Fuel injection typically involves spraying or injecting fuel directly into the engine’s intake ports. Fuel injection has several major advantages over carbureted systems. These include improved drive-ability under all conditions, improved fuel control and economy, decreased exhaust emissions, and an increase in engine efficiency and power. During the 1980s, automotive engineers changed many engines from carburetors or computer-controlled carburetors to electronic fuel injection systems. This action was taken to improve fuel economy, performance, and emission levels. Automotive manufacturers had to meet increasingly stringent corporate average fuel economy (CAFE) regulations and comply with emission standards at the same time. Many of the early EFI systems were throttle body injection systems in which the fuel was injected above the throttles. The TBI systems have been gradually changed to port fuel injection systems with the injectors located in the intake ports. On carbureted and throttle body injected engines, some intake manifold heating was required to prevent fuel condensation on the intake manifold passages. When the injectors are positioned in the intake ports, intake manifold heating is not required. This provided engineers with increased intake manifold design flexibility. Intake manifolds could now be designed with longer, curved air passages, which increased airflow and improved torque and horsepower. Since intake manifolds no longer require heating, they can now be made from plastic materials such as glass-fiber reinforced nylon resin. This material is considerably lighter than cast iron or even aluminum. Saving weight means an improvement in fuel economy. The plastic-type intake manifold does not transfer heat to the air and fuel vapor in the intake passages, which improves economy and hot-start performance. Electronic fuel injection has proved to be the most precise, reliable, and cost-effective method of delivering fuel to the combustion chambers of today's vehicles. EFI systems must provide the correct air/fuel ratio for all engine loads, speeds, and temperature conditions. To accomplish this, an EFI system uses a fuel delivery system, air induction system, input sensors, control computer, fuel injectors, and some sort of idle speed control. Idle speed control is a computer output function on electronic fuel injection systems. It is very important that technicians understand how these systems operate. Since improper idle rpm is often the result of faulty input sensor or switch signals, it is even more important that technicians understand the input sensor signals used by the computer to control the idle air control motor.
Although fuel injection technology has been around since the 1 920s, it was not until the 1980s that manufacturers began to replace carburetors with electronic fuel injection (EFI) systems. Many of the early EFI systems were throttle body injection (TBI) systems in which the fuel was injected above the throttle plates. Recently a similar system, central port injection (CPD, was introduced. In these systems the injector assembly is located in the lower half of the intake manifold. Engines equipped with TBI have gradually become equipped with port fuel injection (PFI), which has injectors located in the intake ports of the cylinders. Since the 1995 model-year, all new cars are equipped with an EFI system. Recently, some engines have been equipped with gasoline direct-injection (GDI) systems. In these systems, the fuel is injected directly into the cylinders. Direct injection has been used for years with diesel fuels but has not been successfully used on gasoline engines until lately. Throttle body injection systems have a throttle body assembly mounted on the intake manifold in the position usually occupied by a carburetor. The throttle body assembly usually contains one or two injectors. On port fuel injection systems, fuel injectors are mounted at the back of each intake valve. Aside from the differences in injector location and number of injectors, operation of throttle body and port systems is quite similar with regard to fuel and air metering, sensors, and computer operation. Most electronic fuel injection systems inject fuel only during part of the engine’s combustion cycle. The engine fuel needs are measured by intake airflow past a sensor or by intake manifold pressure (vacuum). The airflow or manifold vacuum sensor converts its reading to an electrical signal and sends it to the engine control computer, The computer processes this signal (and others) and calculates the fuel needs of the engine. The computer then sends an electrical signal to the fuel injector or injectors. This signal determines the amount of time the injector opens and sprays fuel. This interval is known as the injector pulse width. Port-type continuous injection systems (CIS) deliver a steady stream of pressurized fuel into the intake manifold. The amount of fuel delivered to the cylinders is con- trolled by the rate of airflow entering the engine. An airflow sensor controls movement of a plunger that alters fuel flow to the injectors. When introduced, CIS was a mechanically controlled system. However, oxygen sensor feed back circuits and other electronic controls have been added to the system. CIS that have electronic controls are commonly referred to as CIS-E. For quite some time, diesel engines have been equipped with fuel injection systems. The two basic differences between gasoline injection and diesel injection are that diesel fuel is injected directly into the cylinders and that diesel fuel injection systems are operated mechanically rather than electronically. Although late-model diesel systems use electronic fuel controls, the fuel injection system is a mechanical system that is controlled mechanically.
