Re: Octane vs. MPG

["Jeff Lloyd" <jeff@xxxxxxxxxxxxxxxxxx>]

This reminds me of a certain politically incorrect statement, something 
about arguing on the internet being like the special Olympics......  I 
forget exactly.   
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> ------------------------------
>
> Date: Fri, 20 Sep 2002 09:30:16 -0700 (PDT)
> From: Erik Lotspeich <erikvcl@domain.elided>
> Subject: Re: Octane vs. MPG
>
> Michael,
>
> There are a few issues here and it is important not to confuse them.  The
> statement about higher octane containing more chemical energy IS correct.

Really???  Can you show us evidence???

> 1. Higher octane gasoline DOES contain more chemical energy.  This
> is why jet fuel has higher octane.

What sort of crap is this?  Gas turbines do not suffer from knock, since
they are limited by HP turbine stator temperature, not by effective
compression ratio.  This means that you can get approximately the same
amount of power or thrust out of a gas turbine with any hydrocarbon fuel,
since the critical temperature is a function of amount of heat release, or
the mass of fuel x the calorific value.

Perhaps you are referring to piston engined aircraft engines running on high
Octane Rated fuels.  These are used because the engines have very high
effective compression ratios (to increase cycle efficiency) and therefore
require high Octane Rated fuels to allow stable operation.

> 2. Cars designed to use higher octane have higher-compression engines to
> extract that energy.

Increased compression ratio increases the cycle efficiency.  Higher Octane
Rated fuel allows you to use such a compression ratio without knock,

> 3. Cars that are not designed for higher octane (e.g. lower compression
> engines) will not be able to extract this energy and will perform more
> poorly.

This is incorrect.  The engine will not run poorly if you use a high octane
fuel in a low compression ratio engine, you just wont get the additional
benefits of such a fuel.

> 4. Using low octane gas in an engine designed for higher octane will
> result in knocks and pings since pre-detonation will occur.

Correct

> I hope that this clears things up.

Not really......

> Erik.

Nick Pashley


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