[Diy_efi] The Hunt effect

Wed Oct 5 20:23:54 UTC 2005

You are correct, but the one key thing to keep in mind is that the flash
point does not determine what it takes to begin the burn inside the engine.
The spark plug is not causing the fuel-air mixture to flash (explode), it's
causing it to burn.  What happens when you start blending alcohol-based
fuels with gasoline-based fuels is that you are mixing two fuels that have
different flame front speeds.  When you start changing the flame-front speed
by changing the chemical composition of the fuel you're burning, you will,
by necessity, start having to change ignition timing.

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of dh at busb.com
Sent: Wednesday, October 05, 2005 3:29 PM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] The Hunt effect

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 <mailto:skishop69 at yahoo.com>  

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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