: Re: A couple points ...

Back to the Future's DeLorean is coming back to the future

Michael Wojcik Silver badge

Re: A couple points ...

A high performance engine conversely will be held back by low octane fuel.

That's only true, and then only a little bit, if the octane1 is low enough that the engine suffers from compression ignition. Once you pass that point, the octane rating makes no difference.

And it doesn't matter whether the engine is "high performance". What does matter is whether it's "high compression", because the greater the compression ratio, the more susceptible the vapor is to compression ignition.

In the olden days, when compression ignition happened, you'd get "knock" - the vapor igniting prematurely at the wrong point in the cycle. Obviously that was bad for engine output, among other things.

For many years now, cars have come with knock sensors, and if the engine sees compression ignition it'll retard the timing to compensate. This makes the engine less efficient, but not a lot less powerful.

Higher-octane-rated mixes are less likely to ignite from compression (just as octane ignites at a higher pressure than hexane does). Thus cars with high-compression engines specify higher-octane-rated fuel. Such engines have higher output per unit displacement because they use a higher compression ratio, not because of the fuel they use.

1Really should be "nominal octane". Gasoline / petrol is nominally a mixture of hexane and octane, for octane-reporting purposes; but really it's a hydrocarbon cocktail of various things the refinery cracked out of longer petroleum chains and decided to mix together, along with detergents and oxidizers (where mandated) and whatever other crap they decide to throw in. No one makes gasoline out of mostly octane.

POST COMMENT House rules

Not a member of The Register? Create a new account here.

  • Enter your comment

  • Add an icon

Anonymous cowards cannot choose their icon

Biting the hand that feeds IT © 1998–2019