For some auto manufacturers, TBI served as a stepping stone from carburetors to more advanced port fuel injection systems. TBI units were used on many engines during the 1980s and are still used on some engines. The throttle body unit is similar in size and shape to a carburetor and, like a carburetor, it is mounted on the intake manifold. The injector(s) spray fuel down into a throttle body chamber leading to the intake manifold. The intake manifold feeds the air/fuel mixture to all cylinders. The throttle body assembly is mounted on top of the intake manifold, where the carburetor was mounted on carbureted engines. Four-cylinder engines have a single throttle body assembly with one injector and throttle, whereas V6 and V8 engines are equipped with dual injectors and two throttles on a common throttle shaft. The throttle body assembly contains a pressure regulator, injector or injectors, TP sensor, idle speed control motor, and throttle shaft and linkage assembly. A fuel filter is located in the fuel line under the vehicle or in the engine compartment. When the engine is cranking or running, fuel is supplied from the fuel pump through the lines and filter to the throttle body assembly. A fuel return line connected from the throttle body to the fuel tank returns excess fuel to the fuel tank. The throttle body casting has ports that can be located above, below, or at the throttle valve depending on the manufacturer's design. These ports generate vacuum signals for the manifold absolute pressure sensor and for devices in the emission control system, such as the EGR valve, the canister purge system, and so on. The fuel pressure regulator used on the throttle body assembly is similar to a diaphragm-operated relief valve. Fuel pressure is on one side of the diaphragm and atmospheric pressure is on the other side. The regulator is designed to provide a constant pressure on the fuel injector throughout the range of engine loads and speeds. If regulator pressure is too high, a strong fuel odor is emitted and the engine runs too rich. On the other hand, regulator pressure that is too low results in poor engine performance or detonation can take place, due to the lean mixture.
TBI Advantages
Throttle body systems provide improved fuel metering when compared to that of carburetors. They are also less expensive and simpler to service. TBI units also have some advantages over port injection. They are less expensive to manufacture, simpler to diagnose and service, and do not have injector balance problems to the extent that port injection systems do when the injectors begin to clog. However throttle body units are not as efficient as port systems. The disadvantages are primarily manifold related. Like a carburetor system, fuel is still not distributed equally to all cylinders, and a cold manifold may cause fuel to condense and puddle in the manifold. Like a carburetor, throttle body injection systems must be mounted above the combustion chamber level, which eliminates the possibility of tuning the manifold design for more efficient operation.
lnjectors
The fuel injector is solenoid-operated and pulsed on and off by the vehicle’s engine control computer. Surrounding the injector inlet is a fine screen filter, toward which the incoming fuel is directed. When the injector’s solenoid is energized, a normally closed ball valve is lifted. Fuel under pressure is then injected at the walls of the throttle body bore just above the throttle plate. A fuel injector has a movable armature in the center of the injector, and a pintle with a tapered tip is positioned at the lower end of the armature. A spring pushes the armature and pintle downward so that the pintle tip seats in the discharge orifice. The injector coil surrounds the armature, and the two ends of the winding are connected to the terminals on the side of the injector. An integral filter is located inside the top of the injector. When the ignition switch is turned on, voltage is supplied to one injector terminal and the other terminal is connected through the computer. Each time the control unit completes the circuit from the injector winding to ground, current flows through the injector coil, and the coil magnetism moves the plunger and pintle upward. When this occurs, the pintle tip is unseated from the injector orifice, and fuel sprays from this orifice.
Throttle Body Internal Design And Operation
When fuel enters the throttle body fuel inlet, the fuel surrounds the injector or injectors at all times. Each injector is sealed into the throttle body with O-ring seals, which prevent fuel leakage around the injector at the top or bottom. Fuel is supplied from the injector through a passage to the pressure regulator. A diaphragm and valve assembly is mounted in this regulato4 and a diaphragm spring holds the valve closed. At a specific fuel pressure, the regulator diaphragm is forced upward to open the valve, and some excess fuel is returned to the fuel tank. When the pressure regulator valve opens, fuel pressure decreases slightly, and the spring closes the regulator valve. This action causes the fuel pressure to increase and reopen the pressure regulator valve. In most TBI systems, the fuel pressure regulator controls fuel pressure at 10 to 25 psi (70 to172kPa). The fuel pressure must be high enough to prevent fuel from boiling in the TBI assembly. When the pressure on a liquid is increased, the boiling point is raised proportionally. If fuel boiling occurs in the TBI assembly, vapor and fuel are discharged from the injectors. The computer program assumes that the injectors are discharging liquid fuel. Vapor discharge from the injectors creates a lean air/fuel ratio, which results in lack of engine power and acceleration stumbles.
