3.2 PETROL INJECTION SYSTEM EXAMPLES (MULTI-POINT INJECTION)
3.2.1 Example 1: Simple multi-point
System components and layout
Figure 3.24 shows the main components used in the Bosch LE system, which was widely used by many vehicle manufacturers and was a system from which many others evolved. In addition to the main injection system components, an auxiliary air valve is used to provide a fast idle speed when the engine is cold. The air valve is not controlled by the ECU and its operation is explained later in this section. The LE2 system therefore has several sensors but only one set of actuators: the injectors. The fuel delivery system on the Bosch LE is identical to the example illustrated in Figure 3.20, so it is not described again in this section.
Airflow sensor with combined air temperature sensor
The airflow sensor provides the ECU with an analogue signal that indicates the volume of air being drawn into the engine. Air density changes with temperature, so an air temperature sensor is built into the airflow sensor assembly. The ECU therefore receives airflow and air temperature information.
Airflow sensor operation
The airflow sensor has a flap or vane that is forced to move when the air flows through the sensor body (Figure 3.25). The flap is connected to a hinge or shaft, so the angle of the flap increases as the airflow increases.
A potentiometer (variable resistor) is fitted to the sensor assembly, and the potentiometer ‘wiper’ or
‘slider’ is connected to the shaft. Therefore, when the flap moves, the potentiometer wiper moves around the resistance track of the potentiometer. This changes the voltage at the wiper, with the magnitude of this change depending on the flap position. The voltage reading at the wiper is sent to the ECU, which provides the ECU with an indication of airflow.
Potentiometer
One of the disadvantages of the flap system is the change in angular movement that occurs with increased airflow. When the airflow is low, the flap is almost at right angles to the airflow and therefore the force acting on the flap is relatively high; any small change in airflow will cause a relatively large change in the flap angle. However, when the airflow is already high, the higher forces acting on the flap will have pushed it to a position where it is almost in line with the airflow (almost lying flat in the sensor body), so any further small increases in airflow will not greatly affect the angle of the flap: it will move only a little further.
When the flap is almost at right angles to the airflow, the voltage change at the potentiometer will be quite large for a small change in the airflow. When the flap is almost in line with the airflow, however, small changes in airflow will result only in very small changes
Figure 3.24 Bosch LE injection system
Figure 3.25 Cutaway views of vane type airflow sensor a Air side
b Electrical connection side
in voltage, which does not provide sufficient information to the ECU. The ECU ideally requires large changes in voltage to assess the airflow accurately.
The potentiometer used on the airflow meter is therefore more complex than conventional potentiometers. A thick film resistance track is used, made of several segments, each with a different resistance. The resistance of the segments is designed to compensate for the reducing angular movement of the flap as the airflow increases: as the wiper moves across the track, the output voltage is progressive and linear.
On this type of airflow sensor, 12 volts is applied to the potentiometer, and, as the wiper moves across the resistance track, the voltage at the wiper changes from typically around 5 volts at low air volumes to around 9 volts at high air volumes.
Damping chamber
A second flap (attached to the first flap) is positioned in a small chamber (referred to as a damping chamber).
Air is drawn or induced into the engine in pulses or waves (each cylinder creates a single strong pulse giving as many pulses in one engine cycle as there are cylinders), so the first flap will also tend to pulsate when the airflow passes through the sensor. The second flap is also exposed to the pulsing action of the airflow, but the airflow is directed against the second flap in such a way that the pulsing on the second flap cancels the pulsing of the first flap.
If the compensating flap and damping chamber were not used, the pulsations caused by the airflow would also cause the signal from the sensor to pulsate.
Air temperature (see section 1.5.1)
An air temperature sensor is incorporated within the airflow sensor. The temperature sensor is a thermistor, which is a resistor that changes in resistance with changes in temperature. Because the sensor forms part of a series resistance circuit that has a reference voltage applied to it, when the temperature changes, the resistance and voltage in the circuit also change. The change in voltage is used as a signal to the ECU.
The ECU uses the signal for the change in air temperature in conjunction with the airflow signal (because air mass for a given airflow changes with temperature).
Mixture adjustment
A bypass port is provided on older airflow sensors so that mixture adjustments can be carried out at idle speed. This facility is no longer required with modern engine management systems, but on older engines it was needed to ensure that the idle emissions were within specified limits and to enable the engine to idle smoothly.
Figure 3.25 shows the bypass port, which has an adjusting screw at one end. If the adjusting screw is screwed fully into the port it will block the bypass port, which will force all of the air being drawn into the
engine to flow through the main intake port and therefore move the sensing flap. When the adjusting screw is unscrewed, a small amount of air is able to flow through the bypass port, so the flap will not be affected by all of the airflow: the sensing flap will move back slightly, which will reduce the signal voltage transmitted to the ECU. Altering the adjusting screw will therefore affect the amount of air flowing through the bypass port and thus the amount of air affecting the sensing flap. The signal voltage will alter, which causes the ECU to adjust the quantity of fuel being injected.
Adjusting the bypass port screw will therefore affect the fuel quantity and the mixture at idle speed.
