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KSAE 2009 Annual Conference Copyright c 2009 KSAE 캠리스엔진의가변밸브타이밍에따른성능특성에관한연구 김성수 *1) 최세범 1) 한국과학기술원기계공학과 1) A Study on the Performance Characteristics in Camless Engine According to Variable Valve Timing Seongsoo Kim *1) Sebum Choi 1) 1) Department of Mechanical Engineering, KAIST, 373-1 Gusong-dong, Yusong-gu, Daejeon 305-701, Korea Abstract : Variable intake and exhaust valve timing control in an Internal Combustion engine affects the fuel efficiency, power output, and emission. The typical examples are VVT system. However, the VVT system has a limitation for fully control the variable intake and exhaust valve timing. Recently, A Camless Engine which can control the intake and exhaust valve with a hydraulic actuator excluding camshaft, and enables fully variable valve control has been studied. Because the Camless Engine can fully control the intake and exhaust valve timing, valve duration, valve lift, and valve deactivation, it overcomes the limitations of the existing VVT engine, and improves extremely engine performance and emission characteristics. In this study, Camless system was applied to motorcycle engine using numerical analysis program "Ricardo WAVE" and it was analyzed the Idle stability, engine performance, and emission according to variable intake valve close timing on Idle condition. Key words : Camless Engine( 캠리스엔진 ), Hydraulic Actuator( 유압식액츄에이터 ), VVT(Variable Valve Timing: 가변밸브타이밍 ), Variable Compression Ratio( 가변압축비 ), ISFC(Indicated Specific Fuel Consumption: 순수연료소비량 ) Nomenclature 1. 서론 IVO : Intake Valve Open IVC : Intkae Valve Close EVO : Exhaust Valve Open EVC : Exhaust Valve Close DOHC : Double Overhead Camshaft TDC : Top Dead Center BDC : Bottom Dead Center CA : Crank Angle 1) * 김성수, E-mail: kimseng77@kaist.ac.kr ƒ ¼ w{ v w w{ ˆ xº w v l w{ «v x ˆ«««««««Ž p Ž xºp w{ ˆ xº ³ q v Ž

ˆ Ž ƒˆ Ž p w{p ˆ«p ¼ w w{ ˆ p ««ª v w{ dž±x d m et ƒ p i Ÿk ˆ w{ «v ¼ w{ w w{ wk xºˆ l Ÿ k ˆ l m Ž et ˆ w{ «v x ºˆ l v n g p w { dž±x ƒ n ˆ«l m Ž Ÿk º ˆ sd ˆp d et ˆp zp ˆ«Ž umƒ w k n v w p p ek º ¼ w w{ ¹ «««n v l ¹ˆ m ƒ g ˆp o k ºt v «º w w k ˆ et vk ˆ m ª «ª««d ƒ ºv½ y ƒ ª ƒ m k ««p p s p m ª l m Ž º ƒ p ¼ w{ ½ ˆ p xºˆ Ž p x ˆ g ep º Ž ƒ ƒ v w w ƒ v zƒ yž 2. 시뮬레이션장치및해석조건 가솔린엔진으로시뮬레이션모델및엔진제원은 Fig.1와 Table 1에나타낸것과같다. 또한이모델링은매니폴드를포함하여스로틀과에어클리너등을포함한흡기계시스템과엔진실린더사이의밀접한상호관계를고려한것이다. 2.2 시뮬레이션해석조건 실험조건은상대적으로큰출력을필요로하지않는공회전상태에흡기밸브의닫히는시기를조절하여압축비를변화시켰고, 그에따른연비, 엔진운전특성변화및배기가스의배출물특성을파악하였다. Fig.1 Wave model of Camless Engine system Table 1. Camless Engine Specification Specification Description Single Cylinder, Engine type DOHC, 4 valves Bore (mm) 56.5 Stroke (mm) 49.5 Displacement (cc) 124.1 Compression Ratio 11.8:1 Valve Lift (mm) Baseline IVO / IVC Baseline EVO / EVC Intake Valve: 7.1 Exhaust Valve: 6.8 BTDC 26.5 CA / ABDC 55.0 CA BBDC 66.3 CA / ATDC 23.6 CA 2.1 시뮬레이션장치 본연구에사용한캠리스엔진은단기통 DOHC

