U.S. patent application number 10/556424 was filed with the patent office on 2006-08-17 for extended fan run-on.
Invention is credited to Hans Braun, Ralf Korber, Michael Timmann, Jochen Weeber.
Application Number | 20060180102 10/556424 |
Document ID | / |
Family ID | 33394359 |
Filed Date | 2006-08-17 |
United States Patent
Application |
20060180102 |
Kind Code |
A1 |
Braun; Hans ; et
al. |
August 17, 2006 |
Extended fan run-on
Abstract
The invention relates to a fan run-on control which takes into
account the energy input into the combustion engine in order to
calculate the required run-on time of the fan. If the
characteristics of the fan are known it is possible to calculate
the required fan run-on time from the integral of the energy input
into the combustion engine before the combustion engine was
switched off, and the current operating data and ambient data of
the combustion engine. Furthermore, by comparing the energy input
into the combustion engine with the cooling performance of the
cooling system over a specific period of time before the engine was
switched off, it is possible to predict whether or not running-on
of the fan will be necessary. There is always a risk of further
subsequent heating whenever the energy input into the engine has
been significantly greater than the cooling performance of the
system before the engine was switched off. If the opposite is true,
there may sometimes be no need for the fan to run on, or the fan
run-on time can be much shorter than in previously known
systems.
Inventors: |
Braun; Hans; (Stuttgart,
DE) ; Korber; Ralf; (Stuttgart, DE) ; Timmann;
Michael; (Eutingen, DE) ; Weeber; Jochen;
(Filderstadt, DE) |
Correspondence
Address: |
FITCH, EVEN, TABIN & FLANNERY
P. O. BOX 65973
WASHINGTON
DC
20035
US
|
Family ID: |
33394359 |
Appl. No.: |
10/556424 |
Filed: |
March 25, 2004 |
PCT Filed: |
March 25, 2004 |
PCT NO: |
PCT/EP04/03149 |
371 Date: |
March 6, 2006 |
Current U.S.
Class: |
123/41.12 |
Current CPC
Class: |
F01P 7/048 20130101;
F01P 2031/30 20130101 |
Class at
Publication: |
123/041.12 |
International
Class: |
F01P 7/02 20060101
F01P007/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 9, 2003 |
DE |
103 20 746.5 |
Claims
1. Method for controlling a fan motor (4), in particular for a
motor vehicle, in which at least one logic element (logic) is used
to evaluate operating data and ambient data of a combustion engine
(1) measured by an engine management system and to calculate a fan
run-on time for the fan motor, characterized in that the fan run-on
time is determined from the energy input into the combustion
engine.
2. The method as claimed in claim 1, characterized in that the
energy input into the combustion engine (1) is determined from the
mass air flow (MAF) and the speed of the combustion engine, the
fuel injection quantity, the induced torque or the induced power of
the combustion engine.
3. The method as claimed in claim 1, characterized in that the
energy input into the combustion engine is determined from an
engine-specific air mass/engine speed-dependent temperature
characteristics map (19).
4. The method as claimed in claim 3, characterized in that the
duration of the fan run-on time is calculated by integration of the
energy inputs.
5. The method as claimed in claim 4, characterized in that the
integration is each time performed over a predetermined time
interval and the integration result (27) is stored by intervals,
the number of retroactively recorded interval-specific integration
results being limited and the recorded interval-specific
integration results each time being cyclically overwritten by the
currently calculated integration results.
6. The method as claimed in claim 5, characterized in that an
average is formed from the interval-specific integration results
(27).
7. The method as claimed in claim 1, characterized in that in
addition to the energy input into the combustion engine (2),
characteristics maps for the air temperature and characteristics
maps for the coolant temperature for determining the currently
attainable cooling performance are also introduced into the
calculation for determining the duration of the fan run-on
time.
8. The method as claimed in claim 3, characterized in that the air
mass/engine speed-dependent temperature characteristics map (19)
contains a family of characteristics for multiple
temperature-critical components.
9. The method as claimed in claim 8, characterized in that the
family of characteristics is corrected by correction parameters for
the outside air temperature, vehicle speed, fan activation, water
temperature, intake air temperature, exhaust gas temperature or
radiator shutter position.
