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Basic Overview of Electronic Fuel Injection


The following guide primarily discusses multi-port electronic fuel injection systems since it is the dominate system used in most performance and racing applications.  Most of the theory and operation of multi-port systems is also applicable to throttle-body electronic fuel systems.

In order to avoid being too over whelming with technical descriptions and theories that are beyond the scope of this article, the information discussed here was simplified into understandable, educational terms.




The purpose of Multi-Port Electronic Fuel Injection (MPI) is to supply a precise amount of fuel to an engine’s cylinders in order to properly operate the engine at a particular moment.  Since the engine’s condition is constantly changing (RPM, load, temperature, etc.), the amount of fuel that is injected into each cylinder must change along with the engine’s requirements.  To instantaneously determine the correct amount of fuel to be injected, a computer called an Electronic Control Unit (ECU) is used to calculate how much fuel the engine requires at that moment.  Various engine sensors constantly input information to the ECU so that the fuel requirement is satisfied at all times.

This precise fuel metering capability allows the engine builder/designer to optimize engine operation for a specific result such as fuel economy, exhaust emissions, horsepower, or any combination of the three.



System Overview

The Electronic Fuel Injection System can be divided into three main subgroups:


                                          ●Fuel Supply System

                                          ●Sensing System

                                          ●Data Processing/Fuel Metering System

Fuel Supply System

The Fuel Supply System primarily consists of the hardware used to bring fuel into the engine.  Figure 1 shows the basic fuel system components in a Multi-Port Fuel Injection System.

                                             Figure 1

An electric fuel pump delivers fuel from the tank through a fine element fuel filter to a fuel log or rail.  The fuel rail, usually of bigger volume than the supply line, supplies fuel to the fuel injectors.  The fuel injectors’ nozzle ends are mounted directly into the intake manifold runners and are pointed at the cylinder’s intake port.

Figure 2 (Courtesy Robert Bosch Corp.)

Downstream from the fuel injector is a back-pressure regulator which maintains pressure (usually 30-45 psi) all the way back through the fuel rail to the fuel pump.  Since the electronic fuel pump is capable of producing pressures much higher than required, the regulator incorporates a by-pass feature which bleeds off fuel back to the fuel tank in order to maintain a constant operating pressure.  Many of the newer EFI vehicles are using “returnless” systems which essentially move the regulating function from the engine compartment to the fuel tank.  This eliminates the cost of the return line and has the benefit of cooler fuel temperature since the fuel doesn’t recirculate through a hot engine compartment.



Sensing System

Figure 3 shows the addition of the engine sensing system to the fuel hardware diagram.  Various sensors are used to measure engine operating conditions.  These sensors send information to the ECU regarding engine coolant temperature, intake air temperature, throttle position, exhaust gas composition, engine RPM, manifold vacuum/pressure and, in certain systems, intake air flow.

               Figure 3

Since an internal combustion engine is nothing but an air pump, engine power is dependent on the mass of air which is drawn into the cylinders.  Thus, the computer must know how much air is entering the engine in order to match the air with the correct amount of fuel.

One method of determining the amount of air entering the engine is called the N-Alpha System.  In this system the ECU is programmed to calculate the mass of air which flows through the throttle body based on the throttle opening.  A throttle position sensor (TPS) measures the angle of the throttle blade which is then inputted to the ECU.

The ECU then looks in a table which lists how much fuel should be injected based on that particular throttle angle and other inputs such as coolant temperature,

intake air temperature and engine RPM.  These systems are not very sophisticated but are simple and usually are not overly expensive.  N-Alpha Systems are most effective in racing applications or carbureted engine retrofits where exhaust emissions are not a concern.

A second more sophisticated way to measure the mass of incoming air is with the use of a mass air flow meter (MAF).  Two different types of air flow meters are generally used: hot-wire and flapper door.  A hot-wire meter utilizes intake air that flows past a wire that is heated by voltage and is calibrated to maintain a constant temperature even though the rushing air attempts to cool the wire.  There is direct correlation between how much voltage must increase through the wire to maintain a constant temperature and the amount of air flow that is cooling the wire.  Thus, the ECU can calculate the mass of air entering the engine based on the change in voltage.

Similarly, a flapper door meter sends a signal to the ECU relating the movement of a door that is pushed open by the incoming air.  The larger the quantity of air that enters the meter, the more the door is opened and the greater the angle from the door’s closed position.  The ECU sees the angle position and correlates it to the amount of air that would be required to open the door to that position.

Figure 4 (Courtesy Robert Bosch Corp.)

This system is advantageous in that minor performance upgrades such as headers, intake manifold throttle bodies, etc. do not require a complete ECU recalibration since the flow meter will keep providing the ECU with air flow values.  The drawback to this system is that engine performance is constricted to the size of the meter which limits the amount of air that can pass into the engine.

A third method to determine a value for the mass of air entering an engine is called a Speed Density System.  The ECU in this system primarily relies on the input from a manifold absolute pressure sensor (MAP) which simply measures the vacuum and pressure levels in the intake manifold.  The map sensor’s signal to the ECU is directly proportional to the load on the engine.  Thus, between the MAP information and inputs from engine speed, throttle position, temperature and oxygen sensors, the ECU can look in a table and find a value of how much fuel should be injected based on all those sensor inputs.

Speed Density Systems are well suited for performance applications since their air induction is not limited to the size of an air flow meter.  A drawback to this system is if there is a significant increase in the performance of the powertrain, the ECU must have a new set of fuel calibration tables which in some cases calls for an all new ECU.

 As one can imagine, with the advent of computer power, the auto companies are steadily increasing the sophistication and complexity of electronic fuel injection.  Nevertheless, almost all Multi-Port Systems are similar to that shown in Figure 3 (non-returnless system shown).



Data Processing/Fuel Metering System

The actual processing of the information that occurs in the cells of the ECU is beyond the scope of this article.  In simple terms, the ECU has a series of data tables programmed into its memory which list a value of all the sensors, the ECU will look in the tables and match the sensor values with the proper fuel value.  This value should produce an air to fuel ratio of approximately 14.7:1, called stoichemetric.

Fuel is metered through fuel injectors by pulsing their internal valves to open and close at an extremely rapid pace measured in milliseconds (ms).  The ECU is constantly updating the fuel injector’s open to close time, known as the pulse width (PW) and the time between pulses, known as the pulse interval (PI).  As the engine demand for fuel increases, the sensors relay that requirement to the ECU, which looks in the tables for the corresponding injector pulse width and pulse interval to meet the demand.  In general, the ECU will increase PW and decrease PI to richen the fuel mixture and as well decrease PW and increase PI to lean the mixture.  This way the ECU can infinitely adjust the fuel flow to match engine demand under any possible condition and at any point in time.


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