Now that the inputs to the computer have been covered, it’s time to look at what the outputs can do. Not that many outputs are required to operate most engines. The advantage of EFI is the ability to very precisely control each of these outputs. Think of jet changes in a carburetor as strokes with an axe and injector control as using a razor blade. Obviously, creating fine details in a sculpture is easier with a razor blade.
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The number one priority and function of any injection system is to properly mix the right amount of fuel with the incoming air. This must also be done in such a manner that the fuel is ready to burn as soon as the spark event happens. In a carburetor, the suction of the low-pressure airflow through the venturi draws liquid fuel through a fixed orifice resulting in a misting or suspension of fuel molecule in the incoming air charge in the intake manifold. The manifold design needed to accommodate this “wet” mix features smooth flow paths with gentle bends designed to avoid puddling or uneven fuel distribution.
The fuel injector is basically an electromagnet driven stopper valve for fuel. High-pressure fuel is fed to the top of the injector from the rails. Once the injector is actuated, the force from the electromagnet pulls a pintle upward, opening a small hole to allow the fuel to flow into the manifold. Injectors can use a much higher operating pressure than a carburetor because the spring-loaded pintle design does not leak under pressure. This added pressure also provides for a more aggressive pressure drop as the fuel enters the manifold, making a finer mist of fuel. The smaller droplet size in turn means better ignition since each individual fuel molecule has more surface area contact with the air charge to aid evaporation and combustion. Additionally, the relatively small size of fuel injectors means that they can be located much closer to the intake valve. This frees up manifold design, allowing for longer runners and tighter, more complex turns to fit a smaller package without puddling. When each individual cylinder can have its own fuel injector, fuel distribution can also be optimized.
Not all fuel injectors are created equal. Injectors are typically classified by the resistance of their internal coil as either “high impedance” or “low impedance.” High-impedance injectors are the most common and are used in almost every OEM application today. When measured with an ohmmeter, high-impedance injectors have 10 to 16 ohms resistance. Low-impedance injectors are less common in OEM applications, but readily available in the performance aftermarket. Low impedance injectors typically measure 2 to 6 ohms resistance. It is important to recognize that in most OEM applications, it is not advisable to replace high-impedance injectors with low-impedance injectors. The lower impedance forces the circuit to draw more current through the injector controller, which is really just a transistor. Excess current drawn through these controller circuits often literally cooks them, leading to failure of the injector driver circuit on the board. Since replacement injector driver circuits are so scarce, the result is usually the purchase of a new replacement PCM.
The next division is injector “size,” which is really a misnomer for flow rate rather than physical dimensions. Most injectors are rated in either lb/hr or cc/min. These flow rates are actually the static maximum flow through the injector if it were activated continuously. Keep in mind that if the injector is only open for a very short period of time, a correspondingly smaller amount of fuel can be delivered. The length of time that the injector is open during each cycle is known as “pulsewidth.” Pulsewidth is usually measured in milliseconds. The ratio of pulsewidth to total available time for that injection cycle is known as duty cycle.
It is impossible to have duty cycles exceed 100%. If the commanded pulsewidth equals actual cycle time at a given engine speed, the injector is not allowed any time to close. This is known as “static flow,” as the rate of fuel delivery is no longer changing. This situation typically happens when injectors that cannot support the necessary fuel flow rate for the engine’s power level and fuel consumption rate (BFSC) are used. Uncorrected, this leads to an enleanment of the air/fuel mix. Care must be taken to ensure that this enleanment does not lead to detonation and engine component failure. If more fuel delivery is required, the only available remedies are to either change to injectors with higher flow rate units or increase the fuel rail pressure to artificially increase the flow of the existing injectors.
Fuel rail pressure changes to flow rate are governed by the equation:
Keep in mind that changes in manifold pressure also have an effect on actual injector flow rate. If manifold pressure increases from the use of a supercharger, rail pressure must increase 1:1 with boost to ensure that the injector does not become effectively smaller. It is possible to tune around changes in effective rail pressure and actual flow rate. This is covered later when we discuss forced induction.
