This Vee's for Victory book is a complete education on
     aircraft
     engines.
     Here is a chart showing that detonation is primarily a low
     RPM high
     load
     phenomena.  The white band in the chart is the no
     detonation max HP
     operating range. Cars and airplane engines with variable
     pitch or
     constant speed props are much more likely to suffer from
     detonation
     than an aircraft engine with a fixed pitch prop running at
     max RPM.
     This is because at less than max RPM prop load on the
     engine and
     consequently the boost from the turbo charger, drops off
     drastically.

     NACA did some studies on this back in the late 30's. In no
     particular
     order of importance this is the list of factors that
     affects
     detonation.

     Compression ratio.
     Inlet air temperature.
     Spark advance.
     Fuel air ratio.
     Size and shape of the combustion chamber.
     Temperature of the combustion chamber and cylinder walls.
     Engine speed.

     The chart is limited to 140 F inlet air temperature
     indicating the
     extreme importance of intercoolers. I fully intend to
     install an inlet
     air temp gauge on my turbo 13B.

     An idea suggested by this data is if you have a knock
     sensor it is a
     good idea to not only retard the ignition but richen it up
     instantly
     if detonation occurs. Not possible without electronic fuel
     injection.

     Another idea is to place one or more fuel injectors up
     stream from the
     turbo to cool the mixture. All the Allison's and Merlin's
     had the carb
     up stream from the
     turbos and superchargers for just this reason.

     Paul Lamar

Detonation is affected greatly by combustion chamber design. Not
sure if you can extrapolate these data to the rotary. In my
experience, it is mighty hard to get the rotary to detonate at low
rpm. Much worse at higher rpm. Part of it might also be that the
volumetric efficiency of the rotary allows the engine's IMEP to be
higher at higher speeds, whereas piston engines (and especially
older piston engines) tend to have higher IMEP at lower RPM. I
think that modern piston engines are also somewhat more prone to
damaging detonation at higher RPMs.  For those who don't know what
IMEP is... it is Indicated Mean Effective Pressure. Which is a
measure of how much work is being performed by the combustion gas
directly on the piston or rotor. BMEP is Brake Mean Effective
Pressure, which is a measure of how much work is being performed at
the crankshaft... FMEP is Friction Mean Effective Pressure, which is
a measure of how much work is absorbed irretrievably by friction,
including pumping friction of the intake air and exhaust.
BMEP=IMEP-FMEP.

As an aside on the "normal" process for developing engine ignition
timing curves... the engine is operated at a fixed speed and
throttle setting (may be WOT, or may be less...). The ignition
timing is slowly advanced until the engine torque measured on the
dyno stops increasing. The most retarded timing that provides the
maximum torque is called the MBT timing... for "Minimum Best
Torque." Of course the engine is carefully monitored for detonation
during this testing, and it is possible that the detonation limit
will be reached before the maximum torque (for that given speed and
throttle setting) is achieved. This will generally be the best fuel
economy point, as well -- but sometimes not! Nowadays with emissions
concerns, the MBT timing may not be used in the final calibration,
as it is always a balancing act between performance and emissions.
Also, unless the engine management system is sufficiently
sophisticated, it is often desireable to build in a little "margin"
for non-optimum conditions. This was much more common in the dark
days of distributor ignitions!

Jon Lauter

Is that dyno data Jon or in the boat data. If in the boat you have the
fixed pitch prop situation most of the time. Of course when you
accelerate
the engine is loaded down temporarily. I also assume this is not your
supercharged engine. Do you have any inlet air temp data for your
test?

BTW what BSFC are you seeing with your new supercharged rotary boat
engine?

Paul Lamar
 
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