Coolant temperature sensor
(See section 1.5.1.) The coolant temperature sensor is positioned (usually in the cylinder head) so that it can measure the temperature of the engine coolant. A signal from the sensor is transmitted to the ECU so that the fuel quantity can be altered for cold running (by enriching the mixture) as well as for other minor variations in fuelling that are required at different coolant temperatures (to provide fine tuning of the mixture). Figure 3.26 shows a typical coolant temperature sensor.
The coolant temperature sensor operates in exactly the same way as the air temperature sensor described above. The sensor resistance changes with coolant temperature, resulting in a voltage change in the circuit, which the ECU can then use as an indication of coolant temperature.
The sensor has a negative temperature coefficient (NTC), so its resistance reduces as the temperature
increases. A typical resistance for the sensor is around 7000 ohms (7 kΩ) at 0°C, falling to around 250 ohms at 100°C.
The voltage in the sensor circuit also reduces as the temperature increases. A reference voltage is applied to the sensor circuit, which on the Bosch LE2 system is around 12 volts. However, when the sensor is connected to the circuit, the resistances in the circuit reduce the voltage to a value that depends on the resistance of the sensor, which changes with temperature. For normal operation, the voltage in the circuit is around 9 volts for a very cold engine and around 5 volts for a hot engine.
Throttle position switch
As previously described, the throttle switch consists of two sets of contacts. One set closes when the throttle is closed (the idle position). The second set of contacts closes when the throttle is approximately 60% open (this value will depend on the application).
With the Bosch LE2 system, 12 volts is applied to the centre terminal of the switch. When a set of contacts closes, the 12 volt signal is transmitted back to the ECU.
The ECU uses this signal as an indication of idle position or load position. The air:fuel ratio is usually enriched slightly to stabilise the idle speed and to enable the engine to produce full power. Figure 3.27 shows a throttle switch and its construction.
Timing/trigger reference
The Bosch LE2 system relies on a signal from the ignition system for information on engine speed (to assist in fuelling calculations) and as a reference for triggering the injectors. A signal is taken from the ignition coil or direct from the ignition module. In effect, every time the ignition module switches off the ignition coil (spark timing) a signal is transmitted to the LE2 ECU.
Petrol injection system examples (multi-point injection) 99
Figure 3.26 Coolant temperature sensor Figure 3.27 Throttle position switch
The injectors on an LE2 system operate simultaneously:
they all open and close together on a four-cylinder engine. The ECU uses every alternate ignition pulse (on a four-cylinder engine) as a reference to open the injectors, so the injectors open twice for every engine cycle.
Injectors (actuator) Simultaneous injection
The only true actuators on an LE2 system are the injectors, which are exactly as described in section 3.1.4. On a four-cylinder engine, all four injectors are opened and closed at the same time (simultaneous injection). In fact, all four injectors are connected back to the ECU at one terminal, so the power stage within the ECU switches all four injectors together (more than one power transistor might however be used). With engines of more than four cylinders, the injectors might be triggered and connected in groups of three or four.
LE2 injectors operate with a fuel pressure of around 2.5 bar above the intake manifold pressure (see section 3.1.5).
Figure 3.28 shows the control signal provided by the ECU. Note that the EMF created when the injectors are switched off causes a voltage spike at around 60 volts.
Idle speed control/adjustment
The standard LE2 system did not provide an automated idle speed control system. The idle speed was adjusted manually using a bypass port adjuster on the throttle body (Figure 3.29).
Manual idle speed adjustment
A bypass port was usually formed as part of the throttle body (Figure 3.29). The bypass port allows intake airflow to bypass the closed throttle butterfly (throttle valve). The bypass port has an adjusting screw that can be altered to allow more or less air to bypass the throttle butterfly.
Therefore, if the adjusting screw is unscrewed, more air is allowed to enter the engine, which will increase the idle speed. Screwing in the adjuster will restrict the air, thus reducing the idle speed. This enables the idle speed to be set to the manufacturer’s specifications.
On the LE2 system, when either more or less air is allowed to flow into the engine through the bypass port, the air will still have to flow through the airflow sensor.
An increase in airflow will therefore cause the airflow sensing flap to move, altering the sensor signal to the ECU; the ECU will therefore increase the fuel quantity to correspond with the increase in airflow, thus maintaining the air:fuel ratio.
Auxiliary air valve (cold running)
When an engine is cold, all moving components have higher levels of friction, and the cold oil can also cause additional drag or resistance in the engine. To prevent this additional friction and resistance from stalling the engine when it is cold, the LE system provides additional air to the engine, which results in a slight increase in engine speed at idle (with the throttle closed).
An additional or auxiliary air valve is used which is connected by air pipes to the throttle body. As with the manual idle speed adjuster, the air valve is effectively a bypass port, as shown in Figure 3.29. However, instead of a manual adjusting screw, the auxiliary air valve has a temperature sensitive plate valve which is open when cold and closes when hot (Figure 3.30).
The valve assembly is exposed to two heat sources.
