Basic Overview of Electronic Fuel InjectionThe 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.IntroductionThe purpose of Multi-Port Electronic Fuel Injection (MPI) is to supply a precise amount of fuel to an engines cylinders in order to properly operate the engine at a particular moment. Since the engines condition is constantly changing (RPM, load, temperature, etc.), the amount of fuel that is injected into each cylinder must change along with the engines 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 OverviewThe Electronic Fuel Injection System can be divided into three main subgroups: Fuel Supply System Sensing System Data Processing/Fuel Metering SystemFuel Supply SystemThe 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 cylinders 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 doesnt recirculate through a hot engine compartment.Sensing SystemFigure 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 doors 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 sensors 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 SystemThe 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 injectors 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.Fuel InjectorsA fuel injector is basically an electronic solenoid valve which, when activated, allows pressurized fuel to escape through a nozzle. The fuel injector primarily consists of an armature coil and a needle valve (see Figure1). With the absence of current to the coil winding, the needle valve is forced against the metering orifice (the injectors outlet) by a spring. When the ECU sends a current pulse to the injector coil, the winding is energized which creates a magnetic force strong enough to pull on the steel needle valve uncovering the orifice and allowing fuel to pass. Once the current at the coil is turned off, the internal spring slams the valve shut cutting off the fuel flow.Generally, the fuel flow is metered at the injectors tip by a precisely machined orifice. The injectors flow rate is determined by the relationship of the orifice size and the configuration of the needle valve. This relationship is extremely critical for some injector designs where the needle valves tip actually protrudes through the orifice even when the valve is pulled completely off its seat. This tip, called a pintle, is used to help atomize the fuel spray. This valve-orifice design does not produce a variable flow rate. The fuel that passes through the orifice is constant. The valve just turns the flow on and off. So how is the flow rate increased or decreased to meet engine demand? The ECU controls the on-off time of the injectors. The ECU sends short, rapid current pulses to the injectors that are so fast they are measured in milliseconds. Short current pulses with long pulses in between each pulse will inject less fuel into the cylinders over a given time period than long pulses with short pauses over the same time period. Therefore, the ECU can vary the injector flow rate to meet engine demand.

Figure 1 (Courtesy Robert Bosch Corp.)

Understanding The Injector Flow CurveEvery fuel injector has its own unique flow curve. Injectors are specified at two different points on the curve: static flow and dynamic flow. Static flow is the flow through an injector when it is energized open and left on for a given period of time, usually one minute. This flow rate is the maximum output of the injector at a given pressure. It is this flow rate that is commonly used to describe and classify injectors (i.e. 19 lbs/hr, 30 lbs/hr).The dynamic flow rate is the flow through an injector while it is pulsed on and off for a given number of pulses. The time between the start of each pulse is called the period or pulse interval (PI). The time the injector is actually energized during the period is called the pulse width (PW). Figure 1 shows the relationship of PW to PI.

Figure 1Dynamic flow rate is sometimes expressed in duty cycles which are simply the percentage the injector is turned on during the period. For example, if the PI is 20msec and PW is 5msec, then the duty cycle is 25%. (5/20) x 100% = 25%.Figure 2 shows a basic flow curve for an injector. This particular injector flows 11.5cc/1000 pulses at a PW of 2.5msec and 64.5cc/1000 pulses at a static flow. The static flow value was reduced from its normally measured value of 386.8cc/min to 1000 pulses at a 10msec PW and 10msec PI. This results in the injector being turned on for the duration of 1000 10msec PWs. The dotted line that parallels the flow curve is the theoretical static flow curve which does not account for lost flow during the opening of the injector.

Figure 2Injectors should not be operated below 1.5-2.0msec PWs due to the non-linear portion of the curve. At extremely low PWs, the injectors coil does not have time to reach its magnetic potential, thus the opening and closing action of the injector becomes erratic.At the other end of the flow curve, near static flow, where the duty cycle exceeds 90%, the flow again becomes erratic and non-linear. This time, the opposite is occurring. The injector does not have time to completely close before the coil is re-energized to open once again. Figure 3 shows what happens in the non-linear regions.

