Air to fuel ratios 1 Butterworth filters 1 Catalytic converters 1 Combustion 1 Damage detection 1 Digital signal-processing method Discrete Fourier transforms 1 Engines 1 Environmental conditions 1 Feature identification 1 High-efficiency 1 Incomplete combustion 1 Internal combustion engines 1 Low maintenance 1 Manufacture 1 Mechanical actuators 1 Oxygen sensors 1 Sensors or actuators 1 Signal detection 1 Signal processing 1 Zirconia 1