[Diy_efi] The Hunt effect

[John Gross]

It is a common misconception that higher octane fuels burn slower.  There
are two different phenomena which are commonly interchanged, but they are
not the same.  One is flame front speed.  Flame front speed is the rate of
burn of the fuel air mixture itself, and is determined SOLELY by the
chemical composition of the fuel.  The Octane content makes a negligible
difference in this value.  The other is the rate of burn of the entire
fuel-air charge inside the cylinder.  There are a LOT of things that affect
the rate of burn inside a cylinder, such as the spark plug location and
number, chamber shape and volume, dynamic compression, and turbulence.  

Twin plug heads burn faster simply because you're starting the burn from
multiple locations, so the flame front has less distance to cover to consume
the volume in the chamber.  

Higher turbulence inside the chamber once the valves are closed increases
the rate of burn by increasing the rotational speed and number of eddy
currents, while simultaneous DECREASING their size...

The best way to visually describe the difference with the eddy currents is
to imagine you're watching a show on the History Channel about WWII.  Think
of the aerial videos of the atomic bomb going off....one big flash in the
middle moving outwards.  That's analogous to what a low turbulence chamber
looks like.  Then picture an aerial video of carpet bombing, where there are
a lot of little flames spread out over the same area as the atom bomb.
That's analogous to what a high turbulence chamber looks like.  Yes, the
main flame is big and powerful, but it takes a while to reach the outer
edges of the area.  High turbulence chambers can burn all of the air-fuel
mixture faster because there are more little flames throughout the volume
burning at the same time.  That's why MBT on a street Chevy head may be
36-38 degrees BTDC, but a Chevy SB2.2 NASCAR head (still a 2 valve
wedge-head chamber) only needs 32 degrees of timing.

4/5 valve heads need less timing because the plug is centrally located in
the chamber, whereas the plug in a 2 valve head is offset to one side of the
chamber, resulting in a larger distance for the flame to cover.

Dynamic compression adds to the rate of burn because it increases the
intensity of the turbulence. 

There are a few other minor factors at play, but in the end, the actual
"flame front speed" is determined by the chemical composition of the fuel.
Octane's contribution to that value is minimal.  The overall rate of burn in
the cylinder, however, is affected by a number of things, though octane does
not play into them at all.  What sometimes throws people is when they see
"equivalent octane" ratings for diesel fuel, Methanol, Nitro, and other
fuels which have a totally different chemical compounding than gasoline, and
therefore a completely different "flame front speed".  "equivalent octane
rating" is simply referring to the fuels' resistance to instantaneous,
complete combustion due to heat and pressure without a specific point of
ignition.

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of Gianmarco Rizzo (BE/EAS)
Sent: Tuesday, October 04, 2005 12:08 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] The Hunt effect


i might be totally wong, but i seem to remember that in the gasoline FAQ is
said that certain higher RON fuels tend to burn slower due to their chemical
composition, not because of the higher RON

any comment on this?


gm


> the flame front speed does NOT change with higher octane, and it's actual
burn properties do not change on the global scale Click here to donate to
the Hurricane Katrina relief effort. 
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