My friend RON, the Research Octane Number

[BruceT]

A few people have asked about whether to use standard ULP (91 RON), premium (PULP, 95 RON), or 98 RON. I'll give my view based on my studies in chemical engineering and some reading.

What RON is not:
RON is not a measure of the energy content of a fuel. So there is nothing intrinsically more efficient or powerful about using 98 RON vs 91 RON. In most Japanese and Aussie cars you will notice no difference.

What it is:
RON is the proportion of iso-octane (0-100%) which, when mixed with n-pentane (100-0%) will give the same knocking characteristics as the fuel being tested. Eg. a 91 RON fuel will have the same knocking characteristics as 91% iso-octane mixed with 9% n-pentane.

What is knocking:
A combustible mix of fuel and air can burn 2 ways:
1) smoothly via a flame front - ideal, gives smooth power delivery.
2) explosively like a detonation - which gives a characteristic "pinging" sound called "knocking", damages the engine over time, and gives poor performance. (By the way, diesel fuel burns this way all the time, that's why diesels sound awful.)

Until recently engines were designed for a minimum RON fuel and that was that. As 95 RON is the widespread minimum in Europe, most European cars are built for 95 RON. The WP, WQ Fiesta require 95 RON. There is NO advantage to using 98 RON in such cars, except under heavy load, low temperatures (startup) and a few other conditions.

Recently, engines have been given knock sensors designed to detect the onset of knocking and adjust timing (& perhaps fuel mix & other things) to compensate. See:
http://wiki.answers.com/Q/What_does_a_knock_sensor_do

So what's up with the Aussie WS Fiesta?
I've been running my WS Zetec on 91 RON with no sign of knocking and smooth power delivery. Ford recommends any fuel from 91-98 RON:
http://www.ford.com.au/servlet/Conte...=FOA&c=DFYPage
But the run the mouse over footnote 10 on the same site and it says:
"For optimum performance use fuel of 95 RON or higher"

On this site when they compare the Fiesta to other cars:
http://www.ford.com.au/servlet/Conte...88217&site=FOA
They have a footnote that states (press MORE), "All performance data measured using 95 RON fuel."

So my conclusion is: If you want your full 88kW and best fuel economy then use 95 RON fuel. There is no extra benefit from 98 RON fuel. 91 RON fuel will do the car NO harm whatsoever as the engine will adjust to the lower fuel RON, but in doing this some performance (how much?) will be lost. I've no idea whether the loss of efficiency with 91 RON makes 95 RON more cost effective. I guess you can only experiment - I will.

It would be great if Ford had a little more technical information out there!
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