Energy Loss from Closed Combustion Systems

The process of combustion in a closed system, such as a cylinder in a piston engine, is executed in three steps: (1) ignition, devoid of any pressure change; (2) the dynamic stage, manifested by distinct pressure rise; and (3) decay, associated with pressure drop. This paper is concerned solely with the second step, exhibiting the dynamic behavior of the system. The effectiveness of the exothermic energy generated in a closed system is affected only by the loss incurred as a consequence of heat transfer to the walls. Its evaluation is, then, the principal subject of the reported study. Measurements of heat transfer to the walls were made for this purpose in a constant volume vessel by the use of thin film thermometers, simultaneously with pressure and schlieren records. Profiles of energy loss were determined on the basis of a selfsimilarity theory developed on the basis of the self-similitude exhibited by heat flux profiles. At the same time, the mass fraction of fuel consumed in the course of the dynamic stage of combustion was deduced from the measured pressure trace by thermodynamic analysis based on the balances of mass, volume, and energy. A power-law correlation was thereby derived between the fuel consumed solely to elevate pressure, referred to as effective, and its total amount. As a phase relationship, this correlation is valid irrespectively of the geometry of the enclosure, as well as of its variation in a piston engine, especially in the vicinity of the top dead center where the dynamic stage of combustion takes place. Its utility is then quite general in permitting the ineffective part of consumed fuel, associated with energy lost by heat transfer to the walls, to be evaluated from the pressure record, without taking into account the geometry of the enclosure or its deformation.