-
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
05I2014 Volume 75 5
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
COVER STORY MIXTURE FORMATION AND COMBUSTION
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
Downsizing gen. 228-30 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
COVER STORY MIXTURE FORMATION AND COMBUSTION
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. 228-30 bar
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.
COVER STORY MIXTURE FORMATION AND COMBUSTION
10
-
05I2014 Volume 75 11
1939
2014
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