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Originally Posted by Bosch Fuel Injection & Management by Charles O. Probst,SAE
PULSED INJECTION—THEORY
Lambda Control
You’ll remember from chapter 2 the discussion of the ideal air-fuel ratio (lambda) and its relation to emissions. On most systems, the air-fuel ratio for best emission control is achieved by sensing the oxygen content of the exhaust gas with a lambda sensor, shown in Fig. 2-45. The lambda sensor’s signal is monitored by the control unit, which then adjusts pulse time to maintain the ideal air-fuel ratio. The system operates closed-loop.
Fig. 2-45. Lambda sensor looks something like a spark plug installed in exhaust pipe or exhaust manifold to sense oxygen content of exhaust gas. The closer to the exhaust valves, the faster it heats up.
Lambda Sensor Design and Operation. The lambda sensor is essentially a small battery that generates a voltage signal based on the differential between the oxygen content of the exhaust gas, and the oxygen content of the ambient air.
A cutaway view of the lambda sensor is shown in Fig. 2-46. The tip of the sensor that protrudes into the exhaust gas is hollow, so that the interior of the tip can be exposed to the ambient air. Both sides of the ceramic tip of the sensor are covered with metal electrodes that react to create a voltage only if the ambient air has a higher oxygen content than the exhaust and the ceramic material is hotter than 575°F (300°C)
When these conditions are met, voltage is generated between the two sides of the tip. The voltage is usually about 1 volt. But if the engine is running lean, the exhaust gas has about the same amount of oxygen as the ambient air, so the lambda sensor will generate little or no voltage; if the engine is running rich, the oxygen content of the exhaust will be much lower than the ambient air and the sensor voltage will be larger. See Fig. 2-47.
Fig. 2-46. Cutaway view of lambda sensor.
Fig. 2-47. Rich mixture and low content of oxygen in exhaust causes voltage output from oxygen sensor. Re member, rich = voltage.
Some cars have a lambda sensor that has a heating element built-in to speed warming of the sensor to improve the drivability and reduce the emissions of a cold engine. On a cold engine, it may take 90 to 120 seconds for an unheated lambda sensor to get warm enough to start generating voltage, while a heated sensor may be warm enough after 10 to 15 seconds.
Lambda Closed-Loop Control. Recall the discussion o open-loop/closed-loop systems in chapter 2. The lambda sensor and the control unit form a closed-loop system that continually adjusts the air-fuel ratio by means of the fuel-injector pulse time. For example, the sensor generates a high voltage be cause the mixture is rich, so the control unit reduces pulse time to lean the mixture. Sensor voltage falls, so the control unit increases pulse time to enrich the mixture. Sensor voltage increases, etc....
Fig. 2-48. Since about 1980, fuel-injected engines operate closed-loop most of the time.
The lambda sensor voltage is always fluctuating as shown in 2-49, so it is hard to maintain the exact point at which the air-fuel ratio is ideal. Instead, the ratio tends to oscillate to either side of the ideal ratio, but the oscillation is so fine (about 0.1% change in the air-fuel ratio) that it is not noticeable in engine performance. The rate of the air-fuel ratio oscillation is related to the quantity of exhaust passing the sensor. At idle, the cycle from lean to rich and back again may take 1 to 2 seconds. At cruising speed, the cycle may happen several times a second.
Rich Lean Rich Lean
Fig. 2-49. Closed-loop lambda sensor voltage cycles back and forth from slightly rich to slightly lean. In chapter 4, you’ll measure the effects of this cycling.
This closed-loop system can compensate to some degree changes in the engine over time. For example, if a valve is leaking slightly, or if there is an intake air leak, the lambda sensor senses the change in combustion and brings the system back within its design limits. This has been described as having a skilled technician under the hood, continuously tuning the mixture for best operation under all conditions. Changes beyond the system’s range, though, can still lead to drivability problems.
When the oxygen sensor is cold and not generating a voltage signal, the control unit is programmed to operate open loop at a programmed injection rate. The same thing happens if you disconnect or cut the lambda sensor wire, or if the sensor is fouled by leaded gasoline. This becomes important when you are trying to make closed-loop adjustments at idle, but the sensor cools off because not enough exhaust is passing it. Many service procedures depend on closed-loop operation, so remember that the sensor has to be warm enough.
As you’ll see in the service chapters, the lambda control system is properly adjusted when engine-out CO is the same whether the system is closed-loop or open-loop. This adjust- point allows the system its full range of compensation for operating conditions.