Injector Internal Design And Electrical Connections
The plunger and valve seat are held down by a spring. In this position, the seat closes the metering orifices in the end of the injector. Openings in the sides of the injector allow fuel to enter the cavity surrounding the injector tip. A mesh screen filter inside the injector openings removes dirt particles from the fuel. In some injectors, a diaphragm is located between the valve seat and the housing. The tip of the injector may contain up to six metering orifices, but some injectors have a single metering orifice. Injector design varies depending on the manufacturer. Each injector contains two terminals, across which an internal coil is connected. A movable plunger is positioned in the center of the coil, and the lower end of the plunger has a tapered valve seat. When the ignition switch is turned on, 12 volts are supplied to one of the injector terminals, and the other injector terminal is connected to the computer. When the computer grounds this terminal, current flows through the injector coil to ground in the computer. When this action occurs, the injector coil magnetism moves the plunger and valve seat upward, and fuel sprays from the injector orifices into the air stream above the throttle.
PUTSE WIDTH
The length of time that the computer grounds the injector is referred to as pulse width. Under most operating conditions, the computer provides the correct injector pulse width to maintain the stoichio- metric air/fuel ratio. For example, the computer might ground the injector for 2 milliseconds at idle speed and 7 milliseconds at wide-open throttle to provide the stoichio-metric air/fuel ratio. In many TBI systems, the computer grounds an injector each time a signal is received from the distributor pick-up. This type of TBI system may be referred to as a synchronized system, because the injector pulses are synchronized with the pick-up signals. In a dual injector throttle body assembly, the computer grounds the injectors alternately under most operating conditions.
Air-Fuel Ratio Enrichment
When the coolant temperature sensor signal to the computer indicates that the engine coolant is cold, the computer increases the injector pulse width to provide a richer air/fuel ratio. This action eliminates the need for a conventional choke on a TBI assembly. The PCM supplies the proper air/fuel ratio and engine rpm when starting a cold engine. This eliminates the need for the driver to depress the accelerator pedal while starting the engine. When a TBI-equipped engine is cold, the computer provides a very rich air/fuel ratio for faster starting. However, if the engine does not start because of an ignition defect, the engine becomes flooded quickly. Under this condition, excessive fuel may run past the piston rings into the crankcase. Therefore, when a cold TBI engine does not start, periods of long cranking should be avoided. If the driver suspects that the air/fuel ratio is extremely rich, he or she may push the accelerator pedal to the wide-open position when starting a cold engine. Under these conditions, the computer program provides a very lean air/fuel ratio of approximately 18:1. This may be referred to as a clear flood mode. However under normal conditions, the driver should not push on the accelerator pedal at any time when starting an engine with TBI. When the engine is decelerated, the computer reduces injector pulse width in many TBI systems to provide a lean air/fuel ratio, which reduces emissions and improves fuel economy.
PORT FUEL INIECTION
PFI systems use one injector at each cylinder. They are mounted in the intake manifold near the cylinder head, where they can inject a fine, atomized fuel mist as close as possible to the intake valve. Fuel lines run to each cylinder from a fuel manifold, usually referred to as a fuel rail. The fuel rail assembly on a PFI system of V6 and V8 engines usually consists of a left- and right-hand rail assembly. The two rails can be connected either by crossover and return fuel tubes or by a mechanical bracket arrangement. Fuel tubes crisscross between the two rails. Since each cylinder has its own injector, fuel distribution is exactly equal. With little or no fuel to wet the manifold walls, there is no need for manifold heat or any early fuel evaporation system. Fuel does not collect in puddles at the base of the manifold. This means that the intake manifold passages can be tuned or designed for better low-speed power availability. The port type systems provide a more accurate and efficient delivery of fuel. Some engines are now equipped with variable induction intake manifolds that have separate runners for low and high speeds. This technology is only possible with port injection. The throttle body in a port fuel injection system controls the amount of air that enters the engine as well as the amount of vacuum in the manifold. It also may house the MAP sensor, idle air control motor and the throttle position (TP) sensor. The TP sensor enables the PCM to know where the throttle is positioned at all times. The throttle body is a single cast-aluminum housing with a single throttle blade attached to the throttle shaft. The throttle shaft is controlled by the accelerator pedal and extends the full length of the housing. The throttle bore controls the amount of incoming air that enters the air-induction system. A small amount of coolant is also routed through a passage in the throttle body to prevent icing during cold weather. Port systems require an additional control system that throttle body injection units do not require. Throttle body injectors are mounted above the throttle plates and are not affected by fluctuations in manifold vacuum, but port system injectors have their tips located in the manifold, where constant changes in vacuum would affect the amount of fuel injected. To compensate for these fluctuations, port injection systems are equipped with fuel-pres-sure regulators that sense manifold vacuum and continually adjust the fuel pressure to maintain a constant pressure drop across the injector tips at all times.
