It is very important to supply clean fuel to the fuel rail in EFI systems. Small particles of dirt can block an injection nozzle and cause an irregular spray pattern.
Any water in the fuel will corrode the inside of the injector - especially if the engine stands unused for long periods.
The first filtering occurs with a strainer or fine gauze in the fuel tank.
The next time filtering occurs at the in-line filter, on the high-pressure side of the pump. This is a large-capacity filter encased, in a steel shell or an aluminium housing. This housing must be rigid, to withstand the high pressures in the system.
The filter is a pleated paper type with pore size of about 10 microns. A fluted support plate keeps the filter stable in the housing. The filter is directional, and it must be fitted in the direction of fuel flow.
Final filtering occurs with a small conical filter at the fuel entry to the injector
Where the tank is mounted depends on where the engine is, and on space and styling. Safety demands that it is positioned well away from heated components, and outside the passenger compartment.
Most tanks are made of tinned sheet steel that has been pressed into shape.
Some passenger car tanks are made of non-metallic materials. Aluminium or steel is used on commercial vehicles.
The tank is usually in 2 parts, joined by a continuous weld around the flanges where the parts fit together. Baffles make the tank more rigid.
They also stop surging of fuel and ensure fuel is available at the pickup-tube.
Fuel expands and contracts as temperature rises and falls. So fuel tanks are vented to let them breathe.
Modern emission controls prevent tanks being vented directly to the atmosphere. They must use evaporative control systems.
Vapor from the fuel tank is trapped in a charcoal canister, and stored there, until it is burned in the engine. A vapor or vent line with a check valve connects the space above the liquid fuel with the canister. This valve opens when pressure starts to rise, and lets vapor through, but not liquid.
Liquid fuel closes the check valve and blocks the line, stopping liquid fuel reaching the charcoal.
Some systems have a small container, called a liquid-vapor separator, above the fuel tank. It also prevents liquid fuel reaching the charcoal.
This gasoline tank has a small separator tank and a number of vents.
They’re from different parts of the tank so that for as many vehicle positions as possible, at least one is always above the level of gasoline in the tank.
Fuel lines are usually made of metal tubing or synthetic materials.
A fuel supply line carries fuel from the tank to the engine. A return line may also be provided to allow excess fuel to return to the tank. This helps prevent the formation of vapor that can occur in the fuel supply during hot conditions.
The fuel filler is where fuel enters the system. The filler neck is a pipe that extends above the fuel tank.
On unleaded gasoline vehicles with catalytic converters, the filler neck is designed to prevent leaded fuel being added. Its diameter is smaller than those on leaded vehicles, and a trapdoor inside the filler can only be opened by the nozzle of the unleaded gasoline spout.
The filler cap seals the filler neck to keep out water and foreign objects. Water can corrode internal passages of fuel pumps and carburetors.
The cap on the gasoline tank also stops gasoline vapor escaping and polluting the atmosphere. This is important for gasoline which is very volatile and vaporizes easily, especially in warmer climates.
Some gasoline caps have a low pressure valve built-in. It keeps a balance between the pressure in the tank and the outside atmospheric pressure.
As gasoline in the tank is used, the air space above the fuel increases. This causes a fall in pressure, compared to outside atmospheric pressure. The valve then opens and lets more air into the tank.
A fall in temperature can cause a fall in the pressure in the tank. The valve opens to admit more air till inside and outside pressures equalize again.
The fuel gauge shows how much fuel is still in the tank. It has a gauge unit on the dash, and a sender unit in the tank. This unit indicates the level of fuel in the tank, and transmits this information to the gauge in the dash-panel.
In multi-point injection systems, the fuel pressure regulator is located on the outlet from the fuel rail
Fuel circulates continuously, but the fuel pressure regulator controls the quantity of fuel returning to the tank. This in turn controls the pressure in the fuel rail.
It consists of a metal housing with a spring-loaded diaphragm between the 2 halves of a steel pressed shell. A valve attached to the diaphragm controls the exposure of the opening to the return line.
With the pump operating, fuel pressure builds up on the underside of the diaphragm, and raises it against the force of the spring. The central valve is carried with it, which lets fuel return to the tank. Fuel in the rail is now kept under pressure that’s determined by the force in the spring.
