$3.$ The compression ratio of an engine is the ratio of the $\_\_\_$ volume to the $\_$ volume, and it significantly affects the engine$$s $\_\_\_$ efficiency.

$-($cylinder$,$ combustion chamber, thermal, volumetric, displacement$)$

$4.$ The primary function of the camshaft is to control the $\_\_\_$ and $\_$ of the engine$$s $\_\_\_$ valves.

$-($timing$,$ operation, opening, intake, exhaust, closing$)$