Port Firing Control
While all port injection systems operate using an injector at each cylinder they do not fire the injectors in the same manner. This one statement best defines the difference between typical multiport injection (MPI) systems and sequential fuel injection (SFI) systems. SFI systems control each injector individually so that it is opened just before the intake valve opens. This means that the mixture is never static in the intake manifold and that adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection. In MPI systems, the injectors are arranged together in pairs or groups, and these pairs or groups of injectors are turned on at the same time. When the injectors are split into two equal groups, the groups are fired alternately, with one group firing during each engine revolution. Since only two injectors can be fired relatively close to the time when the intake valve is about to open, the fuel charge for the remaining cylinders must stand in the intake manifold for varying periods of time. These periods of time are very short, therefore the standing of fuel in the intake manifold is not that great a disadvantage of MPI systems. At idle speeds this wait is about 150 milliseconds and, at higher speeds, the time is much less. The primary advantage of SFI is the ability to make instantaneous changes to the mixture. In SFI systems, each injector is connected individually into the computer, and the computer completes the ground for each injector, one at a time. In MPI systems, the injectors are grouped and all injectors with in the group share the same common ground wire.
Some injection systems fire all of the injectors at the same time for every engine revolution. This type of system offers easy programming and relatively fast adjustments to the air/fuel mixture. The injectors are connected in parallel, so the PCM sends out just one signal for all injectors. They all open and close at the same time. It simplifies the electronics without compromising injection efficiency. The amount of fuel required for each four-stroke cycle is divided in half and delivered in two injections, one for every 360 degrees of crankshaft rotation. The fast that the intake charge must still wait in the manifold for varying periods of time is the system’s major drawback.
Port Fuel injection system design
Basically, the same electric in-tank fuel pumps and fuel pump circuits are found on TBI, MFI, and SFI systems. Some MFI and SFI systems, such as those on Ford trucks, have a booster fuel pump on the frame rail in addition to the in-tank pump. A fuel filter is connected in the fuel line from the tank to the injectors. This filter may be under the vehicle or in the engine compartment. The fuel line from the filter is connected to a hollow fuel rail that is bolted to the intake manifold. The lower end of each port injector is sealed in the intake manifold with an O-ring seal, and a similar seal near the top of the injector seals the injector to the fuel rail. Each injector has a movable armature in the center of the injector, and a pintle with a tapered tip is positioned at the lower end of the armature. A spring pushes the armature and pintle downward so the pintle tip seats in the discharge orifice. The injector coil surrounds the armature, and the two ends of the winding are connected to the terminals on the side of the injector. An integral filter is located inside the top of the injector. When the ignition switch is turned on, voltage is supplied to one injector terminal and the other terminal is connected through the computer. Each time the computer completes the circuit from the injector winding to ground, current flows through the injector coil, and the coil magnetism moves the plunger and pintle upward. Under this condition, the pintle tip is unseated from the injector orifice, and fuel sprays from this orifice into the intake port The computer is programmed to ground the injectors well ahead of the actual intake valve openings, so the intake ports are filled with fuel vapor before the intake valves open. In both SFI and MFI systems, the computer supplies the correct injector pulse width to provide the stoichiometric air / fuel ratio. The computer increases the injector pulse width to provide air/fuel ratio enrichment when a cold engine is being started. A clear flood mode is also available in the computer in MFI and SFI systems. On some TBI, MFI, and SFI systems, if the ignition system is not firing, the computer stops operating the injectors. This action prevents severe flooding from long cranking periods when a cold engine is being started. On many MFI and SFI systems, the computer decreases injector pulse width while the engine is decelerating to provide improved emission levels and fuel economy. On some of these systems, the computer stops operating the injectors while the engine is decelerating in a certain rpm range.
Cold start injector
Some older EFI engines were fitted with a cold start injector. A pick-up pipe was connected from the fuel rail to the cold start injector, and the end of this injector was mounted in the intake manifold. Unlike the intake port injectors that are operated by the PCM, the cold start injector is operated by a thermo-time switch that senses coolant temperature. When the engine is cranked, voltage is supplied from the starter solenoid to one terminal on the cold start injector. If the coolant temperature is below 95ºF (55ºC), the thermo-time switch grounds the other cold start injector terminal. Under this condition, the cold start injector is energized while cranking the engine, and the injector pintle opens to spray fuel into the intake manifold in addition to the fuel injected by the injectors in the intake ports. A bimetal switch in the thermo-time switch is heated as current flows through the injector coil. The bimetal switch action opens the circuit through the thermo-time switch in a maximum of 8 seconds. The actual time that the thermo-time switch remains closed is determined by the coolant temperature. In this MFI system, the pulse width supplied by the PCM to the intake port injectors is programmed to operate with the cold start injector and supply the correct air-fuel ratio while cranking a cold engine.
