Which phenomenon has the most adverse effect on volumetric efficiency when engine works at high speeds?

Flow friction and charge heating

Which phenomenon has the most adverse effect on volumetric efficiency

1.	Which phenomenon has the most adverse effect on volumetric efficiency when engine works at high speeds?
A.	Flow friction and choking
B.	Ram effect and chocking
C.	Flow friction and charge heating
D.	Charge heating and back flow
Answer» D. Charge heating and back flow