Robust Oxygen Fraction Estimation for Diesel Engines Utilizing Variable Intake Valve Actuation*

Advanced combustion strategies, such as pre-mixed charge compression ignition (PCCI), homogenous charge compression ignition (HCCI) and low temperature combustion (LTC), can be controlled and enabled through the use of flexible valvetrains. The in-cylinder oxygen fraction serves as a critical control input to these strategies, but is extremely difficult to measure on production engines. Fortunately, estimates or measurements of the oxygen fraction in the intake and exhaust manifold, the in-cylinder charge mass, and residual mass can be utilized to calculate the in-cylinder oxygen fraction. This paper outlines such a physically-based, generalizable strategy to estimate the in-cylinder oxygen fraction from only production viable measurements or estimates of exhaust oxygen fraction, fresh air flow, charge flow, fuel flow, turbine flow and EGR flow. The oxygen fraction estimates are compared to laboratory grade measurements available for the intake and exhaust manifolds. Furthermore, the in-cylinder oxygen estimation algorithm is developed, and proven, to be robust to turbine flow errors. The model-based observer estimates the oxygen fractions to within 0.5% O2 and is shown to have exponential convergence with a time constant less than 0.05 seconds, even with turbine flow errors of up to 25%. The observer is applicable to engines utilizing high pressure cooled exhaust gas recirculation, variable geometry turbocharging and flexible intake valve actuation.