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Electronic Fuel Injection

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Electronic Fuel Injection

Electronic Fuel Injection

The atomization of fuel into the air charge is extremely significant to the functioning of the internal combustion engine. If any single aspect of engine performance can be labeled 'most important,' fuel control is a strong candi­date for the honor. Electronic fuel injection, in particular, can do that job better than any other form of fuel injection or fuel mixing device. Principles, applica­tions, and modification of EFI will be dealt with in the discussion that follows. Neither cis ('continuous injection system'), a type of fuel injection that uses pneumatic and hydraulic controls, nor throttle-body fuel injection is discussed in this book. EFI has proven its superiority all the way from economy shoebox-es to Indy champ cars. It has been a long time since a major road race winner was equipped with a fuel system other than EFI. Surely, then, any serious tur­bo will be accompanied by EFI. Nothing else even comes close. Start with the best there is, and you won't wind up stuck or cornered later on.




Fig. 7-1. The modern engine-management system.

Fig. 7-2. An adaptation of the Electromotive TEC II EFI to an ultra­modern engine in the Acura Integra.

Principle of EFI

An EFI system is composed of electrically actuated fuel valves that open by a voltage signal, permitting fuel to flow. The air/fuel ratio is controlled by the amount of time the injectors are held open per combustion cycle. This is called pulse duration. The EFI computer gathers data from a group of sensors that tell it how fast the engine is running and the load at that instant. With that da­ta, the computer starts looking through its stored information to find how long it should hold the injectors open to satisfy the fuel requirements dictated by those load conditions. When that information is found, it is pulled out of the memory and relayed to the injectors as a voltage pulse of a specific duration. These durations are measured in thousandths of a second, or milliseconds (msec). When that cycle is complete, the programming of the computer tells it to go do it all over again but to be alert for new conditions. All this data acquisi­tion, analysis, and distribution takes about 15% of the computer's attention. The remainder of the time it just sits. Too bad it can't be reconciling your check­book in its off hours. The sensors the computer relies on to keep it informed are an integral part of EFI and are analogous to the eyes and ears of the system:

Air-mass/airflow sensor. An EFI system configured with an air-mass or air­flow sensor is called a 'mass flow' EFI system. The sensor attempts to mea­sure the number of air molecules flowing through the system at any instant. If this number is divided by the speed of the engine, it gives an accurate reflec­tion of the amount of fuel needed per combustion putt in the engine.

Air temperature sensor. Air density changes as a function of temperature. Therefore, the computer must know to change the pulse durations slightly if the air temperature sensor detects a change in the air temperature.

Barometric sensor. Air density also changes with altitude. An atmospheric pressure sensor—a barometer—provides the computer a varying signal with changes in altitude.

Coolant temperature sensor. The amount of fuel the engine needs is inverse­ly proportional to engine temperature. The coolant temperature sensor re­flects the engine's operating temperature. With a cold engine, a huge amount of fuel is required just to get enough to vaporize, so it can burn. The hotter the engine, the easier vaporization becomes, and the less fuel required.

Manifold vacuum/pressure sensor. Not all EFI systems will be equipped with a manifold pressure sensor. Those that are, are properly called ''speed density' EFI systems. When the manifold absolute pressure (MAP) sensor is used, an air- mass sensor or airflow meter is not necessary. The manifold vacu­um or manifold pressure at any given instant is a good reflection of the engine load at that time. Hence, the MAP sensor provides the computer with another bit of operating condition data.

Oxygen sensor. The oxygen sensor measures the amount of oxygen left over from the combustion process. It is mounted in the exhaust manifold and thus becomes the after-the-fact watchdog for the computer. If the sensor detects too much oxygen, the computer will know by referring to its stored information that it is time to lengthen the injection pulses slightly, thus adding fuel and us­ing some of the excess oxygen. By monitoring the leftover oxygen, the comput­er can continuously home the pulse durations in on the air/fuel ratio it was programmed to give. The oxygen sensor's purpose in life is to keep the air/fuel ratio in the ranges needed by the three-way catalytic converter. It is not a pow­er or economy device.

Tachometer circuit. The pulsing of the injectors every combustion cycle must, of course, always be referenced to the engine speed. The tach circuit does this by monitoring the low-voltage pulses to the coil.

