Tuned Port Injection Essay, Research Paper
The first production Tuned Port Injection (TPI) systems appeared on General Motors’ vehicles in 1985. The GM vehicles built with these systems were the Corvette, Pontiac Firebird, Pontiac Trans AM, and the Chevrolet Camaro. Upon their introduction, these systems achieved a 35 % improvement over carbureted systems and a 20% improvement over available forms of fuel injection in horsepower, torque and economy.
The 1985-1988 TPI system utilized the following sensors and devices to control the engine: Mass Air Flow Sensor and Module, Manifold Air Temperature Sensor, Coolant Temperature Sensor, Oxygen Sensor, Throttle Position Sensor, Cold Start Switch, Cold Start Injector, Fuel Injectors, Idle Air Control Valve, Vehicle Speed Sensor, Electronic Spark Timing Sensor and Module, and Knock Sensor.
When the key is in the “on” position the Electronic Control Module (ECM), the main computer that controls all sensors and engine functions, powers up and readies the engine for start-up. When the starter is engaged and the coolant temperature is less than 100 degrees Fahrenheit, determined by the Coolant Temperature Sensor (CTS), the Cold Start Injector provides a spray of fuel to each cylinder via an air distribution system built into the intake manifold. If the engine temperature is greater than 100 degrees Fahrenheit, the Cold Start Injector is disabled by the cold start switch. Upon startup the ECM utilizes information in the Erasable Programmable Read Only Memory (EPROM) to establish the initial pulse rate for the fuel injectors. The ECM pulses, or opens and closes, the remaining 8 fuel injectors in sequence and the engine starts. During this, the Idle Air Control (IAC) valve is completely open to allow as much air as possible to enter the engine to prevent it from dying out. At this point, the engine is operating in open loop mode and will continue to do so until the engine warms up. After the warm up period the ECM scans the sensors, if all sensors are operating and within their proper range, the engine then goes into closed loop operation. This means that the sensors are dynamically controlling the engine. The IAC valve then begins to close, reducing the amount of air entering the engine and thus slowing idle to the value specified in the EPROM. In the event that the information received from a sensor is higher or lower than the normal range, a code will set in the ECM, and the Service Engine Soon light will turn on.
The ECM receives information on airflow, from the Mass Air Flow Sensor (MAFS) or from the Manifold Absolute Pressure Sensor (MAPS), engine temperature, from the Coolant Temperature Sensor (CTS), air temperature, from the Manifold Air Temperature Sensor (MATS), exhaust gas oxygen content, from the Oxygen Sensor, throttle position, from the Throttle Position Sensor (TPS), and vehicle speed, from the Vehicle Speed Sensor (VSS).
The Mass Air Flow Sensor is located in-line with the throttle body and air filter box. It is a hollow cylinder with a 4-wire connector. The blue wire carries a reference voltage of 8-volts, the gray wire returns the variable resistance to the ECM, the black wire grounds the sensor, and the red wire is for the burn-off or cleaning stage. At any RPM above idle, such as during driving or revving, the MAFS begins to function. It directly measures the amount of air entering the engine at any speed. The MAFS accomplishes this with a thin wire strung across a calibrated tube. This wire is fed a reference voltage that causes it to heat up. When the wire has reached operating temperature, the ECM begins to measure the return voltage. As air flows through the MAFS and into the engine, the wire begins to cool and thus changes its resistance. More airflow causes a cooler wire, which results in a lower the resistance, and vise-versa. The ECM then cross-references the return voltage to a table stored in the EPROM to determine the exact amount of air entering the engine. When the engine is shut down, the thin wire is then reheated to “burn-off” any debris that is still on the wire. This sensor was used on the earlier TPI systems from 1985-89.
The Manifold Absolute Pressure Sensor (MAPS) reads engine manifold pressure and translates this reading into a voltage signal that the computer uses to monitor airflow into the engine. These reading are used primarily to calculate spark advance and fuel enrichment by using complex calculations and “fuel maps” to translate manifold pressure into airflow values. “Absolute” refers to the fact that the sensor reads manifold pressure in absolute terms; that is, starting from 0 as opposed to starting at sea-level atmospheric pressure (14.7psi). The MAP sensor has 3 wires, reference, ground, and return. In GM engines, the ECM feeds a 5-volt reference signal to the sensor through one wire. As engine manifold pressure varies, the resistance of the sensor to the 5-volt reference signal varies with it, and the new value is carried by the return wire to the ECM. The higher the manifold pressure becomes, the larger the increase in return voltage, and vise-versa. The key component in a MAP sensor is the strain gauge. A thin square silicon wafer is attached and sealed to a plate. The space between the two is a vacuum and four stretch sensitive resistors are attached to the plate’s edges. As the fluctuating manifold pressure acts against the perfect vacuum inside the plate, the resistors vary the return resistance. This sensor was used on the later TPI systems from 1990-92.
