2014-5-MIller Atkinson Strategy for Future Downsizing(BMEP 29 Bar)

COMBINED MILLER/ATKINSON STRATEGY FOR FUTURE DOWNSIZING CONCEPTSAny 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 down-

sizing showed CO2 savings up to 15.3 %.

4

COVER STORY MIXTURE FORMATION AND COMBUSTION

Mixture Formation and Combustion

MORE PROBLEMS WITH KNOCKING AND AUTO-IGNITION

The planned target of 95g CO2/km for a fleet averaged vehicle mass of 1372kg requires a significant boost in combus-tion engine efficiency. Downsizing com-bined with part-load dethrottling is cur-rently seen as a promising approach to significantly reduce fuel consumption [1]. But in future powertrains, this will lead to an increased complexity of turbo-charged gasoline engines. Although the combination of charging and dethrot-tling caused by shifting load points yields substantial potential in terms of consumption, it does also exacerbate the problems associated with gasoline engine knocking and auto-ignition as the degree of charging rises. Miller or Atkin-son strategies reduce the effective com-pression ratio through the use of variable intake-closure timing. This helps reduce the need for mixture enrichment and takes a step toward satisfying future RDE requirements (Real Driving Emissions).

INFLUENCES ON GAS EXCHANGE AND COMBUSTION

For the purpose of simplification, the Miller method will be referred to hereaf-ter as early intake-valve closure strategy (EIVC) and the Atkinson strategy as the late intake-valve closure strategy (LIVC). The primary objective of both valve con-

trol strategies in gasoline engines is to achieve a reduction in fuel consumption during part-load by dethrottling the gas exchange, and to boost efficiency by cooling the gas and hence to reduce knocking in full-load. 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. Con-versely, the LIVC method involves charg-ing during the entire intake stroke and the excess charge mass is ejected after the gas exchange BDC (GE-BDC) [2]. For full-load operation using EIVC the boost pressure is increased in order to achieve the required air charge at BDC with lower temperature. Analogously, an increase in boost pressure using LIVC compensates for the loss in charge due to backflow into the port.

The low valve lift in part-load, specifi-cally with EIVC, produces tumble and hence turbulence losses with negative consequences for the combustion and the residual gas tolerance, . Further, the early intake closure leads to a sub-stantial extension of the dissipation time and hence to an increased conversion of turbulent kinetic energy (TKE) into heat until the ignition timing.

The LIVC method shows a less pro-nounced loss in charge motion compared with EIVC and also a lower dissipation. Nevertheless, the TKE at ignition timing does not reach the baseline level. In addi-

AUTHORS

DR.-ING. MARTIN SCHEIDTis Senior Vice President R&D in the

Business Division Engine Systems at the Schaeffler Technologies GmbH &

Co. KG in Herzogenaurach (Germany).

DR.-ING. CHRISTOPH BRANDSis Director Advanced Engineering

Analysis R&D in the Business Division Engine Systems at the Schaeffler Technologies GmbH & Co. KG in

Herzogenaurach (Germany).

MATTHIAS KRATZSCHis Executive Vice President

Development Powertrain at the IAV GmbH in Berlin (Germany).

MICHAEL GNTHERis Head of Department Combustion/

Thermodynamics SI Engines at the IAV GmbH in Chemnitz (Germany).

Valv

e lif

t [%

]

100

0

50

40

30

20

10

0

Crank angle [CA]

Crank angle [CA]

300 360 420 480 540 600 660 720

Valve lift

TKE

300

250

200

150

100

50

0

Baseline

EIVC

LIVC

n = 2000 rpm, BMEP = 2 barLow tumble port

Currently achievable TKE levels(combination tumble/swirl)

Turb

ulen

t ki

neti

c en

ergy

(TK

E)

[m2/s

2]

TKE

[m

2/s

2]

630 645 660 675 690 705 720

Turbulent kinetic energy (TKE) in EIVC/LIVC compared with baseline lift

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Mixture Formation and Combustion

tion to the effects of TKE loss on combus-tion stability, the reduced temperature level in both methods has repercussions on the flammability and hence on the residual gas tolerance.