Pressure Regulators
The pressure regulator on MFI and SFI systems is similar to the regulator on TBI systems. A diaphragm and valve assembly is positioned in the center of the regulator and a diaphragm spring seats the valve on the fuel outlet. When fuel pressure reaches the regulator setting, the diaphragm moves against the spring tension and the valve opens. This action allows fuel to flow through the return line to the fuel tank. The fuel pressure drops slightly when the pressure regulator valve opens, and the spring closes the regulator valve. Under this condition, the fuel pressure increases and reopens the regulator valve. A vacuum hose is connected from the intake manifold to the vacuum inlet on the pressure regulator. This hose supplies vacuum to the area in which the diaphragm spring is located. This vacuum works with the fuel pressure to move the diaphragm and open the valve. When the engine is running at idle speed, high manifold vacuum is supplied to the pressure regulator. Under this condition, fuel pressure opens the regulator valve. If the engine is operating at wide-open throttle, a very low manifold vacuum is supplied to the pressure regulator. When this condition is present, the vacuum does not help to open the regulator valve, and a higher fuel pressure is required to open the valve. When the engine is idling, higher manifold vacuum is supplied to the injector tips, and the injectors are discharging fuel into this vacuum. Under wide-open throttle conditions, the very low manifold vacuum is supplied to the injector tips. When this condition is present, the injectors are actually discharging fuel into a higher pressure compared to idle speed conditions, because the very low manifold vacuum is closer to a positive pressure. If the fuel pressure remained constant at idle and wide-open throttle conditions, the injectors would discharge less fuel into the higher pressure in the intake manifold at wide-open throttle. The increase in fuel pressure supplied by the pressure regulator at wide-open throttle maintains the same pressure drop across the injectors at idle speed and wide-open throttle. When this same pressure drop is maintained, the change in pressure at the injector tips does not affect the amount of fuel discharged by the injectors.
SEQUENITAL FUEL INJECTION SYSTEMS
SFI systems control each injector individually so that it is opened just before the intake valve opens. This means that the mixture is never static in the intake manifold and that adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection. In SFI systems, each injector is connected individually into the computer and the computer completes the ground for each injector one at a time. In MPI systems, the injectors are grouped and all injectors within the group share the same common ground wire.
A Typical Sequential Fuel Injection System
In a Chrysler SFI system on a 3.5-L engine, each injector has a separate ground wire connected into the PCM. Many Chrysler engines made before 1992 have multiport fuel injection (MPI) systems with the injectors connected in pairs on the ground side. Each pair of injectors shares a common ground wire into the PCM. Voltage is supplied through the ASD relay points to the injectors when the ignition switch is turned on, and a separate fuel pump relay supplies voltage to the fuel pump. This engine is equipped with an electronic ignition system, and the crank and cam sensors are inputs for this system. Since these inputs are connected to the PCM, the ignition module is contained in the PCM.
Return less Fuel System Pressure Regulators
Some later-model Chrysler SFI systems are referred to as return less systems. In these systems, the fuel pressure regulator and filter are mounted in the top of the assembly containing the fuel pump and fuel gauge sending unit in the fuel tank. The fuel line from the fuel rail under the hood is connected to the filter with a quick-disconnect fitting. Fuel enters the filter through the fuel supply tube in the center of the regulator and filter assembly. Fuel pressure is applied against the regulator seat washer which is seated by the seat control spring. At the specified regulator pressure, the seat is forced downward against the spring, and fuel flows past the seat into the cavity around the seat control spring. Fuel returns from this cavity to the fuel tank. When the pressure drops slightly, the seat closes again. With the return less fuel system, only the fuel needed by the engine is filtered, thus allowing the use of a smaller fuel filter. The Chrysler SFI system has many similarities to the Chrysler TBI system. For example, the voltage regulator and the cruise control module are contained in the PCM board. The SFI system on the 3,5-L engine has a low-speed and a high-speed cooling fan relay. At a specific coolant temperature, the PCM grounds the low-speed relay winding, which closes the relay points and supplies voltage to the fan motor. If the engine coolant temperature continues to increase, the PCM grounds the high-speed cooling fan relay winding, which closes the fan relay points and supplies voltage to the high-speed fan motor. The manifold solenoid controls the vacuum supplied to the intake manifold tuning valve. This solenoid is mounted on the right shock tower, and the manifold tuning valve is positioned near the center of the intake manifold. The manifold contains a pivoted butterfly valve that opens and closes to change the length of the intake manifold air passages. This butterfly valve is mounted on a shaft, and the outer end of the shaft is connected through a linkage to a diaph.ug- in a sealed vacuum chamber. A vacuum hose is connected from the outlet fitting on the manifold solenoid to the vacuum chamber in the manifold tuning valve. Another vacuum hose is connected from the inlet fitting on the manifold solenoid to the intake manifold. One terminal on the manifold solenoid winding is connected to the ignition switch, and the other terminal on this winding is connected to the PCM. While the engine is running at lower rpm, the PCM opens the manifold solenoid circuit. Under this condition, the solenoid shuts off the manifold vacuum to the intake manifold tuning valve, and the butterfly valve closes some of the air pas-sages inside the intake manifold. At higher engine rpm, the PCM grounds the manifold solenoid winding and energizes the solenoid. This action opens the vacuum passage through the solenoid and supplies vacuum to the intake manifold tuning valve. Under this condition, the butterfly valve is moved so that it opens additional air passages inside the intake manifold to improve airflow and increase engine horsepower and torque.
