C OVER STORY MIX TURE FORMATION AND C OMBUSTIONCOMBINED MILLER/ATKINSON STRATEGY FOR FUTURE DOWNSIZING CONCEPTS Any further enhancement in the degree of downsizing in gasoline engines requires the use of dedicated valve control strategies. In this, an interesting approach would be the possibility to apply variable intake-closure timing. Schaeffler Technologies and IAV have come together in a joint project to analyse the potential of a gasoline engine concept in the entire engine map. An optimised Miller/Atkinson strategy combined with advanced downsizing showed CO2 savings up to 15.3 %. 4 Although the combination of charging and dethrottling caused by shifting load points yields substantial potential in terms of consumption. This helps reduce the need for mixture enrichment and takes a step toward satisfying future RDE requirements (Real Driving Emissions). specifically with EIVC. The low valve lift in part-load. Conversely. ❶. the Miller method will be referred to hereafter as early intake-valve closure strategy (EIVC) and the Atkinson strategy as the late intake-valve closure strategy (LIVC). Analogously. In addi- 30 20 10 0 630 Valve lift 150 645 660 675 690 Crank angle [°CA] 705 720 100 100 50 Valve lift [%] AUTHORS TKE 0 300 360 420 480 540 600 660 0 720 Crank angle [°CA] ➊ Turbulent kinetic energy (TKE) in EIVC/LIVC compared with baseline lift 05I2014 Volume 75 5 . The primary objective of both valve con- 300 MICHAEL GÜNTHER is Head of Department Combustion/ Thermodynamics SI Engines at the IAV GmbH in Chemnitz (Germany). this will lead to an increased complexity of turbocharged gasoline engines. Downsizing combined with part-load dethrottling is currently seen as a promising approach to significantly reduce fuel consumption [1]. produces tumble and hence turbulence losses with negative consequences for the combustion and the residual gas tolerance. KG in Herzogenaurach (Germany). For full-load operation using EIVC the boost pressure is increased in order to achieve the required air charge at BDC with lower temperature. CHRISTOPH BRANDS is Director Advanced Engineering Analysis R&D in the Business Division Engine Systems at the Schaeffler Technologies GmbH & Co. and to boost efficiency by cooling the gas and hence to reduce knocking in full-load. MORE PROBLEMS WITH KNOCKING AND AUTO -IGNITION The planned target of 95 g CO2/km for a fleet averaged vehicle mass of 1372 kg requires a significant boost in combustion engine efficiency. Miller or Atkinson strategies reduce the effective compression ratio through the use of variable intake-closure timing.-ING. INFLUENCES ON GAS EXCHANGE AND COMBUSTION For the purpose of simplification. BMEP = 2 bar Low tumble port 40 TKE [m2/s2] MATTHIAS KRATZSCH is Executive Vice President Development Powertrain at the IAV GmbH in Berlin (Germany).-ING. KG in Herzogenaurach (Germany). trol strategies in gasoline engines is to achieve a reduction in fuel consumption during part-load by dethrottling the gas exchange. Baseline EIVC LIVC Turbulent kinetic energy (TKE) [m2/s2] 250 200 Currently achievable TKE levels (combination tumble/swirl) 50 n = 2000 rpm. the LIVC method involves charging during the entire intake stroke and the excess charge mass is ejected after the gas exchange BDC (GE-BDC) [2]. But in future powertrains. the early intake closure leads to a substantial extension of the dissipation time and hence to an increased conversion of turbulent kinetic energy (TKE) into heat until the ignition timing. Further. DR. MARTIN SCHEIDT is Senior Vice President R&D in the Business Division Engine Systems at the Schaeffler Technologies GmbH & Co.DR. it does also exacerbate the problems associated with gasoline engine knocking and auto-ignition as the degree of charging rises. an increase in boost pressure using LIVC compensates for the loss in charge due to backflow into the port. the TKE at ignition timing does not reach the baseline level. The LIVC method shows a less pronounced loss in charge motion compared with EIVC and also a lower dissipation. Nevertheless. EIVC air intake is completed significantly before BDC and the valve timing is selected to trap the charge mass required for the part-load operating point in the cylinder. This is suitable to . An expanded Arrhenius approach is applied to assess changes of the knocking tendency. low tumble port). the drawback associated with a loss in turbulence and reduced residual gas tolerance and therefore a significant drop in intakevalve closure (IVC) potential