[Diy_efi] Lean Cruise

Fri Apr 1 06:22:13 UTC 2005

Reducing the amount of fuel does in essence 'put the flame out'. However, Geoff, you are correct in that the flame will 'look' for another fuel source. In the case of aluminum or steel, the temp required is a few thousand degrees. Well above the normal to higher than normal combustion temps seen. In order to achieve this in an IC engine (and it does happen) the AFR would have to be so far out of wack (I'm guessing 18-20:1) along with some possible timing miscalculations that you would hope the tuner would know better. Everyone makes mistakes though. As for as 'melted' pistons go, if I remember my reading correctly, the most common cause is detonation. Everyone here has valid points and no one is 'dead on wrong'. I think this one got a little too hot and maybe a little too personal. That being said, I'll shut up and go back to my hidey-hole before the pot shots start.     
        Ski  
Geoff Harrison <geoff_h at smartchat.net.au> wrote:
Hi all. If this is an April 1st joke, well done. It "woke up" the list.
If not, a word of warning. Reducing the fuel available during combustion 
does not put the flame out as some one suggested. The flame looks for 
something else to consume. In an IC engine, aluminium may become the fuel. 
This has been documented many times. Phil Irving makes several references to 
holed pistons in his book "Tuning For Speed". Its a bit dated now, but the 
physics are still the same.

The same process is used for oxy-acetylene cutting. Turn up the oxygen and 
burn steel.

Take care on lean burn.

Geoff 
From: "gary" <gas- at charter.net>
To: <diy_efi at diy-efi.org>
Subject: Re: [Diy_efi] OL operation - How lean at cruise?
Date: Thu, 31 Mar 2005 16:51:01 -0600

----- Original Message ----- 
From: "Michael Richards" 
To: ; 
Sent: Thursday, March 31, 2005 10:40 AM
Subject: RE: [Diy_efi] OL operation - How lean at cruise?

> As Hugh mentions EGTs are important to watch as running too lean
> can result in burned valves. I see this a lot in the honda world where
> kids swap their heads and continue to use factory computers that are
> mapped for less VE.
>
Peak EGTs occur at the stoichiometric ideal AFR of 14.65-70: 1.
Making the air fuel mixture richer or leaner from that point, (all other
factors being equal)....will result in LOWER, NOT higher .... EGTs.
With the ignition timing set approopriately, how can you possibly
generate more combustion heat with less fuel? Not logical.
>
> I think you will find the main economy gains under cruise will come
> from ignition and not mixture. Without some special features for lean
> running you will be hard pressed to cruise at 16:1 and leaner without
> mis-firing.
>
Actually, main economy will come from a leaner AFR, coupled with the
correct timing.

AFAIC, misfire from too lean an A/F mixture, is a false myth. In reality,
continual leaning can be conducted until which time, the combustion flame,
due to a lack of heat, just goes out. Sorta like turning down the gas valve
on a kitchen range, less and less heat (not more) is generated until the
flame, from a lack of fuel finally goes out. The misdiagnosed so labeled
'misfire' is actually the result of the flame in one or more cylinders going
out ahead of the others, due to the normal lack of a precise equal AFR
in all cylinders. The physical appearance/feeling of a misfire, is actually
in reality, one or more cylinders going dead, no different than if a plug
suddenly stopped firing.
>
> On many V8's you run alot of advance during cruise. I suspect this
> is related to their archaeic head design (insert other V8 digs here).
> In short if you've got a lot of exhaust left in the cylinder you will need
> more degrees in there.
>
The reason for the need/necessity to advance the ignition timing in a
lean burn application, is due to the slower burning flame front, which
consequently will take longer to complete. Because the point of peak
combustion/heat is now later than what is closer to optimum, more
undue heat is generated/experienced in not only in higher EGTs, but
higher temps in the cylinder as well. The AFR that generates the
quicker flame propagation is that which generates the most power,
appr. 12.5:1. The flame travel is slowed either way, from that point.
>
> Realistically you need to put the car on the dyno to find MBT for
> cruise but you can try adding 3-4 degrees in the cruise part of you
> map and seeing how much of an economy effect it has.
>
> I worked on a 302 powered Ranger some years ago. The customer
> was running 16 degrees of advance during cruise and taking about
> 22l/100km. I leaned it out a bit and found MBT to be up in the 40
> degree advance range. It's been a while but 44 degrees sticks out in
> my mind. End result was 12l/100km. Don't blindly try this but it is an
> example.
>
> -Michael
>
> > -----Original Message-----
> > From: diy_efi-bounces at diy-efi.org
> > [mailto:diy_efi-bounces at diy-efi.org]On Behalf Of hugh at sol.co.uk
> > Sent: Thursday, March 31, 2005 2:59 AM
> > To: diy_efi at diy-efi.org
> > Subject: RE: [Diy_efi] OL operation - How lean at cruise?
> >
> > I read on here a while back that an AFR of 16 : 1 was the most
> > economical cruise mixture. I have experimented with 16 : 1 and
> > 17 : 1 without much to choose between on economy.
> >
> > My Exhaust gas temperature creeps up to 800?C when doing about
> > 90 MPH on the UK motorways at 16 : 1,

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