U.S. patent application number 10/858800 was filed with the patent office on 2005-12-08 for compression ratio mode selection logic for an internal combustion engine having discrete variable compression ratio control mechanism.
Invention is credited to Brehob, Diana, Chen, Yin, Dutcher, William R., Glugla, Chris P., Trumpy, David K., Yang, Woong-chul, Yoo, In Kwang.
Application Number | 20050273245 10/858800 |
Document ID | / |
Family ID | 35405333 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050273245 |
Kind Code |
A1 |
Chen, Yin ; et al. |
December 8, 2005 |
COMPRESSION RATIO MODE SELECTION LOGIC FOR AN INTERNAL COMBUSTION
ENGINE HAVING DISCRETE VARIABLE COMPRESSION RATIO CONTROL
MECHANISM
Abstract
A method and system for operating an internal combustion engine.
The internal combustion engine is operable in a plurality of
compression ratio operating modes. The method includes determining
a relationship between a base engine threshold load where the high
compression and low compression provide substantially the same
engine fuel consumption and engine speed. The determined nominal
relationship is modified by a factor, such factor being a function
of a condition under which such engine is operating, to obtain a
modified relationship. The modified relationship is a function of
engine speed and the condition has an effect on knock generation in
such engine. The modified relationship and engine speed are used in
selecting one of the plurality of compression ratio operating modes
for the engine. A hysteresis load as a function of engine speed is
determined and applied to the determined hysteresis load to the
modified relationship to inhibit switching oscillations between a
pair of the plurality of compression ratio operating modes.
Inventors: |
Chen, Yin; (Dearborn,
MI) ; Yang, Woong-chul; (Ann Arbor, MI) ;
Dutcher, William R.; (Wayne, MI) ; Trumpy, David
K.; (Farmington Hills, MI) ; Yoo, In Kwang;
(Ann Arbor, MI) ; Glugla, Chris P.; (Macomb,
MI) ; Brehob, Diana; (Dearborn, MI) |
Correspondence
Address: |
FORD GLOBAL TECHNOLOGIES, LLC.
SUITE 600 - PARKLANE TOWERS EAST
ONE PARKLANE BLVD.
DEARBORN
MI
48126
US
|
Family ID: |
35405333 |
Appl. No.: |
10/858800 |
Filed: |
June 3, 2004 |
Current U.S.
Class: |
701/111 ;
123/48R; 123/78F; 701/103 |
Current CPC
Class: |
F02D 41/3076 20130101;
Y02T 10/18 20130101; F02D 13/0207 20130101; Y02T 10/12 20130101;
F02D 35/027 20130101 |
Class at
Publication: |
701/111 ;
123/078.00F; 701/103; 123/048.00R |
International
Class: |
G06G 007/70; G06F
019/00; F02B 075/04 |
Claims
1. A method for operating an internal combustion engine, the
internal combustion engine being operable in a plurality of
compression ratio operating modes, the method comprising:
determining a relationship between a base engine threshold load
where the high compression and low compression provide
substantially the same engine fuel consumption and engine speed;
modifying the determined nominal relationship by a factor, such
factor being a function of a condition under which such engine is
operating, to obtain a modified relationship as a function of
engine speed, such condition having an effect on knock generation
in such engine; using the modified relationship and engine speed in
selecting one of the plurality of compression ratio operating modes
for the engine.
2. The method recited in claim 1 including determining a hysteresis
load as a function of engine speed and applying such determined
hysteresis load to the modified relationship to inhibit switching
oscillations between a pair of the plurality of compression ratio
operating modes.
3. The method recited in claim 1 wherein the factor is a function
of at least one of engine coolant temperature, engine air charge
temperature, valve events, humidity, EGR, fuel type and barometric
pressure.
4. The method recited in claim 1 wherein the factor is a function
of at least two of engine coolant temperature, engine air charge
temperature and barometric pressure.
5. The method recited in claim 1 wherein the factor is a function
of coolant temperature, engine air charge temperature and
barometric pressure.
6. The method recited in claim 3 including determining a hysteresis
load as a function of engine speed and applying such determined
hysteresis load to the modified relationship to inhibit switching
oscillations between a pair of the plurality of compression ratio
operating modes.
