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Re: low-octane gas



At 08:08 AM 6/27/00 -0400, I wrote:
Note that low-octane gas is cheaper but contains essentially the same
amount of raw energy, so that if you can make an engine that runs _with
comparable efficiency_ on low-octane gas, it's a good thing.

I should have used the subjunctive:

If you _could_ make an engine that runs with comparable
efficiency on low-octane gas, it would be a good thing.

It turns out there is physics involved. Carnot tells us that high
efficiency (for engines in this category) requires a high ratio of fire
temperature to exhaust temperature. Elementary considerations also favor
low RPM. But high temperatures, the associated high pressures, and low RPM
are exactly what provoke detonation.

Engines that do not premix the fuel and air (e.g. Diesels and turbines) can
use low-octane fuel without detonation, but they have their own problems....