ANALYSIS OF COMBUSTION CHARACTERISTICS OF CAI-ENGINE WITH VARIOUS VALVE STRATEGIES

1. ANALYSIS OF COMBUSTION CHARACTERISTICS OF CAI-ENGINE WITH VARIOUS VALVE STRATEGIES Jin Nam KIM, Ho Young KIM, Sam S. Yoon, Woo Tae KIM and Sang Dong SA Korea University, Hyundai Motor Company 2008. 11. 13.

2. Contents • Introduction • Theoretical Modeling • Numerical Details • Results and Discussion • Concluding Remark

3. Introduction • CAI Engine operation • Homogeneous mixture of air, fuel and residual gases is compressed until auto-ignition occurs at sites distributed throughout the combustion chamber. • CAI combustion in IC engine provides better performance in various aspects compared with both SI and CI combustion. • Operate unthrottled at part load and therefore reduce pumping losses • Overall combustion temperature is significantly reduced by the presence of excess air or internal EGR and produces ultra-low NO emission

4. Control the auto-ignition of CAI Engine There is no direct method to control auto-ignition timing !!!!!! Uniform mixture formation Stable ignition • Fuel: High volatile fuel, Dual fuel supply • Intake air : Intake charge heating auto ignition timing is advanced • Compression ratio : Low smoke and NOx emission with high compression ratio and low intake charge temperature. • Injection time : Achieve the combustion stability by injection timing and multi injection timing. • Recirculation or Trapping of burned gases : Exhaust Gas Recirculation, Residual gas trapping with variable valve timing

5. Trapping of residual gas – Internal EGR in CAI • Trapping of exhaust gases by closing exhaust valves relatively early and opening intake valve late. • Intake valves are opened and fresh charge drawn into the cylinder. • Fresh charge and exhaust mixture is then compressed in the next compression stroke. • The auto-ignition occurs in the final stage of compression stroke.

6. Objective of this study • Poor mixing or stratification between fuel and intake gases often poses a great technical challenge (incomplete combustion, high emission level) • The distribution of the internal EGR affects the mixture formation as well as the combustion characteristics of CAI engine Numerically examine the effects of various valve strategies on the overall performance for the CAIengine To Obtain • Flow and Mixing Characteristics • Combustion Characteristics • Emission Characteristics Various Valve timing and Lift Strategies are applied

7. Modeling • Engine Specification STL format CAD data

8. Modeling • Generalized equation for continuous phase Equations Continuity 1 0 Momentum Turbulent kinetic energy Dissipation rate Species Enthalpy

9. Modeling • Generalized equation for Dispersed phase • fuel droplet Where By Yuen & Chen correlation • mass transfer Where By Ranz correlation • heat transfer Where By El Wakil formulation & Ranz-Marshall correlation • Fuel injection Reitz and Diwakar model • Droplet Break-up Reitz and Diwakar model • Spray injection with atomization

10. Modeling • Ignition and Combustion model • Shell auto ignition model – Halstead et al., 1977 Simplified multi-step reaction mechanism to predict the spontaneous auto-ignition Reactants Equilibrium Constant Product Reaction Type • EBU (Eddy Break UP) Combustion model – Magnussen 1981

11. Numerical Scheme • Time step size – crank angle 0.2° • Time marching calculation – fully implicit scheme • PISO Algorithm • Differencing scheme – MARS scheme • Turbulence model - high Reynolds model

12. Mesh Generation At TDC : 320,000 cells At BDC : 700,000 cells By Pro-am, Es-ice Negative Valve Overlap !!! Exhaust valve Intake valve Section of valve center 3 - Dimensional Grid Generation

13. Parametric Studies The maximum valve lift of intake and exhaust valve is 2 mm. • Case A1 is the benchmarking case • Case A1,A2,A3 are EVO advanced • Case A4,A5,A6 are EVC retarded • Case B1 and B2 are IVO advanced • Case B3,B4 and B5 are IVC retarded • Case B6 andB7 are intake stroke shifted of 20 CAD(advanced and retarded, respectively)

14. Exhaust Valve 2 Intake Valve 2 Exhaust Valve 1 Intake Valve 1 • Parametric Studies • Case C1, C2 and C3 are increasing both intake valve lift. • Case D1,D2,D3 and D4 are various intake valve lift scenario considering swirling motion in cylinder.