7. The method recited in claim 4 including determining a hysteresis
load as a function of engine speed and applying such determined
hysteresis load to the modified relationship to inhibit switching
oscillations between a pair of the plurality of compression ratio
operating modes.
8. The method recited in claim 5 including determining a hysteresis
load as a function of engine speed and applying such determined
hysteresis load to the modified relationship to inhibit switching
oscillations between a pair of the plurality of compression ratio
operating modes.
9. The method recited in claim 1 wherein the factor is a function
of the product of correction factors for at least two of engine
coolant temperature, engine air charge temperature and barometric
pressure.
10. The method recited in claim 1 wherein the factor is a function
of the product of correction factors for coolant temperature,
engine air charge temperature and barometric pressure.
11. The method recited in claim 9 including determining a
hysteresis load as a function of engine speed and applying such
determined hysteresis load to the modified relationship to inhibit
switching oscillations between a pair of the plurality of
compression ratio operating modes.
12. The method recited in claim 10 including determining a
hysteresis load as a function of engine speed and applying such
determined hysteresis load to the modified relationship to inhibit
switching oscillations between a pair of the plurality of
compression ratio operating modes.
13. A system for operating an internal combustion engine, the
internal combustion engine being operable in a plurality of
compression ratio operating modes, the system comprising: a
variable compression ratio apparatus for varying the compression
ratio of the internal combustion engine; and a controller in
communication with the variable compression ratio apparatus for
selecting one of the plurality of compression ratio operating modes
for the engine, such selection being a function of a relationship
between a base engine threshold load where the high compression and
low compression provide substantially the same engine fuel
consumption and engine speed modified by a factor, such factor
being a function of a condition under which such engine is
operating, such condition having an effect on knock generation in
such engine, such controller using the modified relationship and
engine speed in selecting one of the plurality of compression ratio
operating modes for the engine.
14. The system in accordance with claim 13, wherein said controller
adjusts the selection in accordance with a hysteresis load as a
function of engine speed to inhibit switching oscillations between
a pair of the plurality of compression ratio operating modes.
15. An article of manufacture for operating an internal combustion
engine, the internal combustion engine being operable in a
plurality of compression ratio operating modes via a variable
compression ratio apparatus, the article of manufacture comprising:
a computer usable medium; and a computer readable program code
embodied in the computer usable medium for directing a computer to
control the steps of determining a selected one of the plurality of
compression ratio operating modes, such selection being a function
of a relationship between a base engine threshold load where the
high compression and low compression provide substantially the same
engine fuel consumption and engine speed modified by a factor, such
factor being a function of a condition under which such engine is
operating, such condition having an effect on knock generation in
such engine, such controller using the modified relationship and
engine speed in selecting one of the plurality of compression ratio
operating modes for the engine.
16. The method recited in claim 1 wherein the factor is a function
of engine coolant temperature.
17. The method recited in claim 1 wherein the factor is a function
of engine air charge temperature.
18. The method recited in claim 1 wherein the factor is a function
of valve events.
19. The method recited in claim 1 wherein the factor is a function
of humidity.
20. The method recited in claim 1 wherein the factor is a function
of EGR.
21. The method recited in claim 1 wherein the factor is a function
of fuel type.
22. The method recited in claim 1 wherein the factor is a function
of barometric pressure.
23. The article of manufacture recited in claim 15 wherein the
storage medium is a semiconductor chip.
Description
TECHNICAL FIELD
[0001] This invention relates generally to variable compression
internal combustion engines.
BACKGROUND
[0002] As is known in the art, the "compression ratio" of an
internal combustion engine is defined as the ratio of the cylinder
volume when the piston is at bottom-dead-center (BDC) to the
cylinder volume when the piston is at top-dead-center
(TDC)--generally, the higher the compression ratio, the higher the
thermal efficiency and fuel economy of the internal combustion
engine. Unfortunately, compression ratios are limited by the
availability of high-octane fuels needed to prevent combustion
detonation or knock at high engine loads, and therefore a
compression ratio is selected to operate on available fuels, and
avoid knock. So-called "variable compression ratio" internal
combustion engines have been developed, for example, having higher
compression ratios during low load conditions and lower compression
ratios during high load conditions.
