Model-based cylinder-by-cylinder air-fuel ratio control for SI engines using sliding observers

Abstract

Exhaust stoichiometric air-fuel ratio is required for a 3-way catalytic converter to reduce exhaust emissions from an SI engine. Only when the combustion inside cylinders of an SI engine is kept close to its stoichiometric value can the catalytic converter then most efficiently reduce the harmful CO, HC, and NOx in the exhaust mixture. By this method the air pollution from the engine exhaust can be reduced. The purpose of air-fuel ratio (AFR) control is to maintain stoichiometric combustion in an SI engine. Currently one way to control AFR is by using the signal(s) from one or dual oxygen sensors installed on the exhaust pipes as a feedback signal to correct the fuel amount injected into intake manifold runner. The AFR signals obtained by such a method are mean values for the ratios in all cylinders. This makes a cylinder-by-cylinder control difficult. This paper proposes a new method for a cylinder-by-cylinder AFR control. In this method nonlinear state estimation is used to estimate the amounts of fuel and air being injested into each individual cylinder, and these air-fuel ratio signals are transmitted back to the electronic control module to correct the fuel amount injected into each runner port. The fuel mass in a cylinder is estimated by a cylinder pressure and combustion heat release observer, while the air mass into a cylinder is estimated by a runner-by-runner intake manifold pressure observer. This control method not only fulfils the AFR control cylinder by cylinder, but also may eliminate the need for conventional costly oxygen sensors.