toward “early” (∆IVC = 40 °CA) becomes particularly apparent compared with a tumble port or a concept with valve seat masking. caused by charge cooling following expansion. ❸. a target mean effective pressure of BMEPmax = 29 bar occurs with the known shifting of the operating points in the engine map.4-l four-cylinder gasoline engine with direct fuel injection is used as the baseline engine. An empirical friction model is used additionally. The maximum possible shift of combustion phasing toward early is ∆CA50 = 5 °CA at IVC of 487 °CA for the EIVC method within the assessed full-load operating point (n = 1500 rpm. The EIVC and LIVC strategy is assessed across the entire engine map. Analysis of a representative part-load point will lead to nuanced decisions on whether EIVC or LIVC would be the most suitable strategy for different levels of turbulence generation. The decision in this selected part-load operating point is in favour of the combination of EIVC strategy with a masking concept. ❷. 4 LIVC EIVC 2 ∆BSFC [%] 0 -2 -4 Reduction of residual gas due to inflammability Spark timing limited by IVC Low tumble port -6 Tumble port Masking -8 40 °CA GE-BDC -10 360 10 °CA 540 720 Crank angle [°CA] ❷ Part-load fuel consumption depending on the level of turbulence and intake-closure timing tion to the effects of TKE loss on combustion stability. It is thus necessary to initiate measures to increase turbulence in order to achieve the greatest possible dethrottling potential. however it does also require turbulence measures in order to fully exploit the reduction in fuel consumption.8-l turbocharged engine in a medium-sized vehicle (equivalent inertia 1470 kg) in order to increase the degree of downsizing. Conversely. The possible closure timing using LIVC with a tumble port can be displaced by around 10 °CA by reducing the required pre-ignition. In this.8 %. In the EIVC method 6 in particular. The reduced flammability at the lower prevalent cylinder temperature at ignition timing is determined based on the Damköhler number. METHODOLOGY OF DESIGN AND OPTIMISATION Simulation-based assessment of potentials found in EIVC and LIVC strategies in full and part-load operation requires SECOND GENERATION DOWNSIZING STRATEGY In this potential study a modern. optimised for this specific case. the reduced temperature level in both methods has repercussions on the flammability and hence on the residual gas tolerance. The effects of IVC on the charge temperature and turbulence are modelled using a precisely calibrated quasi-dimensional (QD) combustion model as an effective alternative to elaborate optimisation by CFD. This ensures the necessary boost pressure reserves for both valvetrain strategies. BMEP = 29 bar). the LIVC method fundamentally displays a lower degree of dependence on the applied port concept. given that the design of valve lifts (duration and timing) produces a very large number of possible parameter combinations in the engine map. turbocharged 1. ❹ (left). This engine concept replaces a 1. also lead to a greater turbulence level compared to baseline. The masking potential is dependent above all on the relationship between masking height and valve lift and can. in the best-case scenario. stochastic optimisation methods are applied. Surrogate model-supported. In this the level of turbulence generation in the intake port decisively influences the achievable part-load consumption potential. the requirements for the application of EIVC and LIVC strategies differ depending on the map range. making full use of the maximum consumption potential (∆BSFC up to 8 %) necessitates consistent inclusion of the intake port concept. If a LIVC method is applied to a low tumble port.g. DESIGN FOR HIGH ENGINE LOAD The possible intake closure timing is defined primarily by the boost pressure reserve in the charging system. The NEDC range is represented in a simplified form on the basis of 15 relevant speed-mean effective pressure pairs. the required closure timing for maximum dethrottling is so late that the required ignition angle to ensure optimum combustion phasing cannot be set. thus yielding significant consumption potential of up to 7. A two-stage controlled turbocharging system in combination with a tumble port is used in order to satisfy the fullload parameters. However. When the EIVC method is applied to a port with a low level of charge motion (e. and hence the consumption potential is limited.C OVER STORY MIX TURE FORMATION AND C OMBUSTION extended