Rough running/High EGT in Cylinder 2

Brad Asked:

Is it correct that each Lane is exclusively responsible for firing only one of the two spark plugs in each cylinder (as a magneto in a conventional magneto ignition)? Or can each Lane simultaneously take control and be responsible for firing both spark plugs in each cylinder simultaneously on each compression stroke if the other Lane fails completely or is turned off.  So, for example,  during 'run-ups' when one Lane is turned off,  does one spark plug in each cylinder stop firing on each compression stroke, or does the Lane remaining on take control and ensure both plugs continue to fire?

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Each lane and plugs (and sensors and injectors and...) are independent.  They do Not take control of the other side.

The pre-flight lane check simulates a lane failure where you are running on only one plug per cylinder.

If that were NOT the case, the lane check would not be able to reveal dead plugs, coils or bad wires. (Or bad injectors, sensors, etc.)

What does happen, is that the Lane A-B alternation stops and the remaining lane injects and fires on each and every cycle.

Listen very closely the next time you do the lane checks.

You should be able to detect a very slight misfire in the engine during the time it takes for the remaining Lane to detect the failure of its partner and take over the full duty of keeping the power going.

Other than the single misfire, the engine continues to run as if nothing happened.

Thanks Bill.

Further testing today indicated ;

1. resistance of the high tension lead from 1T coil to 2B plug was the same as other leads

2. spark was visible from both 2B and 2T spark plugs removed from cylinders when the motor was cranked over (not running) in dark conditions , but spark at 2B plug was not as bright as 2T.  I wasn't bold enough to run the motor in the dark enclosed hangar alone and search for errant sparking.

3. swapping Coil1 and Coil3 caused exactly the same engine roughness and EGT temperature changes (previously described) to transfer from cylinder 2 to cylinder 3 

Conclusion:  the coil that was positioned at 1 is defective. With the engine is running the coil is supplying sufficient spark to only one of its two spark plugs.  The spark at one plug is not strong enough to cause combustion during the power stroke of the cycle when that spark plug should ignite the mixture.  Perhaps the spark to one of the plugs is going to ground inside the coil when the engine is running.

Therefore when both Lane A and B are switched on, one cylinder is having combustion only every alternate cycle .  When Lane B alone is switched on,  that cylinder is having combustion every cycle.  When Lane A alone is on, no combustion occurs in that cylinder.

A new coil has been ordered and will arrive in the next few days.   I will revert with the outcome after installation of the new coil.

The local distributor says my coil (part number 665612) has now been superseded by a coil with part number 665613.  The local distributor states a coil failure in this engine is uncommon in its experience, based on it needing to supply only one new coil since the IS motor was introduced.  However, this may be more a reflection of few of these IS engines being in Australia. 

A revised coil being introduced by Rotax with a new part number does make me wonder if coil failures of part number 665612 are not uncommon?

Bill has clarified that in contrast to other aircraft engines,  in the IS motor only one of the two spark plugs in each cylinder fires on each power stroke of each cycle.   

My understanding is both spark plugs in each cylinder firing on each power stroke in each cycle maximizes efficiency of combustion and power.  Why is this engine different and why does it not benefit from both plugs firing?  Does the electronically variable ignition timing and fuel injection enhance combustion efficiency and power so significantly that having two plugs fire simply offers no advantage compared with one plug firing?

Bill Hertzel wrote:

I stand corrected:  The Top and Bottom Plugs are paired.

 

You are correct that there is one coil that fires two plugs simultaneously.

When one spark is firing in the compression stroke of one cylinder the other plug is firing harmlessly in the exhaust stroke of the other cylinder.

Nearly every modern engine uses this method commonly called a Wasted-Spark ignition for obvious reasons.

It is simple, reliable, and inexpensive to implement.  No distributor needed!

 

 

Both plugs Fire and Fail simultaneously.

Unless ... UNLESS ...

Unless the spark for the failing plug is being redirected to ground by a  path other than the spark plug.

 

There are going to either be Zero sparks, or TWO sparks.

The question being, WHERE is the spark occurring if it is Not at the Spark plug??? 

Pull the 1B and 2B Plugs.

