Control system of compression-ignition engine

A control system of a compression-ignition engine including a cylinder, an intake passage, an exhaust passage, an intake port communicating the intake passage to the cylinder, an intake valve configured to open and close the intake port, an exhaust port communicating the exhaust passage to the cylinder, an exhaust valve configured to open and close the exhaust port, an injector configured to inject fuel into the cylinder, and a spark plug configured to ignite a mixture gas containing the fuel injected by the injector and air, the engine executing partial compression-ignition combustion in which the mixture gas is spark-ignited with the spark plug to be partially combusted by spark ignition (SI) combustion and the remaining mixture gas self-ignites to be combusted by compression ignition (CI) combustion, comprising:

• an intake variable mechanism configured to change an open timing and a close timing of the intake valve;
• and
• a controller including a processor configured to control parts of the engine, including the intake variable mechanism and the spark plug,
• wherein the controller controls the intake variable mechanism so that, when the engine is operated in a given second operating range, an air-fuel ratio (A/F) lean environment where an air-fuel ratio that is a ratio of air to fuel in the cylinder becomes higher than a stoichiometric air-fuel ratio is formed, and the open timing of the intake valve is at an advanced side of an exhaust top dead center, while causing the spark plug to perform the spark ignition at a given timing so that the mixture gas combusts by the partial compression-ignition combustion, and
• wherein the controller controls the intake variable mechanism so that, when the engine is operated in a first operating range where the engine load is lower than that of the second operating range, the A/F lean environment where the air-fuel ratio becomes higher than the stoichiometric air-fuel ratio is formed, while causing the spark plug to perform the spark ignition at given timing so that the mixture gas combusts by the partial compression-ignition combustion, and the controller controls the intake variable mechanism so that, under the same engine speed condition, the close timing of the intake valve is more retarded within a range on a retarded side of an intake bottom dead center as the engine load decreases, and an absolute value of a rate of change in the close timing of the intake valve to the engine load becomes larger than an absolute value of the rate of change in the second operating range.