Name: Tung Nguyen Group: CAT8
Report: Automotive
Basic operation of an automotive computer control system
Basic Operation
Electronic control systems in a modern vehicle is reponsible for monitoring and managing engine functions. these functions used to be performed by mechanical components, such as a carburetor. System elements are placed in order for proper monitoring and management of the engine functions, they are Input sensors, a Microcomputer (ECU), and output actuators.
ECU receives voltage signals from its various sensors, then conditions and processes the information to give out command signals to its actuators.
1. Inputs
Inputs are voltage signals generated by their sensors and transferred to the command center (ECU). There are some common types of sensors: variable resistors(potentiometer), thermistors, switches , magnetic pickups, voltage generators. But two types of voltage signal received by ECU are analog(variable) and digital(numerical), though ECU’s microprocessor can only read digital signal.
Throttle position sensor is a good example of both analog and digital signal feed to the ECU. It has a variable resistor, which varies whenever the throttle butterfly opens towards the maximum position. E.G at closed voltage reading is 0.69V, as I open, it goes up to a point where it is 2.33V, up to the maximum of 3.91V.
Switch and variable resistor type sensors usually need current limiting resistors in order to
operate. The resistor also serves as a circuit protection device.(EFI theory note) .This is the reason why when voltage supply to TPS sensor is 5V but the output voltage for ECU feed is only around 4V.
TPS also feeds the ECU with digital signal as a ground circuit:
Switch type: The TPS signal is very important to ensure smooth and economic engine running and the TPS is made to be adjusted to ensure optimum operation.(EFI note)
Another common type sensor is thermister, they could be engine coolant temperature(ECT); Intake Air Temperature(IAT); Exhaust Gas Recirculation temperature(EGR). The basic operation of this type is a variable resistor operates in reaction to different value of temperature:
2. Output
Back to the TPS scenario, moving the throttle up and down is not an actuator that is mainly given out by ECU, but ourselves. TPS inputs giving information about different engine loads(Idle; full throttle etc.) affect numbers of actuators. For example, injection system is crucially dependent on the operation of Throttle Body. Because when the butterfly opens, this will let more air into the intake, ECU knows this because it has combined information from places to sense any changes: TPS says open; MAF says more air comes in etc. Therefore, ECU is able to process and give out command to the actuator, the injectors (via main relay) that: “WE HAVE MORE AIR, SO PUMP MORE FUEL IN THE MIX OTHERWISE WE WILL BE RUNNING TOO LEAN!”.
When the ECU receives information from the Input sensors that a change is required, the ECU controls a set of actuators to achieve the desired change. To operate an actuator, the ECU relies on a component called an "output driver". Output drivers are nothing more than a specific on and off switch to control the actuator.
So what actually happens to the injectors is, the solenoid that controls the open and close operation of the armature inside the injector, is switched on and off by the “output driver” to pump and not pump the atomized fuel into the mixture.At different engine loads and conditions, all information about changes is collected by sensors go give ECU a sufficient overall view. In this particular Injectors scenario, the injectors are always switched on and off but the only difference is at different engine loads and conditions, the duty cycle of this switch varies. For example, TPS says open, MAF says more air coming in as we rev the engine, Injectors on time will dominate:
3. Closed loop circuit
In some open-loop operations such as Starting Engine or Cold engine operation, ECU ignores exhaust oxygen sensor information even if the sensor is detecting an excessively rich or lean mixture. Because excessive fuel is needed for example during cold start, therefore it is not desirable to operate the system in closed-loop due to risk of catalyst overheating and driveability concern.
On the other hand, Emission/ fuel control feedback system is the full definition of a closed-loop circuit. ECU is making air/fuel ratio correction based on oxygen sensor information. The oxygen sensor feeds the ECU with information, the ECU then adjusts the fuel injection pulse width (spray time length) with the injectors. This process is continuous, so there should be optimal air to fuel ratio. If an input sensor is defected, then there could be some “fail-safe” scenario happening, in which the vehicle is still be able to make it to the garage for diagnosis. However, it will be running on incorrect fuel efficiency.
The bottom line is, a closed-loop circuit is able to “listen and correct” itself. If there is any knock detected in the combustion chamber, Ignition Timing Advance will fix the problem by the correct timing and air/fuel mixture. Inputs and Outputs have a very dynamic relationship in closed loop system, not only they affect each other, they are also integrated onto a more dynamic relationship. Inputs will not solely determine system outputs but also outputs results will be referenced into correct system input.
THE END.
