9.23.
System Overview
Figure 9.88 represents a typical control layout for a fuel injection system. Based on the sophistication required, idle speed and idle mixture adjustment can be controlled either
mechanically or electronically. A block diagram of inputs and outputs used in most of the fuel injection systems is illustrated in Fig. 9.89. Speed and load are the two most important input sensors used in the system. Similar to the ignition timing, the basic fuelling requirement is determined from these inputs data.
A three dimensional cartographic map (Fig. 9.90) is used to represent how the information on an engine’s fuelling requirements are stored. This information constitutes a part of a read only memory (ROM) chip in the ECU. Once the ECU has evaluated the look up value of the fuel required (injector open time), corrections to this figure can be added for battery voltage, temperature, throttle change or position and fuel cut-off.
Generally the ECU controls the idle speed and fast idle through a suitable actuator. A form of closed loop control is also possible with electronic fuel injection. This requires a lambda sensor to monitor exhaust gas oxygen content. A very accurate control of the mixture strength is achieved through this sensor, since the oxygen content of the exhaust is proportional to the air-fuel ratio. The signal from the lambda sensor is used to adjust the injector open time. The flow chart shows one method of processing the information from the sensors to determine the best injector open duration, as well as control engine idle speed.
