[A4] Need for Premium Gas in 1.8T quattro?

Huw Powell audi at humanspeakers.com
Thu Sep 21 20:44:25 EDT 2006

continuing quoting from the wikipedia entry...

"A counter example to this rule is that ethanol blend fuels have a 
higher octane rating, but carry a lower energy content on a volume basis 
(ie per liter or per gallon)."

and also

"The power output of an engine depends on the energy content of its 
fuel, and this bears no simple relationship to the octane rating. A 
common myth amongst petrol consumers is that adding a higher octane fuel 
to a vehicle's engine will increase its performance and/or lessen its 
fuel consumption; this is mostly false—engines perform best when using 
fuel with the octane rating they were designed for and any increase in 
performance by using a fuel with a different octane rating is minimal."

So it mostly depends on what is being done to raise the octane.  If it 
is being done with the addition of ethanol, for example, it might have a 
lower energy content.

The thing that matters here, I think, is that we are talking about a 
high performance (almost by definition) turbocharged engine, which is 
probably designed to run on higher octane than "regular".  In order to 
run on the lower octane fuel, the engine management compensates in ways 
that also decrease power and fuel economy.

The best way to find out for a given car is to run full, fairly flushed 
tanks of each and calculate the consumption over similar usage patterns.

David Ullrich wrote:

> Higher octane ratings correlate to higher activation energies. Activation
> energy is the amount of energy necessary to start a chemical reaction. Since
> higher octane fuels have higher activation energies, it is less likely that
> a given compression will cause knocking. (Note that it is the absolute
> pressure (compression) in the combustion chamber which is important - not
> the compression ratio. The compression ratio only governs the maximum
> compression that can be achieved).
> It might seem odd that fuels with higher octane ratings burn less easily,
> yet are popularly thought of as more powerful. The misunderstanding is
> caused by confusing the ability of the fuel to resist compression detonation
> (pre-ignition = engine knock) as opposed to the ability of the fuel to burn
> (combustion). However, premium grades of petrol often contain more energy
> per litre due to the composition of the fuel as well as increased octane.
> A simple explanation is the carbon bonds contain more energy than hydrogen
> bonds. Hence a fuel with a greater number of carbon bonds will carry more
> energy regardless of the octane rating. A premium motor fuel will often be
> formulated to have both higher octane as well as more energy.

Huw Powell



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