Fuel injection systems

General information

Fuel injection systems have been used on vehicles for many years. The earliest ones were purely mechanical. As technology advanced, electronic fuel injection systems became more popular. Early mechanical and electronic fuel injection systems did not use feedback controls. As emissions became more of a concern, feedback controls were adapted to both types of fuel injection systems.

Both mechanical and electronic fuel injection systems can be found on gasoline engines. Diesel engines are most commonly found with mechanical type systems, although the newest generations of these engines have been using electronic fuel injection. Following is a description of the most common fuel injection systems.

Multi-port fuel injections
See Figures 21 and 22

This is the most common type of fuel injection system found today. Regardless of the manufacturer, they all function in the same basic way. On these systems an equal amount of fuel is delivered to each cylinder.

These systems all use sensors which transmit operating conditions to the computer. Information from these sensors is processed by the computer which then determines the proper air/fuel mixture. This signal is sent the to fuel injectors which open and inject fuel into their ports. The longer the injector is held open, the richer the fuel mixture. Most fuel injection systems need the following information to operate properly:

1. Temperature sensors - this includes both air and coolant temperature. The computer uses this information to determine how rich or lean the mixture should be. The colder the temperature, the richer the mixture.
2. Throttle position sensors or switches - the computer uses this information to determine the position of the throttle valve(s). Some vehicles use sensors which relay the exact position of the throttle valve(s) at all times. Others use switches which only relay closed and wide-open throttle positions (some may also use a mid-throttle switch). These switches and sensors help determine engine load.
3. Airflow sensors - these sensors also help the computer determine engine load by indicating the amount of air entering the engine. There are several different types of airflow sensors, but in the end, they all do the same job.
4. Manifold pressure sensors - if a vehicle is not equipped with an airflow sensor, it uses a manifold pressure sensor to determine engine load (Note that some vehicles with an airflow sensor may also have a manifold pressure sensor. This is used as a fail-safe if the airflow sensor fails). As engine load increases, so does intake manifold air pressure.
5. Engine speed and position sensors - engine speed/position sensors can be referenced from the crankshaft, camshaft or both. In addition to helping determine engine load, these sensors also tell the computer when the injectors should be fired.

These systems operate at a relatively high pressure (usually at least 30 psi). To control the fuel pressure, a fuel pressure regulator is used. As engine load increases, more fuel pressure is needed. This is due to the richer mixture (more fuel needed) and to overcome the increased air pressure in the ports. Any unused fuel is diverted back to the fuel tank using a return line.

The fuel injectors can be fired as a batch, a bank or sequentially. On batch fire systems, all of the injectors are fired simultaneously, usually at top dead center of the compression stroke for cylinder number one. Bank fire systems are divided into two separate injector banks. The first bank fires when cylinder number one is at top dead center of the compression stroke. The second bank is usually fired when the number one cylinder is at top dead center of the exhaust stroke. On sequential systems, each injector is fired as its cylinder is at top dead center of its compression stroke. This tends to be the most fuel efficient system.

Feedback fuel injection systems use an oxygen sensor to precisely monitor the air/fuel mixture. Using the signal generated by the oxygen sensor, the computer varies the pulse width of the fuel injectors. The longer the injector on time (longer pulse width), the richer the fuel mixture.

Central multi-port injection
See Figures 21, 23 and 24

This system is very similar to the standard multi-port injection system. The main difference lies in the location and construction of the fuel injector(s). Instead of an injector positioned at each intake manifold port, the injector(s) are centrally located in the intake manifold plenum assembly (hence the name central multi-port).

The main component of the system is the fuel meter body. This houses the fuel injector(s), pressure regulator and poppet nozzle/hose assemblies. A hose with a poppet valve extends from the bottom of the fuel injector(s). These hoses are routed to the individual cylinders. The poppet valves handle the atomization of the fuel rather the injector itself as in standard multi-port systems.

Early systems use one fuel injector for all the cylinders and are batch fired. Later systems use an injector for each cylinder and are fired sequentially.

Throttle body injection
See Figure 25

The appearance of throttle body injection system is similar to the carbureted fuel system. Although not as efficient as multi-port systems, it does offer better driveability and lower emissions than carbureted systems.

The fuel injector(s) are mounted vertically above the throttle plate(s). The throttle body assembly also houses the fuel pressure regulator. These systems typically run at lower pressure compared to multi-port systems. This is mostly due to the fact that pressure in the intake manifold does not have to be overcome. Since the injector(s) is mounted above the throttle plate, fuel is actually drawn into the intake system. Other than this, the actual operation of the throttle body injection system is similar to the multi-port system.

