Date: Thu, 13 Jan 2000 23:12:14 -0600
Reply-To: John Rodgers <inua@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Rodgers <inua@HIWAAY.NET>
Subject: Re: Fast German Auto chip and rockers
Content-Type: text/plain; charset=us-ascii
Lawrence, just a few thoughts.
I relate more to aircraft engines but I think the principles apply.
In a piston aircraft engine, the fuel air mixture can be adjusted by the pilot while
in flight. This allows him to control his fuel burn and power which varies
considerably as he goes up and down in altitude. The up and down affects air density,
and the density affects the about of oxygen available for the fuel, so what is the
correct fuel/air ratio on the ground is not the best fuel air ratio at altitude.
Typically most piston aircraft engines are run at maximum rpm for take off to develop
maximum power. As soon as the airplane leaves the ground and various drag inducing
parts are closed up the airplane with all that power applied accelerates rapidly. The
throttle setting is reduced to provide adequate power to climb to altitude. Upon
reaching altitude the pilot will adjust the power for the best performance.
There are three settings generally worked with. Lean Best Power, Peak Power, and Rich
Best Power. In most of todays airplanes the settings for all this can be monitored
very closely with sensitive exhaust gas temperature gages. The pilot has two power
controls available to him. The throttle and the mixture contol. With throttle he
controls the airflow through the engine. With mixture he controls the fuel. With the
throttle set to any given position, the pilot can vary the amount of fuel, and by
watching the exhaust gas temperatue gage, he can tell exactly what the engine is
doing.
In common operation, for best power performance, the throttle is set, and the mixture
leaning adjustment is begun. The throttle is not touched. As the leaning continues,
the exhaust gas temperature gage needle will begin to move, showing hotter and hotter.
Eventually the EGT as it is called, will peak out. That is, it will reach a point
where the needle no longer shows an increase in temperature. This is the hottest
running point of the engine, and efficiency of the fuel burn is at it's absolute peak.
The pilot is getting the max btu's from the fuel at that point. This is Peak Best
Power..
However, the materials in the engine cannot stand continuous operations at those
temperatures. Heat erosion will begin and soon hot spots will develop and detonation
will begin to destroy the engine. So what to do.
Well, the pilot continues the leaning process, As he does so, the temperature begins
to drop. Somewhere between 25 and 50 degrees after peak temperature he will reach a
point where he is running lean, but cool. He is not producing a lot of power but is
getting the greatest fuel efficiency from the engine. This is the mode in which an
airplane is flown in when the concern is not speed but distance. Any further leaning
and the engine will begin to run rough. The fuel air mixture will not produce a
uniform burn in the cylinder. The flame pattern in the cylinder will be ragged, and
will cause the roughness.
The most common practice is to fly an airplane with the power settings at Rich Best
Power or less. The "less" part is at reduced throttle and mixture leaned accordingly.
However the engine is often run at rich best power to get best speed. In this case the
scenario is to set the throttle, then begin to adjust the mixture towards lean. As the
EGT needle rises it is watched closely, When it peaks, the pilot begins to enrich the
mixture until the EGT needle points 25 degrees before Peak Best Power. This is the
cool side of peak power....a little more fuel is flowing through and the extra fuel is
helping to keep things cooler.
These principles apply to our beloved vanagons and their wasserboxers. A slight
richness helps cool things. To rich and black smoke issues forth.....unburned carbons.
The ECU in conjunction with the F/I helps keep mixtures closer towards that peak
temperature, and the O2 sensor is constantly measureing the CO/O2 content to see if
the output is correct. The signals received from it by the ECU are used to tell the FI
how to adjust to keep it right on. If an injector gets dirty or for whatever reason
leans towards that peak power.... that perfect fuel burn, excess heat develops, and
detonation will occur, destroying the engine. REALLY lean, and though it may not
damage the engine, it will hardly run.
Didn't mean to be so wordy with this, just wanted to impart understanding.
Hope it helps.
Pers comment about a bigger fuel/air charge is right on. The bigger the charge, the
greater the total amount of heat released. Could become a big problem trying to get
rid of the extra heat.
Many approaches are made to aid cooling, like using sodium filled exhaust valves. The
sodium converts to liquid when the engine is running, and as the valve goes up and
down, the sodium...liquid when hot in the sealed valve....picks up the heat from the
portion of the valve exposed to the direct heat of combustion, and transferrs that
heat toward the valve stem and valve stem tip. There it is exposed to the circulating
oil and heat is transferred to the oil and upper valve train oil and transported to
the oil cooler. It thereby reduces the valve face temperature. Grinding and Polishing
the exhaust ports to enhance exhaust flow helps in this heat removal effort as well.
John Rodgers
"88GL Driver
Birmingham, AL
Lawrence Johnson wrote:
> Per,
> I may be wrong but I learned that lean burning engines have higher exhaust
> temperatures whereas overly rich burning engines are cooled by the evaporation of
> the injected fuel.
> Any truth to this?
> -Larry
>
> > <snip> The biggest is that because of more fuel
> > injected into the combustion chamber, it will burn hotter. It may not be
> > damaging, but it could be. It has happened on some engines. I would very much
> > like to have a chip installed in my Syncro (112 hp aint enough :-)) but I'd
> > complete it with an exhaust gas temp gauge to see if it runs hotter, or too hot.