Much like any other spring-mass system being driven by an electric current, opening of the injector is not an exact step change. It takes a small amount of time to build up enough energy in the coil to begin to move the pintle of the injector off the seat and allow fuel to flow into the manifold. The initial delay is known as “dead-time,” but is also followed by a period of exponential movement of the pintle until it hits the fully open position. (Figure 5-1) Likewise, closing the valve takes time as well. Once the current to the coil is removed, the pintle is pushed back to its seat by internal spring pressure as well as fuel pressure behind it. The difference between the opening delay and closing delays is called “injector offset.”
Remembering that fuel injectors are actuated by electromagnets, it is important to further understand how their performance can change. The strength of the electromagnet in the injector varies relative to voltage. (Figure 5-2) Having more voltage across the field of the coil increases the strength and allows the injector to open quicker. This in turn means that fuel begins to flow into the manifold slightly sooner if voltage is higher. Knowing that cars almost never have constant voltage, the PCM needs to be able to adjust. A failed alternator, dead battery, or even normal cranking can send voltage to 11.5 or lower. Normal charging usually keeps voltage around 14 volts, and a failed voltage regulator can send output above 17 volts. The bottom line here is that the same injector under these varying conditions can change its actual output by 40% or more. All modern PCMs have tables built into their software code to model this change, even if they aren’t overtly visible to the calibrator. Various injectors exhibit different voltage compensation curves. While all injectors change relative to voltage in a similar manner, the exact offsets at a given voltage are slightly different as internal construction of the injector changes. To best model the actual fuel delivery to the engine, it is ideal to accurately input the voltage compensation for the injector used. A quick Internet search can often yield the exact voltage offsets for most injectors.
To add more complexity, the actual flow rate changes based on pulsewidth. As the injector first opens, more fuel flows for the split-second that pressure differences are the highest. (Figure 5-3) Additionally, when the PCM commands an injector-opening event for a short duration, there is a tendency for the injector’s springmass system to overshoot the desired duration. The net result is that at small pulsewidths, the injector tends to deliver fuel at a rate slightly higher than the static flow rate of the injector. This often leads to modeling the injector with two different flow rates, one for the normal pulsewidths of cruising and power delivery and another for the shorter pulsewidths of idle and starting. This in turn leads to the need to determine where this change, known as the “break point,” occurs. This break point is usually relatively small, on the order of 1 to 3 ms, so the effect is often only seen at idle and very low loads. Again, entering this break point into the PCM routine allows for more accurate modeling of exactly how much fuel can be expected to enter the engine for a given commanded pulsewidth.
Most PCMs model the short pulse phenomenon with an injection time adjustment to the static flow rate model. In this case, the injector is treated as if the actual flow never changes. Instead, the commanded pulsewidth at low duty cycles is adjusted only by a time constant that becomes less dominant as injection time increases. In this case, a small amount of injection time (usually less than one ms) is added directly to the output under all operating conditions. The total fuel mass delivery effect of injector dead time is more pronounced at short pulsewidths, so many PCMs use this as the only means of compensation for short pulse fueling errors. This dead time characteristic is often easier to find than separate high and low injector slopes. It is important to recognize which method is being used when changing the injector characteristics, but either works fine.
Once all aspects of the fuel injectors’ performance have been modeled in the PCM, it becomes much easier to actually deliver a precise amount of fuel to attain an exact A/F ratio. A poorly modeled injector can lead to sloppy A/F ratio control just the same as a faulty sensor. Although not all EFI systems have all of the fuel injector parameters visible to the tuner, most of them at least have some default values that get the calculations close enough to operate. In an ideal world, the injector parameter in the PCM is adjusted to exactly model the performance of the injectors being used. If this is done correctly, it should be possible to change only the fuel injectors and their associated parameters in the software on a perfectly tuned engine and see no change whatsoever in engine performance.