[0003] In an engine with a variable compression ratio mechanism,
the engine compression ratio can be selected to achieve the best
fuel economy of a vehicle. However, drivability and engine knock
issues may occur by changing engine compression ratio while driving
a vehicle in different environmental conditions. To ensure the
switching of compression ratio happens with minimum knock and as
smooth as possible at every possible real-world driving condition,
not only must the engine operating conditions be taken into
consideration but also environmental conditions have to be taken
into considered in the compression ratio selection. The problem is
how to take into account those factors so as to select appropriate
engine compression ratio to obtain optimum fuel economy without
sacrificing drivability.
SUMMARY
[0004] In accordance with the present invention, a method and
system are provided for operating an internal combustion engine.
The internal combustion engine is operable in a selected one of a
plurality of compression ratio operating modes. The method includes
determining a relationship between: (1) a base engine threshold
load providing substantially equal engine fuel consumption for a
pair of the plurality of compression ratio operating modes; and (2)
engine speed. The determined relationship is modified by a factor,
such factor being a function of a condition under which such engine
is operating to obtain a modified relationship. The modified
relationship is a function of engine speed and the condition is one
having an effect on knock generation in such engine. The modified
relationship and engine speed are used in selecting the one of the
pair of the plurality of compression ratio operating modes for the
engine.
[0005] In one embodiment, the method and system include determining
a hysteresis load as a function of engine speed and applying such
determined hysteresis load to the modified relationship to inhibit
switching oscillations between a pair of the plurality of
compression ratio operating modes.
[0006] In one embodiment, the factor is a function of at least one
of engine coolant temperature, engine air charge temperature and
barometric pressure.
[0007] In one embodiment, the factor is a function of at least two
of engine coolant temperature, engine air charge temperature and
barometric pressure.
[0008] In one embodiment, the factor is a function of coolant
temperature, engine air charge temperature and barometric
pressure.
[0009] In one embodiment, the factor is a function of the product
of correction factors for at least two of engine coolant
temperature, engine air charge temperature and barometric
pressure.
[0010] In one embodiment, the factor is a function of the product
of correction factors for coolant temperature, engine air charge
temperature and barometric pressure.
[0011] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0012] FIG. 1 is a diagram of an internal combustion engine having
variable compression ratio and a controller for selecting such
ratio in accordance with the invention;
[0013] FIG. 2A is a curve showing the relationship between a base
cylinder air charge (load) and engine speed at a nominal air charge
temperature (act), nominal engine cooling temperature (ect) and
nominal barometric pressure (bp), such curve showing that for a
given engine speed and cylinder air charge below the curve, the
engine has minimal fuel consumption using a high compression ratio
and that for a given engine speed and cylinder air charge above the
curve, the engine has minimal fuel consumption using a high
compression ratio.
[0014] FIG. 2B is a curve showing a correction factor corr_act, to
be applied to the curve of FIG. 2A as a function of deviation of
the air charge temperature from the nominal air charge temperature
(act) used in generating the curve in FIG. 2A;
[0015] FIG. 2C is a curve showing a correction factor corr_ect, to
be applied to the curve of FIG. 2A as a function of deviation of
the engine cooling temperature from the nominal engine cooling
temperature (ect) used in generating the curve in FIG. 2A;
[0016] FIG. 2D is a curve showing a correction factor corr_bp, to
be applied to the curve of FIG. 2A as a function of deviation of
the barometric pressure from the nominal barometric pressure (bp)
used in generating the curve in FIG. 2A;
[0017] FIG. 2E is a curve showing a bias as a function of engine
speed, such curve being used in the generation of a pair of curves
shown in FIG. 2F and applied to prevent oscillations between the
engine operating in the high and low compression ratio modes;
[0018] FIG. 2F show a pair of curves used in the selection of the
high or low combustion ratio operating modes for the engine of FIG.
