Multipoint - EFI System (Part7. THROTTLE BODY ASSEMBLY)

The throttle body assembly for most modern fuel in­jection systems is used only to control air flow into the engine. A throttle valve, like the butterfly valve in a carburetor, is attached to the throttle cable and gas pedal. When the driver presses the gas pedal, the shaft rotates. This swings the throttle valve open to admit more air. This increases engine power for acceleration or pulling a load. The throttle body, itself, is usually made of cast aluminum. It mounts in the induction system just ahead of the intake manifold. As was shown in Fig below, it sometimes bolts to the inlet of the intake manifold.

Throttle body assembly only controls air flow to engine with most modern fuel injection systems. (Ford)

There are many throttle body designs for fuel in­jected systems. Two variations are shown here. (Ford and Honda)

Air Bypass Valve (idle speed control valve)

An air bypass valve, also known as an idle speed con­trol valve, is frequently used to regulate engine idle speed. The air bypass valve normally mounts on the throttle body assembly. It can be controlled by either a temperature-sensitive device or by the computer. In the figure shows the action of the temperature (bimetal strip) operated idle air control valve. When the control valve and engine are cold, the bimetal strip holds. the air bypass open. This increases engine speed. When the engine warms, the bimetal strip bends and moves to close the idle air valve. This drops engine idle speed to normal.
The picture illustrates the basic action of a computer controlled idle air valve. When the engine is cold, the engine temperature sensor signals the computer. The computer knows that engine rpm should be increased to prevent engine stalling and stumbling. It then sends current to the idle air control valve. This opens the valve and allows air to bypass the throttle valve. The rest of the injection systems react to this increased air flow and engine idle speed increases, just as a carburetor fast idle cam increases cold engine idle rpm.
As the engine warms, the computer operates the idle air control valve to close off the bypass. Then, no extra air flows around the throttle plate and idle speed returns to normal again.

Air bypass valve acts like fast idle mechanism on carbureted fuel system. It speeds up engine idle rpm when it is cold. (Volkswagen)

Expansion and contraction of metals in bimetallic strip open and close air door in this air bypass valve. When engine is cold, metal contracts and opens door admitting more air. As engine warms, warm air, assisted by electric heat element, closes blocking plate to lower idle speed. (Renault and AMC)

shows a cutaway view of one computer controlled type of idle speed valve. When the engine is cold, the computer sends current to rotate the small dc motor in one direction. This turns the rotor and screw to pull the air bypass valve open. When the engine warms, the computer reverses the polarity to the motor so it turns the screw in the opposite direction, closing the valve.

Cutaway of idle air control valve operated by small dc motor and computer. Current from computer energizes motor which turns threaded shaft to open and close air valve. (Toyota)

Multipoint - EFI System (Part6. FUEL RETURN CIRCUIT)

The fuel return circuit is made up of all the components that carry fuel from the outlet of the pressure regulator to the fuel tank, Fig.Below. This normally includes rub­ber fuel hoses, a steel fuel line, and necessary fittings.

The fuel return circuit allows circulation of fuel through the main fuel line, fuel rail, past the inlet to the injectors, through the regulator, return line, and back to the tank. This circulation prevents a heat buildup in the fuel. Heat could cause bubbles in fuel, upsetting fuel mixture.

Fuel Accumulator

A fuel accumulator dampens fuel pressure pulses and helps maintain fuel pressure when the engine is shut off. Fuel pressure fluctuations result from the action of the fuel pump and from the injectors constantly opening and closing. Too much pressure fluctuation could upset the operation of the pressure regulator.
The fuel accumulator acts like a "shock absorber." It can increase component life and help quiet system operation.
The fuel accumulator is simply an enclosed container with a spring-loaded diaphragm. Fuel pressure pushes the diaphragm down and compresses the diaphragm spring. This provides energy to maintain pressure or to counteract a sudden pressure drop.

