EFI Engine Management : EFI operation

• Modes of EFI
• Electronic fuel injection
• Idle speed control systems
• Feedback & looping
• Cold start systems
• Air measurement
• Air-flow monitoring
• Variable intake manifold system
• Electrical functions
• EFI wiring diagram

Modes of EFI

Sequential


Non-sequential

A mode of injection describes the timing, and the sequence, of injecting fuel.
Simultaneous injection means, every injector opens at the same time. Fuel sprays into each intake port, where it stays, until the inlet valve opens. During each engine cycle, the injectors open twice, and each time they deliver half the fuel needs of each cylinder. This happens regardless of the position of the intake valve. The injectors are triggered by the ignition system. So, for a 6-cylinder engine, the control unit triggers the injectors on every third ignition pulse.
Sequential injection means injection for each cylinder occurs once per engine cycle. It is timed to each individual cylinder in the firing order. Fuel spray stays in the intake port until the inlet valve opens. Grouped injection divides the injectors into 2 groups. A 6-cylinder engine can have injectors 1, 2 and 3 in group 1, and injectors 4, 5 and 6 in group 2. The control unit operates the groups in turn, to spray fuel once per engine cycle. Group 1 injects, then, 360°, or one crankshaft rotation later, so does group 2. This happens, regardless of the position of the intake valve. Just one injection provides the full quantity of fuel for each cylinder during that engine cycle.
In some applications, different modes of injection are combined, so that the mode changes according to the operating conditions. Sequential mode may be used for low engine speeds, changing to simultaneous mode at high speeds. The same principle is used in changing from light loads to heavy loads. Similarly, the mode may change from group injection, to simultaneous.
Using different modes for different operating conditions makes the most of how the fuel is used, and that improves power output, fuel economy and emission control.

Electronic fuel injection

EFI systems use electronically controlled injectors to spray the fuel. There are 2 basic systems - throttle-body injection, also called single-point injection. And multi-point injection.
Throttle-body injection sprays fuel into the air as it passes through to the intake manifold.
Multi-point injection has an injector for each cylinder, which sprays fuel directly into the intake valve port. The whole system has

• a fuel tank to store the fuel;
• fuel pump to circulate fuel, and provide pressure in the system;
• fuel filter to clean the fuel and protect the injectors;
• a fuel rail, or pipe, to supply the injectors with fuel;
• injectors which spray into the intake valve ports;
• a pressure regulator to control pressure in the system;
• a throttle-body, with a throttle valve to control the flow of air to the engine;
• an air cleaner, ducting and an airflow meter, to provide clean, measured air;
• and a plenum chamber, or surge chamber, to dampen the flow of air.

There is also an electronic control unit - a computer that receives data from sensors around the engine. It processes this data, and uses the results to operate the injectors.

