[Diy_efi] The Hunt effect

[John Gross]

;-)  Nope, but I did stay at a Holiday Inn Express last night  ;-)  All
kidding aside, I have a master's in Internal Combustion Engine Design.

 

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of Jim Butterfield
Sent: Tuesday, October 04, 2005 3:35 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] The Hunt effect

 

WOW John are you a gas engineer???? great explinations of the reactions
inside an engine

 

jim

 

 



John Gross <jogross3 at hotmail.com> wrote: 

It takes more energy to cause the fuel to "flash" without a specific
ignition point.  The spark from the plug on your car is more than enough to
start the burn on just about any unleaded fuel.  I say unleaded to try to
keep the discussion within reason in terms of octane ratings and appropriate
CRs for the engines.  It takes more spark energy to light off a 115 octane
fuel than it does an 87.  However, when talking about street-driven cars,
any delay in the formation of the kernel (the initial point of combustion
inside the spark plug gap) is so minute, that it is not worth considering.
Additionally, what makes it harder and harder to light the air-fuel mixture
isn't just the fuel, but the dynamic compression.  That's why blower motors
typically need a more robust and powerful ignition system..the turbulence
inside the cylinder in a blown motor (high dynamic compression) can actually
extinguish the kernel, thus stopping combustion.  

 

One thing to keep in mind, though, is that the kernel starts the combustion
at the plug, but once started, there are multiple turbulent pockets of
burning gas traveling around the combustion chamber, like a bunch of tiny
eddy currents.  Quite frequently, especially in engines where the octane
rating of the fuel used is borderline to the dynamic compression of the
engine, some of these eddy currents will surround a pocket of unburnt fuel
and air (end gas), and cause the local temperature and pressure to rise to a
point where the pocket will explode, instead of burn, causing pinging.
Usually, this is not enough to set off the knock sensor, but when it
explodes like that, the release of pressure is instantaneous, and therefore
does not contribute to the overall cylinder pressure..therefore you've lost
some of the potential pressure.  So, from the uniformity of combustion
standpoint, running a little extra octane for WOT, max power may help some.
But also keep in mind that the fluid dynamics inside an engine are chaotic
in the truest sense of the word.  A decent engine will have a cycle-to-cycle
power production variance of 5% for every given cylinder, meaning that
cylinder 1 on a 400hp V8 may make 50 hp on this cycle, but only 47.5 hp on
the next combustion event, or it may make 52.5 hp on the next event.  It's
all very chaotic.  A lot of the R&D that goes into high end racing engines
is in the area of reducing cycle-to-cycle variance and the overall
cylinder-to-cylinder balance, NOT maximizing the output of any given
cylinder.

 

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of Bret Levandowski
Sent: Tuesday, October 04, 2005 11:24 AM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] The Hunt effect

 

But if higher octane fuel takes more energy to light off, then the ignition
point would happen later than it should causing a power loss. Yes??? No???
I'm thinking out loud. Or would the ECM take these factors into account and
advance the timing to compensate? This being the case, older non-controlled
motors would show a loss unless the timing were manually adjusted. As slow
as the older ECM's were (80's), would they be able to adjust fast enough to
compensate? (By slow, I mean compared to todays comm speeds.)  Ski

John Gross <jogross3 at hotmail.com> wrote: 

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