EFI Alphabet Soup
Electronic fuel injection often seems more complicated than it is because of the myriad acronyms used to reference the sensors and controllers included in most systems.
Electronic fuel injection comes in two different flavors, throttle body injection (TBI) and direct port injection. TBI should actually be classified as AFI, almost fuel injection. These systems position the injectors within the throttle body, rather than in each individual port. A TBI unit is similar to a carburetor in that it is bolted to a traditional-style single- or dual-plane intake manifold. However, unlike a carburetor, which requires air flow to draw fuel through it, a TBI unit injects fuel under pressure and is electronically controlled. (Note that direct port systems also incorporate a throttle body, but its purpose is strictly to control airflow. Individual injectors located within a few inches of the intake valves control fuel flow.) Throttle body injection is a lower cost alternative to direct port and has largely been phased out of production as original equipment.
EFI systems are broadly classified according to the manner in which they determine the amount of air entering the throttle body. Most original equipment systems are of the mass air flow persuasion and incorporate a mass air flow (MAF) sensor that actually measures the amount of air flowing through it. The majority of aftermarket systems are of the speed density type. These systems compute the amount of incoming air using calculations that rely primarily on air pressure within the intake manifold (as a measurement of load) and engine speed.
With both types of systems, a variety of other sensors feed input to the engine controller so adjustments can be made for changes in manifold air temperature, throttle position and engine coolant temperature. When a system goes into closed loop operation, an exhaust gas oxygen sensor (usually referred to as an O2 sensor) is used to monitor the oxygen content of the exhaust. Based on input from the O2 sensor, the controller makes the adjustments required to keep the air/fuel mixture at the chemically correct ratio (14.7:1).
The primary electronic component of an EFI system is the ECM— electronic control module or engine control module—the brains of the outfit. Essentially, the ECM receives all sensor input and performs all of the calculations required to establish fuel flow rates, ignition timing and idle speed.
As a means of allowing a single ECM to be configured to control a variety of engines, calibration data is contained in a PROM, which is an acronym for a programmable read-only memory. Once a PROM is programmed, an ECM can read the data it contains, but can’t alter it. Depending on the system, a PROM may or may not be removable. Old school PROMs are removable and must be erased with ultraviolet light before they can be reprogrammed with new data. Newer electronically erasable programmable read-only memory (EEPROM) also called flash memory, can, as their name implies, be erased electronically. EEPROMS typically are not removable and are erased and reprogrammed through the vehicle’s diagnostic port using specialized software installed on a laptop/notebook computer.
Irrespective of a PROM’s type, its purpose is to hold all of the data that the control module needs to match a given set of sensor inputs to fuel and ignition control outputs. The data inside a PROM is arranged in multi-dimensional arrays that look like a topographical map. The reason that computer-controlled engines don’t always respond well to modifications is that the ECM may not be able match the sensor inputs to the data in the map, or the data may not be appropriate for a modified engine. In either case, the computer will continually hunt for output data that will provide the desired engine speed, air/fuel ratio or ignition timing. It’s possible that it may not find the correct data, which makes for erratic engine operation unless the PROM is reprogrammed to account for the operating characteristics of the modified engine.
One of the most confusing aspects of electronic fuel injection is the alphabet soup of abbreviations that pertain to system sensors and controllers. Some of these are:
IAT (Inlet Air Temperature) Sensor (also called manifold air temp or MAT sensor). As its name implies, this sensor monitors the temperature of air entering the intake manifold. This information is sent to the ECM so that adjustments can be made to compensate for changes in inlet air temperature. Depending on the system, fuel flow, ignition timing, both or neither may be altered. In some vehicles, the IAT sensor is incorporated in the mass air sensor.
TPS (Throttle Position Sensor). It doesn’t take a rocket scientist to figure out that the TPS tells the ECM the position of the throttle (usually expressed as percentage of throttle opening). What isn’t so apparent is that this sensor also sends data concerning the rate of throttle opening. This rate is used to calculate the degree of air/fuel enrichment required for acceleration (in a carburetor this function is handled by the accelerator pump). Depending on the system, the TPS may be physically adjustable or non-adjustable and electronically computed. In the latter type of systems, rather than relying strictly on absolute voltage readings to determine throttle position, the system equates the lowest voltage encountered with 0 percent throttle opening (throttle closed). The computer then bases throttle position (percentage of opening) on that voltage. If the PCM subsequently sees a lower voltage, it revises its calibrations with the new voltage equated to 0 percent throttle opening.
