US Patent 7243532 Misfire detector for internal combustion engines

Air fuel ratios of exhaust gases of respective cylinders are estimated based on a detection value of an air fuel ratio sensor mounted in an exhaust-collector. Normal combustion and misfire are determined depending upon whether individual cylinder air fuel ratios of the respective cylinders are within a normal combustion range. Then, whether a fuel system causes misfire is determined based on a difference between an actual injection quantity of the misfire cylinder and an average value of actual injection quantities of all the cylinders. Whether an air system causes misfire is determined based on a difference between an estimated air quantity of the misfire cylinder and an average value of actual air quantities of all the cylinders.