Standard electronic ignition systems rely upon a switching circuit mounted inside the distributor itself to cycle power to the coil. Actual ignition advance is controlled by changing the phasing of the distributor relative to the crankshaft. Simply rotating the distributor changes the delivered spark advance. Additionally, rotating sprung weights inside the distributor can change this phasing relative to engine speed to add timing at high speeds.
Modern EFI systems add another link into this chain. The PCM intercepts the ignition trigger signal from either a sensor inside the distributor or the crankshaft position sensor. It also has the capability to shift the phase of this signal (change actual ignition timing) before it is passed on to the coil. Since the PCM has the benefit of knowing exactly where TDC for cylinder number one is (thanks to the crankshaft position sensor), very precise control of the ignition timing events can be implemented. The PCM can then adjust ignition timing with respect to any other parameter it either has an input for or can calculate. Most PCMs have the main timing adjustments made based on a table of engine speed versus load, MAP, or airflow. Additional adjustments are usually made with respect to coolant temperature and air inlet temperature. When idling, small changes in ignition timing can be used to effectively control idle speed at a steady airflow rate. Dwell time of the ignition event can also be continuously adjusted in a similar manner.
The net output is still a switched ground to the ignition coil. Modern PCMs can also have paired outputs to drive a coil pack of two or more cylinders. Paired ignition outputs are used in what is called a “waste spark” ignition system. A waste spark system has a common coil for two cylinders, each opposite the other in firing order. These systems fire the spark plug on each cylinder once per revolution (twice per cycle). This yields a spark event during both the compression and exhaust strokes to help reduce emissions. A side benefit to this is that each coil doesn’t have to fire all cylinders of the engine. This means slightly more time in between ignition events to allow the coil field to recharge to full energy. The final option is individual output for a separate ignition coil on each cylinder. These allow for more coil saturation time during high engine speeds and highest overall spark energy.
Most EFI systems are run with a “return fuel” style system where a pump in or near the fuel tank moves fuel to the rail on the intake manifold. Pressure is regulated at the rail and unused fuel is returned to the tank. It is important that the pressure regulator is set to deliver the same pressure with which the PCM is programmed to work. Changes in delivered pressure behind the injectors directly change their flow rate, making for inaccurate fuel delivery if the PCM does not compensate.
Some systems such as that found in GM’s LS1 Camaros have a regulator and short return line near the tank with a single regulated line feeding the rail. This shorter return loop exposes less of the fuel to the heat of the engine compartment, reducing evaporative emissions concerns. These systems often have no connection between the intake manifold and fuel pressure regulator. In this case, the PCM assumes a constant rail pressure and calculates a differential based on output from the MAP sensor. The PCM then adjusts pulsewidth accordingly to compensate for changes in manifold pressure.
“Returnless” fuel systems have only a single fuel line from the tank to the rail. In these systems, the fuel pump is not always operating at 100% output. The PCM controls the duty cycle of the pump based on the feedback from a pressure sensor mounted to the fuel rail. These pressure sensors are also usually attached to the intake plenum by a vacuum line to register fuel delta, or pressure drop, across the injectors. Since effective injector size varies with delta pressure, this allows the PCM to monitor conditions very closely. Additionally, the PCM can effectively make the injectors larger by commanding a higher delta pressure. Returnless systems usually have several tables to model fuel pump flow based on demand and available voltage. Fuel pump duty cycle in these systems may be as low as 10% at idle.
Throttle/ETC/Fly by Wire
The majority of vehicles on the road today have a cable connecting the accelerator pedal and the throttle blade. In this simple application, driver input directly equals actual throttle position. The TPS monitors this actual position and allows the PCM to adjust accordingly.