As well, further control is needed, as fuel pressure has to stay at a constant value above manifold pressure. Otherwise, the fuel quantity delivered won’t be accurate.
The spring housing above the diaphragm is sealed, and connected to the intake manifold. Low pressure in the manifold then acts on the diaphragm and helps its movement against the spring. As a result, fuel pressure is continually corrected in step with changes in manifold pressure.
When manifold pressure is low, as at idle, fuel pressure will be lowered. When manifold pressure is high, as at full throttle, fuel pressure rises. As a result, the quantity of fuel injected into the engine is set solely by the duration of the electrical pulse from the control unit.
In engines with multi-point injection, the injectors inject fuel into individual intake tubes, directly in front of the engine intake valves.
The injection valve, or nozzle valve, is held against the valve seat by a helical compression spring. A moveable plunger is rigidly attached to the nozzle, and supported in a guide in the lower section of the injector body. The plunger is acted on by a solenoid winding, wound onto the injector body. Each end connects to a terminal. The control unit completes the injector electrical circuit.
It provides pulses of a set duration, so that the injector valve opens and closes, or pulses, very quickly. Electrical pulses pass through the injector winding, and set up a magnetic field that draws the plunger and valve away from the nozzle seat. Fuel held under pressure in the fuel rail can now pass through the filter, and the centre of the injector, and enter the inlet port.
In a single-hole injector, fuel is injected in a finely atomized, conical spray, at an angle of about 25°.
The exact injector open time depends on the data the sensors give the ECU, but it usually varies from 1 to 20 milliseconds. Valve-lift for the fully open position is about 0.15mm, and response time is about 1 millisecond.
Rubber moldings or ‘O’ rings seal the injector in the manifold and at the fuel rail. This helps prevent the transfer of heat to the injector, and the formation of fuel vapor bubbles under heat soak conditions.
Director-plate injectors operate on the same solenoid principle. 4 minute holes for fuel delivery direct jets of fuel toward each other, so that they collide and break up into a cone-shaped spray.
In some engines with 2 intake valves, the injectors have 4 holes but only 2 jets directed towards each valve. This ensures good fuel distribution for efficient combustion and emission control. The jets are set very precisely, and they must be located accurately by means of the fuel rail.
In side-feed injectors, the injector is located integrally with the fuel rail and fuel flows through the side of the injector near the bottom of the injector body. Fuel vapor is easily carried away by the return fuel, which provides easier starting when the engine is hot. Resin-molded connectors are used to reduce noise, and to retain and locate the injectors in the fuel rail.
All injectors are calibrated so that, in any application, the nozzle valve-lift, and the annular gap between the nozzle valve and the injector body, is the same for each injector.
Fuel pressure is held constant, so the only quantity that can change is how long the nozzle valve is held open, and this is what determines how much fuel is delivered. This is called the pulse width of the injector and it’s decided by the control unit.
The injectors fitted must have the correct calibration. Color coding of the terminal adaptor can be used for identification.
The injector used for throttle-body injection is similar to that used for multi-point. It has a nozzle and spring-loaded plunger, operated by a solenoid. When the injector opens, fuel is sprayed into the intake air passing through the throttle body.
There may be 1 or 2 injectors which open and close at a frequency set by the control unit. At low engine RPM, this frequency is low. As engine speed rises, so does frequency, until at some point, the valve may not close completely, and it then may deliver an almost continuous spray of fuel into the intake air entering the manifold.
In some cases, a cold-start injector is fitted. It is a solenoid-operated valve, on the intake manifold or plenum chamber. It is in the main airstream, on the engine side of the throttle butterfly, and it’s supplied with fuel under pressure from the fuel rail. A turbulence nozzle injects the atomized fuel into the airstream during engine cranking. The operation is controlled by the action of a thermotime switch.
A tachometer is used to indicate engine RPM.
It is normally connected to the negative terminal of the ignition coil. The pulses from the ignition primary circuit are then used as an input to operate the tachometer.
At idle speed, the frequency of the pulse is steady. As the engine speed rises, the frequency of the pulse increases, as the tachometer indicates.
As a safety measure, the tachometric relay uses an input (from the negative side of the ignition coil) to ensure the relay operates only when engine speed is above a specified minimum, say, 350 RPM.