Figure 3Another term widely used when discussing fuel injectors is Dynamic Range. Dynamic Range is the linear portion of the flow curve. It is important to know the Dynamic Range when sizing an injector for a particular application because the same injector is required to supply the minimum fuel requirement for smooth idles and still be able to supply engine demand at wide open throttle. For example, the Dynamic Range of an injector used in a turbocharged 4-cyclinder is much larger than a V-8 engine injector. The 4-cyclinder injector requires a higher static flow rather than the V-8 injector due to the 4-cyclinders higher horsepower per cylinder ratio.Injector Spray StylesThere are several types of Multi-Port fuel injectors. In general, all injector designs are similar. Some are optimized for specific characteristics such as fuel metering, valve response time, low pulse width linearity, and so on. To discuss the actual design differences involves intense injector design theory and is beyond the scope of this article. The focus will be on two areas of injector design that are of interest to most performance enthusiasts, i.e. spray atomization and clogging resistance.Injector spray atomization begins at the metering orifice. Injectors designed by Robert Bosch Corp. help atomize fuel by fanning the fuel into the airstream. The Bosch injector achieves this by using either a pintle valve or multi-hole concept. The pintle design utilizes a needle valve with a formation on the end, called a pintle, which protrudes through the metering orifice. When fuel flows through the orifice it is forced out in the shape of a cone by the pintle. This spray is spread out finely into an approximate 15-30 degree fan.The multi-hole design does not incorporate a pintle-orifice configuration but instead, a series of orifices drilled out at angles which create a similar cone spray when fuel flows through the holes.

Figure 1 (Courtesy Robert Bosch Corp.)Other injectors such as the type once manufactured by Lucas Automotive emphasize disc injector performance over pintle needle valve injectors. The Lucas disc injector design replaced the conventional needle valve with a smaller, lightweight, flat disc. The disc, acting like the needle valve, is pulled off a chimney by the coils electro-magnetic force, allowing fuel to flow through holes in the disc, over the walls of the chimney and out of a single metering hole. Because the fuel is metered through a single hole, the fuel exits the injectors nozzle in a narrow stream, often referred to as a pencil steam. When the magnetic field is turned off, the disc slams shut, forming a flat-surfaced seal with the chimney preventing any fuel leakage. This design is very effective in resisting injector clogging since no fuel is present at the tip to evaporate and the metering orifice is tucked inside the injector body away from any heat source. The low mass of the disc valve allows for quick response times (opening and closing times) which improve the injectors Dynamic Range capabilities. A consequential benefit of the Lucas disc injector design is its low noise operation, which virtually eliminates the traditional injector clicking sound which is often mistaken for engine valvetrain noise. Figure 2 shows the internal components of the old Lucas disc injector design.

Figure 2Non-pintle injectors were developed to fight injector clogging. It was found that most of the deposits would form on the pintles surface which would severely restrict fuel flow through the metering orifice. Removing the pintle from the injector design eliminates a place for the deposits to form thus reducing injector clogging.In many production engine applications, injector spray patterns play a minimal role in combustion performance because the spray is usually not aimed at the intake valve. Instead, the spray is aimed at the intake runner wall or floor. This is done for many reasons such as for emissions or packaging constraints. In these cases, atomization occurs after the airflow picks-up the fuel either in flight or as it splashes off the runner. In performance applications where emissions are not an issue, injector aiming should be optimized towards the valve and atomization techniques should be utilized. Figure 3 shows various spray patterns of the Bosch injector with different types of metering orifices.

