MadSci Network: Engineering

Re: Does premium gasoline really get better milage and to what extent?

Date: Sun Dec 10 15:34:55 2000
Posted By: George Adams, , Chemical Engineering graduate, none
Area of science: Engineering
ID: 973265474.Eg

Would one get enough of an increase in mileage using premium gasoline (as 
opposed to regular unleaded) to justify the difference in price?

The answer is no.  In fact, there would be essentially no difference in 
mileage at all.

Gasoline is currently sold in three categories, regular, intermediate, and 
premium.  These differ by their octane levels, which are required to be 
posted on the pumps.  Generally these are 87, 89, and 93. (There may be 
differences in some areas).  Octane is the measure of resistance to knock - 
the pinging or clicking sound that you hear in the engine, particularly 
when you try to accelerate in high gear at low speed.  

Knocking is caused by uneven combustion in the cylinder.  When your car's 
engine is running each cylinder is filled with a gasoline and air mix and 
fired by the spark plug.  The mix is supposed to burn uniformly across the 
cylinder, but in some engine designs - mostly those with higher compression 
ratios - the mix on the side away from the spark plug will ignite 
prematurely,  causing abrupt and more explosive burning of the remainder, 
which is the knock.  You need a gasoline with a sufficient octane rating to 
prevent this knocking in your car's engine. Most cars made today will run 
on regular gasoline without knocking.  A higher octane gasoline than your 
car requires will do nothing for you. 

The octane rating of a gasoline depends on the components that are blended 
together at the refinery where the gasoline is produced.  The major 
components are:

a) The gasoline that is separated from the crude oil when it is first 
b)The gasoline that is produced from "cracking" the higher boiling point 
portion of the crude oil to break a portion of it down to the gasoline 

The gasoline separated from the crude oil generally has a very low octane 
rating and is put through a "reforming" process to increase its octane.  
The amount of the increase in octane can be varied by operating conditions 
in the reformer, but the greater the octane increase the lower will be the 
yield of the desired gasoline.

To make the finished gasoline the components are blended together at ratios 
that will achieve the desired octane ratings (as well as other 
specifications).  Mileage is not a consideration.

 Oil companies make considerably more profit on the higher octane grades, 
and in their advertisements will often imply that premium gasoline is a 
superior product.  Sometimes there will be an additive in the gasoline that 
is supposed to have some benefit.  Regardless of the putative benefit of 
such additives, there is no reason to expect added mileage from them.

Finally, there is no guarantee that the gasoline you purchase was made by 
the company selling it.  Refineries routinely exchange gasoline with other 
refineries to minimize transportation costs.

It seems only fair to mention, however, that it is amazing how cheaply the 
oil companies can turn crude oil into gasoline and deliver it to their 

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