Throttle position sensor. The actual output of an engine is largely dependent upon throttle position. Full throttle is obviously asking for everything the en­gine has, and fuel flow must rise to the occasion. Therefore, throttle position becomes a significant bit of data for the computer. A further data input that the throttle-position sensor offers is the rate of change of the throttle position. This function becomes the equivalent of an accelerator pump in a carburetor. The accelerator pump offers a sudden rich condition to allow a smoother load transition.

Support pieces for the EFI system are fuel pumps, fuel pressure regulators, fuel lines, air valves, idle controls, and relays.

Sequential Fuel Injectors and Pulse Duration

A good working knowledge of EFI must include an understanding of how injec­tor sizes vary with differing requirements of cylinder size, power output, and operating range of manifold pressure. First it is necessary to understand the intrinsic nature of the timed injector and the available time in which it must work. The available time is limited to the time required for one complete en­gine cycle. In a four-stroke-cycle engine, available injector time is the time re­quired to complete two revolutions of the engine. As the speed of the engine increases, available injector time decreases. Thus the injector inherently takes up a greater and greater portion of the available time as the engine speeds up. Eventually, the point arrives at which engine cycle time is equal to the time the injector needs to deliver the required amount of fuel. This point is the 100% duty cycle point.

Two types of EFI systems are available: sequential and nonsequential. Se­quential, which is the most common, pulses an injector in the same order as the firing order of the engine. In so doing, sequential pulses each injector every other revolution; that is, once per engine cycle. The nonsequential style usual­ly pulses all the injectors at the same time and on every revolution.

Fig. 7-3. Maximum fuel injection pulse time available per revolution is a function of engine rpm.

EFI therefore has a pulse duration twice as long as non sequential, but nonse­quential pulses twice per engine cycle, thereby closely approximating delivery of sequential EFI. A clever variation on sequential injection is the ability to ad­just exactly when the pulse occurs relative to the opening of the intake valve.

The two convenient points to remember are at 600 rpm and 6000 rpm. These two points take 100 msec and 10 msec, respectively, per revolution, or 200 msec and 20 msec for complete engine cycles. Again, it is important to remember that 20 msec total time available, whether it is in two pulses of nonsequential EFI or one pulse of sequential EFI. The fundamental idea behind all this analysis stuff is that the injector must be big enough to deliver all the fuel the cylinder re­quires in 20 msec at 6000 rpm (or even less if the engine runs faster).

Modifying Stock EFI Systems

Within the scope of low-boost-pressure (under 7 psi) turbo systems added to normally aspirated engines, adequate fuel deliveries can be achieved with modification to the stock EFI equipment. The basic requirement of knowing that the fuel delivered through the injector nozzle is the right amount for the conditions still exists and must be satisfied. Increasing fuel flow through the EFT system is limited to one of three choices:



lengthening injector pulse duration

increasing nozzle size

increasing fuel pressure

Lengthening injector pulse duration. Prior to any attempt to increase fuel flow by longer pulse duration, it is necessary to determine the time of an en­gine revolution at redline (peak horsepower) and the maximum duration of an injector pulse. This will allow us to calculate whether additional time is avail-able to lengthen pulse duration. Injector pulse duration can be determined by an oscilloscope or pulse duration meter. This measurement must be taken while the car is moving at full throttle near the torque peak, which is approxi­mately two-thirds of redline rpm.

As rpm increases from about 3000 rpm and injectors are open a larger per­centage of each revolution, sequential EFI reverts to nonsequential. The dis­tinction between the two types can therefore be ignored in calculating additional fuel flow as long as pulse duration is checked above 4000 rpm. Then it is accurate to analyze available pulse increase based on one pulse per revolu­tion.

The time required for one revolution at engine redline determines whether time is available for longer EFI pulses. This can be obtained from figure 7-3 or by calculation:

Example:

Let redline rpm = 5500.

Then

Once the time of one revolution at the redline is known and redline pulse duration has been measured, the available increase can be calculated. In msec,

As a percentage,

Example 1:

Let redline rpm = 5500 and redline pulse duration = 6.2 msec.

Then

Available increase = 10,9 msec - 6.2 msec =4,7 msec

As a percentage,

= 0.758 = 75.8%

Example 2:

Let redline rpm = 7500 and redline pulse duration = 8.0 msec.

