An EMS controls the vehicle engine in response to both vehicle and engine inputs. It consists of an electronic control unit composed of a micro-processor with several electromechanical and electronic actuators and sensors. Sensor devices are used to monitor the vehicle engine and detect engine speed, lambda values, crank angle and other engine parameters. The electronic control unit determines the engine ignition time, the quantity of fuel and other engine parameters that keep the vehicle engine running. The ECU does this by use of input values that are calculated based on the inputs that originate from the sensors. Under this condition, the engine obtains the best operation in terms of performance, fuel consumption, driving efficiency and exhaust gas emission. Below are the sensors that are commonly installed in a vehicle engine:
- Crankshaft position sensor
This sensor uses the technology of inductive pickup. It is used to determine the crankshaft position and produces an analogue signal that is conditioned by use of a high gain amplifier to generate constant voltage pulses. These pulses are produced on all the degrees of crank angle.
- Camshaft position sensor
This sensor works with the crankshaft position sensor discussed above. It informs the electronic control unit which stroke of piston number one is active so that it knows the cylinder in which to inject fuel. It only sends a signal to inform the ECU that piston number one is at TDC. However, it doesn’t inform which stroke, exhaust or compression. The information obtained from both a crankshaft position sensor and a camshaft position sensor is used in control of start of fuel injection and ignition and ignition coil timing.
- Throttle position sensor
This sensor is comprised of a potentiometer that informs the engine management system about the load demand by the driver and it becomes easier to implement strategies to control changes in fuel during transients. The change in throttle position affects air flow. When the throttle opens, an inactive speed control valve enables flow of air around the plate of the throttle to obtain idle speed control.
- Air flow rate sensor
This parameter is measured by either a hot wire probe or pivoting of a van with potentiometer. The hot wire probe is commonly used because it responds quicker compared to the van. Air flow is measured by typically keeping a constant temperature that is above the surrounding wire temperature and the power dissipated by the wire measured. The air flow rate is used to determine the quantity of fuel to fire to the engine and also to define its operating point.
- Inlet manifold sensor for absolute pressure
The measurement of the inlet manifold absolute pressure is aided by piezo-resistor transducer consisting of a silicon diaphragm etched with strain gauges on its surfaces. The temperature, engine speed and manifold pressure enable the approximation of engine mass flow rate that in turn enable determine the circulation level of exhaust gas.
- Air and coolant temperature sensors
Thermistors are the devices used in the measurement of temperature. These devices combine measurements taken by other sensors to help in approximation of engine mass flow rate.
- Lambda sensor
The ECU maintains a specific air-fuel ratio by interpretation of the information gained from the lambda (oxygen) sensor.
- Knock detector sensor
The knock detector is an accelerometer sensor that detects engine structural vibrations. When knocking commences, this sensor detects the vibration signals and ignition is slowed down in an attempt to avert damage to the engine.
- Waste-gate control sensor
This sensor is used in the reduction of turbo lag. It helps in limitation of maximum cylinder pressure. The waste-gate is a flap valve built in the casing of the engine turbine. With the help of an inlet manifold absolute pressure sensor, boost pressure can be sensed and it becomes possible to control turbine speed and boost pressure coming from the compressor.
PART [B] Methods that are used by an EMS in calculation of spark advance and fuel injection amount for a given engine conditions by use of OPEN LOOP Control
Open loop systems employ use of parameters(such as ignition timing) that are set depending on the information stored in the Read Only Memory of electronic control unit, with a selective ignition timing based on the engine speed, manifold pressure and the coolant temperature. The Lambda sensor is mostly ignored at full throttle, implying that “self-learning” cannot be relied upon to take care of full fuel supply that is required by engine modes that need an increment of power and consequently, airflow (Seyfert, 2004).
The ECU controls the ratio of air-fuel using an EFI (electronic fuel injection system). The EFI controls the quantity of fuel to be injected in every cylinder. This task is accomplished by controlling the “on time” period of fuel injectors, which are solenoid and consist of spray nozzles with plungers that are solenoid operated and connected on every cylinder.
The fuel pressure regulator is used to keep a constant fuel pressure from the delivery pipe. The solenoid-operated fuel injectors open and close at intervals between 0.5 and 1 minute. There are two main types of EFI control systems that are commonly used; the mass air-flow EFI and the speed density EFI systems. The difference between these two types is the manner in which intake of air mass is obtained. The main input signals to the systems are intake air mass and engine speed.