L-Jetronics have been used since 1974. Engines adapt easily to L-Jetronic. In 1980, when tighter U.S. emission limits were mandated, virtually every European car imported to the U.S. which had used carburetors in 1979 switched to L-Jetronic. Those that did not, switched to K-Jetronic.
3.1 Air-Mass Sensor
The air-mass sensor is completely electronic. It depends on the measurement of current flowing through heated wires to measure air flow. It is also known as the hot-wire sensor because of its heated-wire design, hence the H” in LH. It has several advantages over the vane-type air-flow sensors of L Jetronic.
1. It measures air mass, or weight, so it requires no correction for changes in density due to temperature or altitude. The air-fuel mixture ratio depends on mass: so much weight of fuel mixed with so much weight of air. Measuring mass eliminates the need for compensation sensors: air temperature, and altitude. It also reduces correcting computations in the control unit.
2. It has no moving parts. That means mechanical simplification. It responds even faster than the moving vane of the air-flow sensor. Measurements follow changes in air-mass in 1—3 milliseconds.
3. It offers insignificant resistance to the passage of air. Even at maximum air flow, drag force on the wire is measured in milligrams.
Air-mass measurement by hot-wire improves drivability, stability, and reliability. It is used in racing. In my opinion, air-mass sensing will probably supplant the measurement of air flow by vane-type sensors.
Air-Mass Sensor Design and Operation
Under hood, between the air cleaner and the manifold, you’ll see a simple black plastic cylinder with an electronic box, as shown in Fig. 3-1. If you remove it and look inside the protective screens, you may be able to see the small platinum resistance, or hot wires, that are suspended inside the cylinder so that the intake air can flow over them. See Fig. 3-2. How fine are these wires? The diameter is 70 micrometers—that’s less than 1/io millimeter, finer than a human hair. By careful design of the sensor and its mounting, the fine wires survive automobile vibration. In the unlikely event a wire should break, the warm engine runs, though without fuel compensation, in a limp-home mode. You can simulate limp-home mode by driving the car after pulling the air-mass sensor connector of a warm engine.
Fig. 3-1. Look for air-mass sensor between air cleaner and intake manifold.
The hot-wire system depends on measurement of the cooling effect of the intake air moving across the heated wires. Suppose you had a fan blowing across an electric heater. With a small movement of air past the heated wires, the cooling effect is small. With more air moving past the heated wires, the cooling effect is greater.
LH control circuits use this effect to measure how much air passes the LH hot wire. The hot wire is heated to a specific temperature differential 180°F (100°C) above the incoming air when the ignition is turned on. As soon as air flows over the wire, the wire is cooled. The control circuits then apply voltage to keep the wire at the original temperature differential. This creates a voltage signal which the control unit monitors: the greater the air flow (and wire cooling) the greater the signal.
Fig. 3-2. Air-mass sensor includes hot-wire assembly and control circuits.
Control Circuits. The LH control circuits use a Wheatstone bridge, as shown in Fig. 3-3. The hot wire is one leg of a bridge circuit whose output voltage is held to zero by regulating the heating current. The hot wire, known as Rh, changes resistance with temperature. Incoming air passes over the hot-wire. Rh, and also over another resistance wire, Rk. The same voltage is applied to both wires. In series with Rk are two fixed resistances, R1 and R2; in series with our hot wire, Rh, is fixed resistance R3.
Fig. 3-3. Wheatstone bridge principle explains maintenance of hot-wire temperature.
The Rh wire is controlled to be 180°F (100°C) hotter than the intake air flowing through. For example, if the air is at freezing, 32°F (0°C), the wire will be heated to 212°F (100°C). On a hot day, if the air is at 86°F (30°C), the control circuits heat the wire to the same 180°F (100°C) difference, to 266°F (130°C).
The air mass changes when the driver changes the throttle opening:
1. more air passes over both resistance wires in the air-mass meter;
2. both Rh and Rk are cooled by the increased air mass;
3. Rh decreases its resistance, because of its Positive Temperature Coefficient;
4. current flowing through Rh increases more than the current through Rk;
5. that unbalances the bridge circuit;
6. comparator increases its output;
7. amplifier increases current (Jh) to bring Rh back to its original resistance, and thus its original temperature back to 180°F (100°C) above ambient temperature of the intake air;
8 the heating current is measured as a voltage drop (Um) across the fixed resistance R3;
9 this voltage drop is a measure the air mass and is used as the output signal to the control unit.
All this happens is 1—3 milliseconds.
The air mass can also change because air temperature or altitude changed its density.
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