Introduction
Cars have become increasingly computerized; and more and more components of the average motor vehicles are getting digital support. Various reasons have precipitated this computerization. These include; an urgent need to control the emissions from internal combustion engines due the effect of green house gasses on the global climate through the process of global warming (Zafonte & Sabatier, 2004); the need to increase safety of traveling in a vehicle; increasing fuel efficiency due to economic incentives; easier design and building of cars; reduction in the extent of bulky hard-wiring; improved efficiency of troubleshooting, diagnostics and repair; and manufacture of more comfortable driving experiences (Cuatto et al, 2000).
The internal combustion engine is now commonly fitted with an engine control unit (ECU); this is an electronic device that control various functions of the engine. The complexity of the ECU varies; the least advanced unit has control only over the volume of gas injected into each cylinder or the timing of the spark. More complicated units have control over many other functions in the engine.
The ECU is able to regulate these functions of the engine in response to the information it receives from a horde of sensors mounted on various parts of the engine. An average ECU has a 32-bit, 40-MHz processor. While this may seem slow compared to the average processor on a desktop computer which carries a 500 to 1,000-MHz processor, the code that the ECU processor runs is more efficient; additionally, compared to the approximately two gigabytes of memory that the PC code requires, the ECU code needs only about one megabyte of storage space.
Components of the Engine Control Unit
The engine control unit has a microprocessor has the capacity to process all the information coming from the peripheral sensors installed in various parts of the vehicle; and is able to produce feedback in real-time. The hardware in the ECU is a circuit board composed of electronic components mounted on a printed ceramic circuit board. The main electronic component on the circuit board is a microcontroller chip which acts as the central processing unit (CPU) of the engine control unit.
The software part of the ECU is stored in the CPU as well as in other electronic components mounted alongside it; commonly, the format of storage is either EPROM or flash memory; consequently, the ECU can be reprogrammed by installing new software; alternatively, changing the microcontroller chip can be the preferred route of reprogramming.
The ECU communicates with many devises within and without the engine; this is achieved through the functions of the Controller Area Network bus automotive network. Some of the devises connected to the ECU through the bus include automatic transmission, traction control systems and other functions that are electronically controlled (depending on the design of the vehicle). This communication standard has the ability to achieve speed exceeding 500Kbps; this is absolutely necessary since some components communicate with the unit through the bus several hundred times in a span of a single second (Cuatto et al, 2000).
Some ECUs incorporate extra features outside the immediate functions of the engine; such include cruise control, electronic control of automatic transmission, anti-theft functions and power braking.
Supporting Components
The processor of the ECU is commonly found packaged together with parts of other circuit boards in form of a module that controls an array of car functions. The ECU is supported by various components including the following.
Analog-to-digital converters
These are functions to interpret some of the information coming from the various sensors located throughout the car from analog signals to digital information that the unit can understand; a good example is the oxygen sensor that measures the amount of oxygen that has not been consumed in the process of fuel combustion; this sensor produces its findings as an analog signal which is a voltage measuring between zero and 1.1 volts which converter changes into a 10-bit digital number.
High-level digital outputs
Most of the components of the engine whose function is controlled by the unit require amount of electric power that the unit circuitry cannot handle; on the other hand, the amount of power that this unit produces as a digital message is so small that they possibly would not activate this functions. Therefore, the high-level digital outputs serve as an interface between the unit and the engine. Some of these components include the cooling system especially the fan, the fuel injectors and the spark plugs.
A digital output has only two possible settings; either on or off. The high level digital output function like relay switches; the very small amount of voltage the unit produces as a signal to the target component activates the transistor in the output; the output, which is able to produce a relatively larger amount of energy activates the relay controlling the components; the relay therefore completes the task by supplying the necessary power to the target function (Cuatto et al, 2000).
Digital-to analog converters
These are necessary because not all components under the control of the unit can be activated by a digital signal. These converters therefore convert the digital data into an analog signal which is then transmitted to the target function.
Signal conditioners
These are installed so as to function in conjunction with the analog and digital converters. Different sensors located in various parts of the vehicles usually produce different levels of analog signals; the main purpose of the conditioners is to ensure that the converters make more accurate measurements than they would if they were directly connected to the sensors.
Communication chips
These support the communication standards that have become more common in the modern cars. Additionally the communication speeds involved in in-car communications have made it necessary for these chips to be of highest possible rating. The chips make it easier to design, construct and repair motor vehicles.
Functions of the Engine Control Unit
Fuel injection
In vehicles that have a fuel injection engine, the ECU controls the volume of fuel injected into the combustion chambers at any one time depending on a number of measurements that the unit receives. The act of pressing the vehicle gas pedal has the effect of increasing the amount of air intake into the engine by opening the throttle body further; depending on the volume of air flowing into the engine at any one time the unit will either inject more or less fuel accordingly. Additionally, there period immediately after the engine has started running when it has not warmed up enough; in response, the ECU injects more fuel and maintain a relatively rich running status until the temperature of the engine has raised satisfactorily and the injection volume reduces in response.