It is thus necessary to initiate meas-ures to increase turbulence in order to achieve the greatest possible dethrot-tling potential. In this the level of tur-bulence generation in the intake port decisively influences the achievable part-load consumption potential. Analy-sis of a representative part-load point will lead to nuanced decisions on whether EIVC or LIVC would be the most suitable strategy for different lev-els of turbulence generation, . When the EIVC method is applied to a port with a low level of charge motion (e.g. low tumble port), the drawback associ-ated with a loss in turbulence and reduced residual gas tolerance and therefore a significant drop in intake-valve closure (IVC) potential toward early (IVC= 40CA) becomes par-ticularly apparent compared with a tumble port or a concept with valve seat masking. The masking potential is dependent above all on the relationship between masking height and valve lift and can, in the best-case scenario, also lead to a greater turbulence level com-pared to baseline. In the EIVC method

in particular, making full use of the maximum consumption potential (BSFC up to 8 %) necessitates consist-ent inclusion of the intake port concept.

Conversely, the LIVC method funda-mentally displays a lower degree of dependence on the applied port concept; however it does also require turbulence measures in order to fully exploit the reduction in fuel consumption. If a LIVC method is applied to a low tumble port, the required closure timing for maxi-mum dethrottling is so late that the required ignition angle to ensure opti-mum combustion phasing cannot be set, and hence the consumption potential is limited. The possible closure timing using LIVC with a tumble port can be displaced by around 10CA by reducing the required pre-ignition, thus yielding significant consumption potential of up to 7.8 %. The decision in this selected part-load operating point is in favour of the combination of EIVC strategy with a masking concept, optimised for this specific case.

METHODOLOGY OF DESIGN AND OPTIMISATION

Simulation-based assessment of poten-tials found in EIVC and LIVC strategies in full and part-load operation requires

extended modelling approaches [3]. The effects of IVC on the charge temperature and turbulence are modelled using a pre-cisely calibrated quasi-dimensional (QD) combustion model as an effective alter-native to elaborate optimisation by CFD. The reduced flammability at the lower prevalent cylinder temperature at ignition timing is determined based on the Dam-khler number. An expanded Arrhenius approach is applied to assess changes of the knocking tendency. An empirical friction model is used additionally. Sur-rogate model-supported, stochastic opti-misation methods are applied; given that the design of valve lifts (duration and timing) produces a very large number of possible parameter combinations in the engine map.

SECOND GENERATION DOWNSIZING STRATEGY

In this potential study a modern, turbo-charged 1.4-l four-cylinder gasoline engine with direct fuel injection is used as the baseline engine. This engine con-cept replaces a 1.8-l turbocharged engine in a medium-sized vehicle (equivalent inertia 1470kg) in order to increase the degree of downsizing. In this, a target mean effective pressure of BMEPmax= 29bar occurs with the known shifting of the operating points in the engine map, .

A two-stage controlled turbocharging system in combination with a tumble port is used in order to satisfy the full-load parameters. This ensures the neces-sary boost pressure reserves for both val-vetrain strategies. The EIVC and LIVC strategy is assessed across the entire engine map. The NEDC range is repre-sented in a simplified form on the basis of 15 relevant speed-mean effective pres-sure pairs. However, the requirements for the application of EIVC and LIVC strat-egies differ depending on the map range.

DESIGN FOR HIGH ENGINE LOAD

The possible intake closure timing is defined primarily by the boost pressure reserve in the charging system. The maximum possible shift of combustion phasing toward early is CA50= 5CA at IVC of 487CA for the EIVC method within the assessed full-load operating point (n= 1500rpm, BMEP= 29bar), caused by charge cooling following expansion, (left). This is suitable to

Low tumble port

Tumble port

Masking

360

40 CA

GE-BDC10 CA

540 720

Crank angle [CA]

BS

FC [

%]

Spark timinglimitedby IVC

Reduction ofresidual gas

due toinflammability

LIVCEIVC

4

2

0

-2

-4

-6

-8

-10

Part-load fuel consumption depending on the level of turbulence and intake-closure timing


6

achieve an acceptable combustion phas-ing in extreme downsizing.

