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Matt Prather wrote:
Maybe the lean mixture limit is being driven by inadequate/not
optimised
spark. More spark energy, bigger gap, multiple discharge, better
fuel
atomization are things that could be looked at. 0.455 isn't bad,
however.
Regards,
Matt-
Maybe. We'll try, we have a system which can adjust spark energy.
Thanks for the suggestion.
Francois
Del Johnson wrote:
The Cafe Foundation's research on Ignition Dynamics (
http://www.cafefoundation.org/research.htm ) found that you had to advance
the ignition as you lean the fuel to achieve adiquate fuel burn. They
stated that as the fuel/air ratio decreases the fuel molecules are further
apart and take longer to ignite. As the engine (Lycoming) was leaned the
EGT would decrease to a point where ignition was starting to fail at which
time the EGT would increase due to the fuel being partially burned in the
exhaust stack. By advancing the spark they could delay the point where
EGT
rose thus permitting further leaning of the engine..
I concur with you Del... and with the CAFÉ Foundation. We have very good
data to quantify this effect. Actually EGT measured right at the exit of the
rotors go down as you lean (we call this measure Rotor Exit Temperature,
RET) , but EGT measured in the exhaust pipe, a couple of feet downstream,
show 100-200°C HIGHER gas temperature and the difference between the 2 EGTs
increase as you lean... an initially counter-intuitive thing. What happens
in between is the post-combustion of the increasing percentage of unburned
hydrocarbons and oxygen. This difference is rock stable at 80-100°C
(depending on RPM) when you vary mixture on the RICH side of peak because
the post-combustion is limited by the amount of unreacted oxygen which
happens to be stable. How come it's stable? We believe the reason is that
on the rich side, well-atomized fuel in the "main" charge burns almost
entirely while fuel either sprayed on the walls or trapped within a thin
boundary layer doesn't burn at all. The thickness of this boundary layer (a
fraction of 1 mm) depends on the efficiency of the injection system and on
wall temperatures and not or very little on the richness of the total
charge.
Well, this is not scientifically proven facts but rather the simplified
model we came up with, but it is amply substantiated by our experimental
data.
Advancing the timing to compensate for the slow-burning mixture has a
limitation. By doing so, you improve the thoroughness of the charge's
combustion, but it widens the pressure curve (proportionally more of the
pressure happens before TDC and before the optimum crank angle for peak
pressure), decreasing the effective power transmitted to the shaft. We
don't have yet the optimal timing/leanness combination, that is why I don't
want to release the number for optimal A/F ratio yet.
... properly tuning an engine doesn't take days but weeks of careful and
thoughtful bench work... Stay tuned.
Francois
PS: everything I may pass on to the ACRE newsletter is correct for our
engine and we decided to do it to help this community better understand the
rotary engine and promote its use in aviation. However, it may differ on
another engine and Mistral is taking absolutely no responsibility for the
possible detrimental consequences of the usage of this data by others.
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