The mass type airflow sensor detects the mass of air flowing into the intake manifold. By measuring the mass of the air, it prevents changes in air density affecting the air-fuel mixture.
The airflow meter has an electrically-heated wire, mounted in the air stream.
A control circuit is linked to the wire, and current is supplied to the wire to keep its temperature constant. The higher the airflow, the more the temperature of the wire falls - and the higher the current needed in the wire to keep its temperature constant. So how this current varies is a measure of what is happening to the air flow.
Current flow variation is then read as an output voltage, and converted by the ECU to an intake air signal. This determines the basic fuel quantity needed for injector pulse duration. The airflow meter can have a self-cleaning function that burns dust and other contaminants from the hot wire. This is done by the control unit heating the wire to 1000°Celsius or 1,832°Fahrenheit for approximately 1 second. This happens 5 to 10 seconds after the ignition is switched off. This function operates only when certain conditions have been met. For this vehicle, the engine must have reached operating temperature, and the vehicle must have been travelling above 10 kilometres an hour or above 6.2 miles per hour.
In some types of sensors, the hot wire is mounted in a sub passage connected to the main passage. This allows maximum airflow through the main passage. The hot wire may be sealed in a glass envelope. This protects the wire, and eliminates the need for burn-off.
In others, the heating element is a ceramic plate.
Changes in engine speed, and load, cause changes in intake manifold pressure. This sensor measures these pressure changes and converts them into an electrical signal. It’s called a manifold absolute pressure, or MAP, sensor.
The signal may be an output voltage, or a frequency.
By monitoring output voltages, the control unit senses manifold pressure and uses this information to provide the basic fuel requirement.
It can use a piezoelectric crystal. If there is a change in the pressure exerted on this crystal, it changes its resistance. This alters its output signal.
The sensor is connected to the intake manifold by a small-diameter, flexible tube. The control unit sends a 5 volt reference signal to the sensor. As manifold pressure changes, so does the electrical resistance of the sensor, and this in turn produces change in the output voltage.
During idling, manifold pressure is low, which produces a comparatively low MAP output.
With wide open throttle, manifold pressure is closer to atmospheric, so output is higher.
The air vortex sensor uses whirlpools of air, or vortices, to measure the volume of air entering the engine. A triangular, vortex-generating rod disturbs the airflow and causes whirlpools of air to form in the body of the sensor.
The vortices alternate from clockwise to anticlockwise rotation, and they are staggered as they pass through a transmitter-receiver area. When there is no airflow, this transmitter emits a constant signal of ultrasonic waves to the receiver.
When the engine is running, the vortices distort the ultrasonic waves. This modulates the ultrasonic waves reaching the receiver. This modulated wave is converted to an electrical pulse signal which is directly proportional to the airflow. This is then used by the ECU to determine the basic fuel requirement.
An air regulator or rectifier is located at the entry to the sensor. It reduces turbulence in the airflow passing the vortex generator.
An air flow meter varies its signal by the deflection of a vane as air enters the engine. Deflecting the vane moves contacts across a potentiometer, to signal its position, and thus, the amount of air entering the system.
The temperature sensor uses material with a negative temperature coefficient of resistance. Its resistance is high when it’s cold, but it falls as its temperature rises. It is called a thermistor. This is the opposite of a normal resistor, which increases its resistance as temperature rises.
The coolant temperature sensor is immersed in coolant , in the cylinder head.
The air temperature sensor is in the air intake - at the airflow meter, or in the manifold.
Throttle position can be signalled by a potentiometer attached to the throttle shaft. It provides a continuous, varying signal, through the entire range of throttle position. Throttle position can also be signalled by a contact-type switch, but it signals the idle and fully open positions only.
Engine speed can be detected by a connection from the ignition system primary circuit, or by a pulse generator-type sensor, on the crankshaft. The pulses are computed by the control unit , into an engine RPM figure. They can also be used to trigger the injectors.
To maintain the air-fuel ratio within an optimum range, the control unit must take account of coolant temperature and air temperature. Extra fuel is needed when the engine is cold, and when the air is colder, and therefore denser.