With LIVC, the latest possible intake-closure timing is achieved with compara-ble intake manifold pressure at 565CA. The potential in terms of reducing the knocking tendency is somewhat lower at CA50= 3CA. 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. Fundamentally, though, the differences between these intake-closure strategies in the operating point exam-ined are insubstantial as concerns the reduction in knocking tendency and required boost pressure.

The potential to achieve acceptable lev-els of enrichment is determined for both strategies at rated power with extreme downsizing. In a high-speed range, how-ever, there are kinetic restrictions in the selection of cam profiles (real lifting cam) for EIVC operation. For constant valve acceleration based on the baseline valve lift suitably broader cam profiles, (right), are needed for a lift reduction, compared to the optimised low-speed cams (idealised valve lift). This leads to a significant increase in flow losses and pumping work. The boost pressure required rises with constant intake-clo-sure timing and the shift of the intake closure is restricted. Compared to LIVC, the reduction in consumption due to a

leaner mixture is lower by BSFC= 3 %. The greatest potential in terms of early combustion phasing at the rated power is achieved using LIVC and amounts to 5CA. Reduction in fuel consumption by 11 % is possible through the application of leaner mixtures.

DESIGN FOR LOW ENGINE LOAD (NEDC OPERATION)

EIVC can be applied to achieve operating point-dependent consumption potential of

between 1.1 and 5.6 % by optimising the compromise between maximum dethrot-tling, turbulence-based combustion losses and friction, . With optimum LIVC valve lift, there are stationary consump-tion benefits of up to 8.8 % at very low engine load. 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. The required LIVC lift duration is substantially greater com-pared to the rated power.

27

29

30

3231

3029

26

21 21

IVC1mm [CA]

480 500 520 540 560 580 600

IVC1mm [CA]

480 500 520 540 560 580 600 620

GE-BDC

n = 1500 rpm; BMEP = 29 bar n = 5000 rpm; BMEP = 23 bar

Valv

e lif

t

Crank angle

pman

pman

BSFC

CA50

CA50

LIVCEIVC

3.2

3.1

3.0

2.9

2.8

2.7

p man

[ba

r]

36

34

32

30

28

26

CA

50

[C

A]

4.5

4.0

3.5

3.0

2.5

2.0

p man

[ba

r]

35

30

25

50

15

10

CA

50

[C

A]

1,0

0,95

0,90

0,85

0,80

0,75

[-

]

340

320

300

280

260

240

BS

FC [

g/kW

h]

299

309

377

300

EIVC* - kinematically viableEIVC - idealised / not speed-resistant

n [rpm]600050004000300020001000

Component protection

Dethrottling

Load shifting

EIVC/LIVC is enabler for downsizing

Downsizing gen. 120-24 bar


Knocking

0

BM

EP

[ba

r]

30

Influence of IVC and cam profile on the engine target parameters

Operating points in the selected engine-vehicle combination in NEDC with increased downsizing

05I2014 Volume 75 7

The EIVC method yields consumption benefits in the middle map section of 1.3 % on average compared to baseline valve lift. These are produced on the one hand due to reduced friction caused by the smaller valve lift, and on the other hand by the effects of higher boost pres-sures on the gas exchange work.

Given that both EIVC and LIVC meth-ods produce benefits depending on spe-cific map ranges, a combination of both strategies to yield the best possible fuel consumption within NEDC is advanta-geous, . In the optimised EIVC/LIVC strategy, the engine is operated using LIVC in the low part-load range and an EIVC above. In consequence, the general concept approach for the examined engine vehicle concept uses specifically optimised LIVC valve lifts both for low load and for rated power, i.e. in upper speed ranges, while optimised EIVC valve lift is applied to the part-load ranges of relevance to the cycle through to full-load with low to moderate speeds, .

Increased downsizing, which is only possible using the EIVC/LIVC strategies, produces a reduction in fuel consump-tion of 11.7 % due to shifting of the oper-ating points and without any further measures to optimise part-load. If only one strategy is used in part-load in each case, there is an additional 2.8 % reduc-tion for EIVC and 2.9 % for the LIVC strategy. A combination of both strate-gies yields an additional saving of 3.6 %

in part-load and hence an accumulated overall potential of 15.3 %. A three-point switch system, combined with cam phasing, on the intake side is required to implement this strategy.