Figure 3 (Courtesy Robert Bosch Corp.)Injector Compatibility With ECUsFuel injectors are controlled by one of two possible injector control circuits called drivers: saturated (high impedance) and peak-and-hold (low impedance or current regulated).An injector used in a saturated driver system requires a high resistance value across its coil (12-16 ohms). The high resistance values enable the injectors to operate at low current levels (.8-1 amp) which keep the circuitry cool, promoting longer component life. Unfortunately, because of the low current levels, injector response time is slow, sacrificing dynamic range. Ohms law (V=IR) can be used to show the relationship between injector resistance and current level, where V=Voltage, I=Current (amps) and R= Resistance (ohms). A high impedance injector (14 ohms) used in a 12 volt system would require an operating current of 0.86 amps. I = 12 Volts/14 ohms = 0.86 amps A peak-and-hold driver circuit utilizes fuel injectors with low resistance coils (2-2.5 ohms) which require more current (4-5 amps) to open (Ohms law: I = 12 Volts/2.5 ohms = 4.8 amps). The driver circuitry will overheat if the injector is constantly operated at 4 amps. Therefore, a switching mechanism is built into the circuit which will turn down the current to a lower, more acceptable level after the injector is opened. Once the injector is opened, it takes far less current to keep it open. Thus the term peak-and-hold is used to describe this type of circuitry. After the initial peak of 4 amps is reached, the driver turns down the current to 1 amp which holds the injector open for the duration of the pulse width. In most peak-and-hold Multi-Port applications, one driver operates two injectors in which case the peak current per injector is 2 amps and the hold current is 0.5 amps. The advantage of this system is the quick response time of the injector. The high initial current instantly creates the magnetic force require to raise the valve. This allows for a wide dynamic range, which is why peak-and-hold systems are used in small displacement, high horsepower engines such as 4 cylinder turbocharged engines.Figure 1 shows the relationship of current and opening times for high and low impedance fuel injectors.

Figure 1It is not recommended to use low resistance injectors in a saturated driver circuit. The additional amperage required to open the injector can overheat the ECU causing permanent damage. In addition, the injectors opening and closing times may become unstable creating rough engine operation and possible lean misfires.It is possible, though, to install high resistance injectors in a peak-and-hold system since the amperage needed to open the high resistance injector is lower and within the limits of the peak-and-hold driver circuit.If you are not sure which injector type will work with which driver circuit, be safe and use the same type that was originally used in the application.Injector Size (Flow Rate) SelectionA formula can be used to help determine an injector flow rate for a particular engine application. This formula is only as accurate as the input values used in the calculation. It is at this point that you must determine an honest and realistic horsepower estimate. If you input a wish horsepower value and reality is nowhere near your wish, then the injector selected will be too rich with the result being a poor running engine. Of course dyno data would give you accurate numbers but for most people, that is a luxury. The following formula works as a good guideline: Q = MAXHP x BSFC No. of CylindersQ = Injector flow rate lbs/hrMAXHP = Estimate of the engines maximum horsepower capabilityBSFC = Brake specific fuel consumption: use .45 naturally for aspirated engines and .55 for turbocharged and supercharged enginesLets use a naturally aspirated V-8 engine which produces 400 horsepower as an example: Q = 400 x .45 = 22.5 lbs/hr 8We are not finished. 22.5 lbs/hr is the total per cylinder fuel required for the engine at that horsepower. This should not be the static flow value of the injector. This value should be 10-20% below the static flow because the injectors are rarely, if ever, operated at full static flow. Injectors operating at pulsewidths near static flow become unstable since the injector does not have time to fully close before it is beginning to reopen. Therefore, it is recommended to use a safety factor of at least 10% (20% on the conservative side).Then the actual injector size should be: 22.5 lbs/hr / 0.90 = 25 lbs/hr 22.5 lbs/hr / 0.80 = 28 lbs/hrTherefore, an injector that flows 25-28 lbs/hr would be ideal for our example engine. There may not be an injector available off the shelf that will flow exactly in that range. If there isnt, one possibility would be to raise or lower the fuel pressure. Four our example, an injector that flows 30lbs/hr at 39 psi fuel pressure will flow 28 lbs/hr at 35 psi fuel pressure and will meet the engine requirements.Read Understanding the Injector Flow Curve to learn more about the injector flow rates..Injector Flow Rates At Different PressuresOnce an injector is manufactured, the flow rate can not be altered. Flow rates can only be changed on a limited basis by raising or lowering the systems fuel pressure set point (the measured pressure with no vacuum or pressure on the regulator to manifold port). By raising fuel pressure, more fuel is forced out the metering orifice per pulse width. Thus, someone who has performed modifications to their vehicle (i.e. low restriction exhaust, improved air induction, increased boost, etc.) may be near the flow limits of their stock fuel injectors. By increasing fuel pressure, a few more lbs/hr of fuel flow may be squeezed through the injectors to meet engine demand.The following formula can help you determine the injectors flow rate with a change in the fuel pressures set point:Q2 = {Square Root (P2/P1)} x Q1Q1 = Original injector flow rate (lbs/hr)Q2 = Injector flow rate at modified pressure (lbs/hr)P1 = Original fuel pressure set point (psi)P2 = Adjusted fuel pressure set point (psi)For example, a Ford Mustang 5.01 uses a fuel injector rated at 19lbs/hr at a fuel pressure set point of 39psi. What would the flow rate be at 50 psi?Q2 = {Square Root (50psi/39psi)} x 19lbs/hr = Square Root (1.28) x 19 lbs/hr = 1.13 x 19 lbs/hrQ2 = 21.5 lbs/hrFigure 1 shows flow rate changes due to pressure of some popular injector applications.