Newer cars are increasingly adding Electronic Throttle Control (ETC), or “fly by wire” features to the PCM. This system includes a Pedal Position Sensor (PPS) mounted directly on a spring-loaded accelerator pedal with no mechanical connection to the engine. The output of the PPS indicates the driver’s intent to the PCM, where an actual throttle position is calculated and commanded to a stepper motor attached to the throttle blade itself. A TPS sensor is still used to monitor actual throttle position and allow for corrections by the PCM. This system allows the PCM to make adjustments to the throttle position to improve idle quality, off idle response, and drivability without the driver’s help.
With the physical connection removed between the accelerator pedal and throttle blade, some redundancy and safety checks are now required. The most obvious is the addition of a second TPS sensor. The additional sensor uses either a different range or slope than the primary TPS and is used to verify actual position as well as perform a rationality check on the primary sensor output. Any errors between the two TPS sensors usually lead to a fault code and possible trigger of a failsafe mode.
The ETC strategy must also have some form of failsafe detection for stuck open throttle or improper operation at part load. Since there are no longer external springs mechanically forcing the cable to retract when the driver lifts off of the accelerator pedal, great care must be taken to make sure that the ETC’s stepper motor is under control at all times. To prevent engine damage and reduce chances of a vehicle crash, this failing condition triggers a “limp mode” for the ETC system where the throttle position is limited to a very small value (usually less than 20%) until reset. ETC systems usually have a table in the ECM of maximum airflow rates at a commanded throttle position and engine speed. (See Figure 5-4) If the airflow exceeds the table’s value, it is determined that something must be wrong with the ETC hardware.
When parts are changed on an ETC equipped vehicle in the name of adding horsepower, we run the risk of exceeding the PCM’s predicted maximum airflow. It’s not unusual to see an ETC equipped car suddenly drop into limp mode on the first test drive after adding a supercharger or larger camshaft. This is due to the added airflow. The solution is to reprogram the PCM to allow for higher airflow rates at high throttle before triggering limp mode. A safety margin of roughly 20% should be sufficient, but each vehicle can be unique. Only actual testing on the specific combination of vehicle and performance parts dictates the proper values to maintain safety without inadvertently triggering limp mode under power. Again, great care must be taken not to simply disable this function. This would leave the potential failure of a stuck throttle and vehicle crash as the responsibility of the person who modified the PCM. If the OEM doesn’t want to be taken to court, it’s doubtful a smaller shop or individual tuner would fare well either.
Two main strategies for ETC control are used today: “pedal follower” and “torque-based.” Pedal follower was the first ETC strategy to be widely employed and is far simpler in its operation. This throttle control strategy takes a PPS input from the driver and gives a reaction that is roughly equal to that at the throttle blade. The main exceptions are usually closed position for idle control and tip in rate to control driveline lash or “clunk.” At idle, the ETC motor makes small corrections to the throttle blade angle to adjust effective throttle area. This change in area performs the function of a traditional IAC valve without the cost or complexity of another piece of hardware. The PCM has the authority to make very fine adjustments that can effectively regulate airflow even at low engine speeds through the throttle blade alone. During upshifts, the throttle position can be reduced to briefly decrease engine power. This reduced output smoothes the transition into the next gear as the clutch engages, reducing driveline shock and noise.
With torque-based ETC control, the strategy is significantly more complex. Actual throttle blade position is no longer directly tied to the PPS. Instead, the PPS is interpreted by the PCM as a torque demand. The PCM has the latitude to choose how it delivers the desired torque. Actual engine torque can be manipulated by throttle position, spark advance, lambda, cam position, or any combination of these. The PCM must be programmed with multiple tables representing the engine’s output capability under all operating conditions. During operation, the PCM is constantly calculating the exact engine torque output. Driver input changes are processed as torque demands the same way accessory inputs are. This makes for easier programming during the development phase for OEM engineers. The PCM can be programmed such that a certain A/C compressor requires 15 ft-lbs of torque to turn once engaged rather than fiddling with IAC or spark individually to smooth engagement.
