The HCCI-spark ignition engine shown in FIGS. 1 a and 1 b is thus attractive in that it uses HCCI where this mode is feasible and advantageous, uses spark-ignition engine combustion at higher loads where HCCI is not feasible, uses a high compression ratio in all operating modes, permits pressure boosting by turbocharging or supercharging or other means to increase the engine's output and requires