$5.$ The thermal efficiency of an engine is defined as the ratio of $\_\_\_.$ It is an important metric for $\_\_\_.$

$-($useful work output to the total energy input, determining the efficiency of fuel use, the engine's power output to the fuel consumed, assessing overall engine performance$)$

$6.$ The purpose of an intercooler in a turbocharged engine is to $\_\_\_.$ This helps to $\_\_\_.$

$-($reduce the temperature of compressed air, increase the air density entering the engine, improve power output, decrease the risk of engine knocking, enhance thermal efficiency$)$