Air amount estimating apparatus for internal combustion engine with supercharger

An air amount estimating apparatus for a supercharged internal combustion engine having an intake path for introducing air drawn in from an outside into a cylinder, a compressor disposed in the intake path, a throttle disposed in the intake path downstream of the compressor, and an intake valve disposed at a connection between the intake path and the cylinder, the air amount estimating apparatus comprising:

• a calculating unit that calculates a compressor flow rate, which is a flow rate of air passing through the compressor, based on a throttle upstream pressure, which is pressure of air in a throttle upstream section which is an intake path section from the compressor to the throttle, and based on a rotational speed of the compressor;
• a calculating unit that calculates the throttle upstream pressure using a physical model developed based on a conservation law of air in the throttle upstream section and based on the compressor flow rate and a throttle flow rate, which is a flow rate of air passing through the throttle;
• a calculating unit that calculates the throttle flow rate;
• a calculating unit that calculates a throttle downstream pressure, which is a pressure of air in a throttle downstream section which is an intake path section from the throttle to the intake valve, the calculating unit using a physical model developed based on a conservation law of air in the throttle downstream section and calculating the throttle downstream pressure based on the throttle flow rate and an intake valve flow rate, which is a flow rate of air passing through the intake valve;
• a calculating unit that calculates the intake valve flow rate based on the throttle downstream pressure;
• and
• a calculating unit that calculates an amount of air drawn into the cylinder based on the intake valve flow rate;
• wherein:
• the throttle flow rate calculating unit includes:
• a first calculating unit that calculates the throttle flow rate using an equation of throttle based on the throttle upstream pressure, the throttle downstream pressure, and an opening of the throttle;
• a second calculating unit that calculates the throttle flow rate using an equation of linear interpolation based on the compressor flow rate and the intake valve flow rate;
• a selection condition determining unit that determines whether or not a predetermined selection condition is satisfied, the predetermined selection condition including a pressure condition that a difference between the throttle upstream pressure and the throttle downstream pressure is smaller than a reference difference or a ratio of the throttle downstream pressure to the throttle upstream pressure is greater than a reference ratio;
• and
• a selection unit that opts to calculate the throttle flow rate using the first calculating unit when the selection condition is determined not to be satisfied and opts to calculate the throttle flow rate using the second calculating unit when the selection condition is determined to be satisfied.