Fundamental to further development of internal combustion engines is the more complete understanding of the combustion processes themselves. The paper develops a theoretical treatment of the total combustion operation based on the first law of thermodynamics and expressed as differentials. Theoretical effects of gaseous fluxes into and out of the combustion chamber are examined.The second portion of the paper describes recently developed equipment which permits simultaneous, electronic measurement of numerous combustion parameters. The measurements provide quantitative values for the qualitative evaluation of the theoretical treatment, thus providing an integrated basis by which to analyze the entire combustion process. In particular, parallel analyses of the injection (or ignition) process of Diesel (Otto) operation provide means of discovering and eliminating causes of irregularities in the energy-conversion process. Integration of the two methods promises engine designs which more closely approximate ideal combustion processes, especially in relation to the final stage of combustion.