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Reduction of noxious emissions

Reduction of noxious emissions

Excess-air factor >.

Rg. 41 Effect of the catalytic exhaust detoxi-caf/on with Lambda closed-loop control. Optimal mixture setting rangę:>. = 0.99... 1.00

Noxious proportion a: without aftertreatment, b: with aftertreatment. CO carbon monoxide. CH carbohydrates, NO, nitrogen oxide

Fig. 42 Functional diagram of the L-Jetronic with Lambda closed-loop control

1 Air-flow sensor, 2 Engine, 3 Lambda sensor, 4 Catalyst. 5 Injection valves, 6 Control unit with regulator. U: Sensor voltage, U_s Vehicle voltage, n Engine speed, Engine temperatura

The noxious emissions from the spark-ianition engine can be effectiyęly re-duced bv means of catalytictreatment

The fuel combustion in the working cylinder of an engine is morę or less in-complete. The morę incomplete the combustion, the greatertheamountof harmful materials in the exhaust gas. There is no such thing as complete combustion of fuel, not even when there is an excess of oxygen. This means that the exhaust will always have a certain harmful content. In order to reduce the effect on the environ-ment, the exhaust gas must be puri-fied.

Ali Chemical procedures necessary for exhaust detoxication can be grouped under the heading “Aftertreatment of exhaust gas”. This includes the Chemical conversion of noxious exhaust con-stituents such as carbon monoxide, hydrocarbons and nitrogen oxide into harmless constituents. For the aftertreatment of exhaust gas there is an ef-fective catalytic process. A prerequisi-te of this process, ho\vever, is optimal mixture composition. An optimal, i.e. stoichiometric mixture composition exists when, with the amount of air drawn in, just enough fuel is gasified or is injected to enable in theory a complete combustion to take place, i.e. a combustion without excess of oxygen orfuel. Such a mixture is characterized by an air ratio X of exactly 1.00.

The actual difficulty in purifying ex-haust gases by means of “catalytic aftertreatment" is in strictly maintaining X = 1.00 for every operating condition of the engine. A deviation of a mere1% is enough to impair the aftertreatment to a considerable degree. It is impos-sible to keep the mixture composition constantly within a limit which should be considerably less than 1% if an open-loop mixture control is used. For this purpose, highly accurate and prac-tically inertia-less closed-loop mixture control is necessary. The reason for this is that the open-loop mixture control, e.g. with L-Jetronic, cannot monitor and feed back the result, although it calculates and meters out the fuel quantity needed. We speak here of an open-loop control. On the other hand, the closed-loop mixture control mea-sures the composition of the exhaust gas and uses the result for correcting the fuel quantity calculated.

The diagram shows how the emission of noxious matter is influenced by the mixture composition and by aftertreatment. The necessity for the highest possible control accuracy can be re-cognized by the considerable increase in the harmful carbon monoxide (CO) just under X = 1.00 as well as by the sudden increase of the equally harmful nitrogen oxide (NO_x) just overż = 1.00.

Lambda closed-loop control

With the Lambda closed-loop control the air-fuel ratio can be maintained ex-tremely accurately.

With the closed-loop control, using a special sensor, the Lambda sensor, deviations from a certain air-fuel ratio can be recognized and corrected. The control principle is as follows: the Lambda sensor measures continually the residual oxygen content in the ex-haust gas, this being a measure for the composition of the air-fuel mixture supplied to the engine. The Lambda sensor in the form of a measuring sensor in the exhaust pipę supplies infor-mation on whetherthe mixture is richer or leaner than X = 1. When there is a deviation from this value, the output signal of the sensor makes a voltage jump which is evaluated by the control unit. In this way the control unit be-comes a (closed-loop) control unit. The Lambda closed-loop control in-fluences the duration of the injection (= quantity), these being calculated beforehand by the injection control. In this manner the fuel can be appor-tioned so exactly that the air-fuel ratio is optimal in all engine conditions. Tolerances and aging of the engine are of no importance here. If, for example, X=1.03 (slightlylean mixture),thenthe Lambda closed-loop control compen-sates for the excess air with an increase in the fuel supply. The closed-loop control works the opposite way when, e.g. ?. = 0.97 (slightly rich mix-ture). This continual, almost inertia-less setting of the mixture at X=1 is the prerequisite for enabling the catalyst switched into the Circuit to burn the harmful particles to an effective degree.

In addition to this basie task the closed-loop control supervises othercon-trol and adaptation duties. Closed-loop operation is impossible, however, until the operating temperaturę of the Lambda sensor has been reached. During this period the system is switched to open-loop control. The closed-loop control monitors further-more the function of the sensor during operation. If a fault should occur, then the system is controlled to give a medium Lambda value.

Fig. 43 Sensor voItage as a factor of the air ratio