$3.$ The brake specific fuel consumption $($BSFC$)$ is a measure of $\_\_\_.$ It is used to evaluate $\_\_\_.$

$-($fuel efficiency, the fuel consumed per unit of power produced, engine performance, the effectiveness of fuel utilization, thermal efficiency$)$

$4.$ In a four$-$stroke engine, the power stroke occurs when $\_\_\_.$ This stroke is responsible for $\_\_\_.$

$-($the piston moves down after the spark plug ignites the air$-$fuel mixture, generating the engine's power output, the exhaust valve opens, reducing emissions$)$