Expressions for the thrust losses of a scramjet engine are developed in terms of irreversible entropy increases and the degree of incomplete combustion. A method is developed that allows the calculation of the lost engine thrust or thrust potential caused by different loss mechanisms within a given flowfield. This method allows the performance-based assessment of the trade between mixing enhancement and resultant increased flow losses in scramjet combustors. An engine effectiveness parameter for use in optimization of engine components is defined in terms of thrust losses.