Feedback & looping

During engine operation, the 3 pollutants enter the catalytic converter. How efficiently they are converted, depends on the composition of the exhaust gases. And that depends on the air-fuel mixture sent into the cylinder for combustion.
If the air-fuel ratio supplied to the engine is too rich, then the nitrogen oxides are converted efficiently, but the carbon monoxide and the hydrocarbons are not.
If the air-fuel mixture supplied to the engine is too lean, the opposite occurs. Carbon monoxide and hydrocarbons in the exhaust gas are converted efficiently, but not the nitrogen oxides.
Highly efficient conversions of all 3 pollutants occur only in a narrow range of air-fuel ratios. This range occurs around the ideal air-fuel ratio by mass, of 14.7 parts of air, to 1 part of fuel. This is called the Stoichiometric Point. It’s also called the operating window of the 3-way catalytic converter.
If the mixture ratio falls outside this range, the efficiency of conversion of either the nitrogen oxides or the hydrocarbons and carbon monoxide will rapidly decrease.
Because of this strict requirement, vehicles with a 3-way catalytic converter have a feedback system, called looping. The ECU monitors the air-fuel ratio by using an exhaust gas oxygen, or EGO, sensor, also known as a lambda sensor. This sensor obtains information about the composition of exhaust gases. It’s located in the exhaust manifold, and it’s connected to the control unit.
Closed Loop can mean the control unit does receive feedback from the EGO sensor AND acts on it, to alter the injection setting.
Open Loop can mean there is feedback to the control unit, but it is ignored, and the fuel settings are then determined from the programmed memory. Alternatively, Open Loop means there is no feedback to the control unit. In that case too, fuel settings are determined from the programmed memory. During cold starting, this sensor is at too low a temperature to provide an output voltage to the control unit. So it operates in Open Loop.
The same applies until the sensor warms up. The control unit is in Open Loop. It is only when the EGO sensor reaches its operating temperature, that a voltage corresponding to the air-fuel ratio is sent to the control unit. During normal operation, Open Loop may also occur during idle, or under maximum power. It can also occur if there is a fault that causes the air-fuel ratio to be excessively rich for long periods. Similarly for a fault that makes the ratio too lean for long periods.
When the lambda sensor reaches its operating temperature, around 350°c or 662°F, it sends an output voltage to the control unit, to signal whether the mixture is richer or leaner than a lambda value of 1.00 - that is, the air-fuel ratio of 14.7 to 1, also called the stoichiometric point. When the mixture deviates from this, the output voltage changes sharply.
The voltage of the signal from the EGO sensor changes sharply when the air-fuel mixture changes from lean to rich. If the control unit sees the voltage as high, then the quantity of fuel injected is reduced.
Similarly if the mixture changes from rich to lean. If the voltage is low, more fuel is injected to enrich the mixture.
The control unit then adjusts the pulse width of the injector accordingly to ensure the most efficient operation of the catalytic converter.
During engine operation, this adjusting is continuous, and almost instantaneous, trying to maintain an air-fuel ratio for lambda equal to 1.

Idle speed control systems

Base engine idle speed may be set by adjusting a screw on the throttle body. This sets how much air flows through a bypass passage, from the intake air side, to the manifold side of the throttle body.
However, if more load is put on the engine during idle, its idle speed may fall to a level where the engine stalls. Higher load can be caused by extra frictional resistance that occurs in a cold engine, and by electrical loads from headlights or the cooling fan. Shifting an automatic transmission into a drive range, or engaging the air-conditioner compressor clutch will also cause a drop in idle speed.
The extra air needed for a cold engine can come from an Auxiliary Air Device. This one has a connecting hose from the intake air side to its controlling passageway, and a return hose to the plenum chamber. It bypasses the throttle plate when it is operation, to provide the extra air. The control unit reacts to this additional air by metering additional fuel. This makes more air-fuel mixture available during the warm-up period.
How much air bypasses the throttle plate can be controlled automatically by the ECU. It receives data on idle speed and idle conditions, and uses it to provide an output to a solenoid-operated taper valve, or to a stepper-motor pintle. The valve varies the opening of the bypass passageway, and changes the idle speed to suit.
The position of the throttle plate can control idle speed automatically. A D-C motor works a plunger in contact with a lever, attached to the throttle spindle. As idle conditions change, the control unit can extend or retract the plunger, which increases or decreases the throttle plate opening. This provides the desired idle speed.
If the control unit is programmed to maintain a fine control of the idling speed - perhaps to within 100 RPM - ignition timing can be used. Advancing the ignition point increases engine speed, just as retarding it decreases it.