15. Results and Discussion

16. Comparison of in-cylinder pressure in 1D and 3D simulation result • 1D Gas Dynamic Engine System Simulation (Ricardo WAVE) The results of 3D simulation is analogous to those of 1D simulation. The maximum pressure difference is about 5%

17. Various Exhaust Valve Timing • Additional pressure phase of residual gas re-compression compared with a standard 4-stroke engine cycle due to the negative valve overlap for all cases. • When EVC is retarded (Case A4,A5 and A6), under-lap period is decreased and less amount of the residual gases remains. The pressure at recompression stroke is decreased • Uniformity Index ( Weltens, 1993) -considering EGR mass fraction and volume at each cell Pressure inside the cylinder The Case A6 has the largest uniformity index because of 30 CAD retarded EVC, resulting in a larger amount of intake air and vigorous mixing Uniformity Index

18. Mass fraction of internal EGR at the valve center - EVO advanced(Case A1, A2 and A3) Case A1(EVO : 140 °CA) Case A2(EVO : 130 °CA) Case A3(EVO : 120 °CA) • The mass fraction of internal EGR is analogous to the result of case A1. The improvement of the mixing characteristics has not occurred even though the EVO is advanced.

19. Mass fraction of internal EGR at the valve center - EVC retarded(Case A1, A4, A5 and A6) Case A4 (EVC : 300 °CA) Case A1 (EVC : 290 °CA) Case A5 (EVC : 310 °CA) Case A6 (EVC : 320 °CA) • The highly accelerated intake flow penetrates the piston head and causes a vigorous mixing in the case A6, unlike the case in A2 and A3. The case A6 has the best mixing characteristics between the internal EGR and thefresh air at the end of compression stroke.

20. Various Intake Valve Timing • Intake valve timing had practically no effect on the pressure during negative valve overlap All other mixing characteristics such as cylinder temperature and flow fields also remained unchanged. However, various backflow patterns were observed !!! • Two dominant temporal increases in the intake port gas concentration, commonly referred to as the “early backflow”, “late backflow”. • The most advanced IVO timing, Case B2 and B6 were shown to the highest values for early backflow. • The most retarded IVC timing, Case B5 was the largest value for late backflow. B2,B6 B5 B6 The heat loss due to late backflow is unfavorable because of the longer residence period of the hot EGR in the intake port; Case B6 presents the most optimal operating condition in terms of improved thermal efficiency

21. Distribution of Scalars - Increasing Intake Valve Lift (Valve 1 & 2) Temperature Internal EGR Fuel O2 at 710 CAD O2 at 715 CAD Case C1 Case C2 Case C3 0 0.04 0.04 1000 1200 0.5 0.8 0.07 0.09 0.09 • The areas with high internal EGR mass fraction correspond to relatively high temperature areas. • Auto-ignition areas correspond to the fuel concentration from fuel and oxygen mass concentration

22. Combustion and Emission - Increasing Intake Valve Lift (Valve 1 & 2) As the intake valve lift increase, more cool fresh air could be induced Lower pressure and temperature Case C1, yielding the highest temperature (2083 K) - The largest NO emission When temperature greater than1800 K, - NO formation increased. Around1800 K NO formation is modeled by Zeldovich’s reaction mechanism

23. Intake valve strategy (different valve lift 1 & 2) Intake Valve 2 Intake Valve 1 To relate the intake valve lift profile to the flow structure

24. Z Y X • Intake valve strategy (different valve lift 1 & 2) Equation by mattarelli et al. Intake valve 1 lift =0Case D3 and D4 • With Intake Vale Lift 1=0 as in Case D3 and Case D4, the swirl intensity substantially increases at the end of the intake stroke. • The x-axis tumble intensity increases as the valve lift increases but gradually loses • The flow direction (from positive, negative value) of y-axis tumblegradually changes the intake stroke.

25. Distribution of Scalars - Intake valve strategy (different valve lift 1 & 2) Temperature Internal EGR Fuel O2 at 710 CAD O2 at 715 CAD Case BM Case D1 Case D2 Case D3 Case D4 1000 1200 0.5 0.8 0 0.07 0.04 0.09 0.04 0.09 • The areas with high internal EGR mass fraction correspond to relatively high temperature areas. • Auto-ignition areas correspond to the fuel concentration from fuel and oxygen mass concentration. • A larger auto-ignition spot in the reaction zones appeared for the most homogeneous mixture as shown in Case D4.

26. Concluding Remark To investigate the effects of various valve strategy and its subsequent combustion for a CAI engine, parametric studies were conducted • The results represent that • When the EVC motion was retarded, the less residual gas was remained and more intake air was supplied The mixing characteristics were improved with the high momentum of the intake air • The advancing EVO motion virtually had no effect on the mixing characteristics • Some areas with high internal EGR mass fraction inside the cylinder correspond to relatively high temperature areas. • High fuel concentration and the auto-ignition spots were affecting each other and Case D4 with the most homogeneous mixture yielded the best combustion efficiency

27. Thank you For your attention