Combustion process in high-speed diesel engines

Transcription

1 Combustion process in high-speed diesel engines Conventional combustion characteristics New combustion concept characteristics Benefits and drawbacks Carlo Beatrice Istituto Motori CNR The Requirements to the Modern Diesel Engine V6 PSA engine FULL LOAD/SPEED CONDITIONS MARKET COST PERFORMANCE FUN TO DRIVE Customer s cost FUEL CONSUMPTION ENVIRONMENT EMISSIONS Low CO2 emiss. NEDC PROCEDURE 1

2 4-Stroke DIESEL ENGINE CYCLE DIESEL SPRAY STRUCTURE Break up length θ Cone angle Final SMR: 5 1µm Sauter Mean Diameter and air/fuel mixing are affected by different spray parameters. The air/fuel mixing process strongly affects the engine fuel consumption and the pollutant emissions. 2

3 6 Diesel engine cylinder pressure cycle Pressione nel cilindro [bar] Alzata dello spillo iniettore [mm] P.M.S. Accensione della miscela formatasi durante in tempo di ritardo all'accensione Area di lavoro attivo 2 1 Pressione di mandata della pompa di iniezione [bar] -4 4 Angoli di manovella dell'albero motore [ ] Tempo di ritardo all accensione Ignition, flame evolution and soot formation in a diesel spray ROHR (J/ms) Injection duration Time after injection (ms) Source: Tokyo University 3

4 Visible combustion evolution in a DI diesel engine Conventional diesel combustion Classical Diesel Combustion Concept: in principle a non-stationary heterogeneous diffusive and partially premixed turbulent combustion NOx controlled by: flame temperature (Zel dovich mechanism); local N 2 and O 2 concentration Source: Sandia National Laboratories PM formation controlled by: over-rich fuel concentration; local O 2 lack; combustion temperature Picture of a almost steady-state burning condition 4

5 NOx formation background NOx formation background 5

6 IM: 1985 LD SC engine with FSV NOx formation background Daimler Benz: 27 HD SC engine and CFD simulation NOx formation background Source: Daimler Benz 6

7 DI Diesel engine combustion system design Adequate spray penetration and fuel atomization control the optimum air/fuel mixing and then the final pollutant emissions. 7

8 8

9 NOx formation background Not for LD engine Source: Daimler Benz NOx formation background 9

10 Effectof EGR Rate on NOxReduction Source: Daimler Benz HD Engine Effectof OxygenConcentrationof IntakeAir on NOxFormation For different engine load there is the same reduction rate for NOX vs O2 concentration Source: Daimler Benz 1

11 Decrease of local Gas Temperatures via EGR In the photos the area with higher luminosity correspond to high sooting area at higher temperature CFD simulation Source: Daimler Benz Decrease of local Gas Temperatures via EGR Source: Sandia Fuel: DGE, C6H14O3 11

12 Diesel combustion control To assure the engine functionality an adequate control of SOC and combustion rate have to be realized. Thermal EGR Ratio (int( ext) Conditions Injection Strategy Boosting Compression Ratio Combustion System Architecture Fuel quality Adequate Control of Air-EGR EGR-Fuel mixing and of ignition delay time Intake Temperature Control of: Comb. Noise; Peak pressure; Efficiency; Pollutant emissions In every engine conditions there is an optimum Air/EGR Fuel mixing that realizes the better compromise among the output characteristics New concepts combustion in diesel engines While for SI engines, the HCCI study is oriented to reduce both FC and NOx formation, for Diesel, the HCCI/PCCI research is oriented to exploit the very low simultaneous level of both PM and NOx, preserving the low Diesel BSFC. Increased premixed level and EGR reduce local over-rich air/fuel ratio. Low Nox Source: SAE paper

13 HCCI/PCCI combustion process Conventional Diesel Combustion Source: Vaglieco et al., SAE paper BTDC TDC 4 ATDC 1 ATDC 15 ATDC 17 ATDC 21 ATDC Nearly HCCI Combustion with diesel fuel in a LD DI Diesel engine 13 BTDC 12 BTDC 11 BTDC 1 BTDC 9 BTDC 8 BTDC 7 BTDC HCCI/PCCI combustion process 1 Soot Yeld under pure pyrolisys vs Temperature Lowering flame temperature below a typical treshold reduces the soot formation. Soot Yeld [a.u.] Low Temperature Combustion regime.2 Tetradecane Source: Beatrice et al., Comb. Sci. & Tech. 21 Temperature [K] 13

14 HCCI/PCCI combustion process Calculated NOx and soot formation rate vs φ-t map for the diesel combustion Source: SAE paper HCCI/PCCI combustion characteristics When fuel burns under diesel combustion, fuel molecules are oxidated under different φ and T conditions Source: SAE paper Reduced NOx and Soot formation 14

15 HCCI/PCCI combustion characteristics HCCI for Diesel fuel can be approached with PFI or very Early injection strategies: PFI leads to very difficult control of global incylinder A/F, oil dilution by fuel, unburned HCs, SOC control and noise limitation; Early injection leads to same problems but less critical, depending on the combustion system and injection strategy. In both cases, due to the high boiling point of heavy fractions of the fuel, the homogenization is never reached. Source SAE Paper HCCI Combustion approach for LD DI Diesel engines With conventional LD DI combustion systems the homogeneous approach is more and more stringent with heavy problems of knocking conditions Cylinder pressure [bar] Syngle-Cylinder LD DI Diesel engine bar IMEP N-Heptane Rate of Heat Release [%/ ] High knocking conditions Crank Angle [ ] Source MTZ 15