Continuous injection systems were used almost exclusively on European imported cars. The basic technology for CIS was introduced in the early 1970s and was continuously updated and refined. Early CIS designs were basically mechanical fuel injection systems. As the system was refined and electronic controls were added, the system became known as CIS-E. In the CIS-E, the injectors in each intake port are injecting fuel continually while the engine is running. These injectors do not open and close while the engine is running. Fuel is supplied from a central fuel distributor to all the injectors. A pivoted air flow sensor plate is positioned in the air intake. With the engine not running, the air flow sensor plate closes the air passage in the air intake. A control plunger in the fuel distributor rests against the airflow sensor lever. When engine speed increases, the air velocity in the air intake gradually opens the air flow sensor plate. This plate movement lifts the control plunger, which precisely meters the fuel to the injectors. The differential pressure regulator winding is cycled on and off by the electronic control unit. This action moves the plunger up and down in the differential pressure regulator, which controls fuel pressure in the lower chambers of the fuel distributor and provides precise control of the fuel delivery to the injectors and the air/fuel ratio. The input sensors connected to the electronic control unit vary depending on the vehicle, but these sensors include an oxygen sensor and coolant temperature sensor. Some CIS-E are complete engine management systems in which the electronic control unit provides fully integrated control of the air/fuel ratio, spark advance, emission control devices, and idle speed.
Metering is done through a mixture control unit. This unit consists of an airflow sensor and a special fuel distributor with fuel lines running to all injectors. A control plunger in the fuel distributor is linked by a lever to the airflow sensor plate. As the airflow sensor’s plate moves, the motion is transferred to the control plunger in the fuel distributor. The plunger moves up or down, changing the size of the fuel metering openings in the fuel lines. This increases or decreases the volume of fuel flowing to the injectors. The amount of intake air is controlled by the throttle. The sensor plate is located in an air venturi in the mixture control unit. The movement of the airflow sensor plate increases with an increase in airflow. Any air that enters. the intake without passing the sensor plate interferes with the proper air/fuel mixture, causing the engine to run lean.
signals from the oxygen sensor are sent to the control unit. The control unit, when in closed loop, modifies the fuel flow in the mixture control unit so that the engine operates with the proper ratio. By reducing the pressure in the lower part of the differential pressure valve, fuel flow to the injector can be increased, enriching the mixture. shortening the time that the control valve is open increases the pressure beneath the differential pressure valve diaphragm. This lessens the amount of fuel injected, leaning the mixture.
The control unit switches to open loop during conditions when the oxygen sensor is cold or when the engine is cold. This open loop operation holds the oxygen sontrol valve open for a fixed amount of time.
The fuel distributor is an assembly containing a fuel control unit, pressure-regulating valves for each cylinder, and a system pressure regulator. The fuel control unit consists of a slotted metering cylinder, which contains the fuel control plunger. Part of the control plunger protrudes past the fuel distributor and rests on the airflow sensor lever. Fuel flows through the slots in the fuel-metering cylinder. There is one metering slot for each engine cylinder. Control plunger movement within the metering cylinder determines the amount of fuel released to the fuel injectors. Each cylinder has its own pressure-regulating valve. These valves maintain a constant pressure differential of approximately 1 .5 psi (10.34 kpa) on either side of the fuel metering slot. This pressure differential remains the same, regardless of the size of the slot opening. Without pressure-regulating valves, the amount of fuel injected would not remain proportional to the size of the metering slot opening. The fuel distributor also contains a pressure relief valve that regulates system pressure.
