While doing a lab for school, I noticed a trend in EGT vs ignition
timing and IMEP. When the EGT was at a minimum, the IMEP was at a
maximum. Is this trend a coincidence, or is it robust enough to tune
the engine's timing by?
The test engine was the 1.6L from a 1991 miata spinning at 2000rpm and a
manifold absolute pressure of 7.68 psi (0.53 bar) and stoichometric AFR.
Aaron Buster
IMEP is mean cylinder pressure. You want that as high as possible. That
is directly related to torque. You want the most torque at any RPM
because HP = T X RPM.
The best BSFC comes leaner than max power and the EGT is about 100 F
lower than peak power.
Paul Lamar
Paul and Aron, The graphs are extremely interesting showing the best
timing at approx 27 degrees BTDC. For over 2 years I have been trying to
eliminate high EGT's of over 1850 degrees. I had already burnt out 2
sets of
EGT probes!. I was told that my tangential muffler was the problem. So
desperate to try anything, I replaced my entire exhaust system down to the
twin mufflers. Nothing that 8 months and a lot of money didn't fix. With
high expectation I fired up the renesis, to quickly find that the EGT's
were
exactly the same. A very expensive exercise to find that my original
exhaust system was not at fault at all. I then decided that I would have
to live with the high EGT's as what else could I do? Then of my own making
Tracy's ECU died which mandated having to return it to be repaired. While
waiting for it's return and having all that perfect flying weather, I had
the offer of another ECU. It took one day to wire it in, very fast, and the
biggest surprise was that my EGT's were now down to 1725 degrees no matter
what I did to get them higher. The spark is showing 27 degrees BTDC at
full
noise and mostly the same from idle up. Unfortunately the original ECU
does
not show the timing. The current ECU has a trigger box to eliminate delays
in firing which the old ECU again did not have. In hindsight now I may
have
achieved the same result by just advancing the timing in Tracy's ECU.
Whatever the cause, I now have an acceptable EGT at 27 degrees BTDC. I
will
be away this week seeing a few experts on timing etc. and will be asking if
I can advance the timing even further to get EGT's under the 1700 degrees.
The things you learn!
Neil.
Do you have a manual leaning knob on that computer?
You will really need it for low fuel burn at altitude.
That will bring the EGT's down another 100 F.
Paul Lamar
I sent an inquiry to Micro Tech about leaning.
A screen capture is attached.
Paul Lamar
Neil,
Advancing the ignition naturally causes more of the mixture to be burned
before the exhaust port opens.
If you can open this attachment it's upper pink line.
http://s287.photobucket.com/user/bbordes/media/1k47atdc.jpg.html
or
[URL=
http://s287.photobucket.com/user/bbordes/media/1k47atdc.jpg.html][IMG]
http://i287.photobucket.com/albums/ll129/bbordes/1k47atdc.jpg[/IMG][/URL]
It shows the gas temperature in the housing and the EO line to the
right is the exhaust opening point. Your egt pick-up is down stream
from there.
Increasing flame speed, which is notoriously slow in a rotary would be
best but it unfortunately is a design feature. What is possible is
optimizing the burn rate by moving the Peak Pressure to 45 degrees after
TDC.
One factor to be considered in the slowing of the burn rate is
controlling the reversion of exhaust gases. Notice that Mazda injected
air into the exhaust sleeve to "pollute" the reversion in order to
increase burn rate.
Reversion consideration would be especially important when changing port
timing (porting) or it's total redesign (from side to peripheral). Time
for some testing to optimize it.
A good strong ignition like Paul's will also help by reliably starting
the burn. And sparkplugs that don't hide the infant kernel.
Barry
And igniting it and igniting it and igniting it :)
Injected air into the exhaust sleeve will help the turbo compound
version of the rotary.
Paul Lamar
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