Topic: Octane Ratings - what's on the pump?

[pops]

I'm having trouble getting comfort with what the octance ratings on our pumps really mean.  Our vehicle manuals often recommend octane ratings of XX RON or YY AKI or ZZ average of RON&MON. I understand RON is 'research octane number' and AKI is 'anti knock index' and MON is 'motor octane number' and recently learned of DON which is 'roaD octane number' - same as average of RON&MON. Makes it confusing for me to know if I'm really putting in the fuel quality that is recommended.  Can anyone give me a convincing education or source for Canadian pump ratings?

[tpl]

from an early thread ( search on  eurosuper) There are MANY threads on pump gas.
our Cdn/US pump octane is (RON+MON)/2  
Euro pumps show RON.  

US          Germany        RON   MON  (RON+MON)/2  

unleaded 87    Normal91     91    82.5    86.75  
midgrade 89    EuroSuper95  95    85      90  
premium 91-94  SuperPlus98  98    88      93  


from http://www.whnet.com/4x4/octane.html


and some more from another recent thread

Octane rating is a measure of a fuels resistance to pre-ignition( detonation or, if you like, making like a diesel)  

Regular Otto cycle gasoline engines are not built to be diesels, they are built to burn a gas/air mixture not explode it. The knock sensors in engines "listen" for a particular set of sounds and then retard the ignition ( detune the engine) until these sounds go away... then they allow some ignition advance until it comes back and so on. So as Artic has pointed out ( very patiently, many times in many threads) if the knock sensors are doing their job with fuels of too low a grade the piston tops are being continually pounded by detonation... in the short term most modern engines will survive this but it will not help the life of pistons, rings, rod bearings etc.  

In the specific case of forced induction engines, instead of the mixture being sucked into the engine it is being blown into he engine and some proportion of the engines compression stroke is already done....so the mixture is closer to detonation anyay... its usually hotter as well intercooler not withstanding.  

So you need fuel with a higher anti-Knock index or octane rating. One excellent additive is ethanol hence the preference around here for Sunoco 94 in turbo engines. Lead compounds are another but no longer allowed etcetera.  

Low Rpms are not necessarily safe. The engines need for octane is highest at the torque peak which is just where granny's automatic transmission will run the engine under acceleration.  


no plagiarism here... these are my posts but there are MANY informed comments on this forum on this topic

(Message edited by tpl on July 02, 2005)

[HeliDriver]

Here's a blurb from Shell Canada's website:

"The value that relates most closely to actual driving conditions is the average of these two values: Road Octane Number = (RON + MON)/2. This Road Octane value is the one referred to in Shell stations: Shell Bronze gasoline has an octane rating of 87, Shell Silver is 89 and Shell V-Power is 91.

Occasionally, less scrupulous Canadian gasoline outlets will use the confusion of these different octane measurements to exaggerate their octane rating claims, by advertising their fuel's Research Octane Number - which will be higher than the Road Octane Number. It is also a common practice in many European countries to advertise the Research Octane Number on their pumps, so you may see unexpectedly high octane values when travelling abroad. In Canada, motorists should always be sure that the octane number a vendor advertises is its Road Octane value, not its RON."

I haven't checked in a while, but I seem to recall that fuel pumps have a sticker on the side that details exactly what the octane rating is, and how it was calculated.

More info at http://www.shell.ca/code/motoring/tips/gasoline.html

[Ed]

Just for reference:

RON = Research Octane Number (Already mentioned)
MON = Motor Octane Number

[tpl]

Just for interest.  They test this in a weird little one cylinder engine  comparing the fuel to a mixture of iso-heptane and iso-octane.  I think heptane always dentonates and octane doesnt or something like that

[Ed]

Arg! All the organic chemistry I learned in high school is trying to resurface!

Heptane is the name of a hydrocarbon containing 7 carbon atoms and (usually) 16 hydrogen atoms. Octane usually contains 8 carbons and 18 hydrogens. But as soon as you start adding prefixes and such, it changes the shape and characteristics of the molecule.

As a general rule of thumb, the higher the number of carbons, the "thicker" and harder the substance is to detonate. Methane and propane (1 and 3 carbons, respectively) are usually found in gaseous state. Heavier atoms like heptane and octane are liquids. As soon as you start getting into the high carbon counts, you end up with waxes and stuff.

This is totally off the cuff, so if someone has a better orgo background than I do, they can offer up a better explanation.

I tried, though.

[pops]

Thanks for the help guys - I actually searched this site and even Shell Canada's site before posting - I guess I didn't look hard enough.  I feel comfortable that road octane [RON+MON)/2] is what is being quoted on the pumps and can relate it to my vehicles' manuals.

[robsaw]

"If the knock sensors are doing their job with fuels of too low a grade the piston tops are being continually pounded by detonation... in the short term most modern engines will survive this but it will not help the life of pistons, rings, rod bearings etc."

I think this is an exageration.  There is a progression from light-ping to heavy knock.  The knock sensor generally backs things off well before anything that can properly be described as "detonation" occurs.


"Just for interest. They test this in a weird little one cylinder engine comparing the fuel to a mixture of iso-heptane and iso-octane. I think heptane always dentonates and octane doesnt or something like that"

Some more info:

The ASTM Standard Procedures for determination of octane numbers require the use of two Waukesha co-operative Fuel Research (CFR) engines (Waukesha Engine Div., Dressier Industries, Inc., Waukesha, WI). The Research Engine runs at 600 rpm, with a controlled air mixture and temperature as defined in ASTM D 2699. The Motor Engine runs at 900 rpm, with a controlled air mixture and temperature and controlled fuel temperature as defined in ASTM D 2700. These engines, or "knock engines," necessarily consume the fuel sample and a series of standard fuels during the test procedure.

There are also octance analyzers (secondary test methods) that uses and Infrared spectrometer to "predict" octane rating.  Much better for field tests than carrying a controlled engine lab around.

And to finish off, a few octane/gasoline myths demolished:

- Gasoline is not a mixture of octane and heptane, it may consist of hundreds of various hydrocarbon molecules.
- Octane is in no way a direct indication of gasoline parameters related to:

energy content
combustion temperature
ignition temperature
speed of flame propagation
fuel economy
emissions
propensity to form deposits

Octane rating indicates one and only one thing, susceptability to spontaenous ignition under the RON/MON test methods.

[tpl]

Right: Robsawatsky  you put it much better than i did  except...

any preignition is forming high frequency pressure waves of varying amplitude within the combustion chamber. Causing an effect similar to if not identical to cavitation  which sooner or later will do damage

[safristi]

Look Ma !!! no cavitations...must be that Sunoco94 dear!!! smile.......

[articsteve]

any preignition is forming high frequency pressure waves of varying amplitude within the combustion chamber. Causing an effect similar to if not identical to cavitation which sooner or later will do damage

I agree.  Knock sensors are for rare occasions when correct octane is unavailable.  For the knock sensor to activate damage to some degree is already happening. Boom Boom Boom