1;
[0019] FIG. 3 is a flow diagram of a process used to control the
engine of FIG. 1 in accordance with the invention;
[0020] FIG. 4A shows the relationship between Brake Specific Fuel
Consumption (BSFC) and spark advance (SA) for an engine operating
at a constant speed, barometric pressure (bp), engine air charge
temperature (act) engine coolant temperature (ect), one curve
therein being generated with the engine operating in the low
compression mode and a second curve therein being generated with
the engine operating in the high compression mode; and
[0021] FIG. 4B shows the relationship between BSFC and load for
both the low compression ratio and the high compression ratio in
which the spark advance is MBT spark timing unless there is engine
knock, in which case, the spark advance is set to borderline knock,
i.e., retarded from MBT spark advance.
[0022] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0023] FIG. 1 shows an exemplary variable compression ratio
internal combustion engine in accordance with the present
invention. As will be appreciated by those of ordinary skill in the
art, the present invention is independent of the particular
underlying engine configuration and component designs, and as such
can be used with a variety of different internal combustion engines
having more than one compression ratio operating modes. The engine
for example can be constructed and arranged as a discrete
compression ratio engine operating for example at a high
compression or at low compression, or as a continuously variable
compression ratio engine capable of operating at an infinite number
of discrete compression ratios. Similarly, the present invention is
not limited to any particular type of apparatus or method required
for varying the compression ratio of the internal combustion
engine.
[0024] Referring again to FIG. 1, the engine 110 includes a
plurality of cylinders (only one shown), each having a combustion
chamber 111, a reciprocating piston 112, and intake and exhaust
valves 120 and 118 for communicating the combustion chamber 111
with intake and exhaust manifolds 124 and 122. The piston 112 is
coupled to a connecting rod 114, which itself is coupled to a
crankpin 117 of a crankshaft 116. Fuel is provided to the
combustion chamber 111 via a fuel injector 115 and is delivered in
proportion to a fuel pulse width (FPW) determined by an electronic
engine controller 60 (or equivalent microprocessor-based
controller) and electronic driver circuit 129. Air charge into the
intake manifold 124 is nominally provided via an electronically
controlled throttle plate 136 disposed within throttle body 126.
Ignition spark is provided to the combustion chamber 111 via spark
plug 113 and ignition system 119 in accordance with a spark advance
(or retard) signal (SA) from the electronic controller 60.
[0025] As shown in FIG. 1, the engine controller 60 nominally
includes a microprocessor or central processing unit (CPU) 66 in
communication with computer readable storage devices 68, 70 and 72
via memory management unit (MMU) 64. The MMU 64 communicates data
to and from the CPU 66 and among the computer readable storage
devices, which for example may include read-only memory (ROM) 68,
random-access memory (RAM) 70, keep-alive memory (KAM) 72 and other
memory devices required for volatile or non-volatile data storage.
The computer readable storage devices may be implemented using any
known memory devices such as semiconductor chip programmable
read-only memory (PROM's), electrically programmable read-only
memory (EPROM's), electrically erasable PROM (EEPROM's), flash
memory, or any other electrical, magnetic, optical or combination
memory devices capable of storing data, including executable code,
used by the CPU 66 for controlling the internal combustion engine
and/or motor vehicle containing the internal combustion engine.
Input/output (I/O) interface 62 is provided for communicating with
various sensors, actuators and control circuits, including but not
limited to the devices shown in FIG. 1. The executable code
instructions for providing the combustion ratio selection will be
described below in connection with FIG. 3. These devices include an
engine speed sensor 150, electronic fuel control driver 129,
ignition system 119, manifold absolute pressure sensor (MAP) 128,
mass air flow sensor (MAF) 134, throttle position sensor 132,
electronic throttle control motor 130, inlet air temperature sensor
138, engine knock sensor 140, and engine coolant temperature
142.
[0026] The engine 110 of FIG. 1 also includes and a variable
compression ratio apparatus 170. In a non-limiting embodiment, the
variable compression ratio apparatus 170 is operated to vary the
effective length of the connecting rod 114, and thus the clearance
volume and compression ratio of the engine. Such an apparatus is
described, for example, in U.S. application Ser. No. 09/682,263,
entitled "Connecting Rod for a Variable Compression Engine," which
is owned by the assignee of the present invention and is hereby
incorporated by reference in its entirety. The actual construction
and configuration of the variable compression apparatus shown in
FIG. 1 is not at all intended to limit the scope of claim
protection for the inventions described herein.