10. A device for calculating the run-on time of a fan motor (4), in
particular for a motor vehicle, having at least one electronic
storage medium and at last one electronic logic element (logic);
characteristics maps (19) of the operating state and operating
conditions of a combustion engine being filed in the storage
medium/media, and calculations to determine the fan run-on time
being performed in the electronic logic element (logic) by means of
software programs or by means of logic elements, characterized in
that the logic element is in communication with signal generators
(17) for the volumetric efficiency of the combustion cylinders and
for the speed of the combustion engine or the fuel injection
quantity or the induced torque or the induced power of the
combustion engine, and the fan run-on time is determined from the
energy input into the combustion engine (2).
11. The device as claimed in claim 10, characterized in that at
least one characteristics map (19) is an air mass/engine
speed-dependent temperature characteristics map.
12. The device as claimed in claim 10, characterized in that the
logic element comprises an integration stage (23) for the time
integration of the energy inputs from the air mass/engine
speed-dependent temperature characteristics map (19). the energy
inputs from the air mass/engine speed-dependent temperature
characteristics map (19).
13. The device as claimed in claim 10, characterized in that the
logic element comprises a software programmed or hard-wired
cyclical loop (29) for the storage of interval-specific integration
results (27).
14. The device as claimed in claim 10, characterized in that the
logic element comprises software programmed or hard-wired averaging
(33) for all registered interval-specific integration results.
15. The device as claimed in claim 10, characterized in that the
logic element is in communication with the engine management system
(17) as signal generator.
16. The device as claimed in claim 1, characterized in that the
logic element (logic) is integrated into an engine management
system (17).
17. The device as claimed in claim 10, characterized in that the
logic element (logic) contains characteristics maps of the air
temperature and characteristics maps of the coolant temperature for
determining the currently attainable cooling performance.
18. The device as claimed in claim 11, characterized in that the
air mass/engine speed-dependent temperature characteristics map
comprises a family of curves of multiple temperature-critical
components in the vehicle.
19. The device as claimed in claim 18, characterized in that the
family of characteristics is corrected by correction parameters for
the outside air temperature, vehicle speed, fan activation, water
temperature, intake air temperature, exhaust gas temperature or
radiator shutter position.
Description
[0001] The invention relates to a method and a device for
controlling a fan motor. The fan motor is preferably used in
cooling systems for motor vehicles. In order to be able to
determine the optimum fan run-on time after switching off the
combustion engine, the energy input into the combustion engine is
registered. The energy input shortly before the combustion engine
was switched off and the specific fan characteristic are used to
calculate the fan run-on time, which is necessary in order to
prevent subsequent overheating of the combustion engine.
[0002] Run-on controls for fan motors in motor vehicles have long
been known. The run-on controls hitherto disclosed operate as a
function either of the temperature or the time. In the case of
temperature-dependent run-on controls a temperature sensor is used
to monitor the coolant temperature and if a critical temperature
value is exceeded the run-on control of the fan motor is activated
and the coolant circuit with an electrical coolant pump is set in
operation. Time-dependent run-on controls operate with timing
elements. The timing element here determines the length of the fan
run-on.
[0003] The German patent specification DE 3424 580 C1 affords a
broad overview of the state of the art hitherto disclosed. The
German patent specification describes a cooling system which is
provided with an electrically driven fan and a run-on control. The
run-on control in this case operates as a function either of the
temperature or the time. The cooling system comprises a second,
electrically driven coolant pump, which is likewise controlled by
the run-on control and which maintains the coolant flow whilst the
run-on control is in operation.
[0004] Known run-on controls for fan motors have the disadvantage
that they cut in regardless of the actual load condition and hence
also regardless of any possible overheating of the engine. They
therefore also cut in when there is no overheating of the engine
whatsoever. Time-dependent fan run-on controls are always bound to
cut in and temperature-dependent fan run-on controls may simply cut
in, for example, because a high ambient temperature results in a
high coolant temperature.
[0005] Conversely, when the engine has been run in the full-load
range immediately before switching off the combustion engine, for
example, it may take several minutes until the overheating of the
engine makes itself felt through a temperature rise on the fan
run-on temperature sensor. This delay before the fan run-on control
cuts in may already be too late for temperature-sensitive
microelectronic components in the motor vehicle.
[0006] The object of the invention, therefore, was to develop a fan
run-on control which avoids unnecessary fan run-on times and on the
other hand detects the risk of a delayed temperature rise and
promptly initiates countermeasures to prevent the temperature
rise.
[0007] The object is achieved by a method and a device according to
the independent claims. Further preferred embodiments of the
invention are contained in the dependent claims and in the
exemplary embodiments.
