Combustion system design of the new PSA Peugeot Citroën EB TURBO PURE TECH engine

Transcription

1 Combustion system design of the new PSA Peugeot Citroën EB TURBO PURE TECH engine Dipl.-Ing. Philippe Souhaite Senior Expert Gasoline Powertrains Dipl.-Ing. Smaïl Mokhtari EB turbo Pure Tech system design supervisor PSA Peugeot Citroën, La Garenne-Colombes, France J. Liebl (Hrsg.), Internationaler Motorenkongress 2014, DOI / _5, Springer Fachmedien Wiesbaden

2 Abstract PSA Peugeot Citroën teams have been involved for a long time to address the two major environmental challenges linked to transport, climate change and air quality. They introduced several original technologies for both gasoline and Diesel engines in order to reduce fuel consumption (thus CO2 emissions) and pollutants emissions such as nitrogen oxides (NOx), Carbon monoxide (CO) or particulate matter (PM and PN). The objective of this paper is to focus on a key brick to reach the 95 g of CO2 per km required by the European Commission by 2020: the new PSA Peugeot Citroën EB TURBO PURE TECH engine. This new PSA Peugeot Citroën 3-cylinder 1.2L turbocharged direct injection gasoline engine, EB TURBO PURE TECH, was developed to achieve a challenging fuel economy target while keeping a high level of performances. To reach that goal, EB TURBO PURE TECH engine offers high specific performance, to get all benefits of the downsizing by gear set optimisation, with a robust combustion system using a central direct injection and dual cam phasers. The ISFC was optimised by using Atkinson cycle at low load and by finding the best trade-off between valve overlap and exhaust opening at mid-load. The high performances of the engine ( rpm and 96 rpm) lead to a real challenge for the design of the combustion system in order to manage risks regarding abnormal combustion, knock and oil dilution. One of the key components of the combustion system is the injector which has been designed to ensure optimal Air/Fuel mixture in all conditions. This was achieved through the use of a 200 bar multi-injection system. Thanks to CFD, in-cylinder spray/mixture formation, injector position and design were optimised and the best designs were extensively tested on engine bench in order to define the best trade-off between: emissions, gasoline/oil dilution, combustion stability with high EGR rate, full load combustion and abnormal combustion, and spark plug fouling. To ensure performance and reliability at full load with a 10.5:1 compression ratio, the water jacket, injection settings and valve overlap have been optimised. A specific work has been also done on the ignition system to perform the A/F mixture ignition at maximum BMEP. As a result, in comparison with the 1.6L NA replaced engine, a fuel economy of 17% is achieved on NEDC cycle with respect to Euro6.1 emissions regulation. Keywords: downsizing, combustion system design, Atkinson, direct injection. 50

3 1 General gasoline challenges The global context of mobility and environmental performance that each car manufacturer has to consider regarding its powertrain development, either thermal or hybrid, is shaped around three major challenges: The infrastructure and mobility challenge with the increase of urbanisation, The environmental challenge through a demanding regulation, declining resources, and more and more eco-responsible customers, The technical and industrial challenge to meet our clients expectations in terms of quality, engine lifetime, safety and quality/price ratio. In the light of this, the reduction of CO2 emissions, to limit global warming, and the reduction of vehicle pollutants, to improve the quality of the air, are the spearhead to offer vehicles that are each time more environment friendly. On all markets, the CO2 regulation induce technical improvements year after year to reach more and more severe targets. Figure 1 : Forecasted CO2 markets regulation

4 Figure 2 : CO2 emissions of passenger cars in Europe Thanks to continuous improvements in technology over the past years, the PSA Peugeot Citroën Group has gained a solid know-how in the field of low-co2 emission vehicles. For the last few years, the Group was the carmaker with largest market share in Europe on the segment of vehicles below 120 g. And our current ranges already include mainstream Gasoline cars emitting less than 100g of CO2. Enhanced technologies are mandatory to reduce fuel consumption and to meet the standards enacted by the European Union in the field of greenhouse gas emissions, while maintaining the driving quality. To address these challenges, the PSA Peugeot Citroën Group develops its thermal motorisations offer based on a balanced strategy in between gasoline and diesel. The new EB PureTech family of three-cylinder gasoline engines is a key brick to reach those targets. This EB PureTech family is composed of: 1.0l and 1.2l naturally aspirated PFI engines, covering the power range from 50 kw to 60 kw launched mid l Turbocharged Direct Injection engines, covering the power range from 80 kw to 96 kw. 52