2.2.1 가변압축비 일반적으로흡기밸브가열려서혼합기가들어오고, 다시흡기밸브를닫고피스톤이상승함에따라혼합기가압축된다. 이때흡기밸브의닫히는시기를조절하게되면실린더내부의압력을변화시킬수있게된다. Fig.2 는흡기밸브의닫히는시기에따라서피스톤이하강할때진공상태가되는시점을조절하게되고, 진공상태에서의피스톤이하강하고상승할때의부하가서로상쇄되어실제로피스톤이혼합기를압축할수있는비율이변하게되는것을나타낸다. b - Valve timing profile Fig.3 A valve timing event for Variable Compression Ratio 3. 결과및고찰 Fig.2 Variable Compression Ratio according to the IVC timing 2.2.2 압축비변화에따른해석조건압축비의변화에따른영향을파악하기위하여 Fig.3에나타낸것과같이배기밸브의개폐시기는고정시키고, 흡기밸브의닫히는시기를 BDC 부터 5 씩진각시켜총 11개의 Event에걸쳐서실험하였다. 또한실린더내부에남아있는잔류가스의영향을최소화하기위하여흡기밸브와배기밸브사이의오버랩을두지않았고, 배기밸브는상사점에서닫힌다. 7) 3.1 가변압축비에따른실린더압력및순간토크변화 Fig.4는배기밸브는고정시키고, 흡기밸브의닫히는시기를크랭크각기준으로 50 까지진각시켰을때의실린더내부의압력변화를나타낸다. 흡기밸브의닫히는시기를크랭크각기준으로 5 씩진각시킴으로서실린더내부의최고압력변화는총 11 개의 Event 동안 17.74bar에서 13.24bar로 25.37% 감소하였다. 이는압축비의변화가실린더내부의압력에직접적으로영향을끼친것으로판단된다. 또한흡기밸브의닫히는시기가진각될수록, 실린더내부의최고압력이감소하게되어결국출력역시떨어지게된다. 이를확인하기위해 Fig. 5와 Fig.6에가변압축비에따른순간토크의변화를나타내었다. 먼저 Fig.5는총 11개의 Event 동안발생한순간토크변화를나타내고있는데, 흡기밸브가진각될수록순간토크가감소하였다. 이는 Event 1과 Event 11을확대해서비교해놓은 Fig.6에서자세히확인할수있다. Event 1에서는최대순간토크가 47.9Nm, Event 11에서는 12.04Nm를기록하였다. 이것은높은출력을필요로하지않는공회전상태에서가변압축비를이용하게되면상당량의연료소모량을개선시킬수있음을의미한다. a - Valve timing chart

3.2 가변압축비에따른공회전속도안전성 Fig.7은가변압축비에따른엔진속도변화를나타낸결과이다. Fig.7을보면흡기밸브의닫히는시기가진각될수록공회전상태에서엔진속도의변화폭이줄어드는것을확인할수있다. 이는실린더내부의압력이감소하여최대순간토크가감소된결과로판단된다. 결과적으로흡기및배기밸브의개폐시기를독립적으로제어할수있는캠리스엔진의장점을이용하여압축비를가변시켰고, 이영향으로공회전 Fig.4 Cy linder pressure fo r V ariable Co mpressio n Ratio 상태에서엔진속도의안정성이향상됨을알수있다. Fig.8은 Fig.7의 Event 1과 Event 11을비교하기위해확대하였다. Fig.5 Engine Torque for Variable Compression Ratio Fig.7 Engine Speed for Variable Compression Ratio a - Event 1 a - Event 1 b - Event 11 Fig.6 Engine Torque for Event 1 and Event 11. Close up #1 and #2 b - Event 11 Fig.8 Engine Speed for Event 1 and Event 11. Close up #1 and #2