modelling approaches [3]. there are stationary consumption benefits of up to 8.6 % by optimising the compromise between maximum dethrottling. This leads to a significant increase in flow losses and pumping work. 1 20-24 bar BMEP [bar] achieve an acceptable combustion phasing in extreme downsizing. The greatest potential in terms of early combustion phasing at the rated power is achieved using LIVC and amounts to 5 °CA. The greater potential in the lowest load range is due to the low impact on combustion and hence the maximum possible dethrottling at the latest possible intake-closure timing.75 377 4. the reduction in consumption due to a 260 GE-BDC 2.0 pman [bar] pman 2. The reason for this is a higher temperature in the cylinder charge due to heating of the ejected charge fraction in the intake port and the intake manifold.8 340 300 Valve lift 3. Compared to LIVC. 2 28-30 bar Knocking pro tec tion Dethrottling 0 2000 1000 3000 5000 4000 6000 n [rpm] ➌ Operating points in the selected engine-vehicle combination in NEDC with increased downsizing leaner mixture is lower by ∆BSFC = 3 %.1 and 5. compared to the optimised low-speed cams (idealised valve lift). Reduction in fuel consumption by 11 % is possible through the application of leaner mixtures. In a high-speed range. ④ (right).5 2.5 309 BSFC 299 3.1 0. The boost pressure required rises with constant intake-closure timing and the shift of the intake closure is restricted. The potential to achieve acceptable levels of enrichment is determined for both strategies at rated power with extreme downsizing.30 Downsizing gen. 35 30 31 30 30 30 29 29 27 26 EIVC CA50 25 50 15 LIVC 21 CA50 21 0. For constant valve acceleration based on the baseline valve lift suitably broader cam profiles.0 n = 5000 rpm.0 2.95 26 λ [-] 32 CA50 [°CA] 32 1. Fundamentally. the latest possible intakeclosure timing is achieved with comparable intake manifold pressure at 565 °CA.5 4. though. are needed for a lift reduction.idealised / not speed-resistant 10 3.8 % at very low engine load. ❺.85 EIVC* .7 480 500 520 540 560 580 IVC1mm [°CA] 600 2. there are kinetic restrictions in the selection of cam profiles (real lifting cam) for EIVC operation. BMEP = 23 bar pman 320 300 280 Crank angle BSFC [g/kWh] 34 CA50 [°CA] ent Load shifting n = 1500 rpm.0 480 500 520 540 560 580 600 240 620 IVC1mm [°CA] ➍ Influence of IVC and cam profile on the engine target parameters 05I2014 Volume 75 7 . With optimum LIVC valve lift. however. The required LIVC lift duration is substantially greater compared to the rated power.90 0.0 0. the differences between these intake-closure strategies in the operating point examined are insubstantial as concerns the reduction in knocking tendency and required boost pressure.9 3.80 λ 0. With LIVC.kinematically viable EIVC . BMEP = 29 bar pman [bar] pon EIVC/LIVC is enabler for downsizing 36 28 Com Downsizing gen. turbulence-based combustion losses and friction. The potential in terms of reducing the knocking tendency is somewhat lower at ∆CA50 = 3 °CA.2 3. DESIGN FOR LOW ENGINE LOAD (NEDC OPERATION) EIVC can be applied to achieve operating point-dependent consumption potential of between 1. These are produced on the one hand due to reduced friction caused by the smaller valve lift.0 -1.2 -6.8 -7.8 0 1200 Transition downsizing gen. a combination of both strategies to yield the best possible fuel consumption within NEDC is advantageous.6 -5. combined with cam phasing.5 -20. i.4 -8. for the rated power range with drawbacks regarding component protection can be combined with a LIVC cam (LIVC1 in ⑧) for part-load.3 %.9 -4. while optimised EIVC valve lift is applied to the part-load ranges of relevance to the cycle through to fullload with low to moderate speeds. The NEDC consumption potential of this combination is 2. ❽ (EIVC*). combining it with an EIVC lift (EIVC in ⑧) optimised for part-load. On the one hand a speed-resistant EIVC cam. although the consumption potential there is reduced. If an optimised LIVC (LIVC2 in ⑧) valve lift is used for the rated output range.3 -5. ➐. 1 1000 Load shifting by downsizing EIVC LIVC part-load strategy part-load strategy ❻ Fuel consumption potential in the NEDC range using different strategies EIVC/LIVC part-load strategy .3 ∆BSFC [%] 0.2 0. on the intake side is required to implement this strategy.8 -7. If only one strategy is used in part-load in each case.2 1400 1600 1800 n [rpm] 2000 12 -6.0 -7.9 % -3.5 -4.9 -1. the engine is operated using LIVC in the low part-load range and an EIVC above.7 % due to shifting of the operating points and without any further measures to optimise part-load.5 -4.1 operation -1. In consequence. 