5 The most demanding targets have been set for PureTech engines: To keep the leadership in terms of CO2. To contribute to car weight decrease. To optimize cost and the Time to Market. To achieve new steps in Quality level. To develop world-wide engine. To meet the Group ambition of moving upmarket thanks to the «fun to drive» of the turbo derivatives. No compromise on NVH requirements of European market and also Chinese market, which is particularly demanding In this paper, we will describe the main features design of the combustion system of the PureTech 1.2l e.thp Turbocharged Direct Injection engine. Figure 3: 1.2 l e.thp

6 2 Mains features of EB PureTech 1.2l e.thp To reach a challenging fuel economy target while keeping a good level of drivability, EB Pure Tech 1.2l e THP engine offers high specific performances, 192 N.m/l and 80 kw/l, to get all benefits of the downsizing and down-speeding by gear set optimisation with a robust combustion system, with a compression ratio of 10.5:1, using a central direct injector and dual cam phasers. Figure 4 shows the full load curve of the EB Pure Tech 1.2l e THP engine : low end torque of 230 N.m at 1750 rpm and maximum power of 96 kw at 5500 rpm. Figure 4: Full load curve of the 1.2 l e.thp The compression ratio for countries with poor fuel quality has been adjusted to 9.6:1 to ensure reliability at full load. That allows to get a maximum torque up to 3500 rpm. Mains features of the EB Pure Tech 1.2l e THP are listed in the Table 1 all focusing on performance and fuel economy targets: A cylinder unit volume of cm3 improves the indicated efficiency Bore/Stoke ratio of 0.83 maximises indicated efficiency and minimised friction For minimizing friction, con-rod length, piston pin offset and crankshaft/liner offset were also optimised 54

7 Table 1 : Main features of the 1.2 l e.thp Mains features 1,2 L e.thp 110 HP 1,2 L ethp 130 HP Type 3 cylinders Emissions regulation Euro 6.1 Consumption on 308 ( g NEDC) C02/k m Max power Kw/H P 81kw/ tr/min 96 kw/ 5500 tr/min Max torque Nm 1500 tr/min 1750 tr/min Fuel system Direct Injection, 200 bars, central mounted injectors 5 holes laser drilling injectors multiple injection mode up to 3 injection per cycle, Turbocharged system single scroll turbocharger : max. boost-pressure,1.4 bar and max. speed tr/mn Electrical management Smart monitoring of the electrical production and consumers, battery load optimisation and stop start system. Displacement Cm Compression ratio 10.5 : 01 Bore/stroke mmxm 75 X 90.5 With 7.5 mm crankshaft offset m Cylinder-block Aluminum vacuum die casting with additional heat treatment. aluminum coating liners inserts during the die casting. Crankshaft/con-rod Steel crankshaft T42 and M42, con rod with high iron material characteristic 38MnSiV4 Balancer shaft Mono anti-rotating shaft, driving unit base on gear mounted on the crankshaft and decoupled counter-gear on the balancer-shaft. Associated with High inertia TVD Pulley Oil pump Sensored regulation oil pump Cylinderhead Sand cast process. Hardened by air soak treatment. Aluminum Alloy: AS7 CU 0.5 Mg 0.3 / Heat treatment: T7. Integrated exhaust manifold Timing System 2 composite camshaft, wet belt driving unit Intake and exhaust Variable Valve Timing with large phase adjustment : IVO = -30 / 40 CA; EVO = -35 / 35 1mm lift Direct tappet ( with DLC coating ) 4 valves per cylinder, stem diam 5.2 mm exhaust valve with sodium "Box size" (L x W x H) mm 637 X X 687 Weigth PSA procedure kg 8O.5 kg W/o oil Fuel RON 91-98