[0008] The object is primarily achieved by means of a fan run-on
control, which takes into account the energy input into the
combustion engine in order to calculate the required fan run-on
time. If the characteristics of the fan are known it is possible to
calculate the required fan run-on time from the integral of the
energy input into the combustion engine before the combustion
engine was switched off, and the current operating data and ambient
data of the combustion engine. Furthermore, by comparing the energy
input into the combustion engine with the cooling performance of
the cooling system over a specific period of time before the engine
was switched off, it is possible to predict whether or not
running-on of the fan will be necessary. There is always a risk of
further subsequent heating whenever the energy input into the
engine has been significantly greater than the currently applied
cooling performance of the system before the engine was switched
off. If the opposite is true, there may sometimes be no need for
the fan to run on, or the fan run-on time can be much shorter than
in previously known systems.
[0009] The invention primarily affords the following
advantages:
[0010] The invention allows the fan run-on time to be optimally
adjusted to the load condition of the engine immediately before it
was switched off. This avoids unnecessary fan run-on times, and the
likely effects of further subsequent heating, which owing to the
thermal inertia of the cooling system would only make themselves
felt after some delay, can be predicted in good time so that
overheating can be promptly counteracted though increased cooling
performance.
[0011] In an advantageous embodiment of the invention the energy
input into the combustion engine is determined using the mass air
flow as a measure of the volumetric efficiency of the combustion
cylinders, and the speed of the combustion engine. This embodiment
has the advantage that the necessary measured values for the
volumetric efficiency and the speed of the combustion engine can be
taken from existing engine management systems. Known engine
management systems, which determine the volumetric efficiency of
the combustion cylinders and the speed of the combustion engine,
include the electronic engine management systems produced by Bosch,
for example. These systems are marketed and used under the name
"Motronic". These systems are described, for example in "Automotive
Handbook", Bosch-23.sup.rd revised edition, Braunschweig: Viehweg,
1999, pages 498-507. Other alternative operating data for
calculating the energy input are the induced torque, the induced
power or, especially in the case of diesel engines, the induced
fuel injection quantity. These alternative operating data are
likewise supplied by engine management systems.
[0012] In a further advantageous embodiment of the invention an
engine-specific air mass/engine speed-dependent temperature
characteristics map is determined from the signals of the engine
management system in road tests using a trial vehicle. This
exemplary embodiment has the practical advantage that in the case
of series production vehicles this engine-specific air mass/engine
speed-dependent temperature characteristics map has to be
determined just once using a representative trial vehicle and that
this engine-specific air mass/engine speed-dependent temperature
characteristics map can then be adopted in all further series
production vehicles of the same type and specifications as the
trial vehicle. The engine-specific air mass/engine speed-dependent
temperature characteristics map can then be used to determine the
fan run-on time in each separate series production vehicle.
[0013] In a further advantageous embodiment of the invention the
length of the fan run-on time is calculated through time
integration of those energy inputs into the combustion engine which
lie above a critical reference value in the air mass/engine
speed-dependent temperature characteristics map. The time
integration makes it possible to average out transient loads, which
do not have any significant effect on likely further subsequent
heating. Introducing a critical reference value that has likewise
to be determined experimentally makes it possible to eliminate from
the calculation of the fan run-on time those load conditions of the
combustion engine which do not require running-on of the fan.
[0014] In a further advantageous embodiment of the invention the
time integration of each of the energy inputs is performed over a
predetermined time interval. The result of the integration is in
this case stored by intervals. The number of integration intervals
registered is limited. For example, at any one time five
integration intervals of one minute each are registered and stored
for the last five minutes. If the operation of the combustion
engine lasts for a longer period of time, the stored
interval-specific integration results are cyclically overwritten.
This means that at any given time the load condition in the last
five minutes before the combustion engine was switched off is
recorded. This saves any excessive retention of data, which is not
needed in order to calculate the fan run-on.
[0015] In a further advantageous embodiment of the invention the
air mass/engine speed-dependent temperature characteristics map
contains a family of characteristics for multiple
temperature-critical components. Not only can the running-on of the
fan thereby be related to a temperature-critical component, but the
temperatures of multiple critical components can be incorporated
into the calculation of the fan run-on time. This has the
advantage, for example, that local irregularities in the heating up
of the engine compartment of a motor vehicle can be taken into
account in the calculation of the fan run-on time.
Temperature-critical components which, for example, are situated in
a heat sink, which does not warm up when the engine is briefly
heated up, can be disregarded when calculating the fan run-on
time.