Fuel accumulator is simply a small canister fitted with fuel inlet and outlet. Canister contains a spring and a diaphragm, the spring and diaphragm absorb pressure pulsations like a cushion. Left. Note names of parts. Center. When engine is running, fuel pressure compresses diaphragm spring. Right. When engine is stopped, spring presses up on diaphragm to maintain system pressure. (Volvo)

Multipoint - EFI System (Part5. FUEL PRESSURE REGULATOR)

The fuel pressure regulator maintains a constant, preset pressure at the injectors. It is usually mounted on the fuel rail. After the fuel flows through the rail, it enters the pressure regulator. Extra fuel flows through an orifice in the regulator, into a return fuel line, and back to the tank.

Engine intake manifold vacuum is connected to the fuel pressure regulator. This allows the regulator to change fuel pressure with changes in engine load.

Fuel Pressure Regulator Construction

Figure below shows a cutaway view of a typical fuel pressure regulator. Study its construction. The basic parts of a fuel pressure regulator are:
FUEL INLET FITTING (allows fuel to enter pressure regulator from fuel rail).

a cutaway view of a typical fuel pressure regulator. Study its construction. The basic parts of a fuel pressure regulator are: 1. FUEL INLET FITTING (allows fuel to enter pressure regulator from fuel rail).

1. FUEL RETURN FITTING (allows excess fuel to flow out of rail and regulator and return to fuel tank).
2. CHECK VALVE (opens and closes to control fuel flow through regulator).
3. DIAPHRAGM (flexible disc that can move with changes in fuel pressure).
4. DIAPHRAGM SPRING (coil spring that pushes diaphragm toward fuel and closes check valve).
5. VALVE SEAT (attached to diaphragm, works with check valve to open and close fuel return).
6. VACUUM CHAMBER (allows engine vacuum to act on backside of vacuum diaphragm), Fig 2.18.
7. VACUUM FITTING (allows vacuum hose from in­take manifold to connect to vacuum chamber).

Vacuum chamber shown at top is sealed by diaphragm. It receives vacuum from intake manifold. When intake manifold vacuum is low (engine accelerating or under load) spring keeps bypass valve closed so more fuel is delivered to injectors. (Ford Motor Co.)

Fuel Pressure Regulator Operation

Whenever the electric fuel pump is operating, fuel flows into the regulator's pressure chamber from the fuel rail. The fuel, being under pressure, pushes on the regulator diaphragm. However, there is still not enough pressure to cause the return valve to open. Additional force must be supplied by engine vacuum.

When the engine is running, vacuum enters the vacuum chamber of the regulator and exerts a "pull" that, together with the force of the fuel in the opposite chamber, causes the diaphragm to flex and open the return valve. Excess fuel pressure is bled from the system to lean the fuel mixture. The excess fuel returns to the fuel tank. See Fig.Below.
Under rapid acceleration, the engine requires a richer mixture. The fuel pressure regulator is designed to help richen the mixture. This is what happens:
As the engine begins to accelerate, engine vacuum drops.
Since fuel pressure alone cannot keep the diaphragm flexed, it returns to its former position, closing the return valve.
This causes fuel pressure to build up higher to richen the mixture for more power.
Keep in mind that the computer and sensors are monitoring fuel mixture and other variables. They work with the pressure regulator to maintain the most efficient air-fuel ratio for the needs of the engine. Fig.Below shows a cutaway view of a fuel rail and its fuel pressure regulator. Note how the regulator acts to maintain fuel pressure in the rail and to the injectors.

Cutaway shows how vacuum affects regulator action. When engine vacuum is high (engine at low speed or idle) diaphragm flexes in direction of vacuum. This opens valve and allows fuel to bypass and return to fuel tank. At low vacuum (engine under load) diaphragm flexes down to close bypass valve and increase fuel pressure. (Honda Motor Co.)

Multipoint - EFI System (Part4. FUEL RAIL ASSEMBLY)

The fuel rail assembly, also called a fuel log, feeds fuel to all of the injectors, Fig. 3.11. The fuel pressure regulator and sometimes the cold-start injector attach to the fuel rail. The fuel rail can be a length of steel tubing or a cast metal block.