Available increase =8,0 msec - 8,0 msec = 0

In this example, redline pulse duration takes up all the available time at the redline rpm; therefore, no increase is available.

If investigation shows an increase in injector pulse duration is available, then the methods of extending those pulses can be examined:

Sensor signal alteration. Pulse durations can be extended by increasing the resistance in the coolant temperature sensor circuit. The amount of resistance is determined by trial and error. The resistance must be added in increments and only when under boost. This requires a messy series of potentiometers and switches and will always prove less than acceptable.

Fig, 7-4. The coolant-temperature-signal-change-based fuel system. Note: This is not a workable fuel system.

Reprogrammed computer chip. Too many problems exist to expect a chip change to offer a means of supplying additional fuel flow. This method is tough to work out on flapper-door-style flowmeters, for example. It will not work on a speed density system unless the MAP sensor is designed to operate at pressures above atmospheric. The tuner with the knowledge to decode an OEM computer program and the equipment to reprogram the system can do the job. These guys are real sharp and real scarce. All in all, this is a tough job to carry out.

Pulse signal interceptor. Currently, the only viable means of extending an injector pulse is to intercept it, modify it based on manifold pressure condi­tions, and send it on to the injector in place of the original pulse. Good technol­ogy and lots of experience are required for success with this approach. Such devices exist in limited applications.

Increasing nozzle size. A change in nozzle size creates a situation wherein, if left alone, the EFI will deliver more fuel all the time under all conditions.

Fig. 7-5. The HKS piggyback computer is designed to operate a factory turbo car at higher-than-stock boost pressures.

Fig. 7-6. The F-CON computer alters the EFI signal based on the magnitude of the boost-pressure signal.

Fig. 7-7. Rising-rate regulator installed in a fuel system

This is not acceptable; thus, a means of returning fuel flow to its original level at low speeds is necessary. It is possible to do this either by modifying the air­flow meter's signal to the ECU or, with flapper-door-style flowmeters, by in­creasing the return spring tension. The latter done inside the flowmeter and is relatively easy. Injector nozzles up to 50% bigger can usually be retuned to good low-speed operation by either method.

Increasing fuel pressure or adding injectors is only practical up to about 9-10 psi (boost pressure), after which larger injectors become necessary Al­though OEM ECUs are difficult to reprogram, aftermarket units, which come with software and instructions, are a cinch.

Fig. 7-8. The rising-rate fuel pressure regulator, invented by Ron Nash in the mid-'70s, raises fuel pressure rapidly as boost increases.

Fig. 7-9. The rising-rate regulator can deliver significantly higher fuel pressures as a function of boost pressure.

With such units, increasing injector size becomes the most potent method of supplying additional fuel. When boost pressure exceeding 9-10 psi is planned, a change of injectors is necessary.




Increasing fuel pressure.

Increasing system fuel pressure as a function of boost pressure is a viable method of increasing fuel flow to accommodate boost pressures up to about 9 psi. Fuel flow changes through a nozzle are proportion­al to the square root of the pressure change across the nozzle. A boost-pres­sure-powered fuel pressure regulator can be made to drive the fuel pressure up rapidly to keep pace with rising boost pressure. This type of mechanism is able to use the original injectors but is limited to fuel pressure available through the stock pump. Bosch or other high-pressure EFI fuel pumps can be substi­tuted or used as supplementary pumps. These pumps generally offer fuel pres­sure up to 130 psi, which give the fuel pressure regulator adequate pressure to work with. Proportioning fuel pressure to boost pressure maintains the timed nature of EFI, keeping fuel delivery proper relative to the air-mass rate of flow.

Extra Injectors

Some systems attempt to increase power by adding one or two injectors over­all, rather than per cylinder. These injectors are customarily placed in the air tube entering the throttle body and can be pulsed by a small control box based on an rpm and boost-pressure signal. As is the case with increasing fuel pres­sure, adding injectors is practical only up to about 9 psi. This is not an ideal system, and, if used, care must be exercised in locating the injectors, to achieve equal distribution of fuel to the cylinders in a manifold designed to flow air only.

Fig. 7-10. One or two additional injectors for the entire system can provide fuel for low-boost applications but should not he consid­ered for serious power.

Fig. 7-11. The inline-six Nissan, manifold as equipped with six staged injectors. Original injectors are to the left; secondaries are further outboard, to the right.

