$7.$ The thermal efficiency of an internal combustion engine is determined by $\_\_\_.$ It measures $\_.$ High thermal efficiency results in $\_\_\_.$

$-($the ratio of useful work output to the energy input, how effectively the engine converts fuel into work, lower fuel consumption, reduced exhaust emissions, increased engine performance$)$

$8.$ The main function of the engine's camshaft is to $\_\_\_.$ It controls $\_.$ Proper camshaft timing is essential for $\_\_\_.$

$-($open and close the engine's valves, the intake and exhaust strokes, optimizing engine performance, ensuring efficient fuel combustion, reducing emissions$)$