Thermal management at Schaeffler: How much water does an ...

22 Thermal management
22 Thermal management
Thermal management
22
Thermal management at Schaeffler
How much water does an engine need?
Elmar Mause
Eduard Golovatai-Schmidt
Markus Popp
Sebastian Hurst
22
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22 Thermal management
Thermal management
22
Thermal management is an important factor for
reducing CO 2
emissions. This article presents
the reasons for the use of a thermal management
system, an analysis of the requirements
and an approach for implementing such a system.
Thermal
management
The term “thermal management” describes the
efficient control of thermal energy flows in the
vehicle in accordance with the specific requirements
and the prevailing operating and load
conditions. As a result, vehicle emissions can be
reduced, and the thermodynamic and mechanical
engine efficiency can be improved. This leads
to lower fuel consumption, a longer engine life
and improved thermal comfort.
0
10
20
30
40
50
60
Load L in %
70
80
90
100
6000
5000
4000
Low load and low speed: high coolant temperature
High load: low coolant temperature
High speed: low coolant temperature
The coolant temperature should ideally be adjusted
depending on the operating condition of the
engine (Figure 1).
During cold start, the combustion engine should
heat up rapidly in order to achieve a significant reduction
in friction. Rapid heating of the engine oil
and the resulting decrease in oil viscosity are the
decisive factors. The heat generated by the engine
must therefore not be dissipated by the coolant
but used for heating the engine oil.
At low and medium loads, high coolant temperatures
(approx. 110 °C) are desirable for further reducing
the engine friction.
In addition to the above advantages, the acoustics
of diesel engines can be improved by reducing the
ignition delay time. Intelligent thermal management
can influence the evaporation rate and thus
the ignition delay.
To prevent knocking of the gasoline engine and
reduce the enrichment of the mixture, the
coolant temperature
should preferably be
reduced (to approx.
80 °C) at high loads
and high speeds.
Intermediate stages
110 must be defined between
the two limit
100
values for the coolant
90 temperature. These
80 vary depending on the
70 combustion engine
and can serve diverse
60
goals (reduced friction,
optimized com-
50
40 bustion, lower raw
30 emissions, increased
comfort etc.).
20
1000 The ideal thermal management
system should
2000
3000
be able to adjust the
relevant coolant temperature
in accordance
with the above requirements.
Thermal management
measures can achieve
fuel savings of up to
4 % in the NEDC (Fig-
eng
Speed n in 1/min
Figure 1 Required coolant temperature depending on load and speed from [1]
Coolant temperature T in °C
ure 2). The blue curve
shows the accumulated
consumption of a
reference engine, the
red curve that of an
engine with a thermal
management system.
The green curve indicates
the savings in
percentage terms that
can be achieved in the
NEDC with the thermal
management system.
Significant savings potentials
of more than
4 % can be expected
particularly in shortdistance
driving operation.
This is mainly
due to a more rapid
heat-up of the engine
and the corresponding
reduction in friction.
Figure 3 gives an impression
of the possible
reduction in engine
friction. If the oil
temperature increases
from 20 °C to 80 °C,
the total frictional
torque of the engine
Cumulave consumpon C in kg
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
Friconal torque M total in %
100
80
60
40
20
0
Speed
v in km/h
-40
-10
20
50
80
110
140
Oil temperature T in °C
oil
0
0
120
60
No
cooling
Cooling
in accordance with requirements
0
0 120 240 360 480 600 720 840 960 1080 1180
Time t in s
Cumulave consumpon without thermal management
Cumulave consumpon with thermal management
Savings with thermal management
0 120 240 360 480 600 720 840 960 1080 1180
Time t in s
NEDC
Figure 2 Savings due to thermal management from [2]
5000
3000
1000
eng
Engine speed n in 1/min
Figure 3 Reduction in engine friction from [3]
decreases by 75 %. At an oil temperature of
110 °C, it decreases by as much as 85 %. This
shows that rapid heating of the engine oil and operating
the engine at the highest possible temperature
make a significant contribution to reducing
friction and therefore fuel consumption.
