Thursday, August 02, 2012

Is it possible to use differences in CHT change when leaning to find engine baffling problems?

aircraft cylinder barrel showing blued surface
Lycoming Cylinder Barrel Blue oxide films -- 550 F.

You lean the engine and the CHT changes as expected. But why does the CHT on some cylinders change more than others?
One reason will be that each cylinder is operating on a different point on the mixture ratio graph. Unless each cylinder is operating at the same mixture ratio, to use our method described below the data needs to be normalized so that each cylinder is at the same mixture ratio point. 

Most publications show how the CHT changes as the fuel/air mixture is leaned or richened.  Also of interest is why some cylinders may experience a greater or lesser temperature change than others. Extending the idea further, why might one engine produce a greater change in CHT than the same engine in a different airplane?
"A cylinder which is shielded from the air blast or improperly air cooled will have a much greater rise in temperature due to leaning of the mixture than one which is properly cooled." An old-old study N.A.C.A. Technical Note No. 388.

Take a rich running engine for example; fuel is used to cool the cylinder. We also have the airflow (and some oil cooling) that also cools the cylinder. But, as we lean the engine we take away fuel's role in cooling the cylinder. We are left with air and oil cooling. Changes in air flow has a greater impact on CHT temperatures on a lean engine.

 Within some rich to lean range those cylinders (or engines) that are not as well air-cooled will experience a greater CHT change. For the first time we now have recording instruments in many cockpits that can capture this data. Can we deduce cowling or baffling problems by comparing the amount of change?

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