TBI Advantages
Throttle body systems provide improved fuel metering when compared to that of carburetors. They are also less expensive and simpler to service. TBI units also have some advantages over port injection. They are less expensive to manufacture, simpler to diagnose and service, and do not have injector balance problems to the extent that port injection systems do when the injectors begin to clog. However throttle body units are not as efficient as port systems. The disadvantages are primarily manifold related. Like a carburetor system, fuel is still not distributed equally to all cylinders, and a cold manifold may cause fuel to condense and puddle in the manifold. Like a carburetor, throttle body injection systems must be mounted above the combustion chamber level, which eliminates the possibility of tuning the manifold design for more efficient operation.
lnjectors
The fuel injector is solenoid-operated and pulsed on and off by the vehicle’s engine control computer. Surrounding the injector inlet is a fine screen filter, toward which the incoming fuel is directed. When the injector’s solenoid is energized, a normally closed ball valve is lifted. Fuel under pressure is then injected at the walls of the throttle body bore just above the throttle plate. A fuel injector has a movable armature in the center of the injector, and a pintle with a tapered tip is positioned at the lower end of the armature. A spring pushes the armature and pintle downward so that the pintle tip seats in the discharge orifice. The injector coil surrounds the armature, and the two ends of the winding are connected to the terminals on the side of the injector. An integral filter is located inside the top of the injector. When the ignition switch is turned on, voltage is supplied to one injector terminal and the other terminal is connected through the computer. Each time the control unit completes the circuit from the injector winding to ground, current flows through the injector coil, and the coil magnetism moves the plunger and pintle upward. When this occurs, the pintle tip is unseated from the injector orifice, and fuel sprays from this orifice.
Throttle Body Internal Design And Operation
When fuel enters the throttle body fuel inlet, the fuel surrounds the injector or injectors at all times. Each injector is sealed into the throttle body with O-ring seals, which prevent fuel leakage around the injector at the top or bottom. Fuel is supplied from the injector through a passage to the pressure regulator. A diaphragm and valve assembly is mounted in this regulato4 and a diaphragm spring holds the valve closed. At a specific fuel pressure, the regulator diaphragm is forced upward to open the valve, and some excess fuel is returned to the fuel tank. When the pressure regulator valve opens, fuel pressure decreases slightly, and the spring closes the regulator valve. This action causes the fuel pressure to increase and reopen the pressure regulator valve. In most TBI systems, the fuel pressure regulator controls fuel pressure at 10 to 25 psi (70 to172kPa). The fuel pressure must be high enough to prevent fuel from boiling in the TBI assembly. When the pressure on a liquid is increased, the boiling point is raised proportionally. If fuel boiling occurs in the TBI assembly, vapor and fuel are discharged from the injectors. The computer program assumes that the injectors are discharging liquid fuel. Vapor discharge from the injectors creates a lean air/fuel ratio, which results in lack of engine power and acceleration stumbles.
Injector Internal Design And Electrical Connections
The plunger and valve seat are held down by a spring. In this position, the seat closes the metering orifices in the end of the injector. Openings in the sides of the injector allow fuel to enter the cavity surrounding the injector tip. A mesh screen filter inside the injector openings removes dirt particles from the fuel. In some injectors, a diaphragm is located between the valve seat and the housing. The tip of the injector may contain up to six metering orifices, but some injectors have a single metering orifice. Injector design varies depending on the manufacturer. Each injector contains two terminals, across which an internal coil is connected. A movable plunger is positioned in the center of the coil, and the lower end of the plunger has a tapered valve seat. When the ignition switch is turned on, 12 volts are supplied to one of the injector terminals, and the other injector terminal is connected to the computer. When the computer grounds this terminal, current flows through the injector coil to ground in the computer. When this action occurs, the injector coil magnetism moves the plunger and valve seat upward, and fuel sprays from the injector orifices into the air stream above the throttle.