3.3 가변압축비에따른연비및배기가스변화 Fig.9는가변압축비에따른연료소비율을나타낸다. 흡기밸브의닫히는시기를진각시킬수록연료소비율이 0.47kg/kwh에서 0.426kg/kwh로약 9.6% 감소하였다. 이는흡입밸브의닫히는시기를진각함에따라혼합기가실린더내부로분사되는양이감소하였고, 높은출력을필요로하지않는공회전상태에서가변압축비를통하여엔진속도의안정화및순간토크의감소로인한결과로판단된다. Fig.1 0 Cy linder temperature fo r V ariable Compression Ratio Fig.9 Indicated Specific Fuel Efficiency(ISFC) for V ariable Co mpressio n Ratio Fig.10은가변압축비에따른실린더내부의온도변화를나타낸다. 총 11개의 Event 동안 2041.98 K에서 1827.9 K로약 10.48% 감소하였다. 이는흡기밸브의닫히는시기가변함에따라서실린더내부의최대압력이감소하였고, 그로인해폭발행정때발생하는최대온도역시감소된결과이다. Fig.11은가변압축비에따른 NOx 및 HC의변화를나타낸그림이다. Fig.11에서보듯이 HC의양이거의변하지않는것을확인할수있는데, 이는흡기밸브와배기밸브사이의오버랩이발생하지않아실린더내부에남아있는잔류가스양이일정하기때문인것으로판단된다. 하지만 NOx의경우압축비가변함에따라서약 38.9% 감소하였다. NOx의감소는 Fig.10에나타난것과같이연소온도가감소함에따라서이루어진것이다. 8)9) Fig.11 Emissions for Variable Compression Ratio 4. 결론공회전시흡기밸브의닫히는타이밍변화에의한압축비의변화및그에따른성능특성에대한분석을수행하였으며, 이를통해다음과같은결론을얻었다. 1) 흡기밸브의닫히는타이밍을진각시킴에따라직접적으로실린더내부의압력이감소하였다. 특히최고압력이눈에띄게감소하는것을확인할수있었다. 2) 가변압축비를통한실린더내부의압력감소는엔진의출력에도영향을미쳐, 순간토크가상당히감소하였다. 3) 공회전상태에서흡기밸브의닫히는타이밍이진각됨에따라엔진속도의변동폭이상당히감소하였다.

4) 가변압축비를통하여연료소비율을약 9.6% 감소시킬수있었다. 5) 실린더내부의온도가흡기밸브의닫히는타이밍이진각됨에따라감소하였고, 그결과 NOx가상당량감소하였다. 7) Haken Sandquist and Johan Wallesten, Karin Enwald and Stefan Stromberg, "Influence of Valve Overlap Strategies on Residual Gas Fraction and Combustion in a Spark-Ignition Engine at Idle", SAE paper 972936, 1997 References 1) D. S. Kim, J. Y. Lee, Y. H. Lee and Y. G. Cho, "Variation of Exhaust Gas Temperature with the Change of Spark Timing and Exhaust Valve Timing during Cold Start Operation of an SI Engine", Spring Conference Proceedings, Vol.I, KSAE, pp. 58-64, 2004 2) Michael M. Schechter and Michael B. Levin, "Camless Engine", SAE paper 960581 8) Lucien Koopmans and Ingemar Denbratt, "A Four Stroke Camless Engine, Operated in Homogeneous Charge Compression Ignition Mode with Commercial Gasoline", SAE paper 2001-01-3610, 2001 9) J. Y. Jang, K. T. Yeom and C. S. Bae, "Effect of Various Intake Valve Timing Conditions and Exhaust Throttle on the Residual Gas Fraction and Exhaust Emission in an SI Engine", KSAE, pp. 65-70, 2004 3) M. Battistoni, L. Foschini, L. Postrioti, M. Cristiani, "Development of an Electro-Hydraulic Camless VVA System", SAE paper 2007-24-0088, 2007 4) J. Allen and D. Law, "Production Electro-Hydraulic Variable Valve-Train for a New Generation of I.C Engines", SAE paper 2002-01-1109, 2002 5) T.G. Leone and M. pozar, " Fuel Economy Benefit of Cylinder Deactivation - Sensitivity to Vehicle Application and Operating Constraints", SAE paper 2001-01-3591, 2001 6) R. Fiorenza, E. Torella, M. Pirelli, P. Pallotti, Paul E. Kapus, G. Kokalj, M. Lebenbauer, "VVT+Port Deactivation Application on a Small Displacement SI 4 Cylinder 16v Engine: An Effective Way to Reduce Vehicle Fuel Consumption", SAE paper 2003-01-0020, 2003