Figure 1: Electronic fuel injector system
- Mass air-flow EFI
In mass air flow EFI, an air flow sensor measures the amount of air that is drawn inside the engine. The common types of air flow sensors used include hot wire, Karman and flap type sensors. Air-flow measured in this way automatically compensates for factors such as engine displacement caused by internal deposits and speed, and varying volumetric efficiency.
- Ignition timing
The primary sensors that control the ignition timing or spark advance are throttle demand, engine air-flow and Crank angle (or TDC position). The data for mapped ignition timing is mapped on the ROM of the ECU. The ECU contains all the values for the engine speed and injection pulse period, and it determines the cylinder that needs fuel and what amount. The requisite injector opens and delivers the fuel and timely causes a spark to burn the fuel (V.A.W., H., 1996).
- Speed-density EFI
In a steady state operation, the ratio of air-fuel must be maintained at a constant since the opening period for fuel injection is in direct relation with the mass of air flow into the engine. The equation below relates the mass of air flow and the manifold pressure.
Ma = (VdnvPi /RTi) Where;
Ma – mass of air flow
Vd – cylinder displacement
nv - volumetric efficiency
Pi _ manifold absolute pressure
R- Constant
Ti – intake air temperature
Volumetric efficiency, nv, is a non-linear function of engine speed and is unique for every design of an automobile engine. The crank triggered ignition systems only recognize the position of the engine and not the position of the cycle and there is a need to ensure that the right cylinder gets the spark. It is normally achieved through three commonly used methods employed by EMS to determine the spark advance in open loop control.
The first way employs use of a distributor cap with a rotor arm attached on the end a camshaft. The rotor arm routes the ignited spark to the right cylinder. This arrangement is popular with earlier EMS systems like the Ford EMS.
The distributor triggers at between 65-70 degrees just before TDC. The EMS then looks the ignition map and calculates the right timing value for engine load and speed, and using the speed as a factor, calculate the time before firing.
The second method uses of two coils paired on fire cylinders. When one coil is fired, a spark is send to both cylinders.
The third method involves the use of an additional sensor attached to one of the camshafts. The EMS knows the position of the engine cycle and therefore, in a position to fire the right cylinder at the appropriate time by use of individual coil for every cylinder.
The EMS knows the position of the TDC from the information sent by the crank sensor and with the information sent from the MAP sensor, it calculates the appropriate ignition time. Each coil is then fired at every engine revolution (Seyfert, 2004).
PART [C] How to modify the OPEN LOOP to a CLOSED LOOP control system using sensor information from the lambda sensor.
Closed loop systems rely on measurement of a varying parameter that is controlled until the target value is obtained. Both EFI types discussed above can be improved by addition of an extra oxygen sensor. The extra lambda sensor is to be used in the establishment of a stoichiometric operation. However, this will require a more complicated exhaust gas sensor for measurement of air-fuel ratio (Seyfert, 2004).
All the EFI control systems can be conveniently used, but if precise engine control is prefered when a catalyst is used, an integrated feedback system has to be used to maintain an air-fuel ratio of approximately 1%. This condition is only possible in a closed-loop control system in combination with mass air-flow or speed-density EFI.
The main factors controlling the combustion process are the ignition timing and the composition of the induced mixture of air and fuel. In extension, this will affect the performance efficiency, economy and amount of exhaust gas produced by the engine. The lambda sensors are used in the determination of whether the air-fuel mixture is weak or rich and using a control system, the air-fuel mixture is closed stoichiometric (chemically perfect). The feedback system only works when the engine warms up since the oxygen sensors work at temperature estimate of 300oC. However, an electric heater installed at the centre of the lambda sensor can reduce this temperature to about 20-30 oC. The sensors are made of three zirconia layers. All the layers are heated; two top layers have electrical connections and platinum electrodes.
Figure 4: Universal exhaust gas oxygen sensor
This sensor provides feedback that indicates the nature of the mixture; whether it is below or above the stoichiometric level.
For a weak mixture, the concentration gradient between measurement cavity and exhaust gas will cause oxygen diffusion in the gas intake that triggers electrical current that is proportional to the amount of concentrated oxygen present in the exhaust gas. For a rich mixture, partial combustion products, carbon (ii) oxide and hydrocarbons, are oxidized leading to increased diffusion across the porous plate.
