Boosted engine

A boosted engine, comprising:

• an engine body formed with a combustion chamber, the engine body having a geometric compression ratio set between 14:1 and 30:1;
• a spark plug disposed in the combustion chamber;
• a fuel injection valve disposed to be oriented into the combustion chamber and configured to inject fuel at least containing gasoline;
• a booster disposed in an intake passage connected to the combustion chamber;
• and
• a control unit connected to the spark plug and the fuel injection valve and configured to output a control signal to the spark plug and the fuel injection valve, respectively, wherein
• the control unit includes a processor configured to execute:
• a combustion mode selecting module to select one of a first combustion mode in which spark ignition (SI) combustion where a mixture gas formed in the combustion chamber combusts through flame propagation starts and compression ignition (CI) combustion where unburned mixture gas combusts by compression ignition is then performed, and a second combustion mode in which only the SI combustion is performed by the flame propagation;
• and
• a compression end temperature estimating module to determine whether a temperature of gas inside the combustion chamber exceeds a given temperature at a top dead center of compression stroke,
• when the combustion mode selecting module selects the first combustion mode, the control signals are outputted to the fuel injection valve and the spark plug so that the mixture gas formed inside the combustion chamber starts combustion through flame propagation by the ignition of the spark plug, and unburned mixture gas inside the combustion chamber then combusts by compression ignition,
• when the combustion mode selecting module selects the second combustion mode, the control signals are outputted to the fuel injection valve and the spark plug so that the mixture gas formed inside the combustion chamber starts combustion through flame propagation by the ignition of the spark plug and the combustion ends still having been caused by the flame propagation, and
• when the combustion mode selecting module selects the first combustion mode and the compression end temperature estimating module estimates that the gas temperature inside the combustion chamber exceeds the given temperature at the top dead center, the control signal is outputted to the fuel injection valve so that an injection end timing of the fuel is on the compression stroke, and the control signal is outputted to the spark plug so that the mixture gas inside the combustion chamber is ignited after the top dead center.