To work correctly, torque-based ETC must start with an accurate representation of engine output. This is accomplished with a table representing actual engine torque (a derivative of airflow, lambda, and ignition timing) versus load and engine speed. To find the predicted load (air mass flow), another table is required to model airflow versus effective throttle area. The physics of this table are unique to the engine configuration. Changes in throttle body area (diameter), camshaft design, intake manifold design, or even exhaust backpressure can require changes to this table to prevent unwanted fault triggering.
The majority of engine calibration must be done prior to the final torque-based ETC calibration. This is accomplished by manually setting the ETC to a fixed position or range and filling in the reference tables with actual values for steady state load versus engine speed and engine torque versus load and speed. Once these reference tables have been calibrated, a map can be built to translate a driver request from the PPS into desired engine output. Actual output as measured by the MAF or MAP sensors is then returned to the ETC strategy again to verify that actual engine load closely matches predicted load to catch any error conditions that may result in “undesirable” engine acceleration.
Modifying the engine, exhaust, or accessories to increase power directly changes the relationship between engine power and throttle position. Torque-based systems often recognize this extra power as a fault and trigger limp mode if the change is large enough. Even though the PCM has been reprogrammed to maintain the proper air/fuel ratio and spark to operate correctly, the ETC system may not allow for so much airflow. In some cases, as little as 5% more power may be enough to force limp mode. The solution is to once again look into the tables controlling ETC function and adjust them to reflect the power the engine is actually making. In some instances, it may be possible to change the ETC strategy from torque-based to pedal follower to more easily accept modifications. Great care must still be taken to retain some form of genuine fault detection and control.
Since many transmission controls are also based on engine torque input, shift strategy is also optimized. Automatic transmission equipped vehicles with ETC usually take the torque control one step further to model instantaneous wheel torque output rather than just engine torque. Driver demands from PPS are calculated as wheel torque requests (vehicle speed or acceleration rate). With a known driveline ratio as indicated by the transmission gear selector and torque converter status, engine output torque can be recalculated as delivered wheel torque. This allows OEM calibrators the luxury of adjusting both engine and transmission conditions to provide a combination that supplies the desired wheel torque the driver has requested while performing earlier converter lockup and upshifts at light load to improve fuel economy. If a driver’s demanded torque exceeds the available reserve in the current gear at WOT, a downshift can then be performed to acquire a higher ratio. Another side benefit is the possibility of executing a transmission shift with zero change to wheel torque by manipulating engine output before, during, and after the shift. This zero torque change shift can yield a seamless driving experience to even the most discriminating consumers, a big benefit for OEMs looking for minimal NVH.
Idle Air Control
The throttle blade is the largest controlling factor in an engine’s airflow, but at low flow rates such as idle it may not be practical to accurately control the airflow rate with one large opening. The idle air control (IAC) valve acts as a small throttle valve with a narrower range of operation but finer control. IAC valves are multiposition devices usually based on a stepper motor that moves a plunger to cover a portion of an alternate flow path around the throttle blade. The PCM has a table showing the transfer function of the IAC as command position versus actual bypass flow. The PCM has a variable output that it uses to command a flow rate through this valve to control idle speed, idle stability, and deceleration. The IAC acts as the “coarse” tuning control of idle speed and stability. Since the valve cannot instantaneously open or close, it is used to control predicted steady state airflow during idle. Faster response to changing idle speed is done by spark advance since this can be done as quickly as the next ignition cycle while the IAC is still moving. During deceleration, the IAC acts as the dashpot control (more on dashpot in Chapter 10). If the PCM commands a lower output for IAC during closed throttle, the engine loses speed more quickly. This can also be adjusted to change the amount of engine braking being done during coast down.
Some engines are equipped with intakes that include multiple possible flow paths for incoming air. Longer runners are used to generate better torque at low and mid RPMs. Shorter runners are used to increase the ram tuning effect at high speed. Changing between the two flow paths is done by opening a butterfly valve attached to a solenoid. The point at which the solenoid opens the runner control valve can be dictated by the PCM as a function of any other input, usually TPS and RPM.
