Well I agree and don't I thinks its more complicated?
John Clark said:
The C/S vs fixed pitch question can be decided easily. I wouldn't try this on approach, but in a decent, power off (and popping, I assume) pull the prop control back and see what happens. Problem is this dosen't do anything for the issue. I know that it is counter intutive, but I think you will find that engines "pop" or backfire in the exhaust when they are lean not overly rich. Comes from the engine missing some when lean and the collected fuel igniting in the exhaust system. This was a common issue in 4 cylinder Porsches. The answer was setting the idle slightly richer.
John Clark
RV8 N18U (not popping
)
KSBA
That is a subtle and good point. Lean mixtures are volatile and combustion will burn very fast and abruptly.
I forget to mention that a leaky GASKET can exasperate the characteristic.
My theory is regardless of what is going on with the mixture going IN to the cylinder (still think is a little rich @ idle), when the exhaust valve starts to open and fresh air mixes with the exhaust gas (which still has un-burnt fuel), it gets super lean suddenly and just POPs off, verses just flaming out or a continued controlled flame burn. Lean mixtures will burn rapidly and abruptly. However its lean because the initial combustion process in the cylinder has burned some fuel and than its injected with air making it super lean and volatile. Similar things happen in diesel engines but fuel is injected at super high pressure into the air already compressed (and hot) and thus you get a explosive spontaneous detonation with no ignition. Diesels are build super heavy duty to take it and get efficiency by the very high pressures they work at. For planes it may seem ideal but the engines are heavy to take the punishment.
One thing that happens at low power (and pressure) is the combustion flame front is slower and you are blowing unburnt fuel out the tail pipe. Also valve timing is a factor where the exhaust valve opens early for idle but fine for 2,500 RPM. That is why hot rods and drag cars with radical cams pop and lope unevenly at idle.
When the existing combustion sees the in rush of air and NOW lean mixture will finish burning abruptly, like detonation. When AIR mixes with lean FUEL mixture it causes an abrupt finish of the combustion that's still going on when the starts to exhaust valve opens.
Add very short pipes, any 4-into-1 or Vetterman than there are two things going on, more air in the pipe (when exhaust rushes out air goes in) and they are just not muffled. The reason other planes don't pop I think is indeed the pipes which are either muffled enough or starved of air (due to restrictions and effective length with muffler).
Detonation is a dirty word as we know and is bad if moderate or severe at high pressure (high power). In the classic sense, denotation occurs in the cylinder. The pipe POP deal its a mini detonation, but happens at such low pressures it's nothing more than a fire cracker noise maker. At high power and lean mixture detonation in the cylinder occurs with such force, abruptness, combustion is like an explosion, a bomb, not a controlled flame combustion. This can cause valve, piston, rod and bearing damage. The boundary layer of air around the combustion is ruined w/ detonation and causes more heat transferred to the cylinder (thus high CHT is a indicator or detonation) verse being expelled out the pipes. If detonation is left long enough and severe enough, it can turn into pre-ignition.
The lesson is lean mixtures burn abruptly. Its ok at low power and in the pipes, but to be a good neighbor it's good to avoid.
I suppose if you run this pop pop a lot, even at low power, idle, you may do some harm over the long term? (Possible erosion of the exhaust port or initial part of the pipe?)
A leaky exhaust gaskets can cause a pop-pop condition in cars or planes.
BY ALL MEANS MAKE SURE the exhaust gaskets on your RV are not leaking. I learned the HARD WAY. If you do have a leak, any, you can erode the exhaust cylinder head seat, but this is more of a worry at high power than idle.
The leaky exhaust gasket I think might make the idle POP worse or more likely. Any way leaks are bad exhaust or intake. SO if you are getting lots of pops check it out. The leak allows more air into the exhaust port and cause the mini pipe detonation in the idle condition.
So bottom line I do think the mixture is RICH to start with at idle, but by the time it gets to the pipes just at the start of (EVO) - exhaust valve open, it is lean and ready to burn or pop abruptly with added oxygen in the pipe.
PS:
As kids, oh yes youth, we'd go down hill in gear and turn the ignition off back-driving the engine and than turn it back on. All that raw fuel in the pipes plus heat and air, pop or boom! It's kind of like what's going on with our Lycs in idle approach; it's a similar thing but different, but in either of these cases, you have combustion in the pipes, not the the cylinder. I didn't do it to my car, but my knuckle head friends thought is was fun.
In the case of drag cars, at idling at the start line w/ large
valve overlap, you hear the, pop, pop-pop, pop. In this case its a HUGE cam over lap.
EVO is happening so early at idle the flame front is still burning as the exhaust valve opens. In the
Porsche case its again a combo of RICH fuel mixture, short pipes and radical cam timing. As the engine decelerates raw fuel is burning in the pipes. It could be a missing ignition but I don't think that is the case, its just normal and avoidable with slow throttle movement and idle position while decelerating or back driving.
