GC-Prevue (Free Gerber Viewer)

GC-Prevue is CAD/CAM software designed to help you work more efficiently with DPF, RS-274 (often called "Gerber") photoplotter and NC drill CAD output data, with a particular emphasis on Printed Circuit Board (PCB) applications. GC-Prevue also supports HPGL and Quest (Marconi Emma) plotter formats.
By verifying your CAD data from the photoplotter's point of view you will save time, money and headaches due to miscommunication with your photoplotting service and fabrication facilities.

CoolPack (A Collection of Simulation Tools for Refrigeration)

CoolPack is a collection of simulation programs that can be used for designing, dimensioning, analyzing and optimizing refrigeration systems.



The simulation programs in CoolPack are divided into six categories - each represented by a tab in the Toolbar above. You can get an overview of the programs in a category by clicking on its Toolbar tab. Clicking on the icons in the Toolbar starts the individual programs.

The CoolPack program is freeware. For downloading the CoolPack tutorial please go to the tutorial page and download it. To install CoolPack you just run the downloaded file: Download CoolPack version 1.46 (app. 12 MB): CoolPack.exe.

You can also download Refrigeration Utilities as a "stand alone" program. The program is self-extracting. When you run this file, all the setup files will be extracted to a temporary folder. If you like, the dialog box gives you a possibility to change the name of the temporary folder. From the temporary folder you should run the SETUP.EXE file to install Refrigeration Utilities.

Download Refrigeration Utilities version 2.84 (app. 2.7 MB) : RefUtil.exe

Multipoint - EFI System (Part10. OPEN AND CLOSED LOOP)

The terms open loop and closed loop refer to the operating mode of a computerized fuel system. When an engine is cold, the computer operates open loop (no feedback from sensors). After the engine warms to operating temperature, the system "kicks in" to closed loop (uses feedback from sensors) to control system operation.

Open Loop Operation

As was mentioned earlier, an oxygen sensor must heat up to several hundred degrees before it will functionProperly. This is the main reason computer systems have an open loop mode. The computer has preprogrammed information (injector pulse width, injector timing, air bypass valve position) that will keep the engine running satisfactorily while the oxygen sensor is warming up. When the engine and oxygen sensor are cold, no information flows to the computer. The computer ignores any signals from the sensors. The "loop of information" is open.

Closed Loop Operation

After the sensor and engine are warm, the oxygen sensor, and other sensors, begins to feed data to the com­puter. This forms an "imaginary loop" (closed loop) as electrical data flow from the engine exhaust, to the oxygen sensor, to the computer, to the injector, and back to the oxygen sensor. Normally, the computer system functions closed loop to analyze the fuel mixture provided to the engine. This lets the com­puter "double-check" itself.

Drawings show difference between closed I and open loop mode of computer operation.

Analog and Digital Signals

The signal from the engine sensors can be either a digital or analog type output. The out­put from the computer can also be analog or digital. Digital signals are instant on-off signals. An example of a sensor providing a digital signal is the crankshaft position sensor which shows engine rpm. Voltage out­put or resistance goes from maximum to minimum, like a switch, to report rpm in number form.

An analog signal progressively changes in strength. For example, sensor internal resistance may smoothly. In­crease or decrease with temperature, pressure, or part position. The sensor acts as a variable resistor.

A-Digital signal is ON-OFF signal, like from wall light switch. B-Analog signal steadily increases or decreases voltage signal. It is not an instant ON-OFF signal.

Multipoint - EFI System (Part9. ENGINE SENSORS)

An engine sensor is a device that changes resistance or voltage output with a change in a condition such as temperature, position, movement, etc. A modern elec­tronic fuel injection system uses numerous sensors to improve efficiency. An Electronic Fuel Injection system might use many of the sensors.

In the Fig below how’s some of the conditions sensed by one computer system. Note that it checks everything from charging voltage to air conditioning operation.

Throttle position sensor

A throttle position sensor is a variable resistor or multi position switch connected to the throttle valve shaft. When the driver presses on the gas pedal for more power, the throttle shaft and sensor are rotated. This changes the internal resistance of the sensor. The change in current signals the computer and the computer can alter the air-fuel ratio as needed. One type of throttle position sensor is illustrated in Fig.3.29. Note how it mounts on the throttle body and that it has several contacts to change output resistance.



Sensors, located in many different locations on engine, may be used to feed information to computer.



Note typical conditions sensed and controlled by electronic means. (Buick)



Throttle position sensor. A-Sensor is mounted on throttle body over throttle shaft. B-View of inside of sensor. As throttle shaft rotates, it turns plate which alters internal circuit connections and resistance, sending electric current signals to computer. (Toyota)

Engine Coolant Temperature Sensor

An engine coolant sensor monitors the operating temperature of the engine. It is mounted so that it is exposed to the engine coolant.
When the engine is cold, the sensor might provide a high current flow (low resistance). The computer would adjust for a richer air-fuel mixture for cold engine opera­tion. When the engine warms, the sensor would supply information (high resistance for example) so that the computer could make the mixture leaner.