Ignition timing
Engines that run on spark ignition require the fuel and air mixture to be ignited by a spark to set off combustion that converts the chemical energy of the fuel to heat and eventually kinetic energy seen as turning of the engine. An engine control unit is enabled to determine the exact moment when the spark is produced.
The spark has to be at the most optimum moment of the compression cycle of the combustion chamber; this ensures generation of maximum kinetic energy and thus optimum consumption of fuel. A failure to ensure the best possible timing of ignition timing results in a knock that can severely damage the engine.
The ECU is also able to detect a possible knock and carry out the necessary adjustments to remedy the situation thus prevent the damage; for example, a knock might be occurring due to the ignition timing being too soon during compression; the ECU responds by delaying the production of the spark to remedy this. Alternatively, the unit shifts downwards the transmission to remedy a situation where the knock is caused by running the engine on a revolution rate that is too low for the amount of work one wants the engine to perform at the time resulting in the piston not being able to move down as quickly as the combusting fuel-air mixture expands.
Engine idling speed
Most of the vehicles that are controlled by an engine control unit have the ability of having the speed of revolution controlled by the unit during idling. The rate of revolution (RPM) is detected by a crankshaft position sensor; this sensor plays a significant part in influencing the rate of fuel injection, the ignition timing and valve timing.
The engine load at idle determines how much power it should be generating at idle; this load can be changed by a number of factors including; power steering, power braking, charging of the battery, temperature of the engine among other functions. The engine control unit has to predict the engine load at any one time and adjust the RPM either through programmable throttle stop or an idle air bypass control stepper motor depending on the make of the vehicle. In some of the engine, the throttle control system is more comprehensive and is able to not only control the idle speed, but also limits top speed (speed governor) and provide cruise control.
Variable valve timing
In engines that have been designed to run on variable valve timing, the ECU determines the moment in the engine cycle when the valves open. The main aim is to increase the power generated by the engine and optimized the consumption of fuel used to produce it; this is achieved by opening the valves later in the engine cycle when the engine is running on a higher speed and much later in the cycle when running at low speed.
Electronic valve control
The need to improve hybrid engines has led to development of electronic control of valves in experimental engine models; these do not have a camshaft and various functions of the valve are fully electronically controlled. These include opening and closing of the intake and exhaust valves; and the extent opening (Austen, 2003).
Such engines that have precision timed electronic ignition and fuel injection can be started and run without the need of a starter motor. This can provide efficiency and reduces the amount of carbon emission for hybrid vehicles; at the same time eliminating the need to having a large starter motor (Kassakian et al, 1996).
Special Engine Control Units
While most engine control units have come already programmed to function according to the design of the vehicles, there are situations where this is not the case or is modified.
Programmable Engine Control Units
The owner of a vehicle can choose to install an ECU that is reprogrammable; thus the responses are not predetermined by the manufacture but are customized according to the requirements of the user. Such are necessary when modifications made to vehicles engine after purchase are incompatible with the preprogrammed ECU responses.
A good example of this is fitting an engine with a turbocharger or modifying the one fitted by the vehicle manufacture. Other includes modifying or adding an intercooler system, the exhaust system; or converting the engine from consuming fossil fuels to running on alternative fuel.
These are very far reaching changes and the ECU would not be expected to provide similar adjustments as it would on the original format. Once the new ECU is installed, it’s programmed to maximum efficiency by using a computer as the engine is running; and the various responses are set accordingly.
Flashing an Engine Control Unit
In the recent past, many vehicles have fitted with an OBD-II engine control unit; unlike earlier models (before 1996) this has a port that allows a person to adjust some parts of the software installed in the factory. This is better than having to replace the entire system of electronically controlling the engine. This is also necessary if there are some additions or modifications in the engine that were not factored in during the manufacture of the car.
References
Austen Ian (2003). WHAT’S NEXT; A Chip-Based Challenge to a Car’s Spinning Camshaft. New York Times. Web.
Cuatto Tullio , Claudio Passerone, Claudio Sansoè, Francesco Gregoretti, Attila Jurecska and Alberto Sangiovanni-Vincentelli (2000): A Case Study in Embedded Systems Design: An Engine Control Unit: Design Automation for Embedded Systems, Volume 6, Number 1.
Kassakian, J.G; Wolf, H.-C.; Miller, J. M.; Hurton, C. J. (1996): Automotive electrical systems circa 2005: IEEE Spectrum, IEEE. Web.
Zafonte Matthew, Paul Sabatier (2004); Short-Term versus Long-Term Coalitions in the Policy Process: Automotive Pollution Control, 1963-1989: Policy Studies Journal, Vol. 32.