[Diy_efi] The Hunt effect

[dh at busb.com]

I could be wrong about some of the following stuff, please correct if you are certain.  But for the sake of discussion.

Flash point is an interesting discussion in itself.  The flash point of commercial gasoline is a combination of many, many factors.  Gasoline is a blend of different distillates.  The variation in chemistry is head spinning.  The flash point of a blended fuel is determined by the constituants but is not necessarily a ratiometric thing.

Thing is in an engine the flash point is WAY more complicated because of time.  For example, alcohol has a very large heat of vaporization and will reduce the heat available to raise the mixture to a particular temperature.  Eventually, the alcohol fuel will get to near the same temperature, but not at the same time. Near is the key word there.

So the flash point of an oxygenated fuel will occur at a later time, even if the flash temp is the same.  This requires greater timing advance for oxygenated fuels.  Here in the south we always have alcohol blends and the 'retarded' fuel increases the need for a higher quality of petro to 'kick off' the combustion.


There is a chart of each constituent of a fuel and that constituent's flash point. Those charts are hard to find, I think primarily because they are difficult to generate.  Where a heavier distillate might have a flash point of 450 degrees a lighter might have a flash point of 375 degrees. (key word might) More of the lighter fuel will decrease the flash point and versa visa.  But the effect is far from linear.  As the lighter fuel kicks off it creates more pressure and temperature for the heavier fuel, so a little of the lighter fuel goes a long way!

dh


  ----- Original Message ----- 
  From: Bret Levandowski 
  To: diy_efi at diy-efi.org 
  Sent: Tuesday, October 04, 2005 10:39 AM
  Subject: Re: [Diy_efi] The Hunt effect


  Octane rating is the fuel's resistance to detonation under heat and pressure with no 'true' ignition source. I believe it equates to a higher flash point. So, running high octane in a lower compression motor that requires low octane could actually lower MPG as the lower comp. would prevent the mixture form lighting off completely. In the case of the caddy, using lower octane would cause detonation which would be picked up by the knock sensor retarding the ignition (which I believe he stated). This would then require more fuel to reach the same 'power' point reducing MPG.  Ski

  Dustin Lof <bubblesjrtwo at yahoo.com> wrote: 
    I have never heard anyone say more octane produces
    better mileage, I was always told to burn, and to
    reccomend to my customers the lowest octane it wont
    ping on....? what is the ethanol content in the 87
    v/s 89. Around here it is 10% corn ethanol in
    everything but the 91 and your not suposed to burn the
    91 unless your driving a collector car or a snowmobile
    and most snowmobiles are jetted to run on 10% now too.
    I am just wondering if the alchol content is what is
    effecting the mileage. I know its eating up a lot of
    fuel pressure regulators. There is also the r*m/2
    factor which is how octane is rated the r factor is
    what the guys in white coats in a lab say the octane
    rating is the m is how it actually performs in a
    single cylinder test motor. If one of those numbers
    is skewed, for example crappy gas doped up with an
    additive package it will make your mileage compairson
    way off. it is all way more scientific than all this
    but that is all I remember from the one day we talked
    about this in college so please dont ask me to explain
    in any more detail. just my $0.02---Dustin Lof 

    p.s. last week Minnesota mandated all diesel be 2%
    bio-diesel. Made from soybeans.
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