As shown in Fig.below, fuel enters the fuel rail from the electric fuel pump. Equal fuel pressure forms inside the rail and at the inlet to each injector. The pressure regulator bleeds off excess fuel to maintain the proper pressure in the system. This allows cool fuel to con­stantly circulate between the fuel rail and the fuel tank.
A service fitting is a threaded orifice for bleeding off pressure and for installing a pressure gauge. One is normally provided on the fuel rail. It is usually covered with a metal cap that keeps out dust and dirt.

Fuel rail assembly. Fuel pressure regulator and sometimes cold-start injector are considered part of this assembly. (Buick)

Electric fuel pump supplies fuel to fuel rail. Note that this rail consists of a length of tubing rather than a casting. (Fiat)

Multipoint - EFI System (Part3. GASOLINE INJECTION)

An injector for a gasoline injection system is simply an electrically operated fuel valve. When energized by the computer, it must open and produce a uniform fuel spray pattern in the intake manifold. When re-energized, it must close quickly, without leakage.

Injector Construction

Most modern injectors are made of metal and plastic. Rubber O-rings seal joints where parts fit together. Usual­ly, the injector fits into a hole machined into the intake manifold. However, as will be discussed in the next chapter, some systems have the injector in the throttle body assembly.

Cutaways show important components of an injector. (Chrysler)

Refer to this illustration.

1. ELECTRIC TERMINALS (electrical connection for circuit between injector coil and computer).
2. INJECTOR SOLENOID (armature and coil that opens and closes valve).
3. INJECTOR SCREEN (screen filter for trapping debris before it can enter injector nozzle).
4. NEEDLE VALVE (end of armature shaped to seal against needle seat).
5. NEEDLE SEAT (machined surface around the hole in end of injector against which the needle valve tip presses to form a seal).
6. INJECTOR SPRING (small spring that returns nee­dle valve to closed position).
7. O-RING SEAL (rubber seal that fits around outside of injector body and seals in intake manifold).
8. INJECTOR NOZZLE (injector outlet that produces fuel-spray pattern).

EFI injector operation. A-Current through injector coil builds magnetic field. Magnetism attracts and pulls up on armature to draw injector needle off its seat. Gasoline sprays out. B-Current flow stops when computer breaks circuit. Injector valve closes stopping fuel spray.

Gasoline Injector Operation

In a simplified way, Above picture illustrates the operation of a gasoline injector. When the computer sends current to the injector coil, the coil develops a magnetic field. Like an electromagnet, the field attracts and pulls on the injector armature. The armature moves up into the coil's field. The needle is then lifted off its seat and let’s fuel spray out the nozzle into the intake manifold.
When the computer shuts off current to the injector coil, the magnetic field collapses. This lets the injector spring push down on the armature forcing the needle against its seat. This blocks fuel flow.

Injector Pulse Width

Injector pulse width refers to the length of time or dura­tion that the injector is open. The computer controls injector pulse width. A long pulse width richens the fuel mixture because more fuel would spray into the intake manifold on each cycle. A short pulse width would lean the mixture because the injector would be kept closed longer between pulses.

Injector pulse width means the amount of time that computer sends current to injector to keep valve open. A-Short pulse causes less fuel spray because injector valve is not open long percentage of time. Mixture is leaner. B-Long pulse keeps valve open more of the time. Mixture is richer

Above picture illustrates short and long injector pulse widths. Note that the square sine wave (sine represent­ing voltage change) denotes the pulse width. When the wave moves up from zero, indicating voltage supply to the injector, the injector is open. When the wave moves back down to zero (base line), the injector is closed because there is no voltage and current flow.
When the square wave is shorter, Fig.A, the injector pulse width is shorter and the fuel mixture is leaner. When the square wave is longer, Fig.B, the pulse width is longer and the mixture is richer.
With many systems, the computer cycles the injec­tors open and closed several times a second. By chang­ing the percentage of ON and OFF times, it can control the air-fuel mixture ratio.