Provisions for a throttle position sensor are not incorporated in electronically controlled throttle bodies. The PCM determines throttle position by monitoring the electric motor that controls the throttle. In essence, the same electronic circuit that controls the throttle feeds back the information so the PCM knows the position it commanded and the position achieved.
ECT (Engine Coolant Temp) Sensor. The name for this sensor is self-explanatory, but the output of this sensor has a pronounced affect on performance. Both air/fuel ratio and ignition timing are altered depending upon coolant temperature. The ECT sensor’s output is used to mimic the effect of a carburetor choke assembly. As an engine warms up, the ECM opens the choke by commanding a leaner air/fuel mixture. However, ECT output can also be used to retard ignition timing and richen the air/fuel mixture as a means of preventing damage to an engine that’s operating with abnormally high coolant temperature.
Knock Sensor. This device helps protect an engine from a driver who’s asleep at the wheel, too cheap to buy gasoline of the required octane or both. The sensor itself is like a small microphone. It listens for the mechanical sounds of engine knock and when it hears them, it signals the ECM, which temporarily retards timing. A knock sensor is always on duty, so it doesn’t simply signal the ECM at the first occurrence of knock. As long as it hears the ugly sounds of knock, it signals the engine controller that all is not well in the combustion chambers. The ECM continually alters spark timing as long as knock is detected. When graphed, spark timing data often looks like a series of saw teeth because almost as soon as the ECM commands spark retard, it cancels that command. If subsequent knock occurs, the ECM again commands spark retard and then cancels the command. This sequence of adjustments continues until knock is no longer detected. Typically, if the conditions that caused the original incidence of knock still exist after the first series of spark adjustment commands, less retard is needed on following adjustments. So a saw-tooth spark pattern typically has teeth that diminish in size as it proceeds from the first occurrence of knock to the last.
IAC (Idle Air Controller). The traditional method of controlling idle speed is through an IAC. This device incorporates a small electric motor that controls an air bypass valve (a separate air channel not controlled by the throttle plates). If the idle speed established by throttle position is too high or too low, the PCM moves the IAC to open or close the bypass valve as required to achieve the desired idle speed.
IACs are not used in systems that incorporate electronic throttle control (such as the 1997 and later Corvettes). Since the throttle plate is under electronic control, the ECM can open or close it, as required, to achieve commanded idle speed.
MAP (Manifold Absolute Pressure) Sensor. This sensor monitors pressure inside an intake manifold. Manifold pressure varies according to the amount of load under which an engine is operating. The PCM alters air/fuel ratio to accommodate varying load conditions, just as a power valve or metering rods perform this function in a carburetor. MAP sensor input may also be used to control ignition timing in speed/density systems.
With all the electronic components that constitute an EFI system, many people lose sight of the fact that the accompanying mechanical equipment must be properly matched to an engine’s requirements. If fuel injectors, fuel pump or fuel filter have insufficient flow capacity, performance will be compromised and engine damage may result.
O2 (Oxygen) Sensor. In spite of all of the information sent to a PCM, it doesn’t always command the ideal air/fuel ratio. Oxygen sensors are incorporated into EFI systems to monitor exhaust gas oxygen content. The PCM alters fuel flow in response to O2 sensor readings so that a stoichiometric (chemically ideal) air/fuel ratio (nominally 14.7:1) is maintained. The O2 sensors used in LS1 systems are known as narrow band sensors, essentially switching devices that tell the PCM whether the mixture is richer or leaner than stoichiometric. The PCM continually varies fuel flow, and O2 sensor voltage typically cycles between 0 and 1,000 millivolts. The end result is that the rich and lean spikes average out to the desired air/fuel ratio.
CKP (Crankshaft Position Sensor). As the name implies, this sensor reports crankshaft rotational position. As the reluctor teeth pass the magnetic crank position sensor, they generate pulses, which are used by the PCM to determine crankshaft position.
CMP (Cam Position Sensor). For an ECM, knowing crankshaft position may not be enough for it to do its job properly. Systems with sequential injector control, or with distributorless ignition, need a means of determining when cylinder number one is at TDC at the end of the compression stroke. (Since the camshaft rotates at half the speed of the crankshaft, its position can be used to define when the crank is in position for cylinder number one to cycle through its power or exhaust stroke.)
Next issue: EFI Alphabet Soup, Part 2— Acronyms in Action