[0027] In a non-limiting aspect of the present invention, the
variable compression ratio apparatus of FIG. 1 is described below
as operating in a "high" compression ratio mode (compression ratio
of 13:1 and above) or a "low" compression ratio mode (compression
ratio of 11:1 and below).
[0028] The compression ratio is basically determined as a function
of engine load thresholds (Th_Load) that are looked up values from
a 2D table using engine speed as an independent variable. More
particularly, the method includes determining a relationship
between: (1) a base engine threshold load (Base Th_Load) which
provides substantially equal engine fuel consumption for a pair of
the plurality of compression ratio operating modes; and (2) engine
speed. That is, while the high compression ratio engine would
always get better fuel economy if it could be operated at MBT
spark, i.e., not borderline limited. However, the high compression
ratio necessitates retard from MBT earlier than low compression
ratio. Eventually the high compression engine will be operating at
retarded spark from MBT and have the same fuel consumption of low
compression ratio at MBT spark (this happens to be the ideal switch
point). The determined relationship is modified by a factor. The
factor is a function of a condition under which such engine is
operating to obtain a modified relationship. The modified
relationship (Th_Load) is a function of engine speed and the
condition is one having an effect on knock generation in such
engine. The modified relationship (Th_Load) and engine speed are
used in selecting the one of the pair of the plurality of
compression ratio operating modes for the engine. (As is known to
one skilled in the art, it is sometimes advantageous to operate at
MBT spark advance minus 1% torque, MBT minus 1%. This is a spark
advance retarded from MBT which provides 99% of the torque of that
produced at MBT spark advance. Alternatively, there are other spark
advance angles, which have a defined relationship from MBT spark
advance, that are used in place of MBT. Throughout this
specification, the term MBT spark advance means actual MBT timing
or another spark advance related to MBT spark advance, such as MBT
minus 1%)
[0029] If, during normal engine operation, the engine load for the
particular engine speed is greater than Th_load, the CPU 66 selects
the low compression ratio operating condition while if the engine
load for the particular engine speed is less than Th_load, the CPU
66 selects the high compression ratio operating condition. As will
be described a hysteresis effect is provided to inhibit switching
oscillations between the pair of the plurality of compression ratio
operating modes.
[0030] During initial engine testing, the Base Th_Load is
determined to be the points where there is substantially equal
engine fuel consumption for the high and low compression ratio
operating modes at each of a plurality of engine speed points at a
nominal air charge temperature (act), nominal engine coolant
temperature (ect), and nominal barometric pressure (bp) operating
condition, such relationship being shown in FIG. 2A at the nominal
act, ect and bp operating condition through engine dynamometer
experimental work. The engine tests are performed in a standard
environmental condition and warm engine temperature (i.e., at the
nominal act, ect and bp operating condition). This compression
ratio information is then converted into a 2D look-up table
generating Base Engine Load Threshold (Th_Load) having the engine
speed as an independent variable, the equivalent curve of such
table being shown in FIG. 2A. It follows then, that if the engine
operated at a particular speed and the calculated engine load,
i.e., ratio of cylinder air charge/maximum cylinder air charge, is
a value less than Th_load, the CPU 66 would select the high
compression ratio mode; whereas, if the engine speed and load at
that engine speed is greater than Th_Load, the CPU 66 would select
the low compression ratio operating mode.
[0031] More particularly, for a particular engine design, during
engine test with such engine operating at a constant load, here
LOAD=LOAD_CONSTANT.sub.--1, and with minimal coolant temperature
(ect), a nominal air charge temperature (act), a nominal barometric
pressure (bp) and a particular engine speed, N1, the spark is
advanced to either: (A) MBT spark advance; or, (B) if the engine
knocks at MBT, then the spark advance at which there is borderline
engine knock. This is performed for the engine operating in the low
compression (LC) mode and then again with the engine operating in
the high compression mode (HC), as shown in FIG. 4A.
