Six Degrees Crankshaft Individual Air Fuel Ratio Estimation of Diesel Engines for Cylinder Balancing Purpose 2006-01-0013

In the context of modern engine control, one important variable is the individual Air Fuel Ratio (AFR) which is a good representation of the produced torque. It results from various inputs such as injected quantities, boost pressure, and the exhaust gas recirculation (EGR) rate. Further, for forthcoming HCCI engines and regeneration filters (Particulate filters, DeNOx), even slight AFR unbalance between the cylinders can have dramatic consequences and induce important noise, possible stall and higher emissions. Classically, in Spark Ignition engine, overall AFR is directly controlled with the injection system. In this approach, all cylinders share the same closed-loop input signal based on the single λ-sensor (normalized Fuel-Air Ratio measurement, it can be rewritten with AFR as they have the same injection set-point. Unfortunately, due to inherent flaws of the injection system (pressure waves, mechanical tolerances, …), the total mass of fuel injected in each cylinder is very difficult to predict with a relative precision better than 7%. Having a sensor in each cylinder would enable an accurate individual control. In practice, cost and reliability of multiple λ-sensors prevent them from reaching commercial products lines. In this context, individual cylinder AFR estimation can give crucial information to get the HCCI running better.

The contribution of this paper is the design and experimental tests of a real-time observer for the individual cylinder AFR using the reliable and available λ-sensor placed downstream the turbine as only measurement. In previous works, the methods used to reconstruct the AFR of each cylinder from the UEGO (Universal Exhaust Gas Oxygen) λ-sensor measurement are based on the permutation dynamics at the TDC (Top-Dead Center) time-scale and a gain identification technique. Here, we propose a higher frequency approach (6 degree crankshaft angle modelling and update instead of 180(TDC)). We design an observer on the balance model of the exhaust and design a high frequency observer to solve the problem. We use a physics-based model underlying the role of periodic input flows (gas flows from the cylinders into the exhaust manifold). The observer is validated experimentally on a 4 cylinder HCCI engine. As a conclusion, we provide results of closed-loop control using the proposed technique to prove the relevance of this approach.