My driver could detonate a NA race engine at will. We had a 70 MPH
first gear, so slipping the clutch was required to move the car up to speed for
the pace lap. The proper procedure is to rev the engine to 3,000 RPM, release
the throttle to zero and the clutch, so the car lunges using only the stored
energy in the rotating assembly. Then repeat until the car is rolling along at a
near zero throttle setting and the clutch is fully engaged. We had either a
crank triggered ignition or an electronic distributor with no advance curve and
about 25 degrees of advance. So with the clutch engaged the engine could not rev
up to reduce cylinder filling, and 25 degrees of advance is way too much for 800
RPM. This is the same exact thing as too much ignition advance.
Detonation is an ignition event remote from the spark plug(s) after the
planned ignition event. Detonation is charge temperature
dependant. Period. Think of turning a gasoline engine into a diesel
engine. So for some reason the fuel air mixture (charge) has been overheated,
and the pressure increase caused by the plug lighting part of the charge has
raised the temperature of the remaining charge too high. Then part of the charge
auto-ignites. Detonation.
Heat of compression. Like a diesel engine, compressing a mass of air
makes it hot. It matters not at all how it is compressed. It only matters how
much. In the diesel perhaps an 18:1 ratio can raise the air temperature to above
the flash point of the fuel to be injected. So no ignition system is required.
The diesel injection period keeps the stresses low.
In the gasoline engine detonating, the fuel is already in the chamber,
so when the auto-ignition takes place
the pressure rise is uncontrolled and may cause damage after only a few
cycles, or in a boosted engine just one event.
The greater the time difference in the split timing, the more
pronounced the collision of the two flame fronts. In piston engines an attempt
is made to keep the plugs firing together. Usually half of the plugs on top and
half on the bottom of each cylinder are fired together. So a split timing would
have assymetrical results.
In the rotary there is little difference in performance of split over
identical timing except at low RPM and for pollution control. The longer the
split period, the more the second event looks like a detonation event.
In high boost engines you want no split at all, and under high boost
very little advance, maybe only 10 degrees of less.
Charge temperatures are affected by chamber temps. So low oil temps
(rotor cooling) and low water temps
(the outer half of the combustion chamber) help resist detonation.
Sudden throttle advances from low RPM (if no retard programming is
available) can cause a problem.
Cold heat range plugs and controlled engine temps eliminate nearly all
problems.
Lynn E. Hanover
In a message dated 8/15/2010 9:03:56 A.M. Eastern Standard Time,
eanderson@carolina.rr.com writes:
Anytime
you increase the internal compression pressure (either by increasing
the
compression ratio or through forced induction - which in effect
increases
the compression ratio) - the more efficiency and power you gain
AND
the closer you move to the detonation regime. The engine doesn't
know
or care how this increase in pressure comes about - well, actually it
does.
If you use a turbo or supercharger you are also heating the air
through
their compression process (which is why you need an intercooler in
many
cases) thereby moving closer to detonation.
Lower compression
engine are further away from the detonation regime to
start with and
therefore can safely take a higher boost level (and benefits
more from it)
than a high compression engine. So as has been mentioned, you
will
see a higher performance enhancement by boosting a low compression
engine
than an high compression engine already running closer to the
detonation
regime.
You can certainly boost high compression engine, but to do it
safely you
generally need some fairly sophisticated "Knock" sensing and
ignition/boost
control to keep it out of detonation. Naturally the
more boost you run the
less margin for any error in controlling the onset
of detonation.
Contrary to what you may have read, it is possible to
get an N/A rotary
engine to detonate - I managed to do that in early
flight (1998) by
inadvertently over-advancing my ignition timing. I
mistakenly set the
static timing to 45Deg BTDC. I took off and after
getting airborne noticed
that if I opened the throttle wide open (more
combustion pressure), the
exhaust sound changed to a staccato, popping
sound. Retard the throttle and
it went away, never liking it when
the engine did anything abnormal I
returned and landed.
Upon
pulling the spark plugs I found I had destroyed them (presumably
through
detonation). The leading plugs had the ceramic cones completely
missing and the electrodes were almost completely eroded away so that the
spark gap was over 3/16", The trailing plugs had not suffer quite as
badly
although the ceramic cone was cracked on both and their electrodes
also
eroded.
So my personal opinion is that unless there is some
flight regime that
requires it (you fly out of mountain valleys, you
normally cruise above
12000 MSL, etc), you need to assess whether the
extra weight, complexity and
cost are worth it. If it is then go for
it.
In my case, after thinking about how I normally fly, I came to the
conclusion that for me a turbo would not be worthwhile - which is why I
still have two sitting on my work bench - well, the other reason is that
they are stock Mazda turbos and really not suited for aircraft use (unless
perhaps used very modestly) as I think John Slade has demonstrated for us
all. Turbocharging an aircraft is a very interesting challenge and I
sometimes wish I had talked myself into it - but, the numbers just didn't
work for me and my typical flying.
Fortunately for those of you who
are going to turbocharge the rotary, you
have some pioneers who are
providing you with knowledge that was gained at
some expense (money,
underwear, seat cusions, etc) {:>). So make use of it
and good
luck.
Ed