The first heat source is an electrical heating element integrated into the air valve body. When the engine is cold the valve plate is in the open position, so when the engine is started, additional air flows through to the engine, thus providing a fast idle speed. However, when the engine is running, the full battery voltage is applied to the heating element, which heats up a bimetallic strip attached to the valve plate. As the bimetallic strip heats up, it bends, which causes the valve plate to progressively close the air bypass port.
Volts
Milliseconds 60
50 40 30 20 10 0 –10 –20
–15 –5
injector open
injector closed back ‘EMF’ spike
0 5 10 15
–10
Figure 3.28 Injector control signal
Manual idle air bypass adjustment
Auxiliary air valve
Figure 3.29 Manual and auxiliary air valve bypass ports to control idle speed
When the engine is switched off, in theory, the bimetallic strip will cool down, allowing the valve plate to reopen the port. However, the air valve is positioned so that it is exposed to engine heat, so the valve body stays hot until the engine cools down. So the port will not open again until the engine is quite cool.
It takes typically around 3 minutes (depending on the vehicle application) for the auxiliary air valve to move from the fully open to the fully closed position. In this time, the engine should have reached an operating temperature that allows it to idle at normal speed.
Electrical systems and wiring (LE2)
Although the Bosch LE system is now an old one, its basic elements are still relevant to today’s injection systems. Therefore, in addition to the wiring circuit shown in this section (Figure 3.31), the operation of the circuit and some of the functions are also explained.
Power supply
Early electronic injection systems generally used battery voltage for all aspects of system operation. On the
Bosch LE system, the injectors used the 12 volt supply and the sensors were also provided with a 12 volt reference voltage. Although all systems largely still use the full battery voltage for actuators such as the injectors, it is now normal practice to use a 5 volt reference voltage for sensors.
A system relay provides the system components with the 12 volt supply. The relay acts as a safety device and will switch off the power supplied to the components unless certain signals are received from the engine, etc.
The relay consists of contacts which, when closed, connect the battery voltage direct to the system components. Energising windings within the relay will cause the contacts to close when a voltage is applied to the windings.
Relay operation
The relay receives the full battery voltage supply direct from the battery (possibly fused on some applications) to terminal 30.
● When the ignition is initially switched on, the battery voltage will be applied to terminal 15 of the relay (the voltage will be applied to the energising winding). A timer circuit within the relay will cause the relay to apply the battery voltage from relay terminal 87b to the fuel pump for a few seconds (allowing the pump to operate, thus ensuring that the fuel system is under pressure). If the engine is not cranked or started, the relay will switch off the supply to the pump.
● When the ignition switch is then placed in the cranking position, the voltage will be applied from the starter circuit to terminal 50 of the relay; this will again cause the energising winding to close the contacts and the battery voltage will now be applied to all of the system components (the fuel pump, injectors and sensors). The engine should now start.
● When the engine starts, an ignition speed signal (from the ignition coil or module) is transmitted to the relay at terminal 1, which indicates that the engine is running. Because the start signal from the starter circuit will now switch off (the engine is no longer cranking), the speed signal acts as a replacement so that the relay will continue to provide battery voltage to the injection system.
● If for any reason the engine were to stop, the ignition signal would disappear and the relay would switch off the power supply to the injection system.
Injectors
All injectors will receive battery voltage from relay terminal 87 during starting and engine running. The second terminal at each injector is then connected to ECU terminal 12, which is the earth path for the injectors. The circuit passes from terminal 12 through the power stage of the ECU to earth. Therefore, when the ECU switches on the injectors, the power stage will complete the earth circuit for the injectors.
Petrol injection system examples (multi-point injection) 101
Figure 3.30 Auxiliary air valve located in the intake system
Fuel pump
The fuel pump receives a power supply from relay terminal 87b while the engine is starting and running.
Throttle switch
The throttle switch receives battery voltage at terminal 18 from relay terminal 87. When the idle contacts or the load contacts in the switch are closed, the battery voltage will then be applied from the switch terminals 2 or 3 to the ECU at terminals 2 or 3. The ECU will then have an indication of idle or engine load.
Coolant temperature sensor
The coolant temperature sensor also has a battery voltage supply from relay terminal 87. As previously noted, the sensor is part of a series resistance circuit, so, as the resistance of the sensor varies with temperature, the signal from the sensor to ECU terminal 10 will change, indicating the temperature to the ECU.
Airflow sensor
The airflow sensor is supplied with the battery voltage at terminal 9 through relay terminal 87. This voltage is applied across the air temperature sensor, which operates in the same way as the coolant sensor described above. The signal from the air temperature sensor passes from terminal 8 to the ECU terminal. The supply voltage is also applied across the potentiometer within the airflow sensor; when the wiper on the potentiometer moves (due to the airflow sensing flap moving), the voltage on the wiper contact will change, and this changing signal is transmitted from terminal 7 of the airflow sensor to terminal 7 of the ECU. Airflow sensor terminal 5 is the earth connection for the potentiometer.
Auxiliary air valve
The valve is supplied with the battery voltage from relay terminal 87 while the engine is starting and running;
this will cause the heating element in the valve Figure 3.31 Wiring diagram for Bosch LE2 injection system