PUTSE WIDTH
The length of time that the computer grounds the injector is referred to as pulse width. Under most operating conditions, the computer provides the correct injector pulse width to maintain the stoichio- metric air/fuel ratio. For example, the computer might ground the injector for 2 milliseconds at idle speed and 7 milliseconds at wide-open throttle to provide the stoichio-metric air/fuel ratio. In many TBI systems, the computer grounds an injector each time a signal is received from the distributor pick-up. This type of TBI system may be referred to as a synchronized system, because the injector pulses are synchronized with the pick-up signals. In a dual injector throttle body assembly, the computer grounds the injectors alternately under most operating conditions.
Air-Fuel Ratio Enrichment
When the coolant temperature sensor signal to the computer indicates that the engine coolant is cold, the computer increases the injector pulse width to provide a richer air/fuel ratio. This action eliminates the need for a conventional choke on a TBI assembly. The PCM supplies the proper air/fuel ratio and engine rpm when starting a cold engine. This eliminates the need for the driver to depress the accelerator pedal while starting the engine. When a TBI-equipped engine is cold, the computer provides a very rich air/fuel ratio for faster starting. However, if the engine does not start because of an ignition defect, the engine becomes flooded quickly. Under this condition, excessive fuel may run past the piston rings into the crankcase. Therefore, when a cold TBI engine does not start, periods of long cranking should be avoided. If the driver suspects that the air/fuel ratio is extremely rich, he or she may push the accelerator pedal to the wide-open position when starting a cold engine. Under these conditions, the computer program provides a very lean air/fuel ratio of approximately 18:1. This may be referred to as a clear flood mode. However under normal conditions, the driver should not push on the accelerator pedal at any time when starting an engine with TBI. When the engine is decelerated, the computer reduces injector pulse width in many TBI systems to provide a lean air/fuel ratio, which reduces emissions and improves fuel economy.
PORT FUEL INIECTION
PFI systems use one injector at each cylinder. They are mounted in the intake manifold near the cylinder head, where they can inject a fine, atomized fuel mist as close as possible to the intake valve. Fuel lines run to each cylinder from a fuel manifold, usually referred to as a fuel rail. The fuel rail assembly on a PFI system of V6 and V8 engines usually consists of a left- and right-hand rail assembly. The two rails can be connected either by crossover and return fuel tubes or by a mechanical bracket arrangement. Fuel tubes crisscross between the two rails. Since each cylinder has its own injector, fuel distribution is exactly equal. With little or no fuel to wet the manifold walls, there is no need for manifold heat or any early fuel evaporation system. Fuel does not collect in puddles at the base of the manifold. This means that the intake manifold passages can be tuned or designed for better low-speed power availability. The port type systems provide a more accurate and efficient delivery of fuel. Some engines are now equipped with variable induction intake manifolds that have separate runners for low and high speeds. This technology is only possible with port injection. The throttle body in a port fuel injection system controls the amount of air that enters the engine as well as the amount of vacuum in the manifold. It also may house the MAP sensor, idle air control motor and the throttle position (TP) sensor. The TP sensor enables the PCM to know where the throttle is positioned at all times. The throttle body is a single cast-aluminum housing with a single throttle blade attached to the throttle shaft. The throttle shaft is controlled by the accelerator pedal and extends the full length of the housing. The throttle bore controls the amount of incoming air that enters the air-induction system. A small amount of coolant is also routed through a passage in the throttle body to prevent icing during cold weather. Port systems require an additional control system that throttle body injection units do not require. Throttle body injectors are mounted above the throttle plates and are not affected by fluctuations in manifold vacuum, but port system injectors have their tips located in the manifold, where constant changes in vacuum would affect the amount of fuel injected. To compensate for these fluctuations, port injection systems are equipped with fuel-pres-sure regulators that sense manifold vacuum and continually adjust the fuel pressure to maintain a constant pressure drop across the injector tips at all times.
Port Firing Control
While all port injection systems operate using an injector at each cylinder they do not fire the injectors in the same manner. This one statement best defines the difference between typical multiport injection (MPI) systems and sequential fuel injection (SFI) systems. SFI systems control each injector individually so that it is opened just before the intake valve opens. This means that the mixture is never static in the intake manifold and that adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection. In MPI systems, the injectors are arranged together in pairs or groups, and these pairs or groups of injectors are turned on at the same time. When the injectors are split into two equal groups, the groups are fired alternately, with one group firing during each engine revolution. Since only two injectors can be fired relatively close to the time when the intake valve is about to open, the fuel charge for the remaining cylinders must stand in the intake manifold for varying periods of time. These periods of time are very short, therefore the standing of fuel in the intake manifold is not that great a disadvantage of MPI systems. At idle speeds this wait is about 150 milliseconds and, at higher speeds, the time is much less. The primary advantage of SFI is the ability to make instantaneous changes to the mixture. In SFI systems, each injector is connected individually into the computer, and the computer completes the ground for each injector, one at a time. In MPI systems, the injectors are grouped and all injectors with in the group share the same common ground wire.
