So my engine may have detonated simply because the engine builder opened up the upper (trailing) spark plug holes to improve the redundant dual ignition idea of an aircraft engine?

As rpm and boost increased the compression of the intake charge? The fire could jump the apex seal at the big hole and light the leading edge of the intake charge just as the intake ports are closing?!

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Well, Yes if that hole were increased in diameter....................

 

That hole is small in order to prevent a loss of compression (Primarily) and to reduce the chance of a preignition event in the following charge. Our friends down under have run some engines with the little hole

made a bit wider in order to make the trailing plug more useful, but with no change in height in order to avoid

the preignition problem. The change in width would probably have little affect on performance, even at low speed. Remember, very low cylinder pressure at or near idle. The trailing plug is set back into a chamber of some volume. The trailing plug is fired some time after the leading plug. Some fuel air mixture is forced into this chamber before the trailing plug fires, and then a bit of burning mixture is shot out of the trailing hole in order to burn more of the hard to light idle mixture. This effect is limited to a very small RPM range and works over a small range of timing splits. Or, the length of time allowed between the leading and trailing firing events. This idea helps in the burning of excess fuel at idle and near idle RPM. This is so the cars could be sold in the Peoples Republic of California. The HC output you see.

 

The trailing plug is of little value at full song in the airplane RPM range. There could be some danger moving the throttles too quickly from a lower RPM, where there is more time (Lower RPM) for a flame front to move across the enlarged hole. The M in RPM is the time problem.

 

A preignition event at higher power settings would be profound and probably unforgettable. (Although one of my drivers would have missed it). A big engine shake and maybe a sheared dowel and, or, a broken dowel hole in the iron. Maybe an oil fire and big dripping mess.

 

In a NA engine probably no problems with slow throttle movements. In a boosted engine a much bigger problem, if boost is available at lower RPM. Even more important to move slower on throttle in those engines.

 

If you have lost an engine with suspect trailing plug holes, I would not use those housings in anything.

 

Racing Beat says to use no split timing in boosted engines. Also, once boost is up not more than 12 degrees of advance is used. They use 10 on the dyno and race on 12 degrees. No split is used, as the engine sees this as a detonation event. As boost goes up, the effective compression ratio goes up. The idea is that you are making the displacement go up while the head space stays the same volume.  

So where a low compression engines may need, for example, 24 to 27 degrees of advance to get maximum cylinder pressure at 50 degrees ATDC. The boosted engine (high compression) may need only 10 to 12 degrees to get maximum cylinder pressure at 50 degrees ATDC.  The flame front speed is much higher so less time (ignition advance) is needed.

 

Lynn E. Hanover

Making boat anchors

out of rotary engines

since 1980