Multipoint - EFI System (Part2. ELECTRONIC CONTROL UNIT (COMPUTER))

The computer, or electronic control unit (ECU), is the "brain" of the fuel injection system. The sensors and wiring harness serve as the "nervous system": check­ing temperatures, positions, and other considerations for proper injection system operation. Fig below shows how electric current is fed to the computer from various sensors and how the computer feeds current to the injectors.
A car's computer is actually a preprogrammed micro­computer (preset, miniaturized electronic circuit). It has microscopic electronic circuits which are formed inside integrated circuits (ICs).
An integrated circuit is an electronic chip or circuit manufactured by photographically reducing a circuit and placing it on a special semiconductor (transistor type) material. This enables the computer to have literally hundreds or thousands of transistors, resistors, capacitors, and similar components in a very small space. Different circuits are provided in the computer for per­forming different functions.

Engine sensors send flow of information to electronic control module (on-board computer) in form of small electric currents. The module, acting on signal received, feeds current that operates injectors.

On-board computer contains thousands of miniaturized circuits.

The picture is showing a photo of the inside of an automobile computer. Note the very small components, especially the integrated circuits.
The on-board computer is about the size of a car radio and is often placed in the passenger compartment. Since computers are sensitive to vibration, extreme tempera­ture change, and moisture, they are sometimes located behind or under the dash panel, Fig.3.7. This places them away from engine heat, moisture, and the elements in the engine compartment.
There are four basic parts or sections to a car's com­puter: input/output devices, central processing unit, power supply, and memories.

Input/output Devices

The input/output devices are electronic circuits that convert signals from sensors into digital (on/off or com­puter) signals for use in the central processing unit (brain or calculator section) of the computer. The devices (cir­cuits) can also change computer language into electrical signals to operate system components.



On-board computer is usually behind instrument panel (dash). In this location, it is shielded from damaging engine heat and vibration. Some computers are mounted on air cleaner or elsewhere in engine or passenger compartment. (Cadillac)

Central Processing Unit

The central processing unit performs mathematical functions or logic functions to deliver the correct air-fuel ratio and to operate other system devices. It uses digital signals from the input devices to determine what is going on during vehicle operation and what should be done to increase efficiency.

Power Supply

The power supply in a car's computer prevents voltage fluctuations that could affect computer operation. A computer relies on very smooth dc current, mainly from the car battery. The power supply simply regulates in­put voltage to other parts of the computer.

Computer Memories

Most computers have three basic types of memory circuits: read only memory, random access memory, and programmable read only memory.

The read only memory (ROM) is programmed data that can only be analyzed by the computer itself. It is infor­mation used by the computer in performing the various functions. The ROM program cannot be changed. If the battery or voltage supply is disconnected from the com­puter, the data in the ROM will remain in the computer.

The random access memory (RAM) is temporary in­formation held in the computer. It is like a "note pad" of inputs and outputs. Data such as self-diagnosis codes can be pulled out of RAM. If battery voltage is removed, all information is erased from RAM.

The programmable read only memory (PROM) has in­formation on the particular make and model car. It has data about engine size, vehicle weight, transmission type, rear axle ratio, etc. As you will learn in the chapter on fuel injection service, the PROM is normally removed and reused when replacing the central processing unit (computer). If voltage is disconnected from the PROM, it will retain its information.

Multipoint - EFI System (Part1. FUEL INJECTION)

Fuel injection is a means of metering Fuel into an internal combustion engine. In modern automotive applications, fuel metering is one of several functions performed by an "engine management system". For gasoline engines, carburetors were the predominant method to meter fuel before the widespread use of fuel Injection. However, a wide variety of injection systems have existed since the earliest usage of the internal combustion engine. The primary functional difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on the vacuum created by intake air rushing through it to add the fuel to the air stream. The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from further back in the fuel supply, from a pump or a pressure container.

A modern gasoline injection system, using pressure from an electric fuel pump, sprays fuel into the engine intake manifold. See Fig below. Like a carburetor, it pro­vides the correct air-fuel mixture for specific engine operating conditions. However, PRESSURE, not engine vacuum (suction), feeds the fuel into the engine cylinders. This makes gasoline injection very efficient.