The coolant temperature sensor is immersed in coolant in the cylinder head. It consists of a hollow threaded pin which has a resistor sealed inside it. This resistor is made of a semiconductor material whose electrical resistance falls as temperature rises.
The signal from the coolant temperature sensor is used to control the mixture enrichment when the engine is cold and is processed in the control unit.
Enrichment occurs during engine cranking, then slowly reduces as the engine warms up. This ensures a steady engine response immediately after releasing the starter. The control unit continually monitors coolant temperature during engine operation.
If the air temperature sensor is installed in the airflow sensor, it’s positioned in the airstream, and it’s called an intake air temperature sensor, or IAT. When it’s installed in the intake manifold it’s located in one of the intake runners, and it’s called a manifold air temperature sensor, or MAT.
In both cases it relays information on temperature, and, therefore, the density of the air. The control unit can then vary the setting accordingly.
This sensor gathers information on throttle positions, to allow the control unit to make adjustments according to operating conditions. It is located on the throttle body, and operated by rotation of the throttle spindle or shaft. Throttle position is sensed by a contact type switch, or a potentiometer. The switch type has 2 contacts - idle and full load. It can be supplied with a voltage to the centre terminal of its connector.
At idling speeds, the full load contacts are open. The idle contacts are closed and the signal to the computer provides for a mixture to maintain idle quality, and stability. Slightly rotating the throttle shaft opens the idle contacts. As the throttle approaches the wide-open position, the full load contacts close. The ECU reads the full-load signal, and enriches the mixture to suit.
The “idle contacts closed” signal can also be used, with the engine RPM signal to the control unit, to stop fuel delivery on engine over-run. This can be done, for example, if engine speed is above 2500 RPM on deceleration. If the ECU reads a figure above this, and also receives a throttle closed signal, it opens the injector electrical circuit, and stops fuel delivery. As RPM falls below, say, 2100 RPM, the circuit is restored. Injection then recommences, to maintain drive ability.
A potentiometer-type sensor monitors throttle position over its full range. One end has a 5 volt reference voltage from the control unit. The other is connected to control unit earth. A third wire runs from a sliding contact in the throttle position sensor, to the input circuits of the control unit. The sensor then works like a variable resistor. As the angle of the throttle valve changes, so does the voltage signal along the third wire.
At closed throttle, the reading is usually below 1.25 volts. As the throttle valve opens, the voltage signal rises. At wide-open throttle, it’s about 4.5 volts. This ongoing monitoring of throttle position provides more data for the control unit. This allows control over a wider range of operating conditions.
Exhaust gas oxygen sensor
The oxygen sensor, also called a lambda sensor, is mounted in the exhaust manifold, or the engine pipe. Its sensing portion is exposed to the stream of exhaust gas. It detects left-over oxygen in the exhaust gas, and sends the data to the control unit.
The control unit uses it to fine-tune the pulse it sends to the fuel injectors.
The sensor consists of a tube, closed at one end, and made of Zirconia ceramic, or Titanium ceramic. Its inner and outer surfaces are coated with platinum. The outer closed end is covered by a louvred metal shroud that protects it from breakage but still lets the exhaust gas contact the tube. Its inner surface is in contact with the air. A wire contacts the inner surface of the tube through a spring and an electrode bush. This provides the electrical link to the control unit.
The inner and outer surfaces of the ceramic tube are coated with porous platinum. The side facing the exhaust gas has a highly porous ceramic layer on top of the platinum, which lets oxygen through.
The ceramic tube with its platinum electrodes is now a porous, solid electrolyte. At temperatures around 350°c or 662°F, it becomes a conductor. One side detects the level of oxygen in the exhaust gas. The other detects its level in ordinary air. If the levels are different, a voltage is generated between the 2 sides.
The control unit compares this voltage to a pre-set level. Below the level indicates a lean mixture, above it means a rich mixture. The control unit may then adjust the pulse to the injectors, to maintain correct mixture. This fine tuning is needed for the catalytic converter to function properly.
Some sensors have a built-in heating element powered by the vehicle’s electrical system. It helps them reach operating temperatures quickly.
