Describes the main types of engine management used today. It also explains the advantages and the appropriate applications for each type of engine management system. The majority of this information was found in the Holden Gen III service manual distributed by General Motors.
There are mainly two predominant types of engine management that exist today. The first of these two types is Speed Density; the other is Mass Air Flow. Both systems have their advantages and disadvantages, and each is better suited for different types of applications.
Speed density calculates the injector pulse width by first calculating the mass air flow from the following inputs: engine displacement, RPM, manifold pressure, 41 manifold air temperature, and volumetric efficiency. Once the mass airflow has been calculated, the engine control unit (ECU) uses it along with RPM, injector flow rating (usually given in lbs.), and the target air/fuel ratio to find the desired injector pulse width. The injector pulse width is simply the time taken in between each firing of the injector.
Mass Air Flow
Mass air flow uses a different technique to calculate the injector pulse width. The mass air flow type of engine management uses a sensing device such as a pivoting vane or headed wire to calculate the mass air flow into the engine. This mass air flow value is then used in conjunction with similar variables as in speed density to calculate the injector pulse width.
Open and Closed Loop Operation
Note that the target air-fuel ratio is used as a factor in calculating the injector pulse width. This is only a factor when the system is operating in a closed-loop manner. In a closed-loop, the ECU compares the actual air-fuel ratio to the desired value. Therefore, the output of the O2 sensor is used in the determination of the final calculation of the injector pulse width.
There are many instances where the closed-loop mode is desirable. One of the most important uses of closed-loop is to meet emissions laws. Simply put, the leaner (high air-fuel ratio) fuel mixtures provide the best possible emissions output due to the fact that the intake charge will burn hotter than a rich mixture.
The hot burning ensures that no unburned fuel gets emitted from the exhaust system and 42 into the atmosphere. There are, times, however, when the closed-loop is not desirable. During cold start-up, an open loop is usually disabled in order to let the engine temperature rise to its normal operating value.
In addition, at cold temperatures, the O2 sensors usually do not produce accurate readings as they can sometimes become lazy when not up to normal operating temperature. At temperatures under operating temperature, the O2 sensor has a very high internal resistance. The ECU usually supplies a constant voltage to the O2 sensor.
With the high internal resistance, the ECU only receives a very low, constant voltage value from the sensor itself. Once it warms, the O2 sensor outputs a very rapidly changing voltage reading that the ECU can use accordingly. The O2 sensors used by General Motors engine platforms range from 100mV (lean mixture) to 900mV (rich mixture).