An internal combustion engine control apparatus includes a cylinder pressure sensor, a driving condition detector, a reference crank angle setter, a reference cylinder pressure calculator, an air-fuel ratio estimator, and a controller. The cylinder pressure sensor detects a cylinder pressure. The driving condition detector detects a driving condition in an engine. The reference crank angle setter calculates a reference crank angle immediately before which an air-fuel mixture starts combusting in accordance with the driving condition. The reference cylinder pressure calculator calculates a reference cylinder pressure in the cylinder at the reference crank angle based on temperature characteristics of a specific-heat ratio of the air-fuel mixture under a condition. The air-fuel ratio estimator calculates an air-fuel ratio based on a pressure difference between the reference cylinder pressure and the cylinder pressure at the reference crank angle. The controller controls the engine in accordance with the air-fuel ratio.

A fuel injection system with a high-pressure fuel injection pomp is provided for pressurizing fuel and delivering it for injection into an internal combustion engine. The system and the high-pressure fuel injection pump have an inlet, a return port, at least one plunger, a suction, channel positioned between the inlet and plunger, and a bleed valve connectively arranged at the suction channel of the high-pressure fuel injection pump, A selector valve is provided between the inlet, and return port.

A fuel injection system with a high-pressure fuel injection pomp is provided for pressurizing fuel and delivering it for injection into an internal combustion engine. The system and the high-pressure fuel injection pump have an inlet, a return port, at least one plunger, a suction, channel positioned between the inlet and plunger, and a bleed valve connectively arranged at the suction channel of the high-pressure fuel injection pump, A selector valve is provided between the inlet, and return port.