Method and system for determining knock control fluid composition

A method for an engine, comprising:

• injecting an amount of water-alcohol blend into an engine at a first intake mass flow rate, and estimating a first alcohol concentration in intake air based on a first change in pumping current of an oxygen sensor;
• then, injecting the amount of water-alcohol blend into the engine at a second, different intake mass flow rate, and estimating a second alcohol concentration in the intake air based on a second change in pumping current of the oxygen sensor;
• learning a diluent concentration in the intake air based on the first and second change in pumping current;
• and
• learning a composition of the water-alcohol blend based on each of the first and second change in pumping currents and the learned diluent concentration.

An engine system, comprising:

• an engine including an intake manifold for receiving intake air;
• a first injector for injecting fuel into an engine cylinder;
• a second injector for injecting a knock control fluid into the intake manifold, downstream of an intake throttle;
• an EGR system including a passage for recirculating exhaust residuals from downstream of the turbine to upstream of the compressor via an EGR valve;
• an oxygen sensor coupled to the intake manifold, downstream of the intake throttle and downstream of the EGR valve;
• and
• a controller with computer readable instructions stored on non-transitory memory for:
• injecting knock control fluid into the intake manifold at a first gaseous volume percentage, then applying a reference voltage to the oxygen sensor and measuring a first change in pumping current;
• then injecting the knock control fluid into the intake manifold at a second, different gaseous volume percentage, then applying the reference voltage to the oxygen sensor and measuring a second change in pumping current;
• estimating a diluent content of the intake air based on each of the first and second change in pumping current and the first and second gaseous volume percentage;
• and
• estimating a composition of the knock control fluid based on each of an injection mass, the first and second change in pumping current, and the estimated diluent content.