Friconal torque M
total
in %
40
30
20
10
3500 1/min
0
-10 20 50 80
Oil temperature T in °C
oil
8
7
6
5
4
3
2
1
75 % 85 %
Reducon in consumpon C in %
red
110
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22 Thermal management
Thermal management
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Components for
thermal management
All components in the cooling circuit as shown in
Figure 4 – radiator, fan, louvers, thermostat, engine
control unit, and water pump – must in principle be
included in the thermal management system.
These components are currently characterized by a
limited variability.
For example, there are thermostats that are controlled
by means of wax elements. Switchable or
electrically-driven water pumps are also being
used. The cooling air flow can be limited by splitting
the radiator into several parts or covering it
with louvers. Solutions have been developed for
the fan that are similar to those for the water
pump (electrically-driven fan, viscous coupling
etc.).
However, almost all vehicles today still use uncontrolled,
mechanically-driven water pumps.
These are permanently linked with the engine
speed via the belt drive and therefore allow no
variability. Presented below is a controllable water
pump that possesses the required variability.
The variably adjustable flow rate enables an additional
degree of freedom for the cooling system.
Controllable coolant pump
Engine control unit (ECU)
Thermostat
Water pump
Figure 4 Components of thermal management from [1]
Pump open
Solenoid
(schemac)
Pump closed
Figure 5
Pulley
Shroud
Impeller
Cover
plate
Push rod
Sealing Floang ring seal
Water pump bearing
Return spring
Design of a controllable water pump
The controllable coolant pump is a centrifugal
pump with a shroud that is integrated in the rotor
as shown in Figure 5. A defined width of the
rotor is exposed when the shroud is moved axially.
This enables adjustment of the volume
flow.
If the shroud is in the left position (Figure 5, top
diagram), the rotor width is completely exposed
and the generated
volume flow achieves
Radiator
a maximum. The solenoid,
which is located
on the left side and
serves as an actuator,
Louvers is not fed with current.
If the volume
flow is to be reduced,
the solenoid is fed
with a defined amount
of current. The armature
is correspondingly
moved to the right,
Fan
presses against the
push rod and thus
moves the shroud to
the right. This reduces
the effective width of the flow channel and cuts
the flow (Figure 5, bottom diagram).
To ensure the fail-safe function in case of a failure
of the solenoid, a compression spring retains
the shroud in the completely opened
pump position. The compression spring is designed
so as to ensure that the water pump is
completely opened when the flow forces reach
a maximum.
Flow rate Q in %
80
60
40
20
0
Figure 6
0 20 40 60 80 100
Closing rao S in %
1500 1/min
3000 1/min
4500 1/min
Flow rate in relation to pump closing ratio
and speeds
Figure 6 shows the flow behavior of the pump at
different closing ratios and speeds. A closing ratio
of 0 % corresponds to a completely opened
pump. The pump is
closed at a closing ratio
of 100 %. The diagram
indicates that
the flow rate decreases
significantly with
increasing closing ratios.
The flow rate can Nominal speed
therefore be adjusted
by the position of the
shroud.
Figure 7 shows the
pump efficiency in
relation to the flow
rate at a speed of
2500 1/min. The diagram
indicates that
Low speeds
the maximum efficiency
at this speed is
achieved when the
pump is approx. 50 %
open. This can be Figure 8
Efficiency η in %
60
50
40
30
20
10
0
Figure 7
2500 1/min
0 10 20 30 40 50 60 70 80
Flow rate Q in l/min
0 % closed
25 % closed
50 % closed
75 % closed
Pump efficiency in relation to flow rate and
closing ratio
attributed to backflows and turbulences that
sometimes occur outside the design point (Figure
8).
At nominal speed, the complete rotor width is
used for pumping the medium. At low speeds,
backflows occur in the conventional pump (Figure
8, left column) and reduce the efficiency.