Some injection systems fire all of the injectors at the same time for every engine revolution. This type of system offers easy programming and relatively fast adjustments to the air/fuel mixture. The injectors are connected in parallel, so the PCM sends out just one signal for all injectors. They all open and close at the same time. It simplifies the electronics without compromising injection efficiency. The amount of fuel required for each four-stroke cycle is divided in half and delivered in two injections, one for every 360 degrees of crankshaft rotation. The fast that the intake charge must still wait in the manifold for varying periods of time is the system’s major drawback.
Port Fuel injection system design
Basically, the same electric in-tank fuel pumps and fuel pump circuits are found on TBI, MFI, and SFI systems. Some MFI and SFI systems, such as those on Ford trucks, have a booster fuel pump on the frame rail in addition to the in-tank pump. A fuel filter is connected in the fuel line from the tank to the injectors. This filter may be under the vehicle or in the engine compartment. The fuel line from the filter is connected to a hollow fuel rail that is bolted to the intake manifold. The lower end of each port injector is sealed in the intake manifold with an O-ring seal, and a similar seal near the top of the injector seals the injector to the fuel rail. Each injector has a movable armature in the center of the injector, and a pintle with a tapered tip is positioned at the lower end of the armature. A spring pushes the armature and pintle downward so the pintle tip seats in the discharge orifice. The injector coil surrounds the armature, and the two ends of the winding are connected to the terminals on the side of the injector. An integral filter is located inside the top of the injector. When the ignition switch is turned on, voltage is supplied to one injector terminal and the other terminal is connected through the computer. Each time the computer completes the circuit from the injector winding to ground, current flows through the injector coil, and the coil magnetism moves the plunger and pintle upward. Under this condition, the pintle tip is unseated from the injector orifice, and fuel sprays from this orifice into the intake port The computer is programmed to ground the injectors well ahead of the actual intake valve openings, so the intake ports are filled with fuel vapor before the intake valves open. In both SFI and MFI systems, the computer supplies the correct injector pulse width to provide the stoichiometric air / fuel ratio. The computer increases the injector pulse width to provide air/fuel ratio enrichment when a cold engine is being started. A clear flood mode is also available in the computer in MFI and SFI systems. On some TBI, MFI, and SFI systems, if the ignition system is not firing, the computer stops operating the injectors. This action prevents severe flooding from long cranking periods when a cold engine is being started. On many MFI and SFI systems, the computer decreases injector pulse width while the engine is decelerating to provide improved emission levels and fuel economy. On some of these systems, the computer stops operating the injectors while the engine is decelerating in a certain rpm range.
Cold start injector
Some older EFI engines were fitted with a cold start injector. A pick-up pipe was connected from the fuel rail to the cold start injector, and the end of this injector was mounted in the intake manifold. Unlike the intake port injectors that are operated by the PCM, the cold start injector is operated by a thermo-time switch that senses coolant temperature. When the engine is cranked, voltage is supplied from the starter solenoid to one terminal on the cold start injector. If the coolant temperature is below 95ºF (55ºC), the thermo-time switch grounds the other cold start injector terminal. Under this condition, the cold start injector is energized while cranking the engine, and the injector pintle opens to spray fuel into the intake manifold in addition to the fuel injected by the injectors in the intake ports. A bimetal switch in the thermo-time switch is heated as current flows through the injector coil. The bimetal switch action opens the circuit through the thermo-time switch in a maximum of 8 seconds. The actual time that the thermo-time switch remains closed is determined by the coolant temperature. In this MFI system, the pulse width supplied by the PCM to the intake port injectors is programmed to operate with the cold start injector and supply the correct air-fuel ratio while cranking a cold engine.