In this way it can operate as a safety device, ensuring for instance that the fuel pump operates only when the engine speed exceeds this figure.
The thermotime switch controls the operation of the cold start injector. It is fitted to the cylinder head casting, with its lower portion immersed in engine coolant. This lower portion houses a bimetallic strip that reacts to coolant temperature. A heater coil is wrapped around the strip. Heating and cooling of the bimetallic strip opens, and closes, a set of contacts. This interrupts, and completes, the circuit for the cold start injector.
When coolant temperature is below the rated value of the switch, the contacts are closed, and so is the circuit for the cold start injector. Then, during cranking the cold start injector will spray fuel into the manifold.
If coolant temperature exceeds the rated value of the switch, the bimetallic strip bends, and opens the contacts. That means that when the engine is above a certain temperature, the cold start injector won’t operate, and this prevents excessive wetting of the spark plugs, and a rich mixture on start-up.
The heater coil also limits the operation time of the cold start injector up to a maximum value, depending upon the application. Typically, this can be about 10 to 12 seconds, at a temperature of minus-20 degrees Celsius.
The heater coil is energized during cranking, and if the engine fails to start within the specified time, the bimetallic strip bends under heating action and opens the circuit. This prevents flooding the engine with excessive fuel.
The time limit is automatically reduced for temperatures up to the rated value of the switch. At that value, the contacts open, under the influence of the coolant temperature.
EFI sensors
Wide band oxygen sensors
An oxygen sensor is positioned in the exhaust pipe, and provides the engine management ECU with an electrical signal that relates to the amount of oxygen in the exhaust gas. The ECU uses this and other information to determine the correct amount of fuel to be injected into the engine.
Originally automotive oxygen sensors were stoichiometric sensors. Although still in use today, they indicate only if the air/fuel ratio is rich (deficient oxygen) or lean (excess oxygen). They do not indicate how rich or how lean.
Their output signal changes almost vertically either side of an air fuel ratio of stoichiometric 14.7 :1.
It is the “Nernst Cell” inside the sensor that produces this signal voltage.
The Nernst cell operates by comparing the amount of oxygen in the exhaust gas to oxygen levels in the outside air. To operate this cell needs to be hot. Exhaust gas can heat the sensor, however when an internal heater is added, the sensor becomes operational quicker. This is most relevant when the sensor is fitted in cooler parts of the exhaust system away from the exhaust manifold.
As emission standards become tighter for both petrol and diesel engines, a more precise signal is required. In these systems the broad band oxygen sensor informs the ECU of a range of air fuel ratios from 9:1 to air.
These systems are ideal for optimum emissions for car petrol engines diesel engines, particularly where the incoming air is unthrottled and not restricted by a throttle butterfly lean running direct injection petrol engines.
The wide band oxygen sensor is far more sophisticated in operation than the earlier type sensors, however it does include some similar parts.
The Nernst cell is still used, however the exhaust gas oxygen levels are referenced to a sealed chamber of air within the sensor and not outside air. These sensors have an electrical heating element that heats the sensor quickly from a cold start, typically in less than 10 seconds. This is a shorter time than that taken by the older sensors. This is achieved because they have far less material within the sensing element.
Current through the heater is controlled by the ECU.This allows the correct operating temperature to be continuously maintained.
A minute chamber within the sensor has access to the exhaust gas, it is this chamber that the Nernst cell samples exhaust gas from.This sensor works by using a solid state pump to add or remove oxygen from the exhaust gas chamber.
The computer controls the current flowing through the pump so that the Nernst cell output is at stoichiometric. Current flowing in one direction through the pump adds oxygen whilst current in the opposite direction removes oxygen.
The value and direction of current required to do this represents the level of oxygen in the exhaust gas. This allows the ECU to control the amount of fuel delivered and maintain correct emission levels.
Twin oxygen sensors
Some manufacturers fit an oxygen sensor before and after the catalytic converter to test for correct operation. For a catalytic converter to change exhaust gases correctly it needs to be capable of storing and releasing oxygen from the catalyst.
When this happens the amount of oxygen entering the converter will differ from that leaving. The ECU compares these two signals to determine if the catalytic converter is functioning. If a malfunction is detected for a predetermined period, the Malfunction Indicator Lamp or MIL will be illuminated.
