Q: What is detonation? What does detonation sound
like in a rotary engine?
Detonation is defined by any uncontrolled ignition of the combustion
charge, usually due to pre-ignition, uncontrolled timing, and/or lower than
acceptable fuel octane. The words "detonation" and "pre-ignition" usually
go hand-in-hand. Pre-ignition is defined by the uncontrolled ignition of
the combustion charge other than triggered by the spark plugs through
the stock ignition system. Pre-ignition can be triggered by localized hot
spots within the combustion chamber and/or lower than acceptable fuel octane
that ignites spontaneously within the combustion chamber. Octane is also
directly related to the entire mix, as octane is defined by a fuel's resistance
Detonation in a rotary engine is bad! Unlike piston engines which
have piston rings what are buried down in the piston skirt (this is the reason
why piston engines can tolerate a little more abuse from detonation), the rotor
apex seals are basically wide-open to the combustion chamber. Damage from
detonation is actually caused by the higher than normal combustion pressures.
Compared to normal, controlled combustion (about 1,000psi), detonation can exceed
normal combustion pressures by ten times - up to around 10,000psi! This
elevated pressure is what causes engine damage; the rotary engine's exposed apex
seals take the brunt of the detonation force, which ends up failing by shattering
into little tiny pieces. Having little tiny piece of steel flying around
in a rotor chamber with a spinning rotor is a recipe for a chewed rotor housing.
So what does detonation in a rotary engine sound like? The best analogy
I've heard is "popping popcorn." Mild detonation sounds like a Jiffy popcorn
popping on the stove. It sounds close to a low, muffled rumble.
Power should significantly decrease due to the uncontrolled ignition of the
combustion charge. Serious detonation sounds like a metal hammer
on a metal pipe; it has a very easily distinguishable metallic "tink" that is very
high pitched in frequency. This is basically the sound of the rotor trying
to reverse direction and spin the opposite way! Bearings and gears take a
tremoundous pounding under serious detonation; you can always tell if a motor has
undergone serious detonation by inspecting the rotor bearings, rotor gears' teeth,
and stationary gears teeth for unusual wear.
As a side note, serious detonation can cause curious indentations on a rotor face,
on opposite ends of the "dish" in the center. Kouki FC3S and FD3S rotors
tend to do this pretty easily due to their lighter weight and, therefore, less mass
and metal thickness at the rotor face surface. Zenki FC3S rotors seem to be
more resistant to this depression damage from serious detonation. The Japan
tuners have seem to have caught onto this characteristic, as demand for Zenki FC3S
rotors (even with the lower compression of 8.5:1 versus 9.0:1 for Kouki FC3S and
FD3S rotors) have skyrocketed for drag racing applications where insane boost
levels are seen, typically over 25psi!
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