Fig. 7-12. The 'add-on injector' fuel supply will indeed add a useful dose of fuel. The add-on is pulsed with engine speed; duration is controlled by boost pressure.

Fig. 7-13. Four staged secondaries can be programmed to operate when under boost.

The injectors must also be sized to deliver the fuel required for the de­sired airflow rates. Ideally, one extra injector per cylinder is required for seri­ous power. Otherwise, consider this a low-boost-power mechanism.

The preceding paragraphs cover the methods by which EFI may be modified to operate under boost. Prior to selecting a method that suits your require­ments, make sure your measurements and calculations are correct. Don't get off on any dopey tangents like turning on cold-start spray nozzles, or any other equally inane schemes, without suitable investigation proving that the scheme meets all the requirements of a properly conceived fuel system.

Calculating Injector Size

The EFI fuel injector has a rating of fuel flow per unit time. A huge variety of sizes exist. An equally huge number of units of volume or mass flow are used to rate injector flow capacity. The following will convert cc/min to lb/hr:

The calculations required to come up with a properly sized injector for a giv­en application are not rigorous. No rocket science here. One simple calculation and the job is done:

The .55 figure is actually the maximum load brake specific fuel consumption (bsfc) of a typical turbocharged engine. In general, the number of injectors is the same as the number of cylinders. Clearly, one should choose the next larger size than the calculated value, to offer some margin for future improvements.

Testing Injectors

An injector can be measured for its flow capability by applying a suitable volt­age (usually 9, but check the manual) to the injector and 36 psi (stock fuel pres-

Fig. 7-14. A simple fuel injector flow-test ring

sure for most cars and standard pressure for measuring injector flow) to the fuel. Let the fuel run into a graduated burette for one minute. The result is the flow capability measured in cc/min. A couple of 1.5-volt dry cells will hold the injector open just fine.

Fuel Pump Requirements

The fuel requirements of any engine system must be backed up by a fuel supply system. The fuel supply system is the fuel pump, fuel pressure regulator, and fuel lines. The fuel supply system must be able to meet the challenge with a rea­sonable margin of extra capability. This margin requires a balance between the pump's flow capability and its pressure capability An odd feature of all pumps is the fact that they produce their greatest flow at their least pressure. The maximum pressure rating of a pump is when your thumb is on the outlet of the pump, not letting anything out. In other words, no flow. On the other side of the coin, the maximum flow of the pump occurs when it is free to pump with no re­striction (no thumb). The EFI fuel pump is a positive-displacement pump driv­en by a dc motor. As the work the pump is asked to do increases, the motor slows down. As the motor slows down, the volume of fuel being pumped falls off. To operate EFI systems, we must have fuel pressures of 40+ psi. Therefore we must know, calculate, or measure the fuel flow rates at these pressures.

Fig. 7-15. Turbo fuel systems, especially those controlled by a rising-rate regulator, require high-pressure/ high-flow fuel pumps. This Bosch pump will supply 130 psi at flow rates supporting 500 bhp.

Fig. 7-16. Typical fuel pump flow versus fuel pressure. Fuel pumps deliver less flow with increasing pressure. The engine's require­ments must always stay below the curve.

Any given pump will have a flow-versus-pressure curve. These can be hard to come by, but it is not a real challenge to measure a particular pump's capability.

Perhaps the simplest method of determing a pump's capability (particularly if it is already there) is an actual field test, to see if it maintains maximum re­quired fuel pressure to the engine redline. If it does, fine. If not, however, this test provides no data about what is needed.



Fig. 7-17. Approximate fuel pump flow require­ments versus engine bhp.

The standard method for measuring an EFI pump's flow capacity at a given pressure is to connect it to an EFI pressure regulator and measure the volume exiting the fuel return line. This is the volume of fuel that can be taken from the fuel system at that pressure without the fuel pressure's dropping off. With the fuel pressure regulator's vacuum reference open to the atmosphere, fuel pressure will be 36 psi. This is the pressure used on the chart to determine flow capacity. It is equally easy to simulate fuel flows when operating under boost. Feed a pressure signal to the fuel pressure regulator equal to the boost desired and again measure flow out the regulator return line. This tan be dons with shop air and an adjustable air pressure regulator Fuel pressure will be equal to boost pressure plus 36 psi. Prom calculations of the injector sizes required un­der maximum load, the total flow required is known. That total is injector ca­pacity times the number of injectors. The number of cc's per minute divided by 1000 is the number of liters per minute. If the point on the chart representing your requirements of flow capacity versus fuel pressure lies beneath the line, all is well. If the point lies above the line, two or more pumps operating in par­allel are required.

