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This year's annual
Ok I am a student who is in the process go getting his A&P License and just finished the portion dealing with what your asking.. When you run your engine at full rich you are giving the engine more fuel than it can fully burn and use to push the cylinder down and drive the other cylinders back up compressing the fuel air mixture. It will continue to push but you won't get the full potential from your compression... A way to see if your running at the correct mixture and see if your engine is functioning to its full potentail you should perform a MAG drop check... To do this... after you have run your engine up and have it warm... enrichen your mixture until it just starts to run rough then pull back slightly... then slowly pull your throttle back to around 1700 rpm then switch briefly while watching your tachometer to your left then back to both and then to right and then back to both... You should see a drop of around 50 to 75 rpm drop if your mixture is set correctly. Daniel Brooks A&P Pending school completion Daniel. You are having an apples and oranges conversation here. We are dealing with compression checks, not reduction in BMEP due to mixture changes. You are correct about what you are describing, but we aren't talking about that subject now. We ARE looking for a reason to explain WHY rich mixtures degrade leakdown tests. With that understood, the reason lean cut numbers are better than rich cut numbers is entirely due to fuel washdown. We have to remember that we are talking about a very small volume of air actually moving thru the oriface of the differential pressure gauge. Rich mixtures wash the oil off the cylinder walls a bit more "efficiently" than lean mixtures. The oil film between the cylinder walls and the rings is the sealing mechanism, NOT the rings running ON THE WALLS. If you diminish oil content at the sealing surface by having excess fuel there, it is fairly obvious that ring "blowby" will increase. A second effect, but much smaller is the effect of excess fuel on exhaust deposits on the exhaust valve seat. Current thinking seems to indicate that fuel will "wet" the deposits and cause them to stick to each other a bit preventing radial movement of the valve into the center of the seat. The guides also use an oil film to seperate the guide from the valve stem, a slight reduction in the oil film thickness can allow the valve to "rattle" to a position other than the center of the guide bore. Since we are only talking tenths here, you can start to see wht there is so much variability in one check to another............ ANYWAY............. These are the main reasons rich idling will adversely effect leakdown test results. They are proven, not conjecture. Earlier I gave reasons from allied fields to make the case in a more united front, but for some reason I forgot to mention that I'm an A&P in the game since 1979 and this particular subject hasn't changed over the years. Scott |
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