16 From HCCI to Premixed Low Temperature Combustion (PCCI/LTC) PCCI with diluted air/fuel charge by high EGR rate can be defined as the middle between HCCI and diesel combustion. They are characterized by almost premixed stratified Air/EGR/Fuel charge with a better link between injection event and SOC as in the diesel combustion. PCCI combustion is a stratified highly diluted quasi-total premixed combustion 7 4-Cylinder LD DI diesel eninge 4 CYLINDER PRESSURE [bar] SOC after EOI Controlled SOC Diesel fuel 14 3 bar IMEP 2 1 SOImain = 11 BTDC C.A.[ ] Source: Neely et al., SAE Paper ROHR [%/ ] Advanced combustion management in modern diesel engines 16

17 PCCI vs Conventional Diesel Combustion Low Load with Early Injection Strategy 4-Cylinder LD DI Diesel engine bar of BMEP.5 Emission Indexes [%] Conventional Diesel PCCI Combustion Emission Indexes [g/kwh] HC raw CO raw NOx raw BSFC Smoke PCCI vs Conventional Diesel Combustion Medium Load with Late Injection Strategy 2 4-Cylinder LD DI Diesel engine 2 5 bar of BMEP.12 Emission Indexes [%] Conventional Diesel PCCI Combustion Emission Indexes [g/kwh] HC raw CO raw NOx raw BSFC Smoke 17

18 Problems of PCCI application to DI Diesel engine Few problems at low load. Heavy problems at medium high load: SOC control; pollutants; η thermod. FC; η comb. engine durability; oil dilution cylinder to cylinder balancement; transient engine control; NOx; HCs; CO; PM. Fouling components mech. stress (knocking) adequate EGR distrb. impingment overleaning low comb. temp. quenching low O 2 local conc. cylind. to cylind. EGR distrib. cylind. to cylind. thermal cond. response of combustion to all above factors durnig transient conditions Conventional combustion application reaction scheme: 44 species and 112 reactions for n-heptane Detailed kinetics CHEMKIN 3.1 Multimethod solvers DVODE /SDIRK solver and time step locally chosen Initial Conditions Fuel Injection Breakup models Fuel evaporation Combustion Model Ignition delay + Combustion Model Numerical Solvers KIVA CFD Computations KIVA3V_Rel2 18

19 Particles formation modeling sectional approach The continuous dimensional distribution function is discretized into classes of fixed molecular weight In the earlier stages of the engine combustion Æ number weighted particle distribution is mono-modal With the progress of combustion the size distributions change from monomodal into bi-modal: after 12 CAD, first mode corresponds to particles smaller than 3nm and second mode to a peak between 3 and 6 nm. D Anna, A., Detailed kinetic modeling of Particulate Formation in Rich Premixed Flames of Ethylene, Energy Fuels 22, 28 Advanced technologies to realize practical application of HCCI combustion: Combustion System Architecture To increase premixing level, reducing unburned compounds, reducing cylinder wall wetting (oil dilution) and extend the HCCI application, an accurate combustion system design is needed low smoke; Reduced FC; CR NOx; High CO, HCs; extension of PCCI application area NADI TM system (IFP) 19

20 Advanced technologies to realize practical application of HCCI combustion: EGR EGR is the main driver for NOx control Source: Imarisio et al., ATA congress, Siracusa 26 The use of an advanced EGR layout with LP+HP EGR can extend the engine tolerability to the high EGR rate increasing the HCCI application area at medium load Advanced technologies to realize practical application of HCCI combustion: Advanced Injection Systems To improve the premixed Air/EGR/Fuel charge inside the cylinder, to employ injection system with injection rate shaping will be useful. Source: Hammer (BOSCH), ATA congress, Bari 24 Improved solenoid injector or Piezo injector Injection flow rate first flat slope final high flow rate low impingment; low oil dilution; low overleaning. time fuel distrib. control; comb. rate control. Source: Gastaldi et al. (RENAULT), ATA congress, Siracusa 26 Source: Imarisio et al., ATA congress, Siracusa 26 2

21 Advanced technologies to realize practical application of HCCI combustion: Advanced Air Layout Systems 2 Source: Lisbona et al., TDCE congress, Ischia 27 VVA system can be a very useful tool to control the in-cylinder temperature during the warm up in the NEDC. valve lift exhaust stroke re-opening exhaust valve during intake stroke cam angle Advanced technologies to realize practical application of HCCI combustion: Closed Loop Combustion Control Closed Loop Comb. Control is the prerequisite for SOC control and EGR effects Source: Lisbona et al., TDCE congress, Ischia 27 PCCI application in a veichle with closed loop comb. control Source: Hűlser et al., SAE paper

22 THE FUTURE HCCI DI CI engines The improvement ok knowledge in the HCCI combustion characteristics is fundamental to define the line-guides for HCCI combustion control technologies. The correct development and application of all technologies will help to bring the LD DI engines to match the future stringent emission regulation preserving the fuel consumption, fun to drive and performance at full load. The practical application of HCCI to the real CI engines will depend on the acquisition of the necessary knowledge to control the desiderate characteristics of the air/egr/fuel charge inside the cylinder before the start of the combustion. 22