We used to do this sort of stuff in College (Mech Eng.). We had a quartz
window in the top of the piston
with a mirror down in the crankcase being looked at by some very, very
bright lights and a
very, very, very fast camera. We could watch the burn.
Only a Diesel engine goes bang hence the loud knocking you hear especially
from the older types of motor. The engines are much stronger than petrol
ones to be able to withstand these
fast explosions.
A petrol engine does not go bang!
If it does it punches holes in your pistons.
As the plug ignites the mixture a flame front propagates outwards at a
fairly constant rate. As Chris said this
fairly constant and the trick is to get the pressure wave to start pushing
the top of the piston just as it has
passed TDC. At idle this may mean we need to ignite 5 degress BTDC to get
the flame front to hit the piston at TDC.
At higher revs the piston becomes faster but not the flame front so we need
to progressively
advance the ignition. All sorts of things affect the burn. For example just
putting bigger valves in bigger ports
can be detrimental as the gas velocity becomes too low and the air fuel
mixture coming from the carbs does
not get swirled around enough in the chamber to give proper ignition
characteristics. This is all great to watch
on the camera.
What happens on the leaner mixtures I cannot remember when I get back to
Ireland I'll dig out my old notes.
If I can remember rightly the trick is to get the charge to burn fully and
uniformly without explosion! This is not
easy and has many variables not least of which as I said is the swirl effect
in the chamber. Hence many mods
with bigger carbs, manifolds, ports etc. work really badly if nothing else
is done. If the charge is not fully
mixed then some of the petrol does not burn properly as the local air/fuel
ratio does not conduce ignition. Raw fuel
then exits the exhaust valve not helping power or consumption and probably
causing destruction of the
catalyser as it gets burnt here. A bigger spark will get the job done better
but it needs loads of energy. I think
the normal Sprite coil draws 4-5 Amps. To get more spark you need more amps!
and the points start to wear
really quickly. Electronic ignition does not wear. A lot of the modern cars
have one coil per plug mounted at
the plug itself. Some cars have twin spark plugs (Alpha Romeo). I think if
you can do it, it is better but the
gains are very cylinder head dependant. What works on one car/mod level may
not bring equal gains on
the next. If your mixture is burning well anyway it won't bring very much.
But you can't just assume that
because it's firing that it's burning well and fully.
If you want power you need to use up all the air you can in the chamber so
you generally run rich. If
you can run leaner and still use all the air then all the better.
(I've just remembered - the flame propagation velocity reduces as the
mixture becomes leaner - I think)
As to the vacuum advance. I thought that this was really only to increase
MPG for road going cars. Race
cars run at WOT most of the time so there is not much vacuum. Road cars
generally run at part throttle.
They are then set up slightly leaner for part throttle settings so the flame
front moves slower so you need
more advance.
OK that's enough for now. It's a bit jumbled but maybe food for thought. I
didn't expect to write so much
but it's a long time since I've done any stuff like that and something new
kept coming back to me.
Sorry if it's long winded or unintelligible.
Steve
> -----Ursprüngliche Nachricht-----
> Von: Chris Kotting [SMTP:ckotting@iwaynet.net]
> Gesendet am: Donnerstag, 25. Juni 1998 18:21
> An: 'Editors, Molecular Vision'; spridgets@autox.team.net
> Betreff: RE: voodoo
>
> Actually, the major factor in having the "bang" happen closer to the
> "spark" isn't the exhaust valve opening. It's that the "bang" has to
> happen when the piston is just about at TDC (actually a tiny bit after).
> Much later than that and you lose power (you don't get the maximum "push"
>
> out of the "bang", making more of the "bang" result in waste heat), much
> before that and you get knocking and engine damage (the "bang" tries to
> turn things the other way). (The reason that you need to advance the
> ignition timing at higher RPMs is that the time lag between "spark" and
> "bang" is relatively constant, but the faster the engine turns, the more
> degrees of rotation happen in that time interval.)
>
> It seems to me that if you can minimize that time lag, you stand a better
> chance of being able to make the "bang" happen consistently at the optimal
>
> moment. Can anybody confirm/correct this supposition?
>
> Chris Kotting
> ckotting@iwaynet.net
>
> On Thursday, June 25, 1998 12:50 PM, Editors, Molecular Vision
> [SMTP:jboatri@emory.edu] wrote:
> > True and I agree with you and Les: throwing in a hodgepodge of smoke and
> > mirrors may do a lot more harm than good. OTOH, like Chris Kotting, if I
> > can avoid gapping points by using a Hall cell, I'll use it if I can
> afford
> > it. Having sold a couple of parts lately, I could afford it. A higher
> > voltage coil allows one to increase spark plug gap, but as Chris points
> > out, the coil and other downstream components (be they Hall cells or
> > points) must operate at higher temps and so could fail, or rather, have
> a
> > lower mean time between failures. The issue of whether increased spark
> plug
> > gap does _anything_ beneficial or harmful appears to be the largest
> point
> > of discussion.
> >
> > >From a practical standpoint, if 0.025 plug gap ignites a real-world
> range
> > of mixtures that vary with winter/summer grade fuel or other fuel
> quality
> > issues, density altitude variations, and acceleration demands, then
> there
> > is little reason to go to a larger gap. But two questions remain: (1)
> does
> > a 0.025 gap always ignite real-world ranges of mixtures? and (2) can
> > further performance (whether economy or power) be realized with an
> > increased gap leading to a shorter lag time between spark and bang (to
> > quote Chris). Would this allow for more of the mixture to be burned
> before
> > the exhaust port is opened? Is a faster burn better inherently, or must
> > many other variables (mixture quality, cylinder and piston design,
> timing,
> > etc) be matched?
> >
> > Finally, let's face it, the 500 lb gorilla here is that those gold Sport
> > coils look really cool...:)
> >
> > At 9:00 AM -0500 6/25/98, Shawn J. Tobin wrote:
> >
> >
> > > I used to think that throwing a kluge of performance parts together
> would
> > > result in a really wicked ride. This kind of thinking was due to a
> vivid
> > > imagination and the naive assumption that advertising claims were
> > > guaranteed to be true because the government kept them to their word.
> > >
> > ...
> >
> >
> >
> > Jeffrey H. Boatright, PhD
> > Senior Editor, Molecular Vision
> > http://www.molvis.org/molvis
> > Mailto:jboatri@emory.edu
> > 404-778-4113
> >
> >
|