dualcycle

Dualcycle, or the dual cycle (also called the mixed cycle), is a thermodynamic model used in air-standard analysis of internal combustion engines. It represents a compromise between the Otto cycle, which assumes all heat addition at constant volume, and the Diesel cycle, which assumes all heat addition at constant pressure. In the dual cycle, heat is added partly at constant volume and partly at constant pressure.

In the ideal dual cycle, the working fluid undergoes four reversible processes: 1-2 isentropic compression, 2-3 heat addition at constant volume, 3-4 isentropic expansion, and 4-1 heat rejection at constant pressure. The engine’s heat input is divided into a fraction f that occurs at constant volume and a fraction (1-f) that occurs at constant pressure. The parameter f varies between 0 and 1, with f = 1 reproducing the Otto cycle and f = 0 reproducing the Diesel cycle.

Thermal efficiency in the dual cycle depends on the compression ratio and the specific heat ratio, as well as the fraction f of heat added at constant volume. For the same compression ratio and heat-input characteristics, increasing the constant-volume portion moves the performance toward the Otto limit, while increasing the constant-pressure portion moves it toward the Diesel limit. The dual cycle is widely used as an educational tool to analyze engine performance and to illustrate how different heat-addition strategies affect efficiency and work output.

Limitations include the idealized nature of the model, such as assuming ideal gas behavior, constant specific heats, and neglecting real engine losses, rate effects, and complex combustion chemistry. Despite this, the dual cycle remains a useful framework for classroom analysis and qualitative comparison of internal combustion engine cycles.