1 to gen.9 -23.6 -3.6 -3. it is essentially conceivable to select between two combinations of these switching steps.3 -18. 2 -1. ❻.9 LIVC 0.0 -1.6 % Downsizing gen.0 -4. A combination of both strategies yields an additional saving of 3. Increased downsizing. produces a reduction in fuel consumption of 11.9 %.0 -9. and on the other hand by the effects of higher boost pressures on the gas exchange work. the general concept approach for the examined engine vehicle concept uses specifically optimised LIVC valve lifts both for low load and for rated power.9 NEDC -1.0 NEDC -1.0 ∆BSFC [%] ❺ Potentials of the EIVC/LIVC strategy on the basis of second generation downsizing in the NEDC range 2200 LIVC part-load lift in addition to downsizing -4. 2 enabled by EIVC/LIVC 2000 Downsizing gen. However.7 % -2. the EIVC valve lift is 6000 used at low speed even in high partload and full-load. there is an additional 2.8 % reduction for EIVC and 2. Given that both EIVC and LIVC methods produce benefits depending on specific map ranges. A three-point switch system.2 -2.9 % for the LIVC strategy. in upper speed ranges. In it.3 % on average compared to baseline valve lift. The EIVC lift in the pairing is also used in high part-load and full-load in the lower speed range.e.C OVER STORY MIX TURE FORMATION AND C OMBUSTION 30 EIVC part-load lift in addition to downsizing NEDC operation ∆BSFC [%] -2. the consequent consumption potential is 3 % in NEDC.1 -17. HARDWARE IMPLEMENTATION The implementation of an early or late intake valve closure requires a mecha- -11.6 -14. if only a two-point system is available.8 % Baseline -2.5 EIVC -1.5 -8. which is only possible using the EIVC/LIVC strategies.3 BMEP [bar] BMEP [bar] BMEP [bar] 12 0 1200 1400 1600 1800 n [rpm] 2000 2200 0 The EIVC method yields consumption benefits in the middle map section of 1. In the optimised EIVC/LIVC strategy.6 % 8 3000 n [rpm] 4000 5000 in part-load and hence an accumulated overall potential of 15.1 operation -5.4 -25. A locking mechanism actuated by oil pressure switches between low and high valve lift. A control groove is also produced into which an actuator pin is inserted. while transmission of the torque takes ➒ Switchable roller finger follower 05I2014 Volume 75 place via a spline. which usually comes with a drawbar spring. a sliding piece and an electromagnetic actuator for each valve pair. The following presents the benefits and drawbacks of both systems [4]. is only feasible using a cam shifting system. The levers are designed with sliding and rolling actuation.❼ Overall concept approach for a combined EIVC/LIVC strategy with three-point switching 30 EIVC Downsizing gen. following the contour of the groove in an axial direction. in order to shift to a different cam profile during a rotation.6 % -14. Additionally. 2 28-30 bar EIVC* / LIVC2 Downsizing gen. Because the small cam lift typically used at low speeds operates with the roller. 2 28-30 bar LIVC LIVC 2 EIVC BMEP [bar] Downsizing gen. The cam shifting system consists of a carrier shaft. A 3/2-way control valve controls the oil pressure. The change in voltage this movement produces on the 9 . a pressureless unlocked finger follower with detachable outer lever is necessary. NEDC potential -14. whereas a three-point combination of one EIVC and two LIVC profiles. A switchable roller finger follower consists of two interlocking levers. the actuator pin is pushed mechanically back into the actuator via a ramp. the inner and outer lever. The oil travels through special ports in the support element and into the lever. The sliding piece is fitted to the carrier shaft and can be moved axially.3 % LIVC LIVC 1 Downsizing Baseline EIVC/LIVC Part-load strategy 0 1000 2000 3000 4000 5000 6000 n [rpm] ➑ Compromise EIVC/LIVC strategies with two-point switching EIVC* – speed-resistant for rated power EIVC – optimised for part-load 30 EIVC* / LIVC LIVC1 – optimised for part-load LIVC2 – optimised for rated power Downsizing gen. Switchable roller finger followers just allow a two-point switching. ➒. fully variable electrohydraulic valve train systems such as the UniAir provide the option of implementing multi-lift switching.1 20-24 bar EIVC NEDC potential EIVC -15. connected by a coupling mechanism. Several adjacent cam lobes per valve are located on the sliding pieces to form the lift curves. Following actuation. A socalled lost motion spring. is fitted to ensure that the deactivated lever returns to its original position after the cam lift. this also offers the greatest advantage in terms of friction. drawing on a variety of technological approaches. hence permitting switching within one camshaft rotation at common speeds. 