[Diy_efi] The Hunt effect, spark energy, films ?

[John Gross]

Mike, thanks.  There is a difference in the energy required to start the
kernel between say 92 and 98.  HOWEVER, the difference in required energy
(in the same engine) is small.  Now, if you are talking about an engine
built to run on 92 or an engine built to run on 98, there will be a much
greater (relatively speaking) difference in required spark energy, however,
that is attributable much more to added compression than to the fuel itself.
In order for the kernel to form at the plug, the gas (whatever blend of air
and fuel that may be) inside the gap must be ionized.  The higher the
ambient pressure of the gasses in the gap, the higher the spark energy
required.  Keep in mind this is dynamic compression, so it would include
such factors as volumetric efficiency of the engine on that cycle, AFR of
the engine at that point, and compressibility of the fuel, among others.  

A naturally aspirated gasoline-based engine running at a lambda of .9 (~13:1
AFR) and 100% VE needs only about 20-30 mJ of spark energy to fire the
cylinder.  A top-fuel dragster engine, however, with a roots-style blower
and nitro fuel (with an AFR of ~2:1) needs about 2-3 *J* of energy to fire
the cylinder...roughly 100x the spark energy.  If you touch a plug wire on a
gasoline engine while it's running and there's a nick in the insulation, you
get a nice little shock, swear a little, hope you weren't grounded through
your pants zipper to the fender, and go on about your business.  If you
touch a plug wire on a top-fuel motor while it's running and there's a nick
in the insulation, you're dead.

Unfortunately, I do not have any high speed films of combustion tests,
however, "Internal Combustion Engine Fundamentals", by John Heywood, has
some great high speed photos.  The book contains photos of kernel formation,
comparisons of flame-propagation in cylinders with different plug
configurations and swirl designs, normal combustion vs. knock, etc, etc.

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of Mike
Sent: Tuesday, October 04, 2005 11:10 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] The Hunt effect, spark energy, films ?

At 12:10 AM 10/5/05, "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. 

I can understand this can be the case for different fuels, ie going from
'normal'  to oxygenated etc. But would it necessarily be the case if simply
changing the mix of a non oxygenated fuel from say 92 to 98 ron ?

Your technical descriptions are great, well articulated and I look forward
to you staying on the group and hope you wont need as much of a thick
skin as others develop here over time ;-)

btw: As you have clearly made significant study into combustion, is there
any chance you have high speed films of combustion from any test
engines specifically designed to show the dynamics ?



Regards from


Mike
Perth, Western Australia
VL Commodore Fuse Rail that wont warp or melt !
Twin tyres for most sedans, trikes and motorcycle sidecars
http://niche.iinet.net.au
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