An evaluation using various operating conditions such as the maximums of: power, power density, ECOP and ECOL optimization functions for the analysis of turbojet engines is reported. Although, these well-known optimization functions were applied for various cycle types by researchers, the application of these methods for aircraft turbojet engines have not been explored. To this effect, a Brayton cycle including diffuser and nozzle together with the compressor, combustion chamber and turbine with various irreversibilities was considered. Engine irreversibilities are taken into account through appropriate efficiencies for each component. Different parameters influencing the cycle performance are examined such as compressor pressure ratio, cycle temperature ratio, compressor and turbine efficiencies, altitude and flight Mach number. The cycle performance was assessed utilizing the maximum power (MP), power density (MPD), Ecological Coefficient of Performance (ECOP) and Ecological Function (ECOL) conditions. In addition, the size variation of individual engine components and their contribution to the overall performance was also assessed under maximum power, power density, ECOP and ECOL conditions. The comparisons show that the design parameters at the maximum ECOL and maximum ECOP conditions may lead to smaller, more efficient and lower fuel consumption for turbojet engines than those at maximum power and maximum power density. However, on a component basis (compressor, combustion chamber, turbine and nozzle) the engine power, thrust, thermal efficiency, size and TSFC can be enhanced when utilizing the MP, MPD, ECOP and ECOL optimization functions. (C) 2021 Elsevier Masson SAS. All rights reserved.