However, if only a two-point system is available, it is essentially conceivable to select between two combinations of these switching steps. On the one hand a speed-resistant EIVC cam, (EIVC*), for the rated power range with draw-backs regarding component protection can be combined with a LIVC cam (LIVC1 in ) for part-load. The NEDC consumption potential of this combina-tion is 2.9 %. In it, the EIVC valve lift is

used at low speed even in high part-load and full-load, although the con-sumption potential there is reduced.

If an optimised LIVC (LIVC2 in ) valve lift is used for the rated output range, combining it with an EIVC lift (EIVC in ) optimised for part-load, the consequent consumption potential is 3 % in NEDC. The EIVC lift in the pairing is also used in high part-load and full-load in the lower speed range.

HARDWARE IMPLEMENTATION

The implementation of an early or late intake valve closure requires a mecha-

220020001800160014001200n [rpm]

-8.8 -7.5

-1.1

0.00.0

-4.2

-8.8-7.3

-6.4

-1.5

-1.2

0.0

LIVC part-load liftin addition to downsizing

BSFC [%]

LIVC

220020001800160014001200n [rpm]

BM

EP

[ba

r]

12

0

-5.3 -5.6 -3.0

-5.5-1.1

-1.9-1.2

-4.6-4.8

-1.9

-2.0

-1.3

BSFC [%]

EIVC part-load liftin addition to downsizing

EIVC

-4.9

-6.0-4.5

-2.6-3.0

-9.6-7.2

-14.1-20.4

-25.3-17.5

-18.9 -23.9

Transition downsizinggen. 1 to gen. 2

NEDC operation

NEDCoperation

NEDCoperation

BSFC [%]

n [rpm]600050004000300020001000

BM

EP

[ba

r]

0

30

BM

EP

[ba

r]

12

0

Potentials of the EIVC/LIVC strategy on the basis of second generation downsizing in the NEDC range

EIVCpart-load strategy

LIVCpart-load strategy

EIVC/LIVCpart-load strategy

Load shiftingby downsizing

Baseline

Dow

nsiz

ing

gen.

1

Dow

nsiz

ing

gen.

2en

able

d by

EIV

C/L

IVC

-11.7 %

-2.8 % -2.9 % -3.6 %

Fuel consumption potential in the NEDC range using different strategies


8

nism to switch between the various lift curves, drawing on a variety of techno-logical approaches. Additionally, fully variable electrohydraulic valve train systems such as the UniAir provide the option of implementing multi-lift switching.

Switchable roller finger followers just allow a two-point switching, whereas a three-point combination of one EIVC and two LIVC profiles, offering the greatest potential to reduce consumption in NEDC, is only feasible using a cam shifting sys-tem. The following presents the benefits and drawbacks of both systems [4].

A switchable roller finger follower consists of two interlocking levers, the inner and outer lever, connected by a coupling mechanism. The levers are designed with sliding and rolling actua-tion. A locking mechanism actuated by oil pressure switches between low and high valve lift. The oil travels through special ports in the support element and into the lever. A 3/2-way control valve controls the oil pressure. It is operated electrically using a map stored in the ECU. This system can achieve switching times of 10 to 20ms, hence permitting switching within one cam-shaft rotation at common speeds. A so-called lost motion spring, which usually comes with a drawbar spring, is fitted to ensure that the deactivated lever returns to its original position after the cam lift. The switching mechanism can be designed for locking or unlocking without application of oil pressure.

For the most beneficial two-point strategy with EIVC/LIVC2, a pressure-less unlocked finger follower with detachable outer lever is necessary, . Because the small cam lift typically used at low speeds operates with the roller, this also offers the greatest advantage in terms of friction.