8 3 Combustion System and Air Loop design 3.1 Global description: Key features of the combustion and air loop system are illustrated Figure 5 Conventional single stage turbocharger with Air Charge Air Cooler (described section 3.7 Turbocharger) Direct injection system with centrally located injectors, up to 200 bar injection pressure and multi-injections capability Intake and exhaust Variable Valve Timing for fuel consumption and performance optimisation Relatively high compression ratio (10.5) considering the high specific torque Low bore/stroke ratio (0.83) High tumble air motion generated by intake pipes shape, enhanced through combustion chamber and piston head shape optimisation High energy ignition system Efficient cylinder head cooling for abnormal combustion limitation Water cooled integrated exhaust manifold. This system helps maintaining the inlet turbine temperature under 980 C (peak) without important mixture enrichment Figure 5: Combustion system overview 56

9 3.2 Method used for system optimisation Intensive CFD investigations were performed to rank the different possible configurations regarding mixture preparation (including spray/wall impingement), air charge motion, combustion process, robustness to knock and auto-ignition. Parameters considered were intake pipes' geometry, injector characteristics (number of holes, targeting, droplet size ), combustion chamber shape (piston head, pent roof ) As an example, Figur shows typical CFD outputs used to compare injectors Figure 6 : Mixture preparation Typical CFD output The selected configurations were then tested on engine bench for fine optimisation and convergence toward the best trade-off between full load behavior (performance, resistance to abnormal combustion), fuel consumption, emissions, gasoline-into-oil dilution, combustion stability, resistance to injector coking and spark plug fouling. 3.3 Combustion chamber and intake ducts The engine displacement has been fixed to 1.2 liter. This has been identified as the right scaling considering the 96 kw performance target and the customer demand for C class vehicles attributes. The combustion chamber uses a relatively low bore/stroke ratio (0.83). This is favorable for combustion efficiency but leads to constraints for valves / injector / spark plug packaging and cooling efficiency.

10 A suitable arrangement has been defined, characterized by: Injector and spark plug aligned on crankshaft axis (asymmetric combustion chamber) Valve inner seat diameter of mm on the inlet side, 22.3 mm on the outlet side M12 spark plug Optimized cooling in the exhaust valve area (water jacket, water bridge between valves, sodium cooled valves, optimized valve seats material as illustrated Figure 7 ) Figure 7 : cylinder head wall temperature before applying cooling measures and water bridge illustration Intake ducts were optimized to rise the tumble ratio up to 1.55 with small penalty on the flow coefficient. The piston head shape allows enhancing the tumble during intake and maintaining it during the compression stroke. The resulting highly turbulent flow field results in: Improved mixture preparation Faster combustion and hence reduced knock sensitivity at high load Acceptance of a high level of residual gas (up to 35%) thus reducing fuel consumption at part load The compression ratio is set to 10.5 (for markets using RON 95 fuels). This value realizes the best compromise between the following objectives and constraints: Fuel consumption emissions minimisation Full load performance and knock limitation Architecture constraints (maximum cylinder pressure and heat flux, exhaust and catalyst temperature) Acyclism and driveability Robustness against abnormal combustion ( pre-ignition, heavy knock) This high value (considering the high low end torque) has been obtained thanks to cooling and air flow optimisation (described above), triple injection strategies and combustion chamber scavenging (see next sections). 58

11 3.4 Injector and spray The above described optimisation process converged toward a 5 holes injector. Figure 8 shows some of its characteristics. Figure 8 : Injector characteristics Figure 9 illustrates how the spray angle has been optimized to achieve a good compromise between oil dilution issues with large spray angle and worsened mixture preparation with low angle leading to bad combustion process and increased abnormal combustion risks ("rumble" / pre-ignition)

12 Figure 9 : Spray angle optimisation Injector protrusion has been optimized to avoid coking, and minimize particulate emission drift as illustrated Figure 10 Figure 10 : Protrusion injector optimisation 60

13 Figure 11 shows the injection strategy: Base strategy: 200 bar injection pressure, single injection phased to optimise particulate emissions Reduced injection pressure (100 bars) at low load to avoid too much dispersion of injected quantities (goal: maintain injection duration above minimum injection duration) Triple injection at low engine speed (up to 3000 RPM) and high load to avoid knock and pre-ignition : main injection during induction, second injection (~25% of total injected mass) near BDC, third (~10% of total injected quantity) around 90 before TDC (depending on spark advance) 3.5 Ignition system Figure 11 : injection strategy The particularly high specific torque and high EGR rate requires special attention on the ignition system to achieve robust ignition and minimize spark plug electrodes wear: The coil delivers a secondary current up to 130 ma to perform the ignition of the mixture at maximum load The primary current is adjusted depending on the engine operating point to obtain the best compromise between combustion quality and electrodes wear Figure 12 shows the influence of the electrodes gap on the ignition of air/fuel mixture.