Water temperature sensor usually is threaded into opening in block or head where it will be in contact with coolant.

Inlet Air Temperature Sensor



Air flow meter shows air temperature sensor. Sen­sor uses thermistor which is extremely sensitive to temperature changes. As temperature rises, resistance of thermistor decreases. (Subaru)

An inlet air temperature sensor measures the temperature of the air entering the engine, Cold air is dense than warm air, requiring a little more fuel. Warm air is NOT as dense as cold air, requir­ing a little less fuel. The air temperature sensor helps the computer compensate for changes in outside air temperature and maintain an almost perfect air-fuel ratio.

Charge Temperature Sensor



unlike air temperature sensor which senses only coldness of air, charge temperature sensor checks temperature of the air-fuel mixture. It is located in intake port just before intake valve. (Chrysler)

A charge temperature sensor, similar to an inlet air temperature sensor, measures the temperature of the air-fuel mixture. It is installed in the intake port, in front of the engine intake valve.

Crankshaft Position Sensor

A crankshaft position sensor is used to detect engine speed. It allows the computer to change injector open­ings as engine speed changes. Higher engine speed generally requires more fuel.
This sensor can be located on the front, rear, or center of engine. Its tip is close to the crank so that it can sense the teeth or notches as they rotate past the sensor. The magnetic field around the sensor, and current flow through the sensor, change as the crank rotates, allow­ing the computer to measure engine rpm.

Flap Air Flow Sensor

A flap air flow sensor measures the air flow into the engine. This helps the computer determine how much fuel should be injected into the intake manifold.
The air flow sensor usually mounts ahead of the throt­tle body assembly in the air inlet duct system.
In the Fig. below shows how a typical air flow sensor operates. At idle, the sensor flap is nearly closed. Sen­sor resistance stays high. This tells the computer that the engine is idling and needs very little fuel.



Flap air flow sensor tells computer whether engine is idling or at higher speed by measuring amount of air moving into engine. A -At idle speed, air flap is nearly closed. Responding to small current, computer produces short injection pulse width for small amount of fuel. B-Throttle open for more power, air flap swings open and sends strong signal to computer. Computer then sends wide pulse width to injectors for richer fuel mixture. C-Potentiometer is attached to sensor shaft. Wiping arm moves across potentiometer as flap moves. Depending on flap position, potentiometer will send weak or strong signal to electronic control unit.

As engine speed and air flow increase, air forces the flap to swing open. This moves the variable resistor to the low resistance position. The increased current flow now tells the computer that more air is flowing into the engine. The computer then increases injector pulse width as needed.
In the Fig. below shows how the flap type air flow sensor and potentiometer (variable resistor) are connected. Note that a weak spring is used to return the flap to the closed, idle position.



Cutaway shows how air sensor flap connects to and controls potentiometer. Strength of signal to computer depends on where wiping arm rests on resistor of poten­tiometer. (Volkswagen)

Mass Air Flow Sensor

A mass air flow sensor performs about the same func­tion as a flap type sensor, but it sends more precise information to the computer. In the Fig. below is a newer type sensor found on some late model cars.
Basically, the mass air flow sensor uses a small elec­trically energized, resistance wire to detect air flow. The wire's temperature drops as air flows over it. The greater the air flow, the lower its temperature. The drop in temperature changes the wire's resistance, signaling the computer of more air intake. The opposite is true for low air flow.
A mass air flow sensor, sometimes called a "hot wire" sensor, is desirable because it automatically compen­sates for changes in air temperature and atmospheric pressure. It eliminates the need for an air temperature sensor and air pressure sensor.



Modern electronic, multi-point fuel injection system. Mass air flow sensor is located in duct ahead of engine. It detects air flow volume with fine resistance wire. (Chevrolet)



Mass air flow sensor with resistance wire. Wire is heated by electric current and is very sensitive to temperature. The greater the air flow past it, the lower its temperature becomes. The lower its temperature, the more the signal to computer changes. (Chevrolet)

Oxygen sensor



Oxygen sensor detects amount of oxygen in engine's exhaust. (Oldsmobile)

The oxygen sensor or exhaust gas sensor measures the oxygen content in the engine's exhaust gases. The oxygen content is an excellent indicator of whether the air-fuel mixture is too rich or too lean. The oxygen sensor is one of the most important sensors in modern electronic fuel injection systems. It actually checks the efficiency of the fuel system with the engine running.

Oxygen Sensor Construction

The oxygen sensor has a special ceramic core made of zirconium dioxide. The surface of the ceramic core is coated with platinum. The coated ceramic core has the ability to produce a voltage output when exposed to heat and a difference in oxygen levels on each side of the ceramic element. The ceramic voltage-producing device is enclosed inside a metal housing. Terminals are provided for connecting the sensor to the computer wiring harness.