[0016] Exemplary embodiments of the invention will be explained in
more detail below with reference to the drawings, in which:
[0017] FIG. 1 shows a cooling system of a combustion engine having
a mapped logic for controlling an electric fan motor,
[0018] FIG. 2 shows a schematic diagram, of the run-on calculation
for the fan motor using operating data from the engine management
system,
[0019] FIG. 3 shows an interval-specific integration diagram for
calculation of the fan run-on control,
[0020] FIG. 4 shows a function diagram for averaging the
interval-specific integration results,
[0021] FIG. 5 shows an experimentally determined multi-component
air mass/engine speed-dependent temperature characteristics map for
calculation of the fan run-on time,
[0022] FIG. 6 shows a reduced characteristics map, extracted from
the characteristics map in FIG. 5, and in which all temperature
measuring points not critical for the fan run-on time have been
extracted by means of a limit value.
[0023] FIG. 1 shows a diagram of a typical cooling system for a
six-cylinder combustion engine 1. In addition to the combustion
engine, a vehicle radiator 2 and a heating system heat exchanger 3
are incorporated into the cooling system. The cooling performance
of the vehicle radiator can be influenced by an electrically driven
fan 4. In order to regulate the fan output the electric motor of
the fan is controlled by a control unit 5. Cooled coolant is drawn
from the vehicle radiator by means of the flow pipe 6 and is fed by
the coolant pump 7 into the cooling pipes 8 for feeding cooling
ducts (not shown further) for the combustion cylinders 9. From the
combustion cylinders 9 the heated coolant is led via return pipes
10 to a three-way thermostat 11. Depending on the position of the
valves in the three-way thermostat 11, the coolant passes out of
the combustion engine via the radiator return pipe 12 back into the
vehicle radiator or via the radiator bypass 13 and the coolant pump
7 back into the cooling pipes 8 of the combustion engine.
[0024] Depending on the position of the valves in the three-way
thermostat 11 the cooling system can here be run in a manner known
in the art in bypass mode, in mixed mode or on the full cooling
circuit. The heating system heat exchanger 3 is connected by way of
a temperature-controlled shut-off valve 14 to the high-temperature
branch of the cooling system in the combustion engine. The rate of
flow through the heating system heat exchanger after opening the
shut-off valve 14 can be adjusted by an additional electrical
coolant pump 15 and a timed shut-off valve 16 in order to regulate
the heating output.
[0025] The temperature level of the coolant in the combustion
engine is here set by the control unit 5, controlled by sensors.
This is achieved in a manner known in the art through actuation of
the control valves in the three-way thermostat 11 and by activation
of the electrical fan 4, if air stream cooling is no longer
sufficient.
[0026] With a sensor-controlled cooling system as described for
combustion engines in motor vehicles satisfactory performances can
be obtained when running. After switching off the combustion engine
critical situations for temperature-sensitive components in the
engine compartment of a motor vehicle can occur should the heat
stored in the combustion engine no longer be dissipated due to the
coolant becoming static. For this reason cooler run-on systems have
already been proposed in the past. These cooler run-on systems
operate, as already described as a function of the time or the
temperature. In the case of purely time-dependent systems,
therefore, it was in the past always necessary to provide a fan
run-on, which as a rule was much too long. Temperature-dependent
systems also had the disadvantage, however, that the temperature
sensors, which usually measure the temperature profile of the
coolant, were able to detect heating up of the combustion engine
only with a considerable time delay. The delay in this case results
from the thermal inertia of the system. The effect of delayed
further heating after switching off the combustion engine is
especially great if, for example, the combustion engine has been
run for a longer period of time in the partial load range and the
engine has been run up to full load only in the last few minutes
before switching off the engine. In this scenario, a
temperature-controlled cooling system will still be operating in
the partial load range, whereas the engine has been fully fired up
shortly before switching off. The engine then holds a large
quantity of heat which still needs to be dissipated. Since a
temperature-controlled run-on system must also cope with this
scenario described above, it was necessary, in order to prevent
localized overheating in the combustion engine, to set the
temperature threshold for starting up of the run-on control very
low. There was no facility for predicting how much energy would
still have to be dissipated and for this reason it was always
necessary to assume the worst case in the event of a rise in the
coolant temperature being detected. This also means that in the
vast majority of cases the temperature-controlled run-on control
responds too often and for too long. This represents the point of
departure for the invention.