Misfire monitoring
Engine misfire can be caused by a variety of faults, however the result is always the same. There is the potential for unacceptable levels of pollution to be produced. Manufacturers can incorporate misfire monitoring to address this.
As an engine rotates through the 4-stroke cycle, the speed of the crankshaft varies. This is due to the change in forces placed on the piston and crankshaft. These changes are gradual and predictable when the engine is under load. By using a high definition (many teeth) crankshaft position sensor/reluctor, the ECU can be programmed to monitor and compare crankshaft position to set predictions.
Misfire monitoring software identifies unacceptable changes in crankshaft speed when a piston is at the relevant position. Depending on the severity of the misfire the ECU may immediately illuminate the MIL or after a delay.
Knock sensors
Engine knock occurs in the combustion chamber when two high-pressure waves collide. This unwanted and damaging event can be caused in different ways.
Two examples are excessive load on the engine and engine overheated
Excessive load on the engine:
- Ignition occurs.
- The expanding gases create a pressure wave designed to push the piston down
- The forces opposing piston movement are too high
- The piston accelerates slowly and maintains a small volume above the piston.
- The unburnt mixture is compressed by the advancing pressure wave.
- This fuel self ignites due to an increase in pressure/temperature and creates its own pressure wave.
- The two advancing pressure waves collide creating engine knock.
- This knock has enough energy to badly damage pistons, rings and valves.
Overheated engine – faulty thermostat:
- Ignition occurs.
- The expanding gases create a pressure wave designed to push the piston down
- The temperature of the unburnt fuel is too high due to the overheated engine
- The unburnt mixture is compressed by the advancing pressure wave.
- This fuel self ignites due to an increase in pressure / temperature and creates its own pressure wave.
- The two advancing pressure waves collide creating engine knock.
One way of overcoming engine knock is to allow the fuel to be ignited later in the compression stroke, this means having less ignition advance. If the fuel is ignited later, then the pressures above the piston will be less during the early stages of the power stroke. This results in engine knock being removed.
The function of the knock sensor is to produce an electrical signal that the ECU can use to determine if knock has occurred. The ECU will then provide less ignition advance until knock is removed.
The sensor is screwed into the engine block where it is influenced by all engine vibrations. Using a piezo crystal the sensor produces a signal voltage proportional to the vibrations applied to it. The ECU interprets the strength, frequency and timing of the signal to determine if knock has occurred.
Once engine knock has been registered, the ECU will gradually reduce ignition advance until Knock is removed.
Twin knock sensors
V configuration engines often have two knock sensors fitted, this allows the ECU to have control over knock on separate banks.
Oil deterioration sensor
The quality of engine oil deteriorates as it is used over time. Additives are used up and contaminants from engine use and wear are added. The net result can be little or no change in oil level but a large reduction in oil quality.
The function of the oil deterioration sensor is to produce an electrical signal that the ECU can use to determine the quality of the oil.
The electrical capacitance value of oil varies with various oil properties, these include Viscosity anti-foaming cleaning.
Electronic circuitry within the sensor converts the capacitance value of the oil to a voltage signal.The ECU monitors voltage signal from sensor and uses this data to determine service requirements.
Exhaust gas recirculation sensors
During lean cruising conditions, combustion temperatures are high due to the slow burning of the lean air/fuel mixture. Under these conditions the pollutant Oxide of Nitrogen or Nox is produced as a by-product of combustion.
To reduce combustion chamber temperatures and resultant Nox, a small amount of the inert exhaust gas can be returned to the combustion chamber via the EGR valve. As emission standards become more stringent it is necessary to have greater control over EGR valve operation. The ECU now has control over this valve and monitors several inputs to determine the amount and timing of operation.
The variables measured can include:
- EGR pressure
- Exhaust gas temperature
- EGR Position
Switches
While at idle, engine speed can be adversely affected by fairly minor changes in load. Loads from transmissions, power steering pumps and AC compressors are examples.
For optimum engine control it is necessary for the ECU to be informed of these loads before the engine speed drops.
Switches are used to inform the ECU when the power steering pump pressure is high, automatic transmission drive gear is selected and the air-conditioning compressor is engaged.
