Report on Inputs, outputs, Closed and open Loop control
Introduction
In this report I will briefly explain how fuel control system work by give an overview of the fuel control system – this will include sensors, actuators, open and closed loop control mode.
In the past 4 weeks we were focused on the Electronic fuel control system which is a sub-system of modern electronic control system. Emission control and fuel efficient is the main focus for this system. A fuel control system has to be able to deal with all possible engine operating conditions, therefore different operating control modes are developed. Each operating modes uses different software to control the operation of an engine. ECU as the brain of the control system will logically determine which operating mode an engine should be in. Data gathered by different sensors will be analyzed by ECU, according to these data ECU will make its decision on what operating engine should be running then actuator will be activate to perform required tasks.
Figure 1. Basic structure of the electronic fuel control system.
As we can see in Figure1. All sensors such as TPS, MAF or MAP, EGO, Crank/cam shaft, Coolant Temperature sensors all provide data to ECU. By analyzing these data ECU select the right operating mode that engine should be operate to provide max amount of emission control and fuel efficiency. For our lab results and discussion, please refer to the CHART OF INPUTS AND OUTPUTS thank you.
Sensors
TPS – throttle position sensor
This sensor will give an indication of how much the throttle valve has been opened by driver. To an ECU this may be the indication of acceleration or loaded etc.
MAP- Manifold Absolute Pressure Sensor
This is a measurement of the pressure in the manifold. It indicates the load of an engine at a particular time.
Crankshaft Sensor
This sensor gives an accurate indication of the position of crankshaft. This kind sensor is perhaps the most accurate indicator of engine speed or cylinder position etc. This is because it is right next to the crankshaft.
Coolant Temperature Sensor
This sensor indicates the temperature of the coolant which means the temperature of the engine. These type sensors are normally NTC (negative temperature Coefficient) which means the voltage it output to the ECU decreases as engine gets hotter. It functions in many areas – for example it helps the ECU to determine the transition between warm up mode and open loop control mode.
EGO sensor
This is main sensor that triggers the closed loop mode control. Once it is fully warmed up and provides reasonable information to ECU, engine will go out from open loop control mode to closed loop control mode – the most preferred control mode due to the fact that emission is very precisely control in this mode because ECU will then adjust air fuel ratio accordingly to achieve very close to 14.7:1 ratio.
Knock sensor
This sensor is the ear of ECU. It detects knock in the engine. Distributor less ignition system normally advance the timing to its limit – this sensor will tell the system where the limit is because engine will start to knock if timing is too far advanced.
Actuators
Fuel Injection
Once ECU determined the right operating mode. The operating mode will switch on and off fuel injectors to provide right air fuel ratio to achieve best emission control and fuel efficiency result.
EGR recirculation actuator
Controlled by certain operating mode EGR actuator will allow exhaust air back into combustion chamber to reduce it heat therefore reduce NOx formation.
Ignition System
The combination of the spark plug, ignition coil and driver electronic unit also formed an equivalent to an actuator. Signals from ECU trigger spark to ignite air fuel mixture at precise time to produce maximum power and efficiency.
How sensor and actuator related
In the paragraphs above I have explained some important sensors and actuators of an EFI system. Now let’s see an example of how ECU use these sensors information to select correct operation control mode in order to activate actuators therefore achieve best engine operation in an electronic fuel control system. This is only a simplified example.
Now let’s start our example. As a driver switch on ignition the ECU will select engine crank mode. This mode of operation will instruct the fuel injectors open longer than normally do – this will provide a rich mixture which will enable engine to start initially. Fuel efficiency and emission control is not considered here.
Once RPM (could be indicated by crank shaft sensor) raised above certain value (this is an indication that car is successfully started). ECU will select warm up mode. Once engine is running in warm up mode fuel injectors will still be instructed to provide rich mixture until coolant temperature sensor indicates that engine has warm up to its operational temperature. Still in this mode of control ECU is only interested in getting engine warmed up as quickly as possible to it operational temperature, fuel efficiency or emission control are not considered.
As temperature of the engine reached it operational temperature. ECU will now select open loop control mode which will be focused on fuel efficiency and emission control by providing a variable air fuel ratio which is very close to 14.7:1 according to engine temperature.
From the above example we could see how ECU getting information from crank shaft sensor and Coolant temperature sensor to select different mode of operation. Then how these mode of operation activated fuel injector as an actuator to perform the goal of that particular operating mode.
Finally before I end this report let do a bit explanations on open loop and closed loop control and do a comparison between these two modes of operation.
Closed Loop vs. Open Loop
As we could see that open loop and closed loop are parts of these seven control modes listed above. Details will be explained in the following paragraphs.
Open Loop Control
This control mode will take over the control of an engine’s operation after warm up mode. The transition will be normally triggered by the fact that coolant temperature has reached a certain value and close loop mode conditions are not met. Open loop mode will try to bring air fuel ratio as close to 14.7:1 as possible. However because at this moment EGO is not providing information back to ECU, ECU could not make adjustment to the air fuel ratio accordingly as in a closed loop. Instead ECU uses the instantaneous air fuel ratio as a function of coolant temperature. A list of air fuel ratio is stored in ECU according to coolant temperature. This may not provide the best emission control result but shouldn’t be too far away either.
Closed Loop Control
Once the EGO is warmed up and start to provide feedback and some other conditions met. Closed loop control will take over full control of the engine operation i.e. open loop control is exited. In closed loop control mode emission is closely monitored by EGO. EGO will repeat sending lean or rich signals back to ECU. ECU will then adjust air fuel ratio according to 14.7:1 ratio. The EGO doesn’t send a constant value because its main focus is the average value – limit cycle control. If EGO goes faulty and provided no feed back or wrong signals over a certain amount of time, ECU will select a limp mode. Therefore closed loop control is exited, open loop mode start to operate. Open loop control and closed loop control are two controlling modes of fuel control system. Open loop mode could be used whenever closed loop conditions are not met. It air fuel ratio is not set to exactly 14.7:1 but instead a varying value according to coolant temperature. A closed loop control with aid of EGO will able to provide a much more accurate emission control.
Conclusion
In this report I explained my basic understanding of electronic fuel controlling system by first did a brief overview of controlling system. Then introduce some sensors, actuators and control modes. Finally made a few examples of how they are related to each to produce efficient engine operations. However this topic is very large and complicated. This is perhaps all the knowledge I have got at the present stage, but hopefully as I do more study I would have much more detailed understanding of engine management system.
References
William B. Ribbens Understanding Automotive Electronics 6th Edition
Chapter 4 - 7
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