A 1.9L four cylinder engine was evaluated for leakage of cylinder charge through the exhaust valve seats. Fast FID HC analyzer traces reveal leakage. Static leakdown tests do not correlate with the Fast FID measurement, unlike previously published reports for a different engine. The causes of exhaust valve seat leakage are likely to be
Flakes of cylinder deposits lodging in the valve seat
Valve seat distortion due to the thermal and pressure loading of the cylinder head structure
Because deposit related effects are very history dependent, it is very difficult to obtain quantitative results.
Some experimental observations:
Static pressure leakage measurements show variation of leakage area with cylinder pressure, caused by flexing of the valve head.
Dynamic leakage results are history dependent. Leakage is reduced after running at high speed/load, and gradually build up during extended light load low speed operation.
Valve closing forces can apparently affect leakage
Particles of cylinder deposits are present in the exhaust port. These particles have composition similar to cylinder deposits but lower in carbon content and higher in oil ash. Particles were observed having 20-200 micron size. These deposits can lodge in the valve seat and contribute to leakage.
A brief survey of several other engines indicated the presence of leaks. It is likely that most engines exhibit exhaust valve seat leakage at least at some times.
It is difficult to assess the effect of these leaks on exhaust hydrocarbon emissions because of the complexity of the engine history dependence. Limited data suggest a 5% contribution is possible.