Fig. 7-18. Fuel pumps in parallel should have separate, dedicated fuel pickups.

Fig. 7-19. An effective example of converting a four-barrel carbureted manifold to an EFI system. A throttle body replaces the carb; fuel injector bosses are installed at the ends of the ports.

Aftermarket EFI Systems

Perhaps not yet recognised for what they really are and for their vast tuning potential, aftermarket. EFI systems will prove the greatest boon for hot rod­ders since the small-block Chevy. This is the equipment that can make a docile lamb and high-economy cruiser out of a twin-turbo Keith Black 600 cid hemi V-8. Aftermarket EFI indeed offers the opportunity to create the 1000 bhp dai­ly commuter automobile. The singular aspect of EFI that permits this is its fine degree of tuning available over huge intake manifold pressure ranges. By comparison, the finest carburetor in the world has four fuel-flow circuits that can be timed over the range in which it is asked to operate. Over this same range, EFI offers literally hundreds of fuel flow circuits—one for virtually ev­ery hundred-rpm band and every inch of manifold pressure. It's equivalent to having 500 main jet circuits in a carb, each one ideally set up for a certain en­gine load and rpm.

Several aftermarket companies have introduced EFI systems in the last couple of years. Air Sensors, in Seattle, seem to have been the pioneers with their units.

Fig. 7-20. Electromotive, of Chantilly, Va., manufactures this high -quality, high-performance engine-management system.

Fig. 7-21. The Australian Haltech EFI has proven durable and versatile for specialty tuners.

More recent developments, like the Haltech, offer a completely programmable EFI Electromotive, in Virginia, and Digital Fuel Injection, of Detroit, offer similar hardware plus the feature of ignition controls.

Hardware for Aftermarket EFI

Setting up a functioning EFI system means creating the air throttling mecha­nism as well as doing the hydraulics. The problems to be solved are exactly the same as the problems discussed earlier in this chapter, plus a few new twists. The hydraulic aspects are the same. Intake manifolding layout must be consid­ered; see Chapter 6. Throttle valving, along with number and positioning of in-jectorsj is also discussed in that chapter.

Fig. 7-22. The laptop computer is a basic tool for creating and tuning fuel curves of aftermarket fuel injection systems.

Fig. 7-23. Integration of a flowmeter into the system can be compact. Note the flexible hose to isolate engine vibrations from the flowmeter.

And Furthermore . . .

Is there any benefit to draw-through throttle designs on fuel injected cars ?

A noticeable throttle response improvement between gear shifts can be achieved by plating the throttle in front of the turbo when no intercooler is used. Slamming the throttle shut downstream of a pumping turbo simply causes a greater loss of turbo rpm. This lost speed must be reacquired before boost can again be achieved. A downstream throttle with an intercooler will ultimately prove superior if accompanied by a compressor bypass valve system.

Why are changes needed to existing fuel systems?

Carbureted turbocharger systems do not have any requirement for extra fuel delivery systems. The more air drawn through a carb, the greater the pressure drop at the venturi, and thus the more fuel pushed through the main jet. A properly sized and calibrated carb is necessary, and that is all.

Fuel injection systems are a completely different situation. It is commonly claimed that fuel injection systems will take care of themselves when a turbo is added. This is decidedly not true. A fuel injection system is sized for a given en­gine. A 2-liter unit will not work on a 4-liter engine. The reason for this is that the airflow meters and fuel injectors are sized for the flow capability of the ac­companying engine, and any substantial increase over stock flow rate will bot­tom out the airflow meter. A 2-liter unit airflow meter subject to an infinite airflow rate might think it's a gorilla 2.2 liter for an instant, but that's about as far as it can stretch. Now add the turbo, and you can easily make a 3-liter en­gine out of a 2-liter with just 7 psi boost. Obviously, the fuel injection airflow meter is again bottomed out and can't cope with the increased flow. A turbo en­gine can never be allowed to run lean; therefore, something must be done to meter fuel to accompany the extra air pushed through the system by the turbo.



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