Bosch continuous injection systems
See Figure 26

CIS System

The Continuous Injection System (CIS) is an independent mechanical system. The basic operating principle is to continuously inject fuel into the intake side of the engine by means of an electric pump. The amount of fuel delivered is metered by an air flow measuring device. Some CIS systems are feedback controlled.

The primary fuel circuit consists of an electric pump, which pulls fuel from the tank. Fuel then passes through an accumulator. The accumulator is basically a container in the fuel line. It houses a spring-loaded diaphragm that provides fuel damping and delays pressure build-up when the engine is first started. When the engine is shut down, the expanded chamber in the accumulator keeps the system under enough pressure for good hot restarts with no vapor locking. Fuel flows through a large, paper element filter to the mixture control assembly.

The mixture control assembly is the heart of the CIS system. It houses the airflow sensor and the fuel distributor. The air sensor is a round plate attached to a counterbalanced lever. The plate and lever are free to move up and down on a fulcrum. Accelerator pedal linkage connects to a throttle butterfly, which is upstream (closer to the manifold and intake valves) of the air sensor. Stepping on the accelerator pedal opens the throttle valve. Increased air, demanded by the engine, is sucked through the air cleaner and around the air sensor plate.

In the air funnel, where the air sensor plate is located, the quantity of intake air lifts the plate until an equilibrium is reached between air flow and hydraulic counter-pressure acting on the lever through a plunger. This is the control plunger. In this balanced position, the plunger stays at a level in the fuel distributor to open small metering slits, one for each cylinder in the engine. Fuel under controlled pressure from the pump goes through the slits to the injectors' supply opening. The slit meters the right amount of fuel.

In order to maintain a precise fuel pressure, a pressure regulator, or pressure relief valve, is located in the primary fuel circuit of the fuel distributor. Excess fuel is diverted back to the tank through a return line. To make sure the amount of fuel going through the control plunger slits depends only on their area, an exact pressure differential must always be maintained at the openings. This pressure is controlled by a differential-pressure valve. There's one valve for each cylinder. The valve consists of a spring loaded steel diaphragm and an outlet to the injectors. The diaphragm separates the upper and lower chambers.

The valve keeps an exact pressure differential of 1.42 psi between upper chamber pressure and lower chamber pressure. Both pressures act on the spring loaded steel diaphragm which opens the outlet to the injectors. The size of the outlet opening is always just enough to maintain that 1.42 psi pressure differential at the metering slit. The diaphragm opens more if a larger amount of fuel flows. If less fuel enters the upper chamber, the diaphragm opens less and less fuel goes to the injectors. An exact pressure differential between upper and lower chamber is kept constant. Diaphragm movement is actually only a few thousandths of an inch (few hundredths of a millimeter). On feedback controlled CIS systems, a frequency valve regulates the pressure differential at the metering slits and as a result is able to control mixture ratio. The frequency valve uses a signal from a control unit which is generated by an oxygen sensor.

The control pressure regulator can alter the pressure on the control plunger according to engine and outside air temperature. For warm-up running, it lowers the pressure so that the air sensor plate can go higher for the same air flow. This exposes more metering slit area, and more fuel flows for a richer mixture. For cold starts, a separate injector is used to squirt fuel into the intake manifold. This injector is electronically-controlled. A thermo-time switch, screwed into the engine, limits the amount of time the valve is open and at higher temperatures, cuts it off.

CIS-E Systems

CIS-E is an electronically-controlled continuous fuel injection system. This system utilizes the basic CIS mechanical system for injection, with electrically-controlled correction functions. The electronic portion of the system consists of an airflow sensor position indicator, coolant temperature sensor, throttle valve switches, idle air stabilizer and the differential pressure regulator.

When the ignition switch is turned on, the electric fuel pump is activated causing pressurized fuel to move from the tank to the accumulator. Fuel pulsations exerted by the fuel pump are then damped or smoothed out by the accumulator. The pressurized fuel is directed through the fuel filter and to the fuel distributor. A differential pressure regulator located on the side of fuel distributor is used to control the air/fuel mixture. The control pressure regulator is not used in the CIS-E fuel injection system. The system pressure regulator valve has been removed from the fuel distributor and replaced by an external, diaphragm type, pressure regulator. This regulator contains an additional port which is used to return fuel from the differential pressure regulator.

