From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Newsgroups: sci.chem
Subject: Re: Super Unleaded Petrol
Date: Sun, 13 Jun 1999 16:48:10 +1300

In article <37631F27.42C01E6E@worldnet.att.net>
Eric Lucas <ealucas@worldnet.att.net> writes:
>Bruce Hamilton wrote:
>> Eric Lucas <ealucas@worldnet.att.net> wrote:
>> >The octane number is related to how fast a flame front propagates
>> >through the air/fuel mixture.
>>
>> No it is not. It is related to the ability of the unburnt end
>> gases ( the hot, turbulent fuel/air mixture that hasn't yet seen
>> the flame front, even though the spark has fired ) to resist
>> autoignition under the conditions in the rating engine.
>
>Think a little, Bruce.  Do you reasonably think that the two phenomena
>are not related to each other?

Thinking hurts, and should be avoided.  The stoichiometric flame speeds
of hydrocarbons under identical conditions don't vary very much
between similar sized members of the groups found in petrol, such as
alkanes, hence flame speeds aren't a major parameter for hydrocarbon-air
octane ratings.

The minimum autoignition temperatures vary significantly, and are the
generally-accepted dominant factor for assessing the octane rating of
hydrocarbon fuels under specific engine test conditions.

Obviously, other properties can affect the octane rating under specific
engine testing conditions, such as their volatility, but remember that
the reference compounds, n-heptane and i-octane, were chosen
because they had similar volatility and physical properties.
Consider the following, from the Gasoline FAQ:-

RON = Research Octane Number
MON = Motor Octane Number
BP = Boiling Point in C at 760mmHg
 d = Density in g/ml at 15C
AIT = Minimum Autoignition Temperature in C under test conditions.

                                      RON   MON    BP      d     AIT
        n-butane                      113 : 114 :  -0.5:  gas  : 370
        n-pentane                      62 :  66 :  35  : 0.626 : 260
        n-hexane                       19 :  22 :  69  : 0.659 : 225
        n-heptane                       0 :   0 :  98  : 0.684 : 225
        n-octane                      -18 : -16 : 126  : 0.703 : 220
        n-decane                      -41 : -38 : 174  : 0.730 : 210
        n-dodecane                    -88 : -90 : 216  : 0.750 : 204
        n-tetradecane                 -90 : -99 : 253  : 0.763 : 200

        2-methylpropane               122 : 120 : -12  :  gas  : 460
        2-methylbutane                100 : 104 :  28  : 0.620 : 420
        2-methylpentane                82 :  78 :  62  : 0.653 : 306
        2,2,3-trimethylbutane         112 : 112 :  81  : 0.690 : 420
        2,2,4-trimethylpentane        100 : 100 :  98  : 0.692 : 415

        cyclopentane                  141 : 141 :  50  : 0.751 : 380
        cyclohexane                   110 :  97 :  81  : 0.779 : 245
        methylcyclohexane             104 :  84 : 101  : 0.770 : 250

        benzene                        98 :  91 :  80  : 0.874 : 560
        toluene                       124 : 112 : 111  : 0.867 : 480
        ethyl benzene                 124 : 107 : 136  : 0.867 : 430
        meta-xylene                   162 : 124 : 138  : 0.868 : 463
        para-xylene                   155 : 126 : 138  : 0.866 : 530
        ortho-xylene                  126 : 102 : 144  : 0.870 : 530

I can't be bothered obtaining and adding the stoichiometric flame
speeds to the above, but there is no significant difference between
the stoichiometric flame speeds of individual hydrocarbons of
the same family found in petrol, but there are some minor
differences between hydrocarbon families, such as alkanes,
olefins, and aromatics.

>If it is only autoignition that higher octane prevents, then how do you
>explain the fact that higher octane can make up for the knocking that
>you get on advancing the spark?  After all, by definition, autoignition
>occurs before the spark has fired, so it shouldn't matter whether or not
>the spark is advanced.

Autoignition that is responsible for Knock ( which is what the Octane
rating scale measures under the engine test conditions ) usually occurs
*after* the spark has fired the fuel/air mixture. The timing of the spark
is not required to describe the ability of the components of the end
gases to autoignite, other than the desired flame front has obviously
not reached those end gases under the knock rating engine test conditions.

Minimum autoignition temperatures is usually measured in a flask on
a hot plate, and no ignition source is involved - other than the hot walls of
the flask. Flame propagation rates in hydrocarbon/air mixtures do not
change much between individual family members usually found in petrol.
Flame propagation rates are very affected by the extremely hot, pressurised,
and turbulent conditions in an engine, hence the use of totally different test
methods to determine the actual octane requirement of engines.

If the fuel/air had ignited *before* the spark, then it would have been
considered *preignition* ( which starts the pressure pulse on the
piston before it should - whereas knock superimposes additional
pressure pulses on top of the normal pulse ). Autoignition does not
usually happen ahead of the spark, provided the spark is reasonably
timed, because it's the hot, turbulent, highly-compressed, fuel-air
mixture near Top Dead Centre that is the breeding ground for knock.

Under ideal conditions, the end-gases gases should be almost ready
to autoignite when the desired flame reaches them, and some engine
management systems with knock sensors can advance the timing
to incipient knock, and then retard the timing slightly. If the spark is
advanced, there is more time for the end-gases to autoignite, so by
retarding the ignition, the opportunity for the end gases to reach
the autoignition point is decreased.

If a fuel is used that can reach a higher temperature before autoigniting,
then the spark can usually be advanced if necessary, but there still is
an optimum advance value for each engine speed and load that is
determined by the engine design.

The Gasoline FAQ points to references, should you wish to pursue
the correlation of autoignition temperature with knock.

                 Bruce Hamilton