The result is excellent ECU control of idle speed as minor loads vary on the engine.
Potentiometer
A potentiometer is a mechanically variable resistor which, in EFI applications, is normally a film-type.
It can be linear, or circular, in construction, and has 3 electrical connecting points. Two are at the ends of the resistor, and a third is attached to a center sliding contact, arranged to move across the resistor. A reference voltage is applied to the resistor, so that a steady current flows through it.
As the center sliding contact moves across the resistor, it measures the voltage at the point it is in contact with, and provides a reference voltage for that position.
In automotive applications, the circular form is commonly used as a throttle position sensor, with the center contact attached to the throttle plate. The throttle plate position can then be monitored by the control unit.
Auxiliary air valves
Auxiliary air valves are part of the cold start system.
The auxiliary air device has a connecting hose from the intake air side of the throttle plate, to its controlling passageway. A return hose then connects from the passageway to the plenum chamber.
The controlling passageway is covered or uncovered by a blocking plate or disc, which is acted on by a bimetallic strip. The strip is heated electrically by a heater coil. The heating current is turned on when the engine starts. The heating effect bends the strip, which turns the blocking plate. As it turns, it changes the cross-section of the controlling passageway, to control the additional air required.
In a cold engine, the passageway is at its maximum set opening. Then, as the engine warms up, it gradually closes.
Idle speed control devices
The solenoid-type air control valve acts on signals from the control unit, to bypass a measured airflow around the throttle plate. The position of the valve depends on how much current the control unit applies to the solenoid.
Maximum current flow opens the valve fully to give maximum airflow. This is generally the assumed position for starting.
Thereafter, the taper valve is positioned by pulsing the solenoid winding circuit at a pre-determined frequency.
The amount of valve opening, and therefore the amount of airflow, depends on the ON time of the pulse. A long ON pulse with a short OFF pulse, will produce a high average voltage, and therefore a large opening of the valve.
A short ON pulse with a long OFF pulse will produce a low average voltage, and a small valve opening.
This ON and OFF time is called the duty cycle, and it’s generally expressed as a percentage.
A variation in pulse width, at the set frequency, is called pulse width modulation.
The stepper motor type of idle control positions its tapered pintle, using a screw and nut assembly. The nut is part of a centrally located, permanent magnet armature. It engages with the screw on the pintle.
Rotating the armature extends, or retracts the pintle, which closes or opens the air passage. Rotation occurs by switching current flow, in 2 coils. The pintle turns in forward, or reverse steps, numbered from zero, with the bypass air passage fully closed, up to 255 for maximum airflow.
Such a large number of steps allow the pintle position to be finely controlled.
When the idle speed is controlled by the position of the throttle plate, a DC motor may be used as a variable throttle stop.
The controlling circuit can let current flow through the motor in either direction, to extend or retract the linear actuator.
Solenoid valves can also be used to bypass a pre-determined amount of air around the throttle plate. These may come into operation for specific load compensation. For instance, when the air conditioning compressor is engaged. They open up a passageway of fixed dimension, and do not vary the opening.
Inertia sensors
These are used by many automotive manufacturers including Daimler Chrysler, Ford, General Motors, Honda, Hyundai, Jaguar & Volkswagen as a safety device. Its purpose is to shut off the fuel pump(s) in case of an accident, preventing fuel from being continually pumped and spilled over the vehicle if there is a leak in the system.
The fuel cut off switch, is designed to instantly stop fuel flow is very similar to a relay. It has a pair of contacts that are linked to the vehicle electrical system and allows current flow through them to run the fuel pump(s). When the switch is closed, the circuit is complete, and the pumps will operate.
The switch is different in that Instead of a solenoid coil or thermal expansion device that a conventional relay uses to break the circuit, this switch uses a mechanical device consisting of a steel ball in a funnel. The ball is held in place by a magnet at the bottom of the funnel. When subjected to shock, the ball breaks away from the magnet and rolls up the side of the funnel, hitting the actuating arm of the switch mechanism.
When the ball pushes against the bottom of the actuating arm, it moves it up, and the linkage in the switch “opens” the electrical circuit and cuts off power to the pump(s). Once actuated the circuit then remains in the “open circuit” position until it is reset.