[0027] According to the invention, therefore, the control unit 5
uses the calculation of the run-on time, based on the energy input
into the combustion engine over a sufficient period of time
immediately before switching off the engine, for controlling the
run-on of the fan 4. For this purpose an engine-specific air
mass/engine speed-dependent temperature characteristics map is
filed in the control unit 5. By monitoring the operating data of
the combustion engine, for example by reading in the operating data
from the engine management system, the energy input into the
combustion engine is determined from the air mass/engine
speed-dependent temperature characteristics map and from this a fan
run-on time is obtained. For example, the energy input into the
combustion engine may be logged for the last five minutes before
switching off the engine, and from the energy input over the last
five minutes a time integration may be performed, the result of
this integration being compared with an experimentally determined
or model-based, calculated reference value. If the integration
result exceeds this reference value, a fan run-on must be
activated. The length of time for which the fan must run on is here
determined from the difference between the integration value and
the reference value. These are basically the activation
characteristic curve of the fan motor, the temperature of the
ambient air and the current temperature of the coolant.
[0028] With all these data an energy balance for the combustion
engine and the cooling system can then be undertaken in a process
computer of the control unit 5 according to the laws of
thermodynamics, and the required cooling performance and hence the
required run-on time of the fan can then be calculated from said
energy balance.
[0029] The system of calculation is explored in more detail in FIG.
2. Here the two reference input variables, engine speed and mass
air flow, are particularly relevant for the operating condition of
the engine. These two reference input variables are provided by
constantly updated engine controls 17 in the form of digital
signals. The engine speed and the air mass are the reference input
variables for that energy which is introduced into the combustion
engine. The "Automotive Handbook" produced by Bosch, as already
cited above, pages 498 to 507, gives a good overview of engine
management systems. The energy introduced is here a measure of the
energy to be dissipated by a cooling system and hence a measure of
the required cooling performance and the necessary run-on time of
the fan.
[0030] For determining the fan run-on, however, it is preferable to
select the anticipated temperature of the engine as intensive
variable to be determined, rather than the energy as extensive
variable to be balanced. A calculation model which is directed
towards the temperature of the combustion engine is in practice
easier to analyze and improve by measuring road tests.
Temperature-intensive variables can likewise be more readily
adjusted to specific engines through measuring road tests. A method
of calculation which is directed towards the temperature of the
combustion engine can thereby be adapted to different engine
variants. The adaptation is here achieved by means of software
program modules 18, which use the operating data of the engine
management system 17 to calculate the delayed temperature curve of
the combustion engine 19. In the program modules 18 the temperature
profile to be expected of the combustion engine is calculated from
the operating data, engine speed and mass air flow, by means of
experimentally compiled calculation equations. The calculated
temperature curve is here adapted to the temperature curve actually
measured through adjustment of the parameter values in the
calculation equations in the program modules. The speed and mass
air flow are the two most important reference input variables for
the engine management and hence also for calculation of the
temperature curve to be expected of the combustion engine. This
predicted temperature curve is adjusted to the prevailing ambient
conditions of the combustion engine by means of a correction
element 20, which likewise takes the form of a software program
module. The most important influencing variables among the
environmental conditions are the air temperature, the temperature
of the intake air, the air pressure and air humidity, the current
cooling performance of the cooling system and the position of any
throttle valve of the combustion engine. The temperature profile
adjusted for the ambient conditions is time integrated by an
integration stage 23 in the form of a so-called moving average. The
integration stage 23 is explored in more detail with reference to
FIG. 3. The integration result from the integration stage 23
undergoes further processing by a further program module 24 for
final actuation of the fan motor 4. For this purpose the program
module 24 compares the integration result from the integration
stage 23 with an experimentally determined reference value 22 and
calculates the required fan run-on time on the basis of the fan
characteristic curve 25. The reference value 22 is here established
experimentally and for the specific variant of the vehicle in
question. The reference value must be established in such a way as
to reliably ensure that the component having the greatest
temperature sensitivity is not damaged.
[0031] FIG. 3 shows a logic flow chart for the integration stage
23. A so-called moving average is calculated. The integration stage
23 is preferably implemented as a program module, that is to say in
the software. In a further preferred embodiment, however, the
integration stage 23 may also be hardwired using logic elements.
The term "moving average" is understood to mean a process of
averaging which progresses over time and in which the averaging is
in each case calculated by way of a fixed number of chronologically
successive sub-averages, the sub-averages each time being
cyclically recalculated and redetermined in chronological order.
For example, if the sub-averages are each calculated over a period
of one minute and five chronologically successive sub-averages are
provided for, via which an overall average is then calculated, the
overall average obtained each time is that for the last five
minutes when the system was activated. Through the cyclical
overwriting and recalculation of the sub-averages this average is
each time updated and adjusted for the last five minutes of the
activated system. This process is represented schematically in FIG.