Gasoline Injection Advantages

Gasoline injection has several advantages over a car­buretor. The most important of these are:
Improved atomization. Fuel is forced into the intake manifold under pressure. This helps break the fuel into a fine mist.
Better fuel distribution. Flow of fuel vapors to each cylinder is more uniform.
Smoother idle. A leaner fuel mixture can be used without rough idle because of better fuel distribution and low-speed atomization.

Simplified cutaway shows action of most common fuel injection system for a gasoline engine.

1. Improved fuel economy. Efficiency is high because of more precise fuel metering, atomization, and distribution.
2. Lower emissions. Lean, efficient air-fuel mixture reduces exhaust pollution.
3. Better cold weather operation. Injection gives bet­ter control of mixture enrichment than a carburetor choke.
4. More engine power. Precise metering of fuel to each cylinder and increased air flow can produce more horsepower.
5. Simpler. Late-model, electronic fuel injection systems have fewer parts than modern computer controlled carburetor systems.

Gasoline Injection Timing

The timing of a gasoline injection system relates engine valve action to the time when fuel is sprayed into the engine intake manifold. There are three basic classifications of gasoline injection timing: intermit­tent, timed, and continuous. An intermittent gasoline injection system opens and closes the injection valves independently of the engine intake valves. This type of injection system may spray fuel into the engine when the valves are open or when they are closed.
Another name for an intermittent injection system is MODULATED injection system. This is one of the most common types of gasoline injection. A timed injection system squirts fuel into the engine right before or as the intake valves open. It is timed to the opening of the engine intake valves. The best example of timed injection is a diesel injection system.
A continuous gasoline injection system sprays fuel into the intake manifold all of the time. Anytime the engine is running, some fuel is forced out of the injec­tor nozzles and into the engine.
The air-fuel ratio is controlled by increasing or decreasing fuel pressure at the injectors. This increases or decreases fuel flow Out of the injectors. A con­tinuous type injection system is frequently used on several foreign cars and on a few American cars.

Note four subsystems of an electronic gasoline injection system (dashed line boxes). Sensor systems feed data to computer. Computer uses this data to operate fuel delivery system. Parts of air system can a/so be controlled by computer.

Fuel Delivery System

The fuel delivery system cleans and meters the right amount of fuel for varied driving conditions. It is made up of an electric fuel pump, fuel filter, fuel rail, pressure regulator, injectors, and connecting lines and hoses.
The electric pump forces fuel out of the tank, through the lines, and into the pressure regulator. When the com­puter energizes the injectors, fuel sprays into the engine. Extra fuel bleeds out of the pressure regulator and back to the tank.
Note: Many of the parts (fuel pump, filters, lines, etc.)

Air Induction System

An air induction system for Electronic Fuel Injection provides clean air and delivers it to the engine cylinders. This system, typically, consists of an air filter, throttle body, throttle valve, intake manifold, and connecting air ducts.

Design of air induction systems will vary depending upon engine and fuel system design. However, they all have an air filter for removing airborne impurities, a throttle valve for controlling air flow into the engine, and other related parts.

Sensor System

The Electronic Fuel Injection sensor system keeps track of engine operating conditions and passes this information to the computer. See Fig below. A typical Electronic Fuel Injection sensor system includes an oxygen (exhaust gas) sensor, engine coolant tempera­ture sensor, air inlet temperature sensor, throttle posi­tion sensor, intake manifold pressure (vacuum) sensor, and other sensors.
Just as your body can sense a change in its surround­ings, (touching a hot stove for example), the sensor system can sense a change in the operation of many vehicle systems. It enables the modern fuel injection system to CHECK and CORRECT itself.

These diagrams summarize computer control system. A-Computer or ECU inputs. B -ECU (electronic control unit] outputs of various system parts. (Renault)

Computer Control System

The computer control system processes information and controls operation of the Electronic Fuel Injection system and certain other automobile systems.
A computer wiring harness carries current to and from the computer. Sensors from the engine, transmission, air conditioning system, and other systems send elec­trical information to the computer through the harness. Then, the computer can operate the injectors, transmis­sion, ignition system, emission control systems, and other components for maximum efficiency.

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