1 20-24 bar BMEP [bar] nism to switch between the various lift curves. offering the greatest potential to reduce consumption in NEDC. This system can achieve switching times of 10 to 20 ms.7 % LIVC1 / EIVC Baseline 0 1000 2000 3000 EIVC*/LIVC1 EIVC/LIVC2 Part-load strategy Part-load strategy 4000 5000 6000 n [rpm] For the most beneficial two-point strategy with EIVC/LIVC2. The switching mechanism can be designed for locking or unlocking without application of oil pressure. The sliding piece is stopped using a spring-loaded detent ball that fits into a groove in the sliding piece. It is operated electrically using a map stored in the ECU. 11 [2] Scheidt.C OVER STORY MIX TURE FORMATION AND C OMBUSTION with a maximum mean pressure of 29 bar. Given that various systems to realise this kind of concept are available. M. Günther. . The three-point cam shifting system is currently in development. Brands. It is not until the high speeds are reached that kinematic limitations cause this method to surrender its benefit. K. 11 [4] Ihlemann. Additional information from the sensors (pressure and lambda probes) and non-uniformity of torque is evaluated in order to satisfy the OBD requirement to be aware of the exact position at all times. Kratzsch. Thomas Spannaus and Christian Vogler from IAV GmbH in Chemnitz also contributed to this article. H. Further. a three-point switching based on a sliding cam system achieves the lowest fuel consumption.6 % in NEDC in an engine concept REFERENCES [1] Kirsten. A LIVC cam profile is most advantageous for maximum dethrottling in the lower load range and moderate turbulence level. 2014 [3] Bühl. causing a switch to LIVC. Kratzsch. less turbulence is required.. the sequence of the cam lobes can be defined in any order. A. M.: Kombinierte Miller-Atkinson-Strategie für zukünftige Downsizingkonzepte.: Potenziale von Schaltsaugrohren zur CO 2Reduktion in der Teillast. Cam shifting systems permit switching of the valve lift for individual cylinders and independent of the oil pressure and also permit a free design of the valve lift curve. KG and Nick Elsner.. H. 2013 THANKS Matthias Lang from Schaeffler Technologies GmbH & Co. Stuttgart.7 % with increased downsizing and secondly provides the substantial advantage of up to 3. No. 6 th MTZ conference “Ladungswechsel im Verbrennungsmotor“. Günther. In: MTZ 74 (2013). M. No. Nitz. N. the NEDC potential falls by a mere 0. If only a two-point system is possible. This is why the EIVC strategy is applied up to full-load in the lower and middle speed ranges. M. In: MTZ 73 (2012). Günther. But as the load increases. M. Pietrowski. C. International Engine Congress.. and so the EIVC method produces the best results...6 %.: Selektive Umschaltung des Ventilhubs beim Ottomotor. In this. C. The combined application of both methods firstly is the basis for achieving a potential of 11. 10 SUMMARY EIVC and LIVC approaches yield different potential in the engine map. A double S-shaped (DS) control groove in combination with a two-pin actuator and three cam pieces each per valve are used in order to achieve the three-point switching. Brands. Baden-Baden. ❿ Three-point cam shifting system actuator’s electrical coil is used to determine the position and is hence used as a feedback signal.. ❿..: Zylinderabschaltung – ein alter Hut oder nur eine Nischenanwendung. M. the combined Miller/Atkinson strategy with increased downsizing represents an outstanding contribution toward achieving the strict CO2 targets... MTZ has always remained true to itself: in its aspiration to offer our readers the ultimate in quality technical journalism. MTZ Motortechnische Zeitschrift has been examining the key issues driving our world: the internal combustion engine and other powertrains.75 YEARS AT TH E C U TTI N G ED G E O F EN G IN E T EC H N O L O G Y. our magazine has pulled off the miraculous feat of staying young and fresh while keeping its finger on the pulse of engine technology.MTZonline.com 05I2014 Volume 75 2014 1939 IT’S OUR BIRTHDAY! 11 . Throughout all those years. And in one point in particular. www. Every month for 75 years.
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