a single-point system is not as high as for multi-point systems, the pressure is higher than that in the intake system. Typical injection pressures for single-point systems are around 1 bar or slightly less.
With single-point injection, all cylinders receive fuel from the single injector. However, because the injector is controlled by an ECU in the same way as on a multi- point injection system, it is possible to use sensors to provide information to the ECU; this therefore provides better control of fuel quantity than a carburettor, but with reduced cost compared with a multi-point injection system.
Disadvantages
The disadvantages of a single-point system are in fact not dissimilar to those of a carburettor; for example, fuel/air separation when the air and fuel mixture flows around corners in the intake system. Additionally, fuel can still condense against the cold manifold walls during cold running.
Single-point injection was quite widely used on four-cylinder engines but these systems were not In the example shown in Figure 3.42, the 5 volt supply
is provided by the ECU to terminal 1 of the sensor.
Terminal 3 is the earth connection (via the ECU) and
suitable on longer engines, such as straight six-cylinder engines, because the different intake manifold lengths result in uneven distribution of fuel. This is the same problem that affected many carburettor engines where the length of the intake pipe from the carburettor or single injector to outer cylinders was much greater than to the central cylinders; this resulted in the outer cylinders running more weakly than the inner cylinders.
A rich mixture was therefore provided to ensure that all cylinders developed reasonable power and could run reasonably efficiently. However, the central cylinders then operated with a slightly rich mixture, which causes high emissions. Single-point injection is therefore suitable for vehicles with smaller engines, although some V8 engines were fitted with a single-point system;
this was possible because the location of the injector within the centre of the V resulted in similar intake pipe lengths to all cylinders.
One other major disadvantage relates to emissions control and emissions control regulations. It is now necessary on modern systems to stop delivery of fuel to a cylinder if that cylinder is operating very inefficiently. If the spark at the plug were very inefficient or failed completely, unburned fuel would flow through the cylinder and into the atmosphere as pollution. Modern multi-point injection systems, can
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Figure 3.43 Single-point injection system
detect which cylinder is operating inefficiently and switch off the injector to that cylinder. This is not possible on single-point systems where the injector supplies fuel to all cylinders.
3.3.2 Operation of a single-point injection system
Injector (actuator)
The injector (Figure 3.44) operates in much the same way as an injector for a multi-point system. The injector is a solenoid that, when energised, causes the needle to lift off the seat (the typical needle lift is approximately 0.06 mm). A control signal from the ECU opens and closes the injector for a calculated period of time (typically 1.25 ms to 8 ms, depending on operating conditions). It is usual for the injector to be opened at every ignition spark; i.e. on a four- cylinder engine, the injector would be opened each time a spark occurred, which equates to four times for every engine cycle.
Idle speed control (actuator)
As with multi-point injection systems, some form of automated idle speed control is provided. A common
method is to use a stepper motor (see section 3.1.6), which acts on the throttle butterfly via some form of linkage. The ECU controls the stepper motor to either maintain or increase the idle speed for cold running or when load is applied to the engine at idle.
Alternatively, the stepper motor can control a valve, which alters the aperture in a bypass port. The port allows air to bypass the throttle butterfly; therefore, when the valve allows more air to flow through the port, the idle speed increases. Bypass port systems are covered in section 3.1.6.
Sensors
The main information required for a single-point injection system to calculate the required fuel quantity is engine speed and throttle position (throttle opening angle). These two signals provide sufficient information for the ECU to calculate the required quantity of fuel to suit the engine load. In effect, the ECU has an indication of ‘air charge’ per cylinder from the engine speed and throttle opening signals. Some systems have a MAP sensor to provide additional information relating to engine load.
Ignition trigger or speed signal
On earlier systems, a speed signal was received direct from the ignition system (ignition coil or ignition module). Later when injection and ignition were combined, a signal was provided by a crankshaft speed/position sensor.
Throttle position
A throttle position sensor (usually a potentiometer) provides information relating to throttle angle opening and the rate at which the throttle is being opened or
closed. The throttle position sensor operates in the same way as those previously described for multi-point injection systems (sections 3.1 and 3.2). It was, however, common practice to use a set of contacts in the throttle sensor to indicate the closed or idle throttle position.
Air temperature
An air temperature sensor is located in the throttle body (Figure 3.43). Because air density changes with temperature, the information from the sensor assists in calculating the required fuel quantity to match the air density. An air temperature sensor operates in an identical way to air temperature sensors previously covered under multi-point systems (sections 3.1 and 3.2).
Coolant temperature
The operation and function of coolant temperature sensors is the same as for multi-point injection systems (sections 3.1 and 3.2). As with all fuelling systems, enrichment (excess fuel) is needed during cold running, and minor fuelling adjustments can be made for minor changes in engine temperature: the coolant temperature sensor provides the relevant information.
Other sensors
Figure 3.43 shows a lambda (oxygen) sensor and other components that are applicable to emissions control.
These components are covered in section 3.5.
Fuel system
The fuel system of a single-point injection system is similar to that of a multi-point system (Figure 3.43). A fuel pump filter and regulator assembly are used, which operate in much the same way as on a multi-point system (see section 3.1.5). However there are two major differences between single-point and multi-point fuel systems. First, single-point systems operate at lower fuel pressures, typically 1 bar.
The second difference is that, because the fuel is injected ahead of (or upstream) of the throttle butterfly, the fuel is injected into a pressure zone that does not change significantly with throttle opening. In section 3.1.5 it was explained that, because a multi-point injector injects fuel into the intake port, the injection pressure is regulated so that it is always at a constant pressure ‘above the pressure in the intake port’. The pressure regulator is therefore connected to the intake system pressure so that the regulator can ‘sense’ intake system pressure.
On a single-point injector, the fuel is injected into a relatively constant pressure zone above the throttle butterfly (which is at atmospheric pressure) and therefore the injection pressure does not need to be altered when the intake pressure changes. The pressure regulator therefore has no connection to the intake pressure.
The fuel supply system and pressure regulator are shown in Figure 3.45.
Figure 3.44 Injector for a single-point system
Manifold heating
If atomised petrol condenses on cold surfaces, problems can occur when the air:fuel mixture flows through the intake manifold when the engine is cold. On some applications, therefore, an electric heater is located at the base of the intake manifold (Figure 3.46) to help prevent the petrol from condensing.
The heater is switched on when the ignition is initially switched on and during starting; the heater can remain switched on for a number of minutes after starting. During cold running, therefore, the air:fuel mixture flowing from the throttle body is heated, which helps to ensure that the fuel remains atomised.
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Figure 3.45 Fuel system for a single-point system
Figure 3.46 Manifold heater on a single-point injection system