The Coolant Temperature Sensor (CTS) is one of the most important sensors under the hood. Nearly every engine management function is dependent upon proper information regarding engine temperature, and the most convenient indicator of this is the temperature of the engine coolant. The sensor is threaded into either the thermostat housing or the intake manifold, and has an oval shaped connector with two wires, one yellow, and a black ground wire. The CTS is known as a negative temperature coefficient thermostatic resistor. This means that as the outside temperature increases, the internal resistance of the CTS decreases. At 0?F, the resistance across the CTS terminals is about 25,000 ohms; at operating temperature of about 210?F, the resistance drops to under 200 ohms. The ECM determines engine coolant temperature by sending a 5-volt reference signal through the yellow wire, and then monitoring the voltage. When the sensor is cold, resistance is high, so the yellow wire voltage is around 3-4 volts; as the sensor heats up, the voltage will drop, “pulled low” by the decreasing resistance of the CTS. To understand the concept of “pulling low””, think of a water pipe with exactly 5psi of pressure or voltage in it when the end of the pipe us closed off. As the end of the pipe is opened or resistance decreases, the pressure in the pipe drops; if the end of the pipe is completely opened or no resistance is present, the pressure or voltage will drop to very near zero. If you were to disconnect the CTS connector and ground the yellow wire, the ECM would see almost 0 volts on the yellow wire. While the CTS is simple in terms of operation, its accuracy is absolutely critical to the proper operation of the engine. Everything from air/fuel mixture to torque converter lockup is controlled to some extent by the CTS. In fact, many engines will not even start if the CTS is disconnected. While the ECM will display a check engine light if the sensor readings are out of range, the CTS is the ECM’s only source for coolant temperature information. In many cases, the CTS can be wildly inaccurate while still staying within range because the ECM has no other reference to decide if the CTS is incorrect.
The Manifold Air Temperature Sensor (MATS) is very similar in both appearance and operation to the coolant temperature sensor (CTS), the major difference being that air, not coolant, is the medium that is being measured. The ECM sends a 5-volt reference signal through the tan wire to the MATS and then monitors the voltage. Hotter temperatures will increase resistance and therefore lower the ECM’s voltage reading, colder temperatures will have the opposite effect. This sensor is used by the ECM to determine the proper air/fuel mixture. While the sensor location varies depending on the engine and model, on TPI equipped cars the sensor is threaded into the underside of the intake plenum.
The oxygen sensor is one of the most critical components in the engine management system. Its primary function is to monitor the oxygen content of the exhaust in order for the ECM to constantly calculate and update the air/fuel mixture for optimum economy and performance. The sensor itself looks roughly like a spark plug, with anywhere from 1 to 4 wires connected to it. It is always threaded into a fitting located between the exhaust manifold and the catalytic converter. Most sensors have a single wire that carries the signal to the ECM; sometimes this type will have a second wire that acts as a sensor ground. Some are heated sensors that have either three or four wires. In three wire sensors the extra wires are for the sensor heating element, and the heater ground wire. Four wire sensors have an additional wire to ground the sensor.
The tip of the sensor usually contains a ceramic/zirconia element that is coated with platinum at the exposed end, and is covered by a fluted metal sheath. The other end of the sensor usually has either a small metal canister or a plastic sheath that contains ambient air. The tip of the sensor protrudes into the exhaust stream and the difference in the amount of oxygen in the exhaust compared to the amount of oxygen in the ambient air “trapped” in the end of the sensor causes a small (0.10 to 0.90 volts) amount of voltage to be generated. The smaller the oxygen content difference between the exhaust and the ambient air, the lower the voltage generated and vise-versa. Therefore, leaner mixtures generate lower voltages and richer mixtures generate higher voltages. Actually, the sensor acts as a small chemical battery. In operation, the sensor voltage will “swing” from rich to lean and back, as the ECM constantly monitors the exhaust oxygen content and alters the air/fuel mixture to compensate. The threshold between rich and lean in 0.45 volts; each time the sensor voltage crosses this figure a “crosscount” occurs. Crosscounts are an important figure for advanced engine management troubleshooting.