The differential pressure regulator is an electromagnetically-operated pressure regulator. It receives an electronic signal in milliamps from the control unit. The higher the milliamp signal the higher the differential between the upper and lower chamber pressures, resulting in a richer mixture. The lower the milliamp signal the lower the differential pressure resulting in a leaner mixture.

In the CIS-E fuel injection system, system pressure is always present in the upper chamber of the fuel distributor. The metering slit in the control plunger regulates the amount of fuel delivered to the upper chamber depending on the airflow sensor position and control plunger position. The amount of fuel delivered to the injectors and consequently fuel mixture, is adjusted by the differential pressure regulator.

Diesel fuel injection
See Figures 27, 28 and 29

There are both electronic and mechanical types of injection found on diesel engines; all are multi-port in design. The electronic types function essentially the same as the gasoline multi-port fuel injection system. There are four main types of mechanical diesel injection systems:

1. Inline or rotary distributor pump
2. Individual control pump
3. Common rail
4. Unit injection

The inline or rotary distributor pump is one of the most common types of diesel injection found. This type of pump pressurizes and distributes fuel for each of the cylinders. Fuel from the fuel filter flows from the transfer pump (usually a vane type pump attached to the distributor pump assembly) to the distributor pump itself. The fuel is pressurized inside the distributor pump to approximately 1800 psi (12,411 kPa). This high-pressure fuel is then directed to an injector at the appropriate cylinder. The injector atomizes the fuel for proper combustion.

The individual control pump systems use a separate high-pressure pump and metering unit for each cylinder. The high pressure pumps are fed fuel from a transfer pump. The plungers have helix cut grooves which allow them to meter fuel. By rotating the plunger, the effective stroke is changed and the amount of fuel fed to injectors is metered. The plungers themselves are cam operated. The injector atomizes the fuel for proper combustion.

On common rail systems, a high-pressure pump feeds fuel to the injectors through a common rail. The injectors are actuated by a cam, pushrod and rocker arm assembly. The amount of fuel delivered depends on how long the injector is open. A wedge mechanism varies the effective length of the pushrod, which controls how long the injector is open. The injector also atomizes the fuel for proper combustion.

Unit injections systems also use an individual injector for each cylinder. The injector also contains a high-pressure pump and metering assembly, fed by a transfer pump. The injectors are cam operated. Rotating the pump plunger in the injector meters the amount of fuel delivered to the cylinder. In addition, the injector atomizes the fuel for proper combustion.

Rochester mechanical fuel injection
See Figure 30

The first hurdle is understanding the design of this fuel injection system. This is best done by thinking of the unit as three separate systems, interlocked to accomplish a common function. The first system is the air meter and this simultaneously furnishes the fuel meter with an assessment of the load demands of the engine and feeds air to the intake manifold. The intake manifold is designed to ram charge the air as it distributes it to the cylinders. The fuel meter evaluates the air meter signal and furnishes the correct amount of fuel to the nozzles where it is injected into the engine.

Air meter
See Figure 31

The air meter consists of three sub-components: the throttle valve, cold enrichment valve and diffuser cone assembly, all of which are contained within the meter housing. Later, air meters were modified to the extent that a choke piston was added and the choke valve stop was relocated in the diffuser cone. This allows an initial choke opening of 10 that increases to 30 after an initial cold start. The throttle valve regulates the flow of air into the manifold and is mechanically actuated by the accelerator pedal. The diffuser cone, suspended in the bore of the air meter inlet, functions as an annular venturi and accelerates the airflow between the cone and the meter housing. The air meter houses the previously mentioned components plus the idle and main venturi signal systems.

The main venturi vacuum signals are generated at the venturi as the incoming air rushes over an annular opening formed between the air meter body and piezometer ring. They are then transmitted through a tube to the main control diaphragm in the fuel meter. The venturi signal measures the flow of air into the engine and automatically controls the air/fuel ratio. The one exception to this is at idle speeds.

Idle air requirements are handled differently by the fuel injection method. Approximately 40% of the idle-speed airflow enters the engine through the nozzle block air connections tapped into the air meter body. Part of the remaining 60% flows past the throttle valve, which is preset against a fixed stop. The remainder enters the idle air by-pass passage that is controlled by the large idle-speed adjusting screw. Idle speed is adjusted by turning this screw in or out.