When using a shroud, the rotor width can always
be adjusted to the required volume flow (Figure
8, right column). This prevents energy loss
caused by backflows and therefore increases the
efficiency of the pump.
Convenonal pump
Controllable pump
Comparison of conventional and controllable water pumps
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22 Thermal management
Thermal management
22
Force on shroud F shroud
in N
40
30
20
10
0
-10
-20
0 10 20 30 40 50 60 70 80 90 100
Closing rao S in %
Figure 9
Actuator
4000 1/min
3500 1/min
3000 1/min
2500 1/min
Forces on the shroud in relation to closing
ratio and speeds
Figure 9 shows the axial forces acting on the shroud
in relation to the pump closing ratio at different
speeds. The negative axial forces resulting from the
flow move the shroud towards “closed”, whereas
the positive axial forces move it towards “open”
(Figure 5).
The diagram in Figure 9 indicates that the force on
the shroud changes its direction at different speeds
and different closing ratios. This point represents
the optimum rotor width at a specific speed. The
volume flow can be reduced at widths lower than
the optimum width.
Due to its physical functional principle, the forces
of the electromagnetic actuator act only in one direction
(push solenoid). A compression spring is
used for compensation to ensure a constantly positive
force level. This way, the shroud is retained in
the “open” position in all operating conditions and
moved towards “closed” by the push solenoid.
Adjusting the rotor width and thus the volume flow
depending on the speed requires an actuator that
allows the setting of defined forces. The simplest
solution for this is a pull solenoid with a modulated
pulse width for influencing the electromagnetic
force characteristic curves. Defined currents that
generate the required forces can be set by means
of pulse width modulation of the voltage. Figure 10
shows the solenoid of the controllable water pump
in full section view.
Figure 11 shows the forces exerted by the solenoid
depending on the magnetomotive force (current)
and the stroke of the solenoid as well as the axial
forces on the shroud. The diagram indicates that
the magnetic force changes with different currents
or magnetomotive forces and the force equilibrium
is achieved in different positions of the shroud.
Thus, the position of the shroud can be set in a targeted
manner and the volume flow can be reduced
to zero. As the forces on the shroud change with
both speed and position of the shroud, each operating
condition requires a suitable current that can
be set by means of pulse width modulation.
Summary and
outlook
This article presented a variable coolant pump
for controlling the engine temperature in accordance
with the specific requirements. Significant
development objectives of thermal management
are reductions in fuel consumption, longer
engine life and increased comfort. Due to the
presented controllable water pump, the coolant
volume flow can be controlled depending on the
current operating condition of the engine, and
efficiency can be increased depending on the
driving situation by adjusting the rotor width.
The development objective in this case was
achieved by intelligently modifying and combining
existing components.
The integration of this controllable water pump
and other variable components in a thermal management
system is another very promising approach
for the future.
Literature
[1] Behr Thermot-tronik GmbH: Kennfeldthermostate
– Höchstleistung für den
Kühlkreislauf, http://www.behrthermottronik.de/produkte/automobil/kennfeldthermostat.pdf,
November 2009
[2] [Maassen, F.-J.; Dohmen, J.; Pischinger, S.;
Schwaderlapp, M.: Engine Friction Reduction
– Design Measures for Reduced Fuel
Consumption, MTZ July 2005
[3] Brinker, M.: Thermomanagement und Motorkühlung
aus Sicht von Opel/GMPT, CTI
Forum: Thermomanagement im Automobil,
February 2008
Force F in N
80
70
60
50
40
30
20
10
0
Speed 4000 1/min
0 10 20 30 40 50 60 70 80 90 100
Closing rao S in %
Magnetomove force
100 %
90 %
80 %
60 %
50 %
40 %
70 %
22
Figure 10 Solenoid
Axial force on shroud
Figure 11 Magnetic forces and flow forces in relation
to closing ratio S and magnetomotive force
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