Tham khảo tài liệu 'understanding automotive electronics 5e episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | THE BASICS OF ELECTRONIC ENGINE CONTROL 5 In the case of fuel control the desired variables to be measured are HC CO and NOx concentrations. Unfortunately there is no cost-effective practical sensor for such measurements that can be built into the car s exhaust system. On the other hand there is a relatively inexpensive sensor that gives an indirect measurement of HC CO and NOx concentrations. This sensor generates an output that depends on the concentration of residual oxygen in the exhaust after combustion. As will be explained in detail in Chapter 6 this sensor is called an exhaust gas oxygen EGO sensor. It will be shown that the EGO sensor output switches abruptly between two voltage levels depending on whether the input air fuel ratio is richer than or leaner than stoichiometry. Such a sensor is appropriate for use in a limit-cycle type of closed-loop control described in Chapter 2 . Although the EGO sensor is a switching-type sensor it provides sufficient information to the controller to maintain the average air fuel ratio over time at stoichiometry thereby meeting the mixture requirements at the three-way catalytic converter. In a typical modern electronic fuel control system the fuel delivery is partly open loop and partly closed loop. The open-loop portion of the fuel flow is determined by measurement of air flow. This portion sets the air fuel ratio at approximately stoichiometry. A closed-loop portion is added to the fuel delivery to ensure that time-average air fuel ratio is at stoichiometry within the tolerances of the window . There are exceptions to the stoichiometric mixture setting during certain engine operating conditions including engine start heavy acceleration and deceleration. These conditions represent a very small fraction of the overall engine operating times and are discussed in Chapter 7 which explains the operation of a modern practical digital electronic engine control system. Engine Control Sequence Referring to Figure the .
