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Lean Of Peak versus Rich Of Peak


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The following thread was taken from the Matronics site:Just as a quick recap of LOP versus ROP, may we specify what it is that we are lean of or rich of?




Generally speaking, most folks like to think of rich or lean in relation to EGT temperature, but the thing we really want to know is whether we are on the rich side or the lean side of best power. At best power, almost all of the air that is available will be used. By checking the temperature with an EGT gauge, we will find that when we are running at "best" power, the EGT gauge will generally read about 60 degrees F cooler than it will at peak EGT. We therefore now know that peak EGT occurs on the lean side of best power. Another way to say that is that best power can be found about sixty to eighty degrees richer than Peak EGT




The hottest internal combustion temperature will occur when just a bit lean of best power. Most of our automobile engines are designed to run right at best power most of the time. Our aircraft engines tend to be designed to run at full takeoff power for a relatively short time and the cooling capabilities are designed to be able to handle full power by adding extra fuel to the mixture which will slow down the rate of burn, move the point of peak cylinder pressure later in the combustion process and allow us to keep the engine temps below the redline while developing high power.




So --- If we are at full or high power running rich and we make the mixture less rich, we are taking away some of the fuel that has been used to move the flame front later in the cycle and the cylinder temperatures will rise. However, if we continue to reduce the fuel flow, the internal temperature will peak at some rather high number. That will be very close to the best power mixture. The problem is that our engines are not designed to cool properly at that high a cylinder pressure. If we continue to make the mixture less rich, it will eventually get to the peak EGT. That will occur with an EGT that is between fifty and eighty degrees Fahrenheit hotter than it was at the peak combustion temperature point. If we now continue to lean the mixture, the EGT will drop. Less fuel means less fire and the cylinder temperatures and pressures will continue to drop. At a cruise power of sixty to seventy percent for most light aircraft engines, the best BSFC (Brake Specific Fuel Consumption) will be found with an EGT about twenty degrees F leaner than Peak EGT. (Or about seventy-five to one hundred degrees away from best power.) It makes sense that when you are burning less fuel the temperatures will be lower, but when we are running at very high power settings, we need the extra fuel to keep the peak combustion pressure and temperatures down.




Thus, it can truthfully be said that there are times when leaning the engine will make the heads run hotter, but there are also times when leaning the mixture will make the heads run cooler.




As Always, It All depends!




Make any sense at all?



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Valve temps, (exhaust) correspond to EGT. The higher the EGT the higher the exhaust valve temp. ALL reciprocating aero engine manufacturers have had a devil of a time getting the valves to last as long as the rest of the engine. (You need a "top", before you reach TBO). also a hot valve can cause detonation and the valve head can detach more easily., as the tensile strength is reduced.


Lean running leaves unconsumed oxygen to burn all trace of lube oil off the cylinder surfaces and oxidise any other thing available, especially if it is hot like the exhaust valve.


Surely all the trouble with the Jabs with the lean kit fitted should put paid to the practice of trying to go to the lean side. Nev



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LOP operation is fine in a fuel injected engine which has the latest calibrated injectors and calibrated EGT analyzer probes for each cylinder, plus a knowledgeable pilot on the end of the mixture control. In a carburetted engine where the mixture varies so much between cylinders, I would not consider it. In RAA machines we are only dealing with relatively low fuel burns anyway, so I would rather use another one or two litres per hour than lunch the engine....



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