Programmed Fuel Injection System Introduction
Originating from the abbreviation of “Programmed Fuel Injection”, PGM-FI is the name given to electronic-controlled fuel injection system. This system supplies fuel to the engine via a set of fuel injectors which is controlled by the engine control module (or ECM). This type of system allows more precise fuel-supply control than is possible using a carburetor.
A fuel-injection system offers the following features over a carburetor system.
• Improved fuel economy
• Improved emission
• Improved response
• Higher power output
2. Carburetor System and Fuel Injection System
In a carbureted engine, the fuel is drawn into the carburetor’s air passage by a pressure difference caused by the flow of air. In other words, the volume of fuel supplied is controlled by the amount of air passing through the carburetor; furthermore, this air volume is regulated by the throttle valve, which is operated by the accelerator pedal.
If rapid acceleration were required, for example, the accelerator pedal would be depressed suddenly and the following steps would then be implemented to increase the volume of fuel supplied:
• The throttle valve will be opened by the action of the accelerator pedal,
• The amount of air flowing through the carburetor will increase, and
• The amount of fuel drawn into the air flow will increase accordingly.
Conditions wherein an appropriate amount of fuel is not supplied to the engine may, however, occur in a transient fashion over the period required for the implementation of these steps. Such situations can lead to impaired engine response and an increase in emission levels.
Although carburetors are provided with a wide range of different functions and capabilities in an effort both to alleviate these conditions and to improve response characteristics, there is no one type of carburetor which can react appropriately to all conceivable situations. Furthermore, carburetors also suffer the fate of being easily influenced by changes in air temperature and various other environmental factors.
In response to the above-mentioned factors, the fuel injection system was developed with the aim of allowing suitable control – under all engine conditions – of more accurate fuelsupply volumes.
Specifically, the PGM-FI system’s ECM determines the current engine condition based on data provided from a range of sensors, and it calculates the required fuel-supply volume accordingly. Then, by injecting this volume of fuel at the most appropriate timing into the intake manifold, the system ensures that the engine is provided with the most suitable airfuel mixture for the corresponding conditions.
3. Basic Construction of PGM-FI System
The PGM-FI system can be divided into the following three sub-systems.
• The air-intake system
This system controls the volume of air which is supplied to the engine.
• The fuel-supply system
This system delivers pressurized and regulated fuel to the injectors.
• The control system
This system uses the ECM to control the fuel volume and injection timing based on signals from various sensors.
4. Types of PGM-FI System
PGM-FI systems may also be classified into three different types of system as follows:
• Multi-point injection (MPI) system
• Groupwise fuel injection (GFI) system
• Dual-point injection (DPI) system
This system is the most widely used of the three. One injector is provided for each of the engine’s cylinders — these being mounted on each of the intake manifold branches. The injectors are controlled independently by the ECM in such a way that fuel is supplied during the intake stroke of the corresponding cylinder.
The basic design of the GFI system is the same as that of the MPI system. However, the GFI system features more simplified control.
This system is used in 4-cylinder engines of vehicles in the lower price range. The four individual injectors are split into two groups, and injection control is performed for each separate group. Injection takes place once during combustion cycle of each of the cylinders, but in the case of three of the four cylinders, this occurs not during the intake stroke. The fuel injected for these cylinders will be drawn into the cylinder during the subsequent intake stroke.
The DPI system is a simplified type of fuel injection system and is similar in nature to a carburetor system. Two injectors (i.e., primary and secondary) are fitted into the throttle body and fuel is injected at the same location as in a carbureted engine.
During normal operation, the primary injector performs the injection of fuel; however, in highspeed or high-load situations where more fuel is required, the secondary injector also operates to add fuel.
5. MPI System
The following system will provide a detailed description of the controls performed by a PGMFI system using the MPI system — the most typical of PGM-FI systems — as an example.
Air Intake System
The main air passage of the air intake system consists of the air intake, the throttle body, and the intake manifold. A throttle valve is provided within the throttle body to allow the intake-air volume to be controlled. Furthermore, the area in direct proximity to the throttle body is also fitted with a number of other necessary devices, such as an idle air control (IAC) valve which performs control of air-intake volumes principally in idling situations, bypass channeling which provides the air required for idling situations, an idle-adjust screw which regulates the volume of bypass air, and a fast-idle valve which increases the volume of air in low-temperature situations so that fast idling may be achieved.
The above-mentioned component parts enable the system to provide a well-suited volume of air in any possible engine-operation condition.
Fuel Supply System
The fuel supply system comprises the fuel tank, the fuel pump, the fuel filter, the pressure regulator, and the fuel injector, in addition to the fuel hoses and pipes which connect these components.
Within this system, the fuel is pressurized to a high degree by the fuel pump and is sent to the injector. Furthermore, the fuel-pressure regulator operates to keep the pressure difference between the intake manifold vacuum and the fuel pressure at a constant level. Accordingly, injection volumes over time are also maintained at a constant level.
Upon the receipt of a signal from the ECM, the injector performs injection of fuel into the intake manifold, the corresponding injection volume being controlled by the injector-valve opening duration.
1 Fuel pump
2 Fuel tank
3 Fuel feed pipe
4 Fuel return pipe
5 Fuel injector
6 Pressure regulator
7 Fuel filter
The control system comprises a number of different sensors, and ECM which generates a range of control signals based on the sensor data and on stored data, and a set of actuators which operate in response to these control signals.
Specfically, the ECM determines the current condition of each of the cylinders (i.e., stroke and piston position) using signals from, among others, the crankshaft position (CKP) sensor, the cylinder position (CYP) sensor, and the top-dead-center (TDC) sensor. Then, as a piston approaches top-dead-center of the exhaust stroke, the corresponding cylinder’s injector is opened for the calculated period of time at the most appropriate timing, thus allowing injection of the optimum fuel volume at the optimum time.
In addition, the ECM also monitors the current conditions of the engine, the transmission, and the electric load among others so that it can perform a range of control tasks such as adjusting the idle speed, managing the operation of the engine auxiliaries, and implementing lock-up control of the automatic transmission.
The actual number of sensors and actuators contained within the system and the control method used will vary widely with respect to vehicle type and other similar factors.
1 TDC/CKP/CYP sensor
2 EGR valve
3 Control box
4 Throttle angle sensor
5 Fast idle valve
6 Air boost valve
7 Injector resistor
8 Intake air temperature (IAT) sensor
9 IAC valve
10 Oxygen sensor
11 Coolant temperature (ECT) sensor
12 Idle mixture adjuster (IMA)
13 Engine control module (ECM)
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