What engine is this???

Sean,

The exhaust is not contained as you suggest. Everything is relative, and back pressure on the exhaust system is reduced as altitude increases.  At sea level there is 14.7 PSI of ambient pressure pushing against the exhaust as it tries to exit, whereas at 60,000’ there is about 1 PSI.  So if you use a turbocharger to keep the cylinder pressure at sea level, and now the exhaust is operating at 60,000’, it’s like having a 14 PSI vacuum on the exhaust pipe at sea level.  That’s going to move a LOT more air. 

RW. 
We were typing at the same time on those last two posts. I did not mean to suggest you were not the expert we needed!  You are.  

Jeff,

"The exhaust is not contained as you suggest."

The hot/exhaust side turbine must provide a significant degree of resistance, as it turns the cold, inlet pressurising turbine. The combination of exhaust manifold system and effort/energy to provide boost pressure on the inlet side, will present as "back pressure" slowing the exit of exhaust gas.  I have no idea by how much but it must be significant.

I don't know how an aircraft turbo works but if is anything like a tractor, it has a wastegate. I imagine in an aircraft the wastegate would be fully closed at altitude, routing all exhaust gas through the turbine. Once on the atmosphere side of the turbine I would expect it to exit with higher velocity than at sea level.The question in my mind is, will this increase/speed the turbine - I think not as the cold side will act to govern the whole assembly.

"...back pressure on the exhaust system is reduced as altitude increases"

I agree but by how much? 

"At sea level there is 14.7 PSI of ambient pressure pushing against the exhaust as it tries to exit,"

And the same pressure on the inlet side, forcing air into the low pressure cylinder, created by the descending piston (inlet stroke).

😈

“And the same pressure on the inlet side…”

Yes, without a turbo or supercharger you are correct these pressures are balanced.  Now add a turbocharger (or three of them in this case) and this balance is no longer true.  With a turbo charger there is always more air being stuffed in.  It’s only when this gets extreme that It causes ignition problems.  At some point you push so much air into the cylinder that the turbulence can interrupt the arc between the spark plug tip and electrode.

I’m an electrical contractor and I’ve seen similar things happen.  I remember one time one of my electricians pulled a fuse out of 600 volt DC solar inverter without realizing there was a load on it.  When he did it started a continuous arc across the fuse holders, effectively bridging the space the fuse had occupied. There was no way to turn off the circuit off quickly (to save the equipment) so I took a deep breath and blew out the arc from about a foot away, like you would blow out candles on a cake. I was kind of amazed it worked at the time, but all you have to do is push the ionized gas particles apart far enough to break the arc.  A plasma cutter is a good example, you need just the right flow of compressed air to creat a plasma, too much and the arc is blown out even though the arc gap is unchanged.  

Thanks Jeff,

You are operating from a knowledge base that I can only aspire too.

Small "point of order":

"........push so much air into the cylinder ....."

If boosting the engines power, you are introducing air at higher density - the volume remains the same. This remains true at sea level and at 80,000 ft above.

I still can not reconcile my limited knowledge, with the spark problem at high altitude, that RW wrote about. 

No matter the boost pressure;

      The cylinder fills with air/fuel mix on the downward stroke - this is when the potential incoming air may produce unusual turbulence.

      Inlet valve closes - you now have a fully contained environment - no more influence from the boost system.

      The piston rises, compressing the mixture

      At a predetermined point, the spark plug fires, igniting the compressed gas, etc etc

Reference to high performance/ revving ground based engines has little relevance to the narrow (relativly slow) rpm range of an aircraft engine.

There must be more to this spark problem than just boost pressure.

Sean,

Let’s say we have two tanks that we want to pressurize to 15 PSI absolute.  The first tank was opened to sea level pressure so it’s sitting at 14.7 PSI absolute (0 PSI gauge pressure at sea level). The other tank was opened to absolute vacuum, so it is at 0 PSI absolute, which is a -14.7 PSI (vacuum) at sea level.  Do yo agree the second tank requires more air to fill to 15 PSI absolute?

As fas as the valves being closed when the spark plug fires, you are correct, and it would seem that the inrush of air would not be a factor.  Buts it my understanding that with that much air being pushed in, turbulance remains after the valves are closed.  Water is a fluid, and I imagine a filling a glass very quickly and watching it settle down after the water is turned off.  It’s not immediate. Anyway, we are getting way out in the weeds, I hope things are going well for you down under!