The Throttle Position Sensor (TPS) is a rotary 3-wire potentiometer. It produces a variable voltage that is used by the ECM to determine the position or angle of the throttle blades. The ECM requires this information to determine a number of engine operating parameters, including air/fuel ratio, torque converter lockup, and ignition timing. The TPS is always located on the throttle body assembly, mounted on the throttle shaft opposite of the throttle linkage. The sensor is grounded through the ECM, which sends a 5-volt reference signal to the TPS. The third wire supplies the variable voltage to the ECM that indicates throttle position.
The Vehicle Speed Sensor (VSS) is an often misunderstood and ignored component of the engine management system, but it is definitely an important one. Depending on the application, the VSS can control any one or more of the following: torque converter clutch lockup, cruise control operation, Idle Air Control (IAC) valve position, ignition timing, and shift light activation. There are two primary types of VSS: models with a speedometer cable, and models with an electronic speedometer.
On models that use a speedometer cable the VSS is mounted behind the speedometer head in the instrument cluster. The smaller black end is the pickup, which consists of an infrared Light Emitting Diode (LED) and a photodiode mounted facing each other. In operation a trigger wheel rotates and alternately exposes the LED to the photodiode and hides it. As the photodiode reacts to the light and dark stages, it sends a signal to a buffer unit at the other end of the VSS assembly. The buffer converts the signal into a square wave for the ECM to process. The higher the frequency of the square wave, the faster the speed interpreted by the ECM.
On models with an electronic speedometer, the VSS is mounted directly to the transmission, where the speedometer cable connection would normally be. This type of sensor outputs directly to the ECM, which uses the signal both to perform the various calculations listed above as well as to send the output to the speedometer. Unlike the speedometer mounted VSS the transmission mounted VSS delivers a magnetic pulse similar to a distributor pickup sensor.
The information from these sensors is used to calculate the proper pulse width for the injectors and the ECM then fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum air/fuel ratio. The electronic spark timing sensor and module provide maximum timing advance to improve engine idle and performance. If engine ping is detected by the knock sensor, the timing is automatically retarded to prevent damage from occurring. This is also a continuous process.
The Erasable Programmable Read Only Memory (EPROM) chip that is installed in the ECM provides specific information on the engine in the vehicle. EPROM chips are programmed with different timing characteristics and injector pulse widths for different engines. To allow for the various engine, transmission, and gear ratio combinations and to meet standards for emissions, a wide variety of these EPROM’s were manufactured. The 1985 TPI calibration information is contained in a 32K EPROM. The 1986-89 ECM contained a 128K EPROM, 90-92 ECM’s use a 256K EPROM. The factory ECM has a learning capability that allows it to make corrections for minor variations in the fuel system to improve performance and drivability. There are two learning features, the Integrator and Block Learn (IBL) and Block Learn Memory (BLM). The IBL feature has a normal value of around 128. If this value is higher than 128, it indicates that the ECM is adding fuel to the base fuel calculation because the system is running lean, a value lower than 128 indicates that the ECM is taking out fuel because the system is running rich. The IBL is a short-term corrective action while the BLM is a long-term correction. The BLM value will change if the IBL has seen a condition that lasts for a longer period of time. There are from two to sixteen different cell locations that the ECM modifies depending on engine RPM, airflow, manifold air pressure, and other conditions such as air conditioning status, etc. The ECM learns how much adjustment is required in each cell, retains it in memory, and applies these adjustments when the engine operates in that RPM or load range. These features of the ECM allow the system to adjust itself automatically to your engine and assure peak performance. If the vehicle power is disconnected for repair or to clear diagnostic codes, the learning process has to begin all over again.
Several modifications were made to the TPI system throughout its implementation. The 1985 system used a Mass Air Flow Sensor (MAFS) and a MAFS module to control the power and cleaning functions for the MAFS. In 1986 two relays replaced the MAFS module and the ECM was changed. In 1989 the cold start injector was deleted. The EPROM provided a wider pulse width on startup to provide a richer mixture for a cold engine. For those years, all other features are the same. In 1990 the MAF was replaced with the MAP sensor. The1990-92 TPI system still operated the same except that manifold air pressure is used to calculate injector pulse width as opposed to airflow.
Tuned Port Injection saw continued use on General Motors’ performance cars from 1992 through 1994. During that period, the TPI system changed greatly but the basic concept behind its operation remained the same. In 1994, GM killed the TPI system and made the switch to a more modern and efficient Sequential Port Fuel Injection (SEFI) system, which is now used all their performance cars.