780205 INTRODUCTION To meet current U.S. emissions standards for automobiles, hydrocarbon (HC) and carbon monoxide (CO) emissions are usually controlled by a catalytic converter in the automobile exhaust. Federal standards for nitrogen oxides (NOx) emissions are met mainly by exhaust gas recirculation (EGR) techniques. However, in 1981, when NOx emissions standards are further tightened from the current limit of 2.0 grams per mile to 1.0 gram per mile, these techniques alone may no longer be sufficient. Although many schemes are under investi­ gation for meeting the 1981 emissions standards for all three emissions, one of the most promising employs a three-way catalyst in conjunction with a closed loop air-fuel ratio control system -(1-4).* To gain production experience with this system, General Motors is introducing it on some 1978 model cars sold in California. The combination of closed loop air-fuel ratio control and 3-way catalyst on General Motors automobiles is known as the Phase II Catalyst System. This paper describes the 1978 systems. a Numbers in parenthesis designate references at end of paper ABSTRACT Three-way catalysts provide a means of catalytically achieving lower NOx emission levels while maintaining good control of HC and CO emissions. However, very accurate control of air-fuel ratio is necessary. The precise air-fuel ratio control required is accomplished by employing a closed loop fuel metering system in conjunction with an exhaust gas sensor and an electronic control unit. To gain production experience with this type of system, General Motors is introducing it on two 1978 engine families sold in Cali­ fornia. One is a 2.5 litre L-4 engine and the other is a 3-8 litre V-6 engine. Closed loop controlled carburetors are used on both systems. This paper discusses these 1978 systems. The components used on both systems are des­ cribed and emission and fuel economy results are reviewed. 843 0096-736X/79/8701-0843$02.50 Copyright © 1979 Society of Automotive Engineers, Inc. General Motors Phase 11 Catalyst System Raymond P. Canale Engineering Staff Samuel R. Winegarden Buick Motor Division Clifford R. Carlson Pontiac Motor Division Donald L. Miles Engineering Staff General Motors Corporation Downloaded from SAE International by North Carolina State Univ, Saturday, September 15, 2018844R.P.CANALE, ETAL. BASICSYSTEM Figure 1showsaschematic ofthePhaseII Catalyst SystemasappliedtotheGeneral Motors2.5litreL-4enginebuiltbyPontiac MotorDivision andFigure 2showsitasused onthe3.8litreV-6enginebuiltbyBuick MotorDivision. Theelements ofabasic systeminclude athree-way catalytic converter thatsimultaneously actsonHC,COandNOx exhaustpollutants, anexhaustgassensorwhich givesanindication ofrichorleanexhaust, anelectronic controlunitwhichprocesses this information andconverts itintoacommand signaltothecarburetor, andtheclosedloopcarburetor whichcontrols theair-fuel ratio fedtotheenginetotheproperlevelsrequired forgoodoperation ofthecatalytic converter. Thesecomponents arediscussed inmoredetail below.THREE-WAY CATALYST Beforedescribing theclosedloopcompon ents,operation ofthethree-way catalyst should beexplained. Catalytic controlofHCandCO emissions requires anoxidizing atmosphere in theexhaust whereascatalytic controlofNOx emissions requires areducing atmosphere inthe exhaust- (5)·However, avarietyofcatalysts have been developed thatcancontrolallthree emissions simultaneously, hencethenamethree waycatalyst. But,inordertodoso,thecon stituents intheexhaust mustbemaintained ata veryprecisevalueclosetothechemically cor rectair-fuel mixture orstoichiometric mixture. Figure 3showsthecharacteristics ofa three-way catalyst. Thecurveofefficiency as afunction ofair-fuel ratioindicates thatwhen theair-fuel ratioislean(excessofoxygen), thecontrolofHCandCOisverygood,butcon trolofNOxispoor. Ontheotherhand,when theair-fuel mixtureisrich(deficiency ofoxy gen),thecontrolofNOxisverygoodbutcon t