TRUE thermodynamic interpretation of the indicator card must be based upon the properties of the actual medium working in the engine and must take into account the actual nature of the heat liberation.
The temperature-energy diagram for the working combustible mixture and for the resultant combustion products provides for this interpretation a foundation that is universally applicable to engines using a given type of fuel. This diagram automatically includes the effect of variation in specific heat with temperature, because the entire energy content of a gas at any temperature is the energy required to raise it, at constant volume, from absolute zero to that temperature. The work done during the actual changes of state, as determined from the indicator card, can readily be represented on the same diagram, and the heat interchanges involved can be determined quantitatively by comparison with the adiabatic criterion.
By the use of this diagram, a heat balance can be drawn up which accounts for the distribution of the entire heat input and output, not only as to the manner of its utilization or loss but also as to the time of each item in the cycle. A complete thermodynamic analysis is thus presented, from which the virtues and shortcomings of the engines are readily apparent.
The item of outstanding importance in the interpretation of the card is the nature of the heat liberation. By the procedure herein presented, this process is quantitatively evaluated, and the heat liberation during the normal combustion period and during expansion are separated so that the extent of the loss due to after-burning is revealed.
The procedure is illustrated by complete analysis of a sample card, and forms for tabulation and calculation are given whereby the work of interpretation can be reduced practically to routine.
Another advantageous use of the indicator is in determining the tendency toward explosion roughness. This depends primarily upon the maximum acceleration in the rate of pressure rise during combustion and upon its location in the burning period. The pressure-time record of combustion can be readily analyzed to determine relative roughness tendency on this basis.