The Influence of Compression Ratio and Dissociation on Ideal Otto Cycle Engine Thermal Efficiency 620557

THIS PAPER PRESENTS a theoretical analysis of the ideal adiabatic Otto cycle engine. The analysis was made to examine the influence of compression ratio and dissociation on engine thermal efficiency over an extreme range of compression ratios (that is, 4–300) to see if chemical dissociation could limit Otto cycle engine thermal efficiency.

Assuming isooctane, benzene, ethyl alcohol, and nitromethane to be the fuels being consumed, the effects of compression ratio and mixture strength on the thermodynamic properties and equilibrium species concentration of the working fluid at every step in the ideal Otto cycle were computed. The calculations were made using a mathematical model of the ideal adiabatic engine which had been programmed to an IBM 704 digital computer. With the model, the effect of compression ratio on engine thermal efficiency was calculated over a wide range of operating conditions. The results of the study showed that engine thermal efficiency continued to increase with compression ratio at least up to 300:1 for all the fuels considered.

The effect of compression ratio and mixture strength on chemical dissociation was studied using two parameters termed: (1) “Extent of Dissociation” and (2) “Net Energy Loss.” The “Extent of Dissociation” was defined as the deviation from the theoretical concentration of product species when a given fuel air mixture is burned. The “Net Energy Loss” associated with chemical dissociation was defined as the change in internal energy between a burned mixture at chemical equilibrium and the frozen theoretical burned mixture when both are at some selected temperature and pressure level. Both dissociation parameters were examined after combustion at top dead center in the Otto cycle engine at various levels of compression ratio and mixture strength. It was noted, in general, that the “Extent of Dissociation” and “Net Energy Loss” increased with increasing compression ratio. The increase, however, was small.

The paper demonstrates that under theoretical and ideal conditions, there is no upper limit to which the compression ratio of the Otto cycle engine can be raised. The reason for this is that any increase in chemical dissociation brought about by increasing temperature is substantially suppressed by the very rapidly increasing pressure levels which result when the compression ratio is increased.