Fuel injection system for internal combustion engine

A fuel injection system for an internal combustion engine, the internal combustion engine including:

• a port injector that injects fuel into an intake port;
• and
• an in-cylinder injector that injects fuel into a cylinder, the in-cylinder injector including a needle valve, and the in-cylinder injector being able to execute partial lift injection while the needle valve is not fully opened,
• the fuel injection system comprising
• an electronic control unit configured to
• control the port injector and the in-cylinder injector by controlling energization of the port injector and the in-cylinder injector,
• calculate a first fuel pressure required in one fuel injection cycle during which a maximum partial lift injection is executed once,
• calculate a second fuel pressure required in one fuel injection cycle during which the maximum partial lift injection is executed twice, the second calculated fuel pressure being less than the first calculated fuel pressure,
• compare a pressure of the fuel supplied to the in-cylinder injector to the first calculated fuel pressure and to the second calculated fuel pressure in order to determine that an injection amount of the maximum partial lift injection in one fuel injection cycle is equal to or less than a target injection amount for one injection cycle,
• set the number of times that the maximum partial lift injection is executed in one fuel injection cycle to zero when the pressure of the fuel supplied to the in-cylinder injector is greater than the first calculated fuel pressure,
• set the number of times that the maximum partial lift injection is executed in one fuel injection cycle to one when the pressure of the fuel supplied to the in-cylinder injector is less than the first calculated fuel pressure and greater than the second calculated fuel pressure,
• set the number of times that the maximum partial lift injection is executed in one fuel injection cycle to two when the pressure of the fuel supplied to the in-cylinder injector is less than the second calculated fuel pressure,
• cause the in-cylinder injector to execute the maximum partial lift injection the set number of times, and
• calculate a shortfall of fuel with respect to the target injection amount and cause the port injector to inject the fuel in an amount equal to the calculated shortfall.