Fuel Injectors
CIS fuel injectors open at a set fuel pressure. Once the engine is started, each injector continuously sprays finely atomized fuel into the intake port of the cylinder. A vibrator pin or needle inside each injector helps break up and atomize the fuel. This vibrating action also helps keep the injectors from clogging. When the engine is stopped, the pin and spring assembly seal off the injector to retain fuel pressure in the lines. This helps assure quick starting. CIS are normally equipped with a cold-start injector and auxiliary air valve system to control cold starting and engine idling.
BASIC EFI
In an EFI system ,the computer must know the amount of air entering the engine so it can supply the stoichiometric air/fuel ratio. In EFI systems with a MAP sensor the computer program is designed to calculate the amount of air entering the engine from the MAP and rpm input signals. The distributor pick-up supplies an rpm signal to the computer. This type of EFI system is referred to as a speed density system, because the computer calculates the air intake flow from the engine rpm, or speed, input, and the density of intake manifold vacuum input. Therefore, the computer must have accurate signals from these inputs to maintain the stoichiometric air/fuel ratio. The other inputs are used by the computer to “fine tune” the air/fuel ratio.
Powertrain Control Moduls
The heart of the fuel injection system is the computer or powertrain control module (PCM). The PCM receives signals from all the system sensors, processes them, and transmits programmed electrical pulses to the fuel injectors. Both incoming and outgoing signals are sent through a wiring harness and a multiple-pin connector. Electronic feedback in the PCM means the unit is self-regulating and is controlling the injectors on the basis of operating performance or parameters rather than on pre-programmed instructions. A PCM with a feedback loop, for example, reads the signals from the oxygen sensor, varies the pulse width of the injectors, and again reads the signals from the oxygen sensor. This is repeated until the injectors are pulsed for just the amount of time needed to get the proper amount of oxygen into the exhaust stream. While this interaction is occurring, the system is operating in closed loop. When conditions, such as starting or wide-open throttle, demand that the signals from the oxygen sensor be ignored, the system operates in open loop. During open loop, injector pulse length is controlled by set parameters contained in the PCM's memory.
Fuel Injectors
Fuel injectors are electromechanical devices that meter and atomize fuel so it can be sprayed into the intake manifold. Fuel injectors resemble a spark plug in size and shape. O-rings are used to seal the injector at the intake manifold, throttle body, and./or fuel rail mounting positions. These O-rings provide thermal insulation to prevent the formation of vapor bubbles and promote good hot-start characteristics. They also dampen potentially damaging vibration. When the injector is electrically energized, a fine mist of fuel sprays from the injector tip. Two different valve designs are commonly used. The first consists of a valve body and a nozzle or needle valve. A movable armature is attached to the nozzle valve, which is pressed against the nozzle body sealing seat by a helical spring. The solenoid winding is located at the back of the valve body. When the solenoid winding is energized, it creates a magnetic field that draws the armature back and pulls the nozzle valve from its seat. When the solenoid is de-energized, the magnetic field collapses and the helical spring forces the nozzle valve back on its seat. The second popular valve design uses a ball valve and valve seat. In this case, the magnetic field created by the solenoid coil pulls a plunger upward, lifting the ball valve from its seat. Once again, a spring is used to return the valve to its seated or closed position. Fuel injectors can be either top fuel feeding or bottom fuel feeding. Top feed injectors are primarily used in port injection systems that operate using high fuel system pressures. Bottom feed injectors are used in throttle body systems. Bottom feed injectors are able to use fuel pressures as low as 10 psi (68.95 kpa). There have been some problems with deposits on injector tips. Since small quantities of gum are present in gasoline, injector deposits usually occur when this gum bakes onto the injector tips after a hot engine is shut off. Most oil companies have added a detergent to their gasoline to help prevent injector tip deposits. Car manufacturers and auto parts stores sell detergents to place in the fuel tank to clean injector tips. Some manufacturers and auto parts suppliers have designed deposit-resistant injectors. These injectors have several different pintle tip and orifice designs to help prevent deposits. On one type of deposit-resistant injector the pintle seat opens outward away from the injector body and more clearance is provided between the pintle and the body. Another type of deposit-resistant injector has four orifices in a metering plate rather than a single orifice. Some deposit-resistant injectors may be recognized by the color of the, injector body. For example, regular injectors supplied by Ford Motor Company are painted black, whereas their deposit-resistant injectors have tan or yellow bodies. Each fuel injector is equipped with a two-wire connector. The connector is often equipped with a spring clip that must be unlocked before the connector can be removed from the injector. One wire of the connector supplies voltage to the injector. This voltage supply wire may connect directly to the fuse panel or it may connect to the PCM, which in turn connects to the fuse panel. In some systems, a resistor at the fuse panel or PCM is used to reduce the 12-volt a needle valve that has a specially ground pintle for precise fuel control. battery supply voltage to 3 volts or less. Most other injectors are fed battery voltage. The second wire of the connector is a ground wire. This ground wire is routed to the PCM. The PCM energizes the injector by grounding its electrical circuit. The pulse width of the injector equals the length of time the injector circuit is grounded. Typical pulse widths range from 1 millisecond to 10 milliseconds at full load. Port fuel injection systems having four, six, or eight injectors use a special wiring harness to simplify and organtze injector wiring.
