EGT rise during leaning the mixture.

3 Answers

1. 
   

   
   

   Micah on Oct 16, 2011

   It’s easy to explain, but I don’t know anything about thermodynamic rules. I’ll give a short answer and let others take stab at the second question. Generally speaking, EGT rises when leaning the mixture because the previous mixture was at a sub-optimal setting (too much fuel). Approaching the optimal setting increases the power generated by the (explosion) reaction and therefore RPM (do you hear this increase also?) and EGT likewise increase.

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   Earl Kessler on Oct 17, 2011

   Remember, there are 3 things that cool the engine; airflow, oil circulation and fuel.  You can enrichen your mixture in flight (i.e. in a climb) if you find the engine termperature is creeping up to help cool the engine.  The opposite will occur when you lean the engine, more heat is produced by the efficient burning of the fuel as indicated by rising engine temps.

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3. 
   

   
   

   John D. Collins on Oct 17, 2011

   EGT is not a real temperature of the exhaust gasses, but an average of the temperature over the engine cycle. When the fuel-air mixture is ignited, it burns the fuel and generates the pressure to move the piston downwards. This occurs over a period of time, starting from the combustion event that is typically fixed at approximately 20 degrees before top dead center on the compression stroke. The pressure in the cylinder increases until it reaches a peak and for best mechanical advantage this occurs on the down portion of the power stroke.  During the exhaust stroke, the exhaust valve opens and the hot gasses are pushed out of the cylinder. At this point, they are still over 2000 degrees. They cool by conduction with the cylinder head and the exhaust valve along with expansion. By the time they reach the EGT probe, the temperatures have cooled significantly. The thermal mass of the probe is such that it can’t follow the instantaneous temperature as the gasses pass by it and they reflect a stabilized temperature where the probe is heated during the exhaust cycle and cooled during the other 3/4 ths of the time.

    

   The EGT reading is affected by several factors. One is the time that has elapsed since the end of the combustion burn. The longer it has been since the burn has been completed, the more time the gasses have had to cool. A richer mixture will burn faster than a leaner mixture, so this tends to increase the EGT going from rich to lean up to a point. A richer mixture will have un-burnt fuel that will have the effect of cooling the exhaust.  A richer mixture will also tend to move the peak cylinder pressure to occur later in the cycle.  Since a rich mixture has excess fuel, power is controlled by how much air is provided which is primarily controlled by a combination of the throttle position, the RPM, the altitude, and the temperature of the induction air.

    

   Temperature of the EGT will increase to a point as the mixture is leaned until the fuel and air mixture are in balance.  This occurs roughly at the peak EGT point on the EGT gage.  Beyond this point, the power generated by the engine is controlled by the amount of fuel provided to the engine and is directly proportional to the fuel flow.  As the mixture is leaned beyond the peak EGT, the EGT temperatures will go down.  This is due to the fact that a lean mixture will complete the combustion burn sooner and therefore the gasses will have had more time to cool and that the engine is producing less power. The peak cylinder pressure will also be retarded and provide better mechanical advantage than a richer mixture.  This shows up as a reduced cylinder head temperature.  When operating with a lean mixture, the unused air also provides for cooling of the engine.  It is typical to see a 20 to 30 degree F reduction in cylinder head temperature at 50 degrees lean of peak EGT when compared to the same EGT on the rich side of peak EGT.  In fact, the worst place in terms of CHT to run the engine is at 50 degrees rich of peak EGT.  Not all engines will run smoothly at mixture setting lean of peak EGT because of imbalance in the distribution of fuel to the cylinders.  It usually takes a fuel injected engine with a balance injector system to run smoothly on the lean side of peak EGT. 

    

   When you have a bad spark plug, between the ignition of the fuel until the burn has completed takes a longer amount of time because there is only a single flame front instead of the normal two, so the gasses are hotter by the time they pass the EGT probe.  This is why a bad plug will show up as a high EGT reading on the affected cylinder.  When you do your runup, you can see this effect during the magneto check. When you are on a single magneto, you will see all the EGT values increase together, and when back to both, they will all come down together.  If they don’t show this characteristic, there is a problem in the system.  In particular, if there is no tell tale increase, it is very likely that the magnetos are badly miss-timed and you should investigate before flight.

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