System Sensors


System Sensors

The ability of the fuel injection system to control the air/fuel ratio depends on its ability to properly time the injector pulses with the compression stroke of each cylinder and its ability to vary the injector "on" time, according to changing engine demands. Both tasks require the use of electronic sensors that monitor the operating conditions of the engine.

Airflow Sensors

To control the proportion of fuel to air in the air/fuel charge, the fuel system must be able to measure the amount of air entering the engine. Several sensors have been developed to do just that.

VOLUME AIRFLOW SENSOR The airflow sensor measures airflow, or air volume. The sensor consists of a spring-loaded flap, potentiometer, damping chamber, backfire protection valve, and idle bypass channel. As air is drawn into the engine, the flap is deflected against the spring. A potentiometer, attached to the flap shaft, monitors the flap movement and produces a corresponding voltage signal. The strength of the signal increases as the flap opens. The signal voltage is relayed to the electronic control module.

DAMPING CHAMBER The curved shape of the airflow sensor is the damping chamber. The damping flap in this chamber is on the same shaft as the airflow sensing flap and is also about the same area. As a result, the damping flap smooths out any possible pulsations caused by opening and closing of the intake valves. Airflow measurement can be a steady signal, closely related  to airflow as controlled by the movement of the flap.

BACKFIRE PROTECTION The airflow sensor flap provides for backfire protection with a spring-loaded valve. If the intake manifold pressure suddenly rises because of a backfire, this valve releases the pressure and prevents damage to the system.

IDLE BY-PASS The airflow sensor assembly includes an extra air passage for idle, bypassing the airflow sensor plate. When the throttle is closed at idle, the opening and closing of intake valves can cause pulsations in the intake manifold. Without the idle by-pass, such pulsations could cause the flap to shudder, resulting in an uneven air/fuel mixture. The idle by-pass smooths the flow of the idle intake air, ensuring regular signals to the electronic control.

Another design of airflow sensor, called a Karman Vortex sensor, works on a different operating principle. Air entering the airflow sensor assembly passes through vanes arranged around the inside of a tube. As the air flows through the vanes, it begins to swirl. The outer part of the swirling air exerts high pressure against the outside of the housing. There is a low-pressure area in the centre. The low-pressure area moves in a circular motion as the air swirls through the intake tube. Two pressure-sensing tubes near the end of the tube sense the low-pressure area as it moves around. An electronic sensor counts how many times the low-pressure area is sensed.

The faster the airflow, the more times the low-pressure area is sensed. This is translated into a signal that indicates to the combustion control computer how much air is flowing into the intake manifold.

Air Temperature Sensor

Cold air is denser (weighs more) than warm air. Cold, dense air can burn more fuel than the same volume of warm air because it contains more oxygen. This is why airflow sensors that only measure air volume must have their readings adjusted to account for differences in air temperature.

Most systems do this by using an air temperature sensor mounted in the throttle body of the induction system. The air sensor measures air temperature and sends an electronic signal to the control computer. The computer uses this input along with the air volume input in determining the amount of oxygen entering the engine.

In some early EFI systems, the incoming air is heated to a set temperature. In these systems an air temperature sensor is used to ensure that this predetermined operating temperature is maintained.

Mass Airflow Sensors

A mass airflow sensor (MAF) does the job of a volume airflow sensor and an air temperature sensor. It measures air mass. The mass of a given amount of air is calculated by multiplying its volume by its density. As explained previously, the denser the air, the more oxygen it contains. Monitoring the oxygen in a given volume of air is important, since oxygen is a prime catalyst in the combustion process. From a measurement of mass, the electronic control unit adjusts the fuel delivery for the oxygen content in a given volume of air. The accuracy of air/fuel ratios is greatly enhanced when matching fuel to air mass instead of fuel to air volume.

The mass airflow sensor converts air flowing past a heated sensing element into an electronic signal. The strength of this signal is determined by the energy needed to keep the element at a constant temperature above the incoming ambient air temperature. As the volume and density (mass) if airflow across the heated element changes, the temperature of the element is affected and the current flow to the element is adjusted to maintain the desired temperature of the heating element. The varying current flow parallels the particular characteristics of the incoming air (hot, dry, cold, humid, high/low pressure). The electronic control unit monitors the changes in current to determine air mass and to calculate precise fuel requirements.

There are two basic types of mass airflow sensor: hot wire and hot film. In the first type, a very thin wire (about 0.2mm thick) is used as the heated element. The element temperature is set at 100C to 200C above incoming air temperature. Each time the ignition switch is turned to the off position, the wire is heated to approximately 1000C for 1 second to burn off any accumulated dust and contaminants.

The second type uses a nickel foil sensor, which is kept 75C above ambient air temperatures. It does not require a burn-off period. Thus, it is potentially longer lasting than the hot wire type.

Manifold Absolute Pressure Sensor

Some EFI systems do not use airflow of air mass to determine the base pulse of the injector(s). Instead, the base pulse is calculated on manifold absolute pressure (MAP).