14 Figure 12 : Impact of the electrodes gap on the mixture ignition 3.6 Dual Variable Valve Timing The two camphasers are used to: Improve fuel consumption by reducing pumping and thermal losses at part load through suitable combination of "late Atkinson" settings (delayed intake and exhaust valve closing See illustration Figure 13 ) and valve overlap optimisation Improve engine performance at full load, including combustion chamber scavenging at low engine speed (possible thanks to direct injection) 62

15 Figure 13 : Atkinson cycle Figure 14 and Figure 15 illustrate how the different valve setting strategies are used depending on engine and load: At low load: late Atkinson settings and EGR control through exhaust valve closure for fuel consumption optimisation At mid load: high valve overlap to obtain high EGR rate for fuel consumption and particulates emission reduction At low engine speed and mid / high load (where induction pressure is higher than exhaust pressure): reduction of the valve overlap to minimize fuel consumption under emission and catalyst temperature constraints At full load: optimized settings for performance, including scavenging control at low engine speed Besides, special valve settings are used during transients to minimize torque response

16 Figure 14 : valve setting strategies Figure 15 : Typical low engine speed valve settings (indicative) Figure 16 shows the fuel consumption for different EVC and IVO values at 2000 rpm and 2 bar BMEP. For this operating point the minimum ISFC is reached with Atkinson settings by reducing pumping and thermal losses. 64

17 Figure 16 : Optimal settings at 2000/2 (ISFC MAP and Intake pressure MAP) These valve setting strategies coupled with tumble enhancement allowed up to 35% Exhaust Gas recirculation at part load and "dethrottled" operation on a large operating range as shown on Figure 17. Figure 17 : EGR rate and inlet pressure maps 3.7 Turbocharger The exhaust manifold integrated to the cylinder-head allows to compact the exhaust side of the engine. Besides, the volume between the exhaust valves and the turbine wheel is reduced: this configuration optimises the use of the pressure pulsation in order to increase the low end torque.

18 Figure 18 : 1.2l e.thp Turbocharger The turbine housing material is a stainless steel in order to guaranty the robustness for a max exhaust gas temperature of 980 C. The semi-floating radial bearing system with axial bearing integrated was optimized to obtain the best compromise between robustness and reduction of the friction losses. The aerodynamic characteristics are optimized in order to guarantee the robustness of the max power operating point without negative impact on the transient behaviour. The main characteristics of the turbocharger are listed in table 2. Table 2 : 1.2l e.thp Turbocharger main characteristics Compressor side Turbine side Wheel diameter 41 mm 37 mm AR TRIM Max speed rpm rpm From 1750 rpm to 6000 rpm engine speed, the relative boost pressure lies between 1 and 1.4 bar which positions this 3-cylinder engine as a reference compared to our competitors. The turbocharger is equipped with an Arm&Valve monoblock. This monoblock design presents the particularity of a sphere / cone sealing contact interface between valve and seat which is a first worldwide. This technology allows to eliminate the metallic noise usually generated by the shock between the arm and the valve, while keeping the sealing quality and so the low end torque and transient behaviour. The global 66

19 dimensioning of the wastegate kinematic and the material choices were reworked and adapted to resist to the high instantaneous exhaust gas pressure specific to this high performance 3-cylinder engine. Figure 19 : Monoblock arm & valve design Figure 20 : Clearance compensation An additional spherical shape integrated to the valve head was adapted in order to obtain a better progressivity of the discharge. This allows improving the engine load controllability. The second effect is a 20% reduction of the aerolic force transferred to the wastegate kinematic. The external part of the wastegate kinematic was the object of acoustic improvement. Usually on gasoline engine, the kinematic noises are limited by the strategy which consists to close the valve, even on certain atmospheric functioning points, to put the different clearances under constraint and avoid the metallic shock. This type of strategy generates a CO2 drawback impact due to increase the pumping losses and so it is

