Given data BP = 80 kW mf = 20 kg /Hr CV = 43 MJ / kg = 43000 kJ / kg To Find ηbth = ? The brake thermal efficiency of an engine is, ηair = BP mf 3600 × CV ηair = 80 20 3600 × 43000 ηair = 0.3348 ηair = 33.48 %