Boyle’s law does not apply, since nowhere in its strokes is there an isothermal process Charles’ law only applies in the case of the diesel cycle’s combustion process at constant pressure In a spark-ignition Otto cycle engine, Gay-Lussac’s law applies in place of Charles’ law Boyle’s law, Charles’ law, Gay-Lussac’s law, and Avogadro’s law are all really part of the same law, the ideal gas law, just each with different conditions held constant. It is the ideal gas law which actually applies. And its constant quantity changes for each different type of stroke. The compression and expansion processes are nearly constant entropy (adiabatic) processes. The combustion in the Otto cycle, and the exhaust-intake in either cycle are constant volume processes.