Methods and system for central fuel injection

An engine system, comprising:

• an engine intake manifold;
• an engine cylinder;
• a direct injector configured to direct inject a fuel into the cylinder;
• a port injector configured to port inject the fuel into the cylinder;
• a central fuel injector to inject the fuel into the engine intake manifold, upstream of the cylinder;
• a knock sensor coupled to the cylinder;
• and
• a controller with computer-readable instructions stored on non-transitory memory for:
• indicating pre-ignition, different from knocking, in the cylinder responsive to on an output of the knock sensor in a first crank angle window, before a spark ignition event of the cylinder, being larger than a pre-ignition threshold;
• and
• in response to the indication of pre-ignition, selectively increasing a ratio of fuel delivered to the cylinder via the central fuel injector relative to the direct injector for a first duration, while operating the cylinder with a stoichiometric air-fuel ratio.