Magneto/ Propeller pitch.
 

No, it will never help an engine running rough due to magneto problems, although when doing runnups, it can help clear oiled/fouled plugs, by increasing the workload on the engine and making it burn a little hotter.

It helps clear any gunk out of the sphincter valves in the primary thronomister section.

I had an expert instructor, he must have been coz he had lots of gold on his shoulders, well he told me to always use one magneto only at a time, that way if it failed you'd always have a good spare. If you run them both at the same time they are both likely to break at the same time.:}

I believe that the change in pressure in the cylinder may help to dislodge carbon deposits on the plugs.

I think the pressure in the cylinder is changing quite rapidly every revolution - even at a constant speed and load.

A load of Bollocks, a rough running mag ( as opposed to a U/S mag, lead or plug ) is generally one of two things :

1) a little moisture in the Mag causing arching, or,
2) Carbon / Oil deposit on the plug, this is best removed by increasing the burn temperature in the cyclinder, for example run up to full power and gently lean this mixture for about 30 seconds.

So in other words, having run the engine up to do the prop check has infact increased the cylinder temperature and probably burn't off the crap on the plug.

It had to be a rough running prop then. :hmm:

Actually its not a load of bollocks.

I used to use that technique all the time to clear the plugs on C210's to save running up engines over loose gravel. If the prop was cycled a few times after locating a fouled plug, at around 1700 rpm, then nine times out of ten it would fix the situation without having to resort to high power runs. Obviously that is only good when it is a fouled plug not a magneto problem. I heard about the technique at a forum with Richard Collins (from Flying magazine) at Oshkosh a few years ago. He said it increases the pressure inside the cylinder effectively blowing the plug clean. I think that was a very simplified explanation but I could care less - it works.

Gudday Harry,

I am not a LAME or any sort of mechanic, but I've been around a few engines ...

Not disputing that the technique you described works, however, I suspect that the engine cylinder pressure does not change by raising or lowering the rpms, after all if an engine has a (for example) 12 to 1 compression ratio, it will produce it (12/1) at 500 rpm or 2500 rpm.

I tend to favour the higher engine temps burning off combustible contaminants and also increased engine vibes shaking loose any solid stickies that may come loose, such as carbon (before it burns off) or oxidisation etc.

Obviously 1700 rpm is enough in your example and I guess if it can be done at that rpm all the better.

Just my two cents, now standing by for correction.

Cheers,

ABX

Fair call.

Until I had seen that forum I had never heard of that technique to clear a plug - beats destroying a good propellor for nothing.

As for how it works I am unsure. By cycling the prop at those mid-range RPM's you still gain a fair wad of extra Manifold Pressure, effectively I would think, increasing the volumetric flow through that or those cylinders - if more air or in this case charge is entering the cylinder then I gather that the operating pressure would be higher at the power stroke. Maybe this technique "mimics" higher operating pressures and temperatures by cycling the prop at lower RPM's. I'd like to find out more though if anyone has any ideas?

These engines are 'fixed timing'....this means that if you slow the RPM down significantly the peak power pulse happes closer to top dead center.

In simple terms x amount of fuel and y amount of air are introduced through the inlet manifold and a spark occurrs at a fixed time..at normal RPMs that spark lights off the mixture with the piston at the ideal point, so the peak power pulse happens about 16 degrees of rotation past TDC from memory, to get maximum leverage applied to the piston to spin the crankshaft which spins the prop and the knee bone is attached to the leg bone.

That mixture is igniting into a combustion chamber that is relatively large and getting larger relatively quickly as the piston accelerates downward.

If you slow the RPM the combustion chamber is relatively small, as the piston is closer to TDC, and it is not accelerating away, initially, as quickly so the Peak Power Pulse, the maximum pressure in the cylinder, is much greater and that creates more heat as well...clearing crap off your plugs and blowing it out the exhaust valve (you hope). I think it is fairly easy to understand how cycling the prop briskly (not brutally or violently) from fine to coarse pitch at a middle of the road power setting would tend to loosen crap off the plugs.

It should also be realised that doing this at very high power settings could put you into detonation...the reason why the spark is timed to occurr when it does is because at full power on takeoff if the peak power pulse occurred with the piston much closer to TDC the detonation margins would be reduced to near nothing...just in case someone reading this is tempted to think that if doing this at modest power settings works then doing it at high power settings works better:ugh:

Hope that helps explain it novicef.

Edited to add some highlighting and restructure one sentence just a tad for jamair the pedant:ok:

I would have thought 1700rpm is certain to pick up loose stones as the vortices under the prop tips can be seen quite clearly on a wet surface during a normal run up at that rpm.

Chuck - 16 degrees PAST TDC??!!:mad: If the Pelican were dead, that one would make him spin in his grave - if he hears it it might just knock him off his Perch anyway!!:eek: I'll put it down to temporary alcohol induced madness.....? See Deakins PP #18 - Mixture Madness.

Ignition happens BEFORE TDC.:= PPP (peak pressure point) occurs about 16 degrees AFTER TDC, as it takes a consistant time for the flame proagation to fully develop.

As for the rest of the topic - FIIK. Running up to 1500-1700 or whatever on a leaner mixture sure does clean oily plugs; never tried the prop cycling thing for that purpose though. Damn sure it does sod-all to the maggies.:rolleyes:

Sorry Chuck, would have to disagree with you on that one.

Spark goes in at usually 20 or 22 before TDC ( retard points on the LH mag are usually about 10 degrees less ).

The increase of RPM to 1700 increases the burn temperature, even when fully rich, therefore will generally burn off carbon/oil.

The 540 or 520 does not have vacumm advance system or similar ( that i am aware of ).

The volumetric efficiency does not vary all that much ( excluding ring and valve leak ) with RPM.

Quite frankly I find that leaning brutally on the ground as soon as the engine is running smoothly stops me getting fouled plugs...I simply never get them in the Bo.

None the less my previous post, as amended to completely satisfy my old mate jamair:E , is the theory behind why cycling the prop might, if the fouling wasn't too bad, clear it.

Jeez Chuck you must be bored - I've just got home from a late shift & I'm winding down from dealing with some 'difficult' clients - WTF are you still doing up? You should be getting some shut-eye in prep for your Captains Course tomorrow....:ok:

weekend off, except I will be studying for Flight Time Limitations exam on Monday, and it's 2 hrs earlier here than there.:ok:

What will not change if the prop is cycled at constant manifold pressure is brake mean effective pressure (BMEP), this is equivalent to torque.

I would have thought that the way to increase combustion temperatures would be to increase manifold pressure at constant RPM. This would increase BMEP and temperature within the cylinder.

In other words, assuming the governor is governing (>2000 rpm?) playing with rpm aint going to do anything because the engine isn't going to "see" any change.

Messing with the power level via the manifold pressure (ie throttle) should change things.

Personally, leaning does it for me.

OK all you experts, tear me to shreds.

The governor is being over ridden by using the prop lever to change pitch. thats why the engine slows down when the prop is cycled. the engine will try to maintain the speed it was running at due to the manifold pressure being set and therefore the engine is required to do more work, resulting in a higher burn temprature as the piston is harder to push downwards and the combustion process is held in the cylinder longer before being released.