20 not adapted to the small gasoline engine. To avoid this type of strategy, we integrated a spring which compensates the clearance during the engine life. 3.8 Lambda MAP Thanks to the integrated exhaust manifold and the inlet turbine temperature allowed up to 980 C (peak), a wide operating range at lambda one is possible: this limits the fuel consumption penalty due to the exhaust thermal protection. Figure 20 : Lambda MAP 4 1,2 L ethp 130 HP Engine attributes 4.1 Full load performance: Figure 21 shows the specific torque curve compared to 3 "best in class" competitors. Remarkable features are: A high maximum torque of 230 Nm (192 Nm/l) at 1750 RPM and even 200 Nm (167 Nm/l) reached at 1500 RPM 96 kw maximum power (80 kw/l), with 90 kw (75 kw/l) already at 4000 RPM 68

21 Figure 21: specific torque compared to competitors Despite the high boosting rate, the transient torque response is comparable to a 1.6 l TGDI engine with the same maximum torque, as shown on Figure 22 Figure 22 : Transient response compared to a 1.6 l boosted engine

22 As shown Figure 23, Peugeot 308 vehicle acceleration and (mainly) elasticity are significantly improved compared to former version with 1.6 NA, comparable or better than equivalent "best in class" current vehicles of the same category Figure 23 : New Peugeot 308 acceleration and elasticity compared to replaced vehicle and competitors 4.2 CO2 emissions and fuel consumption: Figure 24 shows the BSFC map obtained, on which have been plotted engine running conditions (RPM x BMEP) under different driving patterns: NEDC cycle, future "WLTC" cycle, typical highway use. The key characteristics are: A minimum BSFC value of 237 g/kwh, comparable to state of the art NA engines of the same unit displacement A large area where the BSFC is under 240 g/kwh, allowing near to optimal fuel consumption in most of driving conditions without requiring too high gear ratios 70

23 Figure 24: BSFC MAP Together with the downsizing and down-speeding effects allowed by the high specific performance and Stop Start functionality, BSFC map contributes to 21% improvement of CO2 emissions compared to former 1.6 NA powertrain, as shown Figure 25 Figure 25 : CO2 improvement compared to replaced powertrain

24 5 Conclusion The new PSA Peugeot Citroën EB TURBO PureTech 1.2l 3-cylinder engine, by means of significant technological breakthroughs, provides outstanding performance and fun to drive coupled to dramatically reduced CO2 emission whilst preserving a high level of components and industrial commonality with the Naturally Aspirated versions. Its maximum power of 96 kw, peak torque of 230 Nm, high torque at low rpm and fuel economy improvement of 21% vs. the 1.6 NA Valvetronic replaced engine, make it a perfect balance between fuel economy and fun to drive. A remarkable property of this engine, among others, is its wide low BFSC operating range, ensuring robust fuel saving for a wide range of driving profiles. 6 Acknowledgement The authors acknowledge the contribution of our colleagues and/or team members at PSA Peugeot Citroën, especially: Arnaud TELLIER, Stanislass DESSARTHE. 7 Bibliography a) Fabien GOUZONNAT, Philippe MERCKX, Rodophe CAZENAVE, Stephane LE COQ, Fabrice DEMESSE : New challenges encountered when designing highly downsized gasolines engines ( trough new PSA Peugeot Citroën powertrain examples ) SIA France Strasbourg December 5 Th, b) Patrice MAREZ, Smaïl MOKHTARI, Arnaud TELLIER: New PSA PEUGEOT CITROEN EB Turbo PureTech 1.2l gasoline engine RWTH AACHEN December 9 th, 2013 c) Caroline DOGNIN Renault, Smaïl MOKHTARI PSA Peugeot Citroën, Alexandre PAGOT, Franck VANGRAEFSCHEPE, Jean-Marc ZACCARDI IFP Energies nouvelles: Optimal design for a highly downsized gasoline engine, SAE Technical Paper