The MAP sensor measures changes in the intake manifold pressure that result from changes in engine load and speed. The pressure measured by the MAP sensor is the difference between barometric pressure (outside air) and manifold pressure (vacuum). At closed throttle, the engine produces a low MAP value. A wide-open throttle produces a high value. This high value is produced when the pressure inside the manifold is the same as pressure outside the manifold, and 100 percent of the outside air is being measured. This MAP output is the opposite of what is measured on a vacuum gauge. The use of this sensor also allows the control computer to adjust automatically for different altitudes.

The control computer sends a voltage reference signal to the MAP sensor. As the MAP changes, the electrical resistance of the sensor also changes. The control computer can determine the manifold pressure by monitoring the sensor output voltage. A high pressure, low vacuum (high voltage) requires more fuel. A low pressure, high vacuum (low voltage) requires less fuel. Like an airflow sensor, a MAP sensor relies on an air temperature sensor to adjust its base pulse signal to match incoming air density.

In EFI systems with a MAP sensor, the computer program is designed to calculate the amount of air entering the engine from the MAP and engine rpm input signals. This type of EFI system is referred to as a speed density system because the computer calculates the air intake flow from the engine speed input and the density of the intake manifold vacuum input. Many EFI systems with MAF sensors do not have MAP sensors. However, there are a few engines with both of these sensors. In these cases, the MAP is used mainly as a backup if the MAF fails. When the EFI system has a MAF, the computer calculates the intake air flow from the MAF and rpm inputs.

Other EFI System Sensors

In addition to airflow, air mass, or manifold absolute pressure readings, the control computer relies on input from a number of other system sensors. This input further adjusts the injector pulse width to match engine operating conditions. Operating conditions are communicated to the control computer by the following types of sensors.

COOLANT TEMPERATURE The coolant temperature sensor signals the electronic control unit when the engine needs cold environment, as it does during warm-up. This adds to the base pulse, but decreases to zero as the engine warms up.

THROTTLE POSITION The switches on the throttle shaft signal the electronic control unit for idle enrichment when the throttle is closed. These same throttle switches signal the electronic control unit when the throttle is near the wide-open position to provide full load enrichment.

ENGINE SPEED The ignition system sends a tachometer signal reference pulse corresponding to engine speed to the electronic control unit. This signal advises the electronic control unit. This signal advises the electronic control unit to adjust the pulse width of the injectors for engine speed. This also times the start of the injection according to the intake stroke cycle.

CRANKING ENRICHMENT The starter circuit sends a signal for fuel enrichment during cranking operations even when the engine is warm. This is independent of any cold-start fuel enrichment demands.

ALTITUDE COMPENSATION As the car operates at higher altitudes, the thinner air needs less fuel. Altitude compensation in a fuel injection system is accomplished by installing a sensor to monitor barometric pressure. Signals from the barometric pressure sensor are sent to the ECU to reduce the injector pulse width (or reduce the amount of fuel injected).

COASTING SHUTOFF Coasting shutoff can be found on a number of control systems. It can improve fuel economy as well as reduce emission of hydrocarbons and carbon monoxide. Fuel shutoff is controlled in different ways depending on the type of transmission (manual or automatic). The ECU makes a costing shutoff decision based on a closed throttle as indicated by the throttle position or idle switch or on engine speed as indicated by the signal from the ignition coil. When the ECU detects that power is not needed to maintain vehicle speed, the injectors are turned off until the need for power exists again.

ADDITIONAL INPUT INFORMATION SENSORS Additional sensors are also used to provide the following information on engine conditions. NOTE: This list does not attempt to cover all of the sensors that are used by all manufacturers. It contains the most common.

- Detonation

- Crankshaft position

- Camshaft position

- Timing of ignition spark

- Air conditioner operation

- Gearshift lever position

- Battery voltage

- Amount of oxygen in exhaust bases

- Emission control device operation

ELECTRONIC CONTROL COMPUTER

The heart of the fuel injection system is the control computer or electronic control unit (ECU). The ECU is a small computer that is usually mounted within the passenger compartment to keep it away from the heat and vibration of the engine. The ECU includes solid state devices, including integrated circuits and a microprocessor.

The ECU receives signals from all the system sensors, processes them, and transmits programmed electrical pulses to the fuel injectors. Both incoming and outgoing signals are sent through a wiring harness and a multiple-pin connector.

Electronic feedback in the ECU means the unit is self-regulating and is controlling the injectors on the basis of operating performance or parameters rather than on preprogrammed instructions. As ECU with a feedback loop, for example, reads signal from the oxygen sensor, varies the pulse width of the injectors, and again reads the signals from the oxygen sensor. This is repeated until the injectors are pulsed for just the amount of time needed to get the proper amount of oxygen into the exhaust stream. While this interaction is occurring, the system is operating in the closed loop. When conditions, such as starting or wide-open throttle, demand that the signals from the oxygen sensor be ignored, the system operates in open loop. During open loop, injector pulse length is controlled by set parameters contained in the ECU memory banks.

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