Engine system

An engine system, comprising:

• an engine having a cylinder and a piston reciprocatably accommodated in the cylinder;
• an injector attached to the engine and configured to inject fuel into the cylinder;
• a spark plug attached to the engine and configured to ignite a mixture gas of fuel and intake air, the intake air containing fresh air and burnt gas;
• a variable valve operating device connected to an intake valve and an exhaust valve, and configured to control opening and closing of the intake valve and the exhaust valve to adjust a filling amount of the intake air;
• and
• a controller electrically connected to the injector, the spark plug, and the variable valve operating device, and configured to control the injector, the spark plug, and the variable valve operating device according to a demanded load of the engine,
• wherein the controller estimates a mass ratio of the intake air inside the cylinder to fuel,
• wherein while the engine operates at a given speed and the demanded engine load is a first load, when the mass ratio (G/F) is estimated to be below a first G/F, the controller controls the injector and the spark plug so that at least part of the mixture gas inside the cylinder combusts by flame propagation, and when the mass ratio is estimated to be above the first G/F, the controller controls the injector so that the entire mixture gas inside the cylinder combusts by compression ignition, and
• wherein while the engine operates at the given speed and the demanded engine load is a second load higher than the first load, when the mass ratio is estimated to be below a second G/F lower than the first G/F, the controller controls the injector and the spark plug so that at least part of the mixture gas inside the cylinder combusts by flame propagation, and when the mass ratio is estimated to be above the second G/F, the controller controls the injector so that the entire mixture gas inside the cylinder combusts by compression ignition.