Crank angle sensor
Crank Angle Sensing uses information on the speed and position of the crankshaft to control ignition timing, and injection sequencing. The control unit can then trigger the ignition, and injection, to suit operating conditions.
The position sensor may be mounted externally on the crankcase wall, or it may be inside the housing of the ignition distributor.
There are different kinds of crankshaft position sensors.
Inductive-type sensors sense the movement of the ring gear teeth on the flywheel, or a toothed disc on the crank pulley. These sensors do not make physical contact. The sensor is mounted on the crankcase wall. It consists of a stator, with a central permanent magnet, and a soft iron core surrounded by an induction winding. The housing around all of these components is insulated from them.
The stator is positioned so that it has a very small clearance, or air gap, between the end of the soft iron core, and the flywheel teeth.
As the flywheel rotates, the teeth approach, and leave, the stator, and the air gap changes.
As this occurs, the strength of the magnetic field changes. The winding is part of a complete circuit, so changing the magnetic field produces an alternating voltage and current.
As the tooth approaches the stator, the strength of the magnetic field is increasing. This induces a voltage, and current flow, in the winding. The polarity of the voltage is said to be positive, as it produces a current flow in the winding in a certain direction.
When the tooth aligns with the stator, the magnetic field is at its strongest, but at that point, it is not changing. Voltage and current flow fall to zero.
As the tooth moves away from the stator, the strength of the magnetic field changes again, and once again voltage and current flow is induced in the winding. This time, current flow is in the opposite direction, and the polarity of the voltage is now said to be negative.
Since polarity changes every time a tooth approaches and leaves the stator, the voltage produced is an AC voltage, and the current flow in the winding is an alternating current.
Similarly with the toothed disc. As the tooth approaches the stator, the magnetic field is changing. It reaches a maximum when the tooth aligns with the stator, then changes again, decreasing as the tooth moves away. An alternating voltage is produced.
It is the frequency of this alternating voltage that is used by the control unit to calculate engine RPM.
Crankshaft position is detected by a separate sensor, also an inductive-type. It sends a signal to the control unit when a pin or bore passes, and generates one pulse per revolution. It signals the control unit that the number 1 piston is, for example, 80° before top dead centre.
Ignition timing is then decided according to the operating conditions, and triggered to occur a certain number of degrees from that point.
When only 1 sensor is used, the pulse inductor is shaped to provide information on both crankshaft speed and position. A disc attached to the crankshaft pulley has a number of equally spaced ribs around its circumference, but 2 ribs are omitted. The frequency of the pulse from each rib gives engine speed in RPM, but on each revolution, the pulse alters as the gap from the 2 missing ribs passes the sensor. This again gives the position of the number 1 piston.
Hall effect voltage sensor
Hall-effect sensors can also provide a voltage signal, and like the inductive-type, can be mounted on the crankcase wall, or inside the housing of the distributor.
The sensor has a permanent magnet, and a Hall switch, as part of its assembly, and an air-gap between the magnet’s North and South poles. The switch is on 1 pole of the magnet, and an interrupter ring, with a number of square-shaped blades or segments, rotates through the gap formed by the poles.
When it’s used in a distributor, this interrupter ring has the same number of blades as engine cylinders, and a corresponding number of windows, or gaps, between the blades. The magnetic field is strongest when the gap is aligned with the poles. This allows the switch to earth a low-current signal voltage that is applied to it.
When the interrupter ring rotates so that a blade is in line with the poles, the magnetic field is shielded, and the signal voltage is not earthed.
With continuous rotation, the blades repeatedly move in and out of the air gap, and the signal voltage will appear to turn on and off repeatedly. The control unit uses this on-off signal to detect engine RPM, and to control ignition timing.
If a sequential injection mode is used, the position of the camshaft also must be signalled to the control unit. This is done by making 1 blade of the interrupter ring shorter than the others. It is called a signature blade. It passes through the sensor, and alters the signal, so that injection commences at the correct time in the cycle. Since the distributor rotates at camshaft speed, the sensor in the distributor provides camshaft position readily.
When the sensors are on the crankshaft, a separate sensor is needed for camshaft position. It identifies when to commence injection for the number 1 cylinder. Injection for the other cylinders then occurs in the same sequence as the firing order.