Cold start systems

When a cold engine starts, some of the fuel injected by the main injectors condenses on the cold intake port, or the cylinder walls. Less fuel stays mixed with the air, which weakens the mixture. To overcome this and ensure a rapid start, an extra supply of fuel must be provided.
In some cases, during engine cranking, extra injection pulses in each revolution can provide the extra fuel. It depends on engine temperature, and there is a time limit to prevent flooding. The cranking period is followed by an after-start enrichment. Over about 30 seconds, this slowly reduces to normal warm-up. The engine then responds steadily, immediately after releasing the starter.
More air comes from an auxiliary air valve or bypass air control valve. This bypasses the throttle valve, to raise the engine’s idle speed when it is cold, and during warm-up.
Extra fuel can also come from a separate cold-start injector, normally mounted centrally on the plenum chamber. It’s supplied with fuel under pressure from the fuel rail, and only operates when the engine is cranking. A switch called a thermotime switch, immersed in engine coolant, completes the electrical circuit. It controls the operation, according to engine temperature. This ensures the injector operates under cranking conditions only when the engine is actually cold.
The control unit can help cold-starting and provide cranking enrichment by increasing the pulse width of the injectors. This is in addition to the cold-start injector operation, and is again, temperature-controlled.
Some sequential systems use a pre-injection of fuel. This means all injectors open simultaneously, to provide an initial injection of fuel. This happens only during cold cranking, and there is a time delay, to prevent pre-injection occurring again within a certain time if the engine does not start. The system reverts to sequential injection when the engine starts.
Alternatively, simultaneous injection may be used during cranking, and also in the warm-up period. Once a pre-set temperature is reached, the control unit changes to sequential mode.

Air measurement

The amount of fuel needed for each engine cycle varies, depending on pressure changes in the manifold. That means manifold pressure needs to be measured. One way to do this is to measure airflow.
Measuring airflow allows for changes in an engine that can affect volumetric efficiency, such as wear, combustion chamber deposits and valve clearance.
The vane-type air-flow sensor is located between the air filter and the intake manifold. All the air drawn in by the engine is measured by this sensor. It’s the main measurable variable for fuel control and metering.
The sensor measures the force from the air-stream, on an airflow sensor flap. It overcomes the opposing force of a coil spring on the flap, and deflects it. As the air flow increases, so does the deflection of the flap, and its angle. A potentiometer converts the angular position of this flap into a corresponding voltage. This voltage becomes a signal, and it is transmitted to a control unit.
The sensor operates in a wide range of conditions. At light throttle openings, only a small amount of air passes, but the sensor must be able to measure it.
The engine’s intake strokes can cause the sensor to oscillate. This is counter-acted by a compensation flap. It operates in a dampening chamber and moves in unison with the air sensor flap. Since intake manifold pulsations have the same effect on both flaps, the forces cancel each other out, and measurement isn’t affected.
An adjustable bypass diverts a small quantity of air past the sensor flap. It is used to adjust the air-fuel mixture at idle speeds.

Air-flow monitoring

Manifold-pressure sensors measure pressure in the manifold. One common type has a small connecting hose, from the manifold to a sealed diaphragm within the sensor. Changing manifold pressure alters the position of the diaphragm, and the pressure it exerts on a sensing resistor. This provides an electrical signal, relative to the manifold pressure, which is sent to the control unit. Other devices can also be used to provide the air measurement, but all result in an electrical signal to the control unit.
Variations in air density according to temperature are measured by the air intake temperature sensor. It is installed in the main passageway, protruding into the air stream.
During cold starting, the engine needs extra fuel, and extra air also, to provide a suitable mixture, and increase idle speed. When the engine is cold, an auxiliary air valve lets air by-pass the throttle plate. As the engine warms up, the valve gradually closes. This keeps idle speed fairly close to the basic setting of the idle speed adjusting screw, which is on the throttle body.
Another method uses an idle speed control valve, with the bypass passageway controlled by the ECU. The passageway is varied for cold starting, and if the engine has extra loads such as headlights or air-conditioning. A pre-determined, programmed, idle speed can thus be maintained.
Variable inertia manifolds can vary the effective manifold length in 2 or 3 stages, to extend the torque curve over a wider RPM range. Generally, long intake pipes improve cylinder filling, and therefore engine torque, at lower engine RPM; shorter pipes improve cylinder filling at higher RPM. Cylinder filling can also be improved by using engine-driven supercharging, and by turbocharging.
In throttle-body systems, the air supply is led from the air filter into the central throttle body system. Here, the air mixes with the fuel from the injector, or injectors, and the mixture is carried through the manifold pipes into the cylinders.
Manifold design, however, is usually a compromise in order to keep the mixture vapourized. Heating the manifold by engine coolant is necessary, and the pipes tend to be smaller in cross-sectional area. This tends to limit the advantages of EFI compared to a carburetted system, but useful gains can still be achieved in power, and in reduced emissions.
As in a multi-point system, air entering the engine must be measured. This, together with the engine speed signal, provides the ECU with the basic information needed, to determine the pulse width of the injector.

