Details on "Rotary Only" mode for MAP/RPM engine protection limits?

[Pete_89t2]

Greetings, I'm curious to learn a bit more on what goes on behind the scenes when the "Rotary Only" limit mode is engaged. Per the help file (see excerpt below) it looks like it combines a Ignition % and Fuel % cut in its implementation, but doesn't say much else with regard to how the cuts are implemented.

 

Limit Mode

The limit mode is used to turn the system on and off. When turning the system on, there are four available limit methods.

• OFF - Limiting is turned off.
• Ign %Cut - An ignition only progressive cut.
• Fuel %Cut - A fuel only progressive cut.
• Rotary ONLY - (Ign+Fuel) A special ECU controlled fuel and ignition limit for rotary engines only. MUST have Injection Mode set to Sequential Staged.

Assuming that I'm using the default Rotary Only limit mode (i.e., advanced mode OFF, and dual MAP limit OFF), can someone explain what the ECU does in sequence when the limit is engaged? For example, is a progressive ignition cut applied first followed by the fuel cut? Or are the ignition/fuel cuts simultaneous, or are there any other variants involved, like perhaps alternating the ignition/fuel cuts between the 2 rotors?

[Adamw]

Many assume because there is a spark every crank rev that the otto cycle of the wankel engine is 360 deg - often it is thought of like a two stroke engine.  However many forget the Wankel has 3 combustion chambers and the rotor revolves at 1/3 of crank speed so it actually 1080 deg for a full otto cycle.

When you inject fuel into a fresh chamber on the intake stroke, it takes nearly a further 2 crank revs before that charge reaches the other side of the engine compressed at TDC ready for spark. 

So you cant just cut fuel and spark at the same time - There is fuel and air already sitting in the chamber from 2 revolutions ago...

The simplified sequence:  The rotary limit mode starts by cutting fuel first, but continues to spark for the next two events since there is already fuel in those chambers, then it cuts spark.  When RPM has dropped below the limit and we want to restore power, then it enables fuel first, then waits 2 spark events before enabling spark again.  

This strategy is the same as the Mazda factory ecu's use, it is the safest for stock engines as very low risk of backfires or lean combustion events, but it is also relatively "slow" and granular so not commonly used for motorsport type applications where the seals have been replaced with something more robust and can handle a more conventional limiter strategy.  

[Pete_89t2]

Thanks Adam, that's a great explanation. I was aware of how a Wankel requires 1080 degrees of crank revs to complete a full 4-stroke Otto cycle, hence the question. Glad it uses the same strategy the Mazda ECU uses.