Figure 1It must be understood that raising fuel pressure is fine if you are near the limit of the injectors capability. Raising fuel pressure to overcome a major fuel deficiency problem is a band-aid solution and should be avoided. High fuel pressures will add more strain to the entire fuel system, including the injectors internal spring. In addition, high pressures can make some injectors to become unstable, or in some cases, completely shut-off.Finally, it must be realized that when the fuel system pressure is increased, fuel pump volume is decreased. So if you though you richened your fuel system by raising the pressure with an adjustable regulator, but the lean problem persists or becomes worse, then you most likely have a fuel volume problem. The fuel pump is not pumping a sufficient volume of fuel to feed the system.In general, fuel pressures should not exceed 65 psi except during boost in special applications such as turbocharged or supercharged engines with high boost pressures.Fuel Pressure And Manifold Vac/Press InterfaceOne of the most commonly confused functions of the fuel supply system is the interaction of manifold Vac/Press and the fuel pressure regulator. Simply stated, the function of the fuel pressure regulator is to maintain a constant pressure at the fuel injectors metering orifice.It is important to the calibration of the EFI system that the ECU know how much fuel will be injected into the cylinders per a given pulse width. Therefore, the ECUs calculations are based on the injectors flow curve which is established at a constant fuel pressure. As was show earlier, fuel pressure changes will change the injectors flow rate. If the pressure was allowed to change continuously, the ECUs calibration tables would be incorrect since the flow rate versus pulse width values would be constantly changing due to the pressure fluctuations. Therefore, fuel pressure at the injectors metering orifice must be kept constant.Most EFI fuel systems achieve constant pressure regulation through a back-pressure regulator. The regulator receives a reference signal from the intake manifold and will raise or lower the fuel pressure to maintain a constant pressure (called Delta P) at the injector metering orifice. The regulator function can be shown by Figure 1 and the following formula:Press @ METERING ORIFICE (Delta P) = Fuel System Pressure Manifold Pressure