Another more recent innovation has been the implementation of charge motion plates or “port flaps.” These are similar in construction to runner controls, except that they only restrict flow across a single runner rather than direct flow from a different runner altogether. The purpose of a charge motion plate is to direct the intake charge toward one side of the intake port, improving the amount of mixing that happens once the air enters the cylinder itself. These plates are usually closed at idle and low speeds, leaving a small opening that results in a higher velocity stream of air heading toward the intake valves. Once engine speed (and subsequently, port velocity) picks up, the plates are opened similar to a throttle blade to allow virtually unrestricted flow. Generally speaking, very little extra power is gained by removing these from the intake tract, and the low-end torque loss is usually significant enough to be noticed by the average driver.
Camshaft controls are quickly becoming commonplace in modern engine controls. The ability to change valve-timing events on the fly allows automakers to tune engines more optimally for horsepower, emissions, and fuel economy simultaneously. These systems work by placing an oil-fed valve and spring assembly on the camshaft drive mechanism. A remote PCM-triggered solenoid directs oil flow to the assembly overcoming spring tension and changing actual valve timing.
Honda’s VTEC system was an early pioneer where a change from one set of cam lobes to another made for a distinct step change in engine output. Essentially, operating on a small camshaft at idle, cruise, and lower RPMs made for good mileage and emissions, and changing to larger cam lobes as speeds increased made more power.
BMW’s VANOS system was a leader in the more widely used cam phasing approach. This system uses a single set of cam lobes where the indexing of the cam relative to the crankshaft can be adjusted. By shifting the cam phasing relative to the crankshaft, it becomes possible to keep the ram tuning effect more powerful across a broader RPM range, increasing overall efficiency. Cam phasing has the benefit of being linear as opposed to the older binary VTEC system. With finer control and more adjustment points, camshaft performance can be optimized rather simply by switching between two set points.
On DOHC engines, it becomes possible to rotate intake and exhaust camshafts independently of one another to allow for another degree of freedom. With dual cam phasing, not only can each cam be indexed relative to the crank, but now overlap can also be incremented. With independent control, the exhaust duration can be adjusted at cruise for increased EGR or the intake cam can be adjusted to increase cylinder pressure as speed and loads change. Overlap can be added at high RPM to increase ram-tuning magnitude without creating unnecessary EGR flow or loss of compression at idle where the lobe centers can be separated more. Some variable cam timing systems also increase overlap during extreme cold start conditions to reduce pumping effort and use the warm exhaust to impart heat to the frozen intake manifold. Once a predetermined temperature is reached, the system reverts to a wide lobe separation to stabilize idle and reduce the amount of intake charge blowing from the intake to the exhaust without combustion.
BMW has further upped the ante lately with the addition of complete lift control. By adding another solenoid on a cam that changes the valve lift with respect to camshaft lobe lift, anywhere from 0 to 100% of the camshaft’s lift can be applied to the valves themselves. This has the potential to eliminate the need for a throttle blade since the intake valves themselves become the throttling mechanism to limit engine airflow. Although not currently in use for highperformance vehicles, the technology shows incredible potential that even the horsepower junkie can appreciate.
On supercharged and turbocharged engines the throttle blade is not the only method of load control. The amount of positive manifold pressure (boost) that an engine makes is directly proportional to the total engine load at a given operating point. Most OEM vehicles with forced induction include some form of PCM controlled boost limit. On a supercharged engine this is usually a bypass valve that can recirculate air around the compressor. Turbocharged engines typically employ a PCM-controlled wastegate to dump excess exhaust energy, limiting shaft speed. Either way, a PCM-triggered solenoid usually resides in series with the vacuum line between the reference (intake manifold or compressor outlet) and control (wastegate or bypass diaphragm). Most OEM PCMs employ some form of boost control to soften torque onset for driveline durability and NVH, manage “abuse,” or provide for a limp mode without boost. These situations can prove detrimental to making more power than stock and usually require changes to the PCM to overcome.
Written by Greg Banish and Posted with Permission of CarTechBooks