Fuel Meter

The fuel meter's float-controlled fuel reservoir is basically the same as that found in conventional carburation. The fuel meter receives fuel from the regular engine fuel pump. The incoming fuel is routed through fuel filter before entering the main reservoir of the fuel meter, where the high-pressure gear pump picks it up. This high-pressure spur-gear type pump is completely submerged in the lower part of the fuel meter main reservoir. A distributor-powered, flexible shaft drives the pump at one-half engine speed. Fuel pressures span a range of near zero to 200 psi, according to engine speed. Fuel not used by the engine reenters the fuel meter through a fuel control system. Some fuel meters contain a vent screen and baffle which helps to stabilize the air/fuel mixture.

Fuel Control System
See Figure 32

The fuel control system regulates fuel pressure (flow) from the fuel pump to the nozzles. This flow is controlled by the amount of fuel that is spilled or recirculated from the high-pressure pump, through the nozzle block and back to the fuel meter spill ports. This is accomplished by a three-piece spill plunger or disc that is located between the gear pump and the nozzles.

When high fuel flow is required, it moves downward, closing the spill ports to the fuel meter reservoir and concentrating the flow to the nozzle circuits. Correspondingly, the spill plunger or disc must be raised to allow the spill ports to be exposed when a low fuel flow is required. This causes the main output of the gear pump to by-pass the nozzles circuits and reenter the meter reservoir through the now opened spill ports.

The accelerator pedal does not mechanically control the spill plunger. Fuel control is accomplished by a precisely counterbalanced linkage system sensitive to fuel pressure and diaphragm vacuum. Thus the slightest change in venturi vacuum signal on the main control diaphragm will activate the linkage. One end of the fuel control lever pivots on the roller end of an arm called the ratio lever. When the increased vacuum above the diaphragm forces the control lever upward, the lever pivots on the ration lever's and pushes the spill plunger or disc downward. This closes the spill ports and steps up fuel flow to the nozzles. When decreased vacuum above the diaphragm reverses the pivot action, fuel pressure forces the spill plunger upward and permits the spill ports to by-pass fuel into the reservoir, thus fuel flow to the nozzles is reduced.

The diaphragm vacuum-to-fuel pressure ration, and subsequent air/fuel ratio, is regulated by the position of the ratio lever. As the ratio lever changes position, the mechanical advantage of the linkage system also changes, thus providing the correct air/fuel ration for each driving condition. As long as engine manifold vacuum exceeds 8 in. Hg, the ratio lever remains at the economy stop and fuel flow follows the dictates of the main control diaphragm vacuum. A sudden decrease in manifold vacuum moves the ratio lever to the power stop. The resulting increase in the mechanical advantage of the linkage system closes the spill ports and increases fuel flow to the nozzles.

Kugelfischer mechanical fuel injection
See Figure 33

In the Kugelfischer mechanical fuel injection system, fuel and air are inducted separately through the injection pump and the throttle manifold butterfly. Fuel is injected into the intake manifold behind the open intake valve under high pressure.

The electric fuel pump pumps fuel from the tank through a fine-mesh filter in the tank and a filter in the fuel line. The fuel flows through the expansion container, the main fuel filter, and into the injector pump. Excess fuel and any air bubbles are routed back to the tank via a return line. This ensures that the fuel is always kept cool and free of bubbles.

The injection pump camshaft is belt-driven from the engine crankshaft. Four pumping pistons, operating in firing order sequence, inject the required amount of fuel. The amount of fuel injected depends on engine load and speed.

Fuel injection volume is regulated by engine load. The accelerator pedal is connected with throttle butterfly and the lever on the injection pump. When the pedal is depressed, the throttle butterfly moves and the stroke length of the pump piston is governed by the regulating cam, depending on throttle opening. Fuel injection volume is also regulated by engine speed. The stroke of the pump piston is governed by the injection pump governor.

When the engine is started, fuel is injected into the intake manifold by a solenoid valve. The duration time of injection depends on the coolant temperature.

When the injection pump pressure reaches approximately 435-551 psi, each injection valve opens. Intake air flows through the air cleaner and the throttle manifold butterfly to the manifold plenum chamber, and from there through the 4 manifold branches to the combustion chambers.

Fuel system maintenance

The major components of the fuel system are usually quite reliable in themselves. Fuel system maintenance basically consists of a routine visual inspection and fuel filter replacement. If equipped, the fuel/water separator should be periodically drained.

System inspection
See Figure 34

The fuel system should be routinely inspected for leaks. Check all the fuel lines for cracks, leaks and deformation. Fittings are usually the most common points for leaks to develop. Leaks sometimes also develop at the fuel injectors as the sealing O-rings age. Any type of damage or leaks should be fixed immediately.

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