If the 2B plug is cold and the 1B plug is sparking, the 2B spark is occurring somewhere that needs to be located.

Is could be within the ignition coil itself and a simple coil swap could confirm that possibility.

Or it could be jumping out of the plug wire to ground somewhere.

 

Bill this sounds similar to what you and I troubleshooted on my engine. I was having that nominal current not reached issue at one of the Cylinders. Turned out to be it jumping from the ignition coil wire where it was not shielded to the manifold cover! Fixing that w/ a slit piece of fuel line fixed my issue. Not sure if that is something that might help out here!

Thanks Michael.  I take it your problem was caused by spark jumping from the high tension spark plug lead exiting the coil rather than the low tension leads entering the coil.  It transpires that this was not the cause in my engine as the coil swap with same high tension leads left in the same positions has corrected the problem .

A new coil, (now part number 665613) was installed today, with complete resolution of the problem. Coil with part number 665613 has superseded coil with part number 665612.

The local distributor says coil with part number 665612 is not available now, having been superseded by coil with part number 665613.  Externally these coils appear identical (apart from the different number).  However, I wonder if the new part number implies the internals have been modified because of a sufficiently high incidence of reported failures of Coil 665612 to justify internal modification?  The local distributor states a coil failure in this engine is uncommon (in its experience), based on it needing to supply only one new coil since the IS motor was introduced.  However, this may be more a reflection of few of these IS engines being in Australia? 

Conclusion:  Even though some spark was visible in both plugs when cranking the engine in the dark, with the engine running the defective coil was not producing sufficient spark in one of the two plugs it supplied to cause combustion during the cycle when that plug normally fires combustion.  The spark must have been going to ground inside the coil.

Bill has clarified that in contrast to other aircraft engines (including the 912 ULS motor) only one of the two spark plugs in each cylinder fires during each power stroke of each cycle in the 912IS motor.   

My understanding is both spark plugs in each cylinder firing on each power stroke in each cycle is necessary for maximum efficiency of combustion and maximum power in most aircraft engines, as revealed by the RPM drop when one of the two spark plugs doesn't fire (eg magneto check) .  What is unique about this engine that it does not benefit from or need both plugs firing during each power stroke each cycle?  Does the ECU controlled variable ignition timing and fuel injection timing enhance combustion efficiency and power so much that having two plugs fire together simply offers no advantage compared to one plug firing?

Brad Asked:

My understanding is both spark plugs in each cylinder firing on each power stroke in each cycle is necessary for maximum efficiency of combustion and maximum power in most aircraft engines, as revealed by the RPM drop when one of the two spark plugs doesn't fire (eg magneto check).  What is unique about this engine that it does not benefit from or need both plugs firing during each power stroke each cycle?  Does the ECU controlled variable ignition timing and fuel injection timing enhance combustion efficiency and power so much that having two plugs fire together simply offers no advantage compared to one plug firing?

 In order to obtain the maximum efficiency out of an engine, it is required that the fuel mixture is ignited in the later part of the compression stroke. (Before Top Dead Center- BTDC)

This allows the fuel time to ignite and produce the pressure peak at the optimal location in the downward power stroke and consume all of the fuel before the exhaust valve starts to open.

Igniting two plugs simultaneously at two different locations within the fuel charge is the equivalent to igniting the fuel at a single location a few degrees earlier BTDC.

When you perform a MAG Check on a conventional, simultaneous dual spark ignition, you are also causing the virtual timing to be retarded slightly.

When you perform a LANE(Mag) check on an Alternating Dual Spark Ignition, the location of the ignition changes slightly but the virtual timing does not change significantly.

The timing BTDC of each type of ignition has been selected to produce the optimal performance based on each engine's specific fuel burn characteristics.

And then, whereas the 912ULS engine's timing is fixed by the location of the ignition trigger coils relative to the alternator Rotor, the 912is timing can be freely varied by the ECU as needed.

And the advantage of firing one plug alternately is that it becomes possible to identify the plug failure and remove it from the rotation as opposed to just being masked by the other plug.

This allows a Plug/Coil failure to be identified during the current flight and can be corrected after landing and before the next flight as opposed to being identified during the MAG check before the next flight after all the family and luggage is already on board.