Variable intake manifold system

The air intake manifold for an EFI multipoint engine normally has long branches of equal length. The long branches increase the pulsing effect of the airflow in each pipe, and help charge the cylinders. The more air drawn into the cylinder, the denser is the air-fuel mixture when the inlet valve closes. And it’s this density of the air-fuel mixture that determines how much pressure develops in the cylinder during combustion, and the level of thrust on the piston to turn the crankshaft.
The characteristic torque curve of a naturally aspirated engine depends mainly on how the engine’s mean pressure changes across the RPM band. The design of the inlet system largely determines the mass of air that can be drawn into a cylinder at a given engine speed, so that means the inlet system largely determines the engine’s torque curve. In general, a long intake manifold produces high torque at lower engine RPM. And higher torque is obtained at higher engine RPM with a shorter intake manifold.
Manifolds that respond to changes in engine load and speed by changing their effective length, are called variable inertia, or intake charging systems. They can have controlling valves operated by the engine management system, which can extend the primary type manifold into 2 or 3 stages.
The primary section is made long and narrow for the low range of RPM. The secondary section is shorter and wider for the high range of RPM. This combination maintains a high-speed airflow in the system.
The 3-stage manifold extends the torque curve, so that the torque curves overlap each other as advantageously as possible.

Electrical functions

In the electrical system, detecting elements sense engine operating conditions, and relay this information in the form of electrical signals to the electronic control unit, or ECU.
The ECU is a small computer, programmed with the operating characteristics of each individual application. This information is stored in its memory. The control unit processes input signals, to determine the amount of fuel required by the engine at that instant. It then grounds the injection valve circuit, to open the solenoid-operated injection valves. The length of time injectors stay open, in milliseconds, determines how much fuel is injected.
A basic EFI system needs input signals on:

• ignition switch operation,
• engine cranking,
• the amount of air entering the engine,
• engine coolant temperature,
• throttle position and engine speed,
• and battery voltage.

The varying data received from all of these inputs allows the ECU to arrive at its injection setting.
This vehicle began from a cold start, and has been running only a short time. It is now operating in top gear, with a light throttle only. What influences its fuel requirements? A relatively small volume of air is entering the engine. The throttle position is associated with economy, or cruising conditions. Engine speed is in the low to mid-range. Coolant temperature is still below normal.
These factors combined produce a fuel setting. It is less than if the throttle were opened suddenly for acceleration, or to overcome increased load. However, it is more than it would be if coolant temperature were at its normal level.
The varying signals can be supplied by variable resistors, by switch contacts, by voltage pulses from the ignition system, or by sensors.

EFI wiring diagram

The battery is grounded to the vehicle frame at the negative terminal. Its positive terminal is connected to a tachometric relay, and to the ignition switch.
A connecting wire runs from the tachometric relay, to the positive terminal on the fuel pump. A fuse in the connecting wire protects the circuit.
The fuel pump circuit is completed by grounding the fuel pump negative terminal to the frame of the vehicle.
When the fuel pump operates, current flows from the battery, to the pump. It flows through the pump to rotate the armature, and completes the circuit by flowing from the negative terminal, through the vehicle frame, to the battery negative terminal.
Similarly for the other components in this simplified system. Positive connections are made to a number of components from the tachometric relay. The relay only operates when the engine is cranking, or running, so no current flows in any connecting wires at other times.