Pressure Regulators
The pressure regulator on MFI and SFI systems is similar to the regulator on TBI systems. A diaphragm and valve assembly is positioned in the center of the regulator and a diaphragm spring seats the valve on the fuel outlet. When fuel pressure reaches the regulator setting, the diaphragm moves against the spring tension and the valve opens. This action allows fuel to flow through the return line to the fuel tank. The fuel pressure drops slightly when the pressure regulator valve opens, and the spring closes the regulator valve. Under this condition, the fuel pressure increases and reopens the regulator valve. A vacuum hose is connected from the intake manifold to the vacuum inlet on the pressure regulator. This hose supplies vacuum to the area in which the diaphragm spring is located. This vacuum works with the fuel pressure to move the diaphragm and open the valve. When the engine is running at idle speed, high manifold vacuum is supplied to the pressure regulator. Under this condition, fuel pressure opens the regulator valve. If the engine is operating at wide-open throttle, a very low manifold vacuum is supplied to the pressure regulator. When this condition is present, the vacuum does not help to open the regulator valve, and a higher fuel pressure is required to open the valve. When the engine is idling, higher manifold vacuum is supplied to the injector tips, and the injectors are discharging fuel into this vacuum. Under wide-open throttle conditions, the very low manifold vacuum is supplied to the injector tips. When this condition is present, the injectors are actually discharging fuel into a higher pressure compared to idle speed conditions, because the very low manifold vacuum is closer to a positive pressure. If the fuel pressure remained constant at idle and wide-open throttle conditions, the injectors would discharge less fuel into the higher pressure in the intake manifold at wide-open throttle. The increase in fuel pressure supplied by the pressure regulator at wide-open throttle maintains the same pressure drop across the injectors at idle speed and wide-open throttle. When this same pressure drop is maintained, the change in pressure at the injector tips does not affect the amount of fuel discharged by the injectors.
SEQUENITAL FUEL INJECTION SYSTEMS
SFI systems control each injector individually so that it is opened just before the intake valve opens. This means that the mixture is never static in the intake manifold and that adjustments to the mixture can be made almost instantaneously between the firing of one injector and the next. Sequential firing is the most accurate and desirable method of regulating port injection. In SFI systems, each injector is connected individually into the computer and the computer completes the ground for each injector one at a time. In MPI systems, the injectors are grouped and all injectors within the group share the same common ground wire.
A Typical Sequential Fuel Injection System
In a Chrysler SFI system on a 3.5-L engine, each injector has a separate ground wire connected into the PCM. Many Chrysler engines made before 1992 have multiport fuel injection (MPI) systems with the injectors connected in pairs on the ground side. Each pair of injectors shares a common ground wire into the PCM. Voltage is supplied through the ASD relay points to the injectors when the ignition switch is turned on, and a separate fuel pump relay supplies voltage to the fuel pump. This engine is equipped with an electronic ignition system, and the crank and cam sensors are inputs for this system. Since these inputs are connected to the PCM, the ignition module is contained in the PCM.
Return less Fuel System Pressure Regulators
Some later-model Chrysler SFI systems are referred to as return less systems. In these systems, the fuel pressure regulator and filter are mounted in the top of the assembly containing the fuel pump and fuel gauge sending unit in the fuel tank. The fuel line from the fuel rail under the hood is connected to the filter with a quick-disconnect fitting. Fuel enters the filter through the fuel supply tube in the center of the regulator and filter assembly. Fuel pressure is applied against the regulator seat washer which is seated by the seat control spring. At the specified regulator pressure, the seat is forced downward against the spring, and fuel flows past the seat into the cavity around the seat control spring. Fuel returns from this cavity to the fuel tank. When the pressure drops slightly, the seat closes again. With the return less fuel system, only the fuel needed by the engine is filtered, thus allowing the use of a smaller fuel filter. The Chrysler SFI system has many similarities to the Chrysler TBI system. For example, the voltage regulator and the cruise control module are contained in the PCM board. The SFI system on the 3,5-L engine has a low-speed and a high-speed cooling fan relay. At a specific coolant temperature, the PCM grounds the low-speed relay winding, which closes the relay points and supplies voltage to the fan motor. If the engine coolant temperature continues to increase, the PCM grounds the high-speed cooling fan relay winding, which closes the fan relay points and supplies voltage to the high-speed fan motor. The manifold solenoid controls the vacuum supplied to the intake manifold tuning valve. This solenoid is mounted on the right shock tower, and the manifold tuning valve is positioned near the center of the intake manifold. The manifold contains a pivoted butterfly valve that opens and closes to change the length of the intake manifold air passages. This butterfly valve is mounted on a shaft, and the outer end of the shaft is connected through a linkage to a diaph.ug- in a sealed vacuum chamber. A vacuum hose is connected from the outlet fitting on the manifold solenoid to the vacuum chamber in the manifold tuning valve. Another vacuum hose is connected from the inlet fitting on the manifold solenoid to the intake manifold. One terminal on the manifold solenoid winding is connected to the ignition switch, and the other terminal on this winding is connected to the PCM. While the engine is running at lower rpm, the PCM opens the manifold solenoid circuit. Under this condition, the solenoid shuts off the manifold vacuum to the intake manifold tuning valve, and the butterfly valve closes some of the air pas-sages inside the intake manifold. At higher engine rpm, the PCM grounds the manifold solenoid winding and energizes the solenoid. This action opens the vacuum passage through the solenoid and supplies vacuum to the intake manifold tuning valve. Under this condition, the butterfly valve is moved so that it opens additional air passages inside the intake manifold to improve airflow and increase engine horsepower and torque.
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