The cam shifting system consists of a carrier shaft, a sliding piece and an electromagnetic actuator for each valve pair. The sliding piece is fitted to the carrier shaft and can be moved axially, while transmission of the torque takes

place via a spline. Several adjacent cam lobes per valve are located on the sliding pieces to form the lift curves. A control groove is also produced into which an actuator pin is inserted, in order to shift to a different cam profile during a rota-tion, following the contour of the groove in an axial direction. The sliding piece is stopped using a spring-loaded detent ball that fits into a groove in the sliding piece. Following actuation, the actuator pin is pushed mechanicallyback into the actuator via a ramp. The change in voltage this movement produces on the

30

1000

EIVC Downsizing gen. 228-30 bar

Downsizing

NEDC potential

Downsizing gen.120-24 bar

EIVC

EIVC

LIVC 2

LIVC1

EIVC

LIVC

LIVC

2000 3000 4000

Baseline

-15.3 %

EIVC/LIVCPart-load strategy

5000 6000

0

BM

EP

[ba

r]

n [rpm]

LIVC1 / EIVC

EIVC* / LIVCEIVC* / LIVC2


Downsizing gen.120-24 bar

EIVC* speed-resistant for rated powerEIVC optimised for part-load

LIVC1 optimised for part-loadLIVC2 optimised for rated power

BM

EP

[ba

r]

30

01000 2000 3000 4000 5000 6000

n [rpm]

Baseline

-14.6 % -14.7 %

EIVC/LIVC2Part-load strategy

EIVC*/LIVC1Part-load strategy

NEDC potential

Compromise EIVC/LIVC strategies with two-point switching

Overall concept approach for a combined EIVC/LIVC strategy with three-point switching

Switch able roller finger follower

05I2014 Volume 75 9

actuators electrical coil is used to deter-mine the position and is hence used as a feedback signal. Additional informa-tion 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.

The three-point cam shifting system is currently in development. A double S-shaped (DS) control groove in combi-nation with a two-pin actuator and three cam pieces each per valve are used in order to achieve the three-point switch-ing, . 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. Further, the sequence of the cam lobes can be defined in any order.

SUMMARY

EIVC and LIVC approaches yield differ-ent potential in the engine map. A LIVC cam profile is most advantageous for maximum dethrottling in the lower load range and moderate turbulence level. But as the load increases, less turbulence is required, and so the EIVC method pro-duces the best results. This is why the EIVC strategy is applied up to full-load in the lower and middle speed ranges. It is not until the high speeds are reached that kinematic limitations cause this method to surrender its benefit, causing a switch to LIVC. The combined applica-tion of both methods firstly is the basis for achieving a potential of 11.7 % with increased downsizing and secondly pro-vides the substantial advantage of up to 3.6 % in NEDC in an engine concept

with a maximum mean pressure of 29bar. In this, a three-point switching based on a sliding cam system achieves the lowest fuel consumption. If only a two-point system is possible, the NEDC potential falls by a mere 0.6 %. Given that various systems to realise this kind of concept are avail able, the combined Miller/Atkinson strategy with increased downsizing represents an outstanding contribution toward achieving the strict CO2 targets.

REFERENCES[1] Kirsten, K.; Brands, C.; Kratzsch, M.; Gnther, M.: Selektive Umschaltung des Ventilhubs beim Ottomotor. In: MTZ 73 (2012), No. 11[2] Scheidt, M.; Brands, C.; Gnther, M.: Kom-binierte Miller-Atkinson-Strategie fr zuknftige Downsizingkonzepte. International Engine Congress, Baden-Baden, 2014[3] Bhl, H.; Kratzsch, M.; Gnther, M.; Pietrowski, H.: Potenziale von Schaltsaugrohren zur CO2-Reduktion in der Teillast. In: MTZ 74 (2013), No. 11[4] Ihlemann, A.; Nitz, N.: Zylinderabschaltung ein alter Hut oder nur eine Nischenanwendung. 6th MTZ conference Ladungswechsel im Verbren-nungsmotor, Stuttgart, 2013

Three-point cam shifting system

THANKS

Matthias Lang from Schaeffler Technologies

GmbH & Co. KG and Nick Elsner, Thomas

Spannaus and Christian Vogler from IAV GmbH

in Chemnitz also contributed to this article.


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05I2014 Volume 75 11

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2014

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2014-5-MIller Atkinson Strategy for Future Downsizing(BMEP 29 Bar)