PART [D] How control can be extended using sensor information from a knock sensor
A knock sensor is an accelerometer sensor that detects engine structural vibrations. When knocking commences, this sensor detects the vibration and ignition is slowed down in an attempt to avert damage to the engine. Pressure oscillation on cylinders as a result of combustion causes engine vibrations. The detector consists of a mass mounted spring, with a means of detecting any slight motion of the mass on the spring in case of vibrations. The figure below shows a basic piezo-electric knock detector.
Figure 5: A basic piezo-electric knock detector
It is normally convenient to combine both spring and sensing element by mounting of the mass on the piezo-electric crystal. A good spot is identified by carrying out tests on the engine body where a good vibration signal can be tapped. The vibration signal is observed to identify the knocking cylinder in order to obtain the ignition timing selectively.
The advantage of the piezo-electric crystal is its high stiffness so that it becomes easier to design and use a high natural frequency transducer. Its main disadvantage is that it generates an electrical charge that is proportional to the engine acceleration, and it has to pass through an amplifier to obtain a voltage signal. The knock detector senses structural vibrations at a specific frequency and if correspondence with the transducer frequency is noted, the resonance gives a dynamic amplification which now enables the transducer to give out an improved signal-noise ratio. One advantage of the knock detector is its ability to offer a safety edge that could be alternatively be found by having a reduced compression ratio or a retarded ignition.
PART [E] Discuss the benefits and issues of EMS control regarding wide-band lambda sensors
Most vehicles use normal lambda sensors that are stoichiometric and only indicate whether the air-fuel ratio is lean or rich, though there is no specification of how rich or how lean the air-fuel ratio should be. The output is a binary, vertically with either side of the stoichiometric ratio.
Emission standards for automobiles are being raised higher and higher with advancement in control in emission and signal measurement is needed for this reason. A wide-band lambda sensor technology is one of the advancements that have been made and used in the world of today. The sensor can measure the real lambda values and deliver the information to the ECU over a range of air-fuel ratios (JONSSON, 1999 p. 47)
These sensors are much more sophisticated than the predecessor sensors. The exhaust gas levels are in a sealed air chamber that is within the chambers and not outside as their predecessor sensors. They are also fitted with a heating element that heats up the sensor quickly when the temperatures are very low.
The ECU controls the current through the heater element to ensure maintenance of the right operating temperature. A small chamber inside the sensor is access enabled to the gas chamber. The sensor uses a solid state pump to increase or decrease oxygen in the exhausted chamber. The programmable logic controller on a computer monitors and controls the current through the pump. Current flow in one direction increases oxygen concentration while current flow in a reverse direction reduces the level of oxygen (Adamides 2004 p. 966) The current direction and value required for this process is a representation of the level of oxygen in the exhaust to enable the electronic control unit to monitor, deliver and maintain the amount of fuel required as well as control the emission level (V.A.W., H., 1996s).
Appendix:
- An example of an ignition map.
- Graph of engine air-fuel ratio against engine speed.
References
Adamides, E.D., Stamboulis, Y.A. & Varelis, A.G. 2004, "Model-based assessment of military aircraft engine maintenance systems", The Journal of the Operational Research Society, vol. 55, no. 9, pp. 957-967. Accessed on 19 Nov 2014 from http://search.proquest.com/docview/231332451/B93F504A7E354EC6PQ/19?accountid=1611
JONSSON, K., 1999. The ins & outs of European engine management systems. Motor, 192(6), pp. 42-47. Accessed on 19 Nov 2014 from http://search.proquest.com/docview/227814832?accountid=1611.
Knowles D, Erjavec J., 2005. TechOne: Basic Automotive Service and Maintenance.. New York: Thomas Delmar . Accesssed on 19 Nov 2014 from http://www.amazon.com/TechOne-Basic-Automotive-Service-Maintenance/dp/1401852084
Seyfert, K. 2004, "GETTING INSIDE EUROPEAN ENGINE MANAGEMENT SYSTEMS", Motor, vol. 202, no. 6, pp. 40-42,44-46.
V.A.W., H., 1996. Hillier’s Fundamentals of Automotive Electronics. UK. Cheltenham: Nelson Thornes Ltd.. Accessed on 19 Nov 2014 from http://books.google.co.ke/books/about/Hillier_s_Fundamentals_of_Automotive_Ele.html?id=ho9kKynnWHkC&redir_esc=y