Figure 1The formula shows the relationship of manifold vacuum or pressure with fuel line pressure (the pressure you would see on your gauge). As can be seen from the examples in Figure 1, when no vacuum or pressure is applied to the regulator, fuel is metered at the set pressure. When vacuum is applied, the orifice is operating in a negative pressure environment. Less pressure in the fuel line is required to maintain the constant pressure of 40 psi at the metering orifice. Likewise, when there is positive pressure (boost from turbos and superchargers) more pressure is needed in the fuel line to overcome the additional pressure surrounding the metering orifice.When checking or adjusting the pressure regulator, it is always best to disconnect the manifold line to the regulator before taking a reading. This reading is the true fuel system pressure set point.Importance Of Injector Flow-MatchingInjector flow-matching, also called blueprinting, is an excellent means to improve the engines performance and efficiency. Since each cylinder is essentially metered individually, it only makes sense that the engine would perform better if each cylinder received the exact same amount of fuel.This flow-matching theory can be demonstrated by the following example. A GM 350 TPI fuel injector is rated 22 lbs/hr at 43.5 psi. Due to manufacturing tolerances, it is extremely rare to find that all 8 injectors from a particular engine would flow exactly 22 lbs/hr. If one cylinders injector flowed 5% less, then that cylinder would only receive 21 lbs/hr of fuel (0.95 x 22 = 20.9). 1 lb/hr less fuel roughly translates to a potential loss of 2 horsepower for that cylinder. If 4 injectors were 5% lean, that would translate to a potential loss of approximately 8 horsepower.In reality, it is not unusual to see flow rate differences as high as 10% between cylinders. The situation can be aggravated further for in-use injectors which are susceptible to deposit formations which clog metering orifices.A flow-matched injector set should be matched at more than one point on the flow curve. Flow-matching at the injectors static flow alone does not produce a matched set. As was discussed earlier, injectors are rarely, if ever, operated at static flow. Therefore, the set should be matched at a dynamic flow point, preferably the manufacturers calibration set point (usually around pw of 2.5 msec). With a set of injectors that are matched at their static and dynamic points, you can be assured that they will flow almost identically through the entire flow curve.So a flow-matched injector set will not only help produce more efficient power at W.O.T., it will help engine performance through the entire rpm range. In many cases idle quality, throttle response and overall driveability are noticeably improved with a flow-matched set of fuel injectors.EFI Fuel Pump SelectionVarious factors must be considered when selecting an electric fuel pump for an EFI system, or if you are just trying to determine whether your existing fuel pump can handle horsepower increases to the powertrain.The most important criterion in sizing an electric fuel pump is calculating the total fuel requirement for the engine. The formula used to determine total fuel required is similar to the one used to calculate injector sizing.Q = MAXHP x BSFCUsing a 400 HP, naturally aspirated engine with a BSFC of 0.45 four our example (.55 BSFC for turbocharged or supercharged engines), we can calculate the total fuel needed to feed this engine:Q = 400 x 0.45 = 180 lbs/hrNow we need to express this value in volume flow rate such as gallons per hour (gph). Divide 180 lbs/hr by 6, which is approximately the weight of 1 gallon of gasoline, and the result is 30 gph. Therefore, we need a fuel pump that can supply a minimum of 30 gph for our example engine.The analysis is not complete. Fuel pump flow capacity decreases as fuel system pressure increases. Figure 1 shows flow curves of 2 fuel pumps at various pressures.

Figure 1When selecting a fuel pump for a particular application, you must know what fuel system pressure the pump will see. Notice, if you selected pump 1 it would be adequate if you ran your fuel system in the 20-230 psi range. One final consideration in making our fuel pump choice is the pumps performance at various voltages. Voltage has a direct effect on pump output. Apply more voltage and the pump will spin faster, pumping more fuel. Apply less voltage and the pump will slow down, pumping less fuel. Figure 2 demonstrates the voltage sensitivity of the fuel pumps output performance.

Figure 2For our example, do not choose a pump that flows 30 gph at 13 volts since a slight voltage drop of even 1 volt will cause the engine to run out of fuel at peak horsepower (the worst time to run out).So when selecting an electric fuel pump, keep in mind the engines fuel requirement, system operating pressure and voltage inputs in order to prevent fuel starvation under all driving conditions.