Idle speed control is a function of the PCM. Based on operating conditions and inputs from various sensors, the PCM regulates the idle speed to control emissions. In throttle body and port EFI systems, engine idle speed is controlled by bypassing a certain amount of airflow past the throttle valve in the throttle body housing. Two types of air by-pass systems are used; auxiliary air valves and idle air control (IAC) valves. LAC valve systems are more common. Most TBI units are fitted with an idle speed motor. The IAC system consists of an electrically controlled stepper motor or actuator that positions the IAC valve in the air by-pass channel around the throttle valve. The IAC valve is part of the throttle body casting. The PCM calculates the amount of air needed for smooth idling based on input data such as coolant temperature, engine load, engine speed, and battery voltage. It then signals the actuator to extend or retract the idle air control valve in the air by-pass channel. If the engine speed is lower than desired, the PCM activates the motor to retract the IAC valve. This opens the channel and diverts more air around the throttle valve. If engine speed is higher than desired, the valve is extended and the by-pass channel is made smaller. Air supply to the engine is reduced, and engine speed falls. During cold starts, idle speed can be as high as 2,100 rpm to quickly raise the temperature of the catalytic converter for proper control of exhaust emissions. Idle speed that is attained after a cold start is controlled by the PCM. The PCM maintains idle speed for approximately 40 to 50 seconds even if the driver attempts to alter it by kicking the accelerator. After this preprogrammed time interval, depressing the accelerator pedal rotates the throttle position (TP) sensor and signals the PCM to reduce idle speed. Some engines are equipped with an auxiliary air valve to aid in the control of engine idle speed. The major difference between an IAC valve and an auxiliary air valve is that the auxiliary air valve is not controlled by the PCM. But like the IAC system, the auxiliary an valve provides additional air during cold-engine starts and warm-up. The auxiliary air valve allows air to bypass the throttle plate, thereby increasing the idle speed. The opening and closing of the auxiliary air valve is controlled by a bimetallic strip. As the strip heats up, it bends to rotate the movable plate, gradually blocking the opening. When the device is closed, there is no auxiliary airflow. The bimetal strip is warmed by an electric heating element powered from the run circuit of the ignition switch. The auxiliary air device is independent of the cold start injector. It is not controlled by the PCM but is continuously powered when the ignition key is set to the run position.
Some engines are equipped with an auxiliary air valve to aid in the control of engine idle speed. The major difference between an IAC valve and an auxiliary air valve is that the auxiliary air valve is not controlled by the PCM. But like the IAC system, the auxiliary m valve provides additional air during cold-engine starts and warm-up. The auxiliary air valve allows air to bypass the throttle plate, thereby increasing the idle speed. The opening and closing of the auxiliary air valve is controlled by a bimetallic strip. As the strip heats up, it bends to rotate the movable plate, gradually blocking the opening. When the device is closed, there is no auxiliary airflow. The bimetal strip is warmed by an electric heating element powered from the run circuit of the ignition switch. The auxiliary air device is independent of the cold start injector. It is not controlled by the PCM but is continuously powered when the ignition key is set to the run position.
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