3, as follows:
[0032] The temperature profile adjusted for the ambient conditions
is each time integrated over a period of one minute by a time
integration element 26, and stored. The time division is set and
the integration result from the integration intervals is stored in
that a cyclical cascade causes the integration to recommence once a
period of one minute, for example, has elapsed, and the integration
result after each minute is registered in a memory area 27. The
duration of the intervals for the individual integration stages is
in principle freely selectable and is defined by a time constant or
a delay element 28. The cyclical storage of the integration results
from the integration intervals is preferably embodied in the
software in the form of a recurring loop. It is also possible,
however, to provide for hard-wired switching of the integration
results to the memory areas. Both embodiments are represented
schematically in FIG. 3 as a cascade 29 of successive AND elements
and an OR element, by means of which, among other things, the
activation signal for the integration stage is also inputted. The
summing of the subsequence results registered in the memory areas
27 is performed in a summation stage 33, which is preferably
embodied as a software program module. An embodiment of the
summation stage 33 as a hard-wired AND element 32, as represented
in FIG. 3, for example, is less preferable.
[0033] FIG. 4 again shows a function diagram for the calculation of
an overall average. In this case the sub-averages denoted by the
FIGS. 1 to 5 are summed either by a software program or by a logic
circuit to obtain an overall average. The individual sub-averages
are cyclically overwritten.
[0034] FIG. 5 shows a table of measured values obtained from road
tests. Fitted to the test vehicle in this case were various
temperature sensors, which registered and plotted the oil
temperature, the temperature of the integral carrier, the
temperature of the steering rack, the temperature of the steering
gaiter, the temperature of the axle half-shaft, the catalytic
converter temperature and the temperature of the electronic
injection system as a function of the two reference input
variables, mass air flow (MAF) and engine speed (Eng-Spd).
[0035] The temperatures tabulated as a function of the two
reference input variables serve the program modules 18 in FIG. 2 as
reference points for the calculation of the temperature
characteristics map 19. The main function of the program modules 18
is interpolation of the reference points, in order that a
continuous temperature characteristics map can be calculated.
[0036] Table 1 in FIG. 5 clearly illustrates that various
temperatures in the engine compartment of a motor vehicle can be
used in determining the temperature characteristics map. This means
that a run-on calculation according to the invention may serve not
only to safeguard an isolated local temperature but also a
temperature distribution or even different localized temperatures
of multiple components. The fact that the program modules 18 resort
to experimentally obtained reference points in order to calculate
the temperature characteristics map also makes it possible to
readily design the fan motor run-on control according to the
invention for specific engines or for specific variants. For the
various vehicle variants or the various engine variants, a test
vehicle is used to determine the temperature reference points
specific to each variant or each engine, and these experimentally
plotted temperature reference points are fed into the program
modules 18 as reference points. The fan run-on according to the
invention can thereby be easily adapted to different vehicle
variants.
[0037] FIG. 6 shows a further Table 2, in which the temperature
reference points from Table 1 in FIG. 5 have already been evaluated
with reference to a critical reference temperature. In Table 2 the
mass air flow is entered in the horizontal direction and the engine
speed in the vertical direction. The table values themselves
contain the temperature values for the steering rack, as measured
in the test results in FIG. 5. An evaluation of the temperature
distribution in FIG. 5 has in fact revealed that the temperature of
the rack as a function of the two reference input variables, mass
air flow and engine speed, is the temperature, the level of which
is most likely to lead to damage. For this reason the steering rack
temperature is most suited to determining a reference temperature,
as is used in FIG. 2 for the calculation of the fan run-on time.
The temperature values for the steering rack in Table 1 have here
been rescaled by the factor 1000 and entered in Table 2. Rescaling
for the calculation of the temperature values does not constitute
an essential element of the invention. Knowing the design data of
the steering rack, the exceeding of a non-dimensional reference
value for the temperature of 109 or 0.109 as in FIG. 6 had to be
regarded as critical. These are the temperature values represented
in bold type in FIG. 6. A run-on control for the fan motor is
therefore to be provided for the associated operating conditions of
the combustion engine in FIG. 5. In this exemplary embodiment these
are the operating conditions for mass air flow and engine speed
likewise shown in bold type in the table in FIG. 5. For the
remaining operating conditions of the combustion engine a fan
run-on may be dispensed with.
* * * * *