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Methods of exhaust-gas im- reduction in the combustion tomperaturo and provement lipak pres,;tiri%. Die-advantnon: combustion is allvoild. .t:, a iu!,uli the engitiu duou nul run All measures aiming at reducing pollutant smoothly, power output is reduced and fuel emissions limited by the various legal regu- consumption increases. lations are designed to maintain optimum ex- haust-gas quality with the smallest possible Ignition timing changes in performance values (fuel consump- The ignition point has an essential influence on l tion, vehicle performance, driveabdity, price) combustion efficiency. ` The greatest problems in this respect are the Advanced nitron } opposing concentrations of CO and C„Hm on g produces high torque with favorable consumption figures. Due to the the one hand and NO, on the other hand, high combustion-chamber temperature how- which are a functfon of the air-fuel ratio. ever, the formation of oxides of nitrogen :s also sign:ficantly promoted. With advanced ignition ; 1. On-tho-engine measures combustion is largely effected in the corn- bustion chamber, so the exhaust remains t Engine design cool. with little afterburning of unconsumed A favorable combustion-chamber configu- hydrocarbons. ration (piston shape, spark plug location) and Retarded ignition causes a reduction in good mixture swirl in the intake passage im- the combustion-chamber temperature, and prove combustion. hence the formation of oxides of nitrogen. Due , to the hotter exhaust gases. unburnt hydro- F Induction of fuel carbons are subsequently afterburnt in the ex- ; Combustion is improved by prehoating the haust to a greater degree. intake air using the heat of the exhaust mini- An incorrect ignition timing (over-ad- fold. vanced or over-retarded) damages the engine An ignitable mixture is always achieved and impairs the quality of the exhaust gases. t during overrunning (engine braking) by means If the ignition is over-advanced by 6* for ro- of a vacuum limiter (a valve provides a con- stance. the proportion of hydrocarbons in- ' nection between the ambient air and intake creases by more than 25%, while that of oxides manifold). A throttle dashpot has a similar of nitrogen rises by 20 ... 30%. For this reason effect. the prescribed ignition-timing settings must be strictly observed. ' Idle-mixture adjustment The exhaust gas can be cleaned up by setting 2. Exhaust-gas aftertreatment a lean idling mixture. But if the mixture is too Lean, the engine power fails dramatically, the The toxic substances In the exhaust gases can engine runs unevenly and the nitrogen content be reduced by means of auxiliary devices rises again due to misfiring (combustion miss). downstream of the combustion chamber. Fixed settings are prescribed by the law and Basically the following reactions must take obtained by adjusting the mixture with the aid place of an exhaust-gas tester. Infection systems provide for optimum mixture preparation (see a) C„H,,, + O; -CO. + H:O Oxidation, nacre p.283 and Bosch Technical Instruction Manual s""c"� 'c` ”" "'� VDT-UBP 74111 "K-Jetronic" and VDT-UBP b) CO + O, --CO_ deficiency. 75111 "O- and !_-Jetronic"). c) NO -f- CO -N_ + CO. Reduction, there should be no ex- Exhaust-gas recirculation (EGR) cess air is a further possibility for reducing pollutant A proportion of the exhaust gases ,emissions. :s recirculated from the exhaust side to the Tliermal afterburning intake side (controlled by pneumatic or eiectric in the thermal rCACtOr or so-called hot tube. To exhaust-gas recirculation vaive), causing a be allie to operate, moth systems must reach