Oxygen sensor is placed in exhaust system either at exhaust manifold or in pipe leading from exhaust manifold, as shown here. (Renault and AMC)



Cutaway of exhaust sensor. Note how it operates. Graph at right shows how oxygen content changes voltage of sensor's signal to computer. (Fiat)

Oxygen Sensor Operation

When the sensor is cold, it produces no voltage. The system then operates on data preprogrammed into the computer. When the oxygen sensor is heated above about 300 °F (149°C), it begins to produce a voltage signal.
When the fuel mixture is too rich, there is a small amount of oxygen in the engine exhaust gases. This produces a large difference in the oxygen levels on each side of the ceramic sensing device. Negative oxygen ions flow through the ceramic device and a voltage output is produced for the computer, Fig.3.41A. About a ONE VOLT signal is fed to the computer. The computer can then shorten injector pulse width to lean the air-fuel mixture slightly.
When the engine's fuel mixture is too lean, there is an excess amount of oxygen in the engine exhaust. This reduces the difference in the oxygen levels on each side of the sensor's ceramic element. Very few oxygen ions flow through the sensor and the sensor's voltage out­put drops to a fraction of a volt. this signals the computer to increase injector pulse width. This helps maintain an almost perfect air-fuel mixture.



A -Small amount of oxygen causes high voltage signal from sensor. B - Large amount of oxygen reduces signal strength. (Volkswagen)

Idle Speed Control

Most Motronics control idle rpm by a combination of the idle-speed stabilizer and ignition timing. The stabilizer is de­scribed in the section on LH-Jetronic. Inputs include rpm, closed-throttle signal, and engine temperature. The control unit sends on-off or digital signals to the idle-speed stabilizer. Early Motronics use the auxiliary air valve to increase air flow during warm-up, also described in the L-Jetronic section. In these, cold-engine idle rpm is increased according to temperature; it is an open-loop system.



Idle-speed control by idle air bypass. Idle-speed stabilizer handles coarse rpm corrections.

Timing Signals: RPM, TDC

For the most accurate measure of engine timing and speed, Multi – pint EFI systems read the position of the crankshaft directly, instead of from the ignition system as in L-Jetronic. Special sensors, shown in Fig. below, pick up signals from the flywheel teeth. Taking RPM and TDC timing signals from the crankshaft avoids inaccuracies from gear-lash or belt-drive such as when rpm and timing are determined in a camshaft-driven distributor, causing "spark scatter".

The rpm sensor (also called the engine-speed sensor) is an inductive-pulse sender that picks up pulses from a toothed wheel, usually the flywheel. The rpm signal can be displayed on a scope just as it is sent to the control unit, one blip or spike for each tooth as shown in Fig. below. It is so accurate it can sense an rpm change while the crankshaft turns only a few degrees.



RPM sensor (1) sends engine-speed signals from flywheel teeth; TDC sensor (2) sends one pulse per passing of set screw (arrow) each crankshaft revolution.

The TDC, or reference-mark sensor (reference from cylinder 1 TDC), is triggered by a set screw on the flywheel. Each time the screw passes the TDC sensor, the sensor signals one blip for each crankshaft revolution as shown in Fig. below. Both sensors are magnetic, with a soft iron core that stores the magnetic field. When a tooth in the flywheel or the reference pin moves through the magnetic field, the change induces an electrical voltage in the winding. This voltage is the input signal to the control unit. The sensor is known as a passive diffusion-field sensor because it does not require a current supply.



RPM sensor scope pattern shows one pulse per flywheel tooth



TDC sensor scope pattern shows one pulse per crankshaft revolution

One of these flywheel sensors provides input of rpm to the control unit. The other sensor provides input of TDC reference. The air flow sensor provides input of engine load. From the control unit ROM, an output signal to the coil primary sets timing advance and dwell for the next spark firing. Some engines operate with only one sensor that combines both functions, using a toothed timing wheel instead of the flywheel. Two missing teeth signal TDC, as shown in Fig. below.



In some Motronics, special timing wheel replaces starter gear ring or is mounted on front of crank­shaft. Single sensor picks up rpm from teeth, and picks up TDC from gap in teeth.

Other Sensors

Other sensors besides those just covered can be used in a computer control system. Some of them include sen­sors checking the operation of the transmission, air con­ditioning system, brake system, and emission control systems. When information is required on any of these sensors, refer to a service manual. It will give the exact details of the specific system. It will explain its opera­tion and how the sensor should be tested or serviced.

Multipoint - EFI System (Part8. AIR INLET DUCTS)

Air inlet ducts on most fuel injected engines carry air from the air cleaner to the throttle body. Sometimes, the air flow sensor is mounted in this duct. The duct is usually made of plastic to reduce weight.

Clamps at duct fittings prevent leakage and keep air­borne dirt out of the engine. Where ducts are placed between the air flow sensor and the intake manifold leakage could upset the air-fuel ratio. Fig.3.25 also shows where to look for components just discussed in the chapter. Note the locations of the fuel pressure regulator, fuel rail or manifold, throttle cable, throttle body, air bypass valve, cold start valve, and air flow sensor.

This air intake uses flexible duct to move air from air cleaner into intake manifold. Note location of air flow sensor, Leaks in duct could upset air-fuel ratio. (Fiat)

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.