Original: In thermodynamics , the thermal efficiency ( eta _ { th } , ) is a dimensionless performance measure of a device that uses thermal energy , such as an internal combustion engine , a boiler , a furnace , or a refrigerator for example .
Simplified: The thermal efficiency ( eta _ { th } , ) is a dimensionless performance measure of a thermal device such as an internal combustion engine , a boiler , or a furnace , for example .