[0032] FIG. 4A shows the relationship between Brake Specific Fuel
Consumption (BSFC) and spark advance (SA) for an engine operating
at a constant speed, barometric pressure (bp), engine air charge
temperature (act) engine coolant temperature (ect), one curve
therein being generated with the engine operating in the low
compression mode and a second curve therein being generated with
the engine operating in the high compression mode. The Brake
Specific Fuel Consumption (BSFC) is reordered for the low
compression operating engine (i.e., BSFC_LC_LOAD_CONSTANT.sub.--1)
and The Brake Specific Fuel Consumption (BSFC) is reordered for the
high compression operating engine (i.e.,
BSFC_HC_LOAD_CONSTANT.sub.--1) as shown in FIG. 4A.
[0033] By reordered it is meant that the data is presented in a
different manner. More particularly, when the engine is mapped in
each compression ratio, the MBT timing is determined for each
compression ratio with a high octane fuel to establish a function
of the spark advance an fuel curve which has a parabolic type shape
with a minima of fuel consumption (y-axis) at the MBT spark advance
(x-axis). This map is performed over light to heavy loads, and over
the entirety of engine speeds. A borderline survey is then
performed over the same speed and load, in both compression ratios,
using a fuel with an octane level that corresponds to the octane
level that is targeted for the production vehicle. In this manner,
borderline spark advance is established for the operating
condition. At light load it is possible that MBT is obtainable
without knock. At heaver loads, knock is reached before MBT, and at
a higher fuel consumption rate. This data can now be plotted at a
constant engine speed, having load along the x-axis, and BSFC or
brake specific fuel consumption along the y axis. The points that
are plotted are taken at the mapping data of the production intent
octane fuel where they use the MBT spark if possible, or the
borderline spark where applicable. These two curves are usually
parabolic like in shape and at light loads parallel to each other.
However as the high compression ration starts to employ retard from
MBT due to knock, the fuel curve starts to "hook" up more steeply,
and eventually crosses the low compression fuel curve where the low
compression ratio is still operating at MBT spark. The crossing
point load corresponds to the switch point for that constant engine
speed.
[0034] The process is repeated for the particular engine, during
engine test with such engine operating at a different constant
Load, here LOAD=LOAD_CONSTANT.sub.--2, and with minimal coolant
temperature (ect), a nominal air charge temperature (act), a
nominal barometric pressure (bp) and a particular engine speeds.
Again the spark is advanced to either: MBT or borderline spark,
whichever of these two points is at the least spark advance (SA).
This is performed for the engine operating in the low compression
(LC) mode and then again with the engine operating in the high
compression mode (HC) and the Brake Specific Fuel Consumption
(BSFC) is reordered for the low compression operating engine (i.e.,
BSFC_LC_LOAD_CONSTANT.sub.--2). The process is repeated to generate
a table represented by the curves shown in FIG. 4B.
[0035] FIG. 4B shows the relationship between BSFC and load for
both the low compression ratio and the high compression ratio in
which the spark advance is MBT spark timing unless there is engine
knock, in which case, the spark advance is set to borderline knock,
i.e., retarded from MBT spark advance.
[0036] Thus, BSFC is plotted as a function of load for the high
compression engine and for the low compression engine. The two
curves cross at a point where both the high compression operating
mode and the low compression operating mode produce substantially
the same fuel consumption (i.e., BSFC). The load at the cross over
point is the Base threshold (Th_Load) for the operating engine
speed, N1.
[0037] The process described above is performed over a range of
engine speeds to thereby generate the curve shown in FIG. 2A, i.e.,
the base engine threshold load (Base Th_Load) which provides
substantially equal engine fuel consumption for a pair of the
plurality of compression ratio operating modes; Base Th_Load as a
function of engine speed.
[0038] As will be described, during normal operation of the engine,
this determined nominal relationship (Base Th_Load) is modified by
a factor, such factor being a function of a condition under which
such engine is operating, to obtain a modified relationship as a
function of engine speed. The condition is one that has an effect
on knock generation in such engine. Here, for example, wherein the
factor is a function of at least one of engine coolant temperature,
ect, engine air charge temperature, ect, and barometric pressure,
bp. For barometric pressure, bp, such can be measured or inferred
from a measurement of the air mass, throttle angle, and engine
speed.
[0039] Preferably, the factor is a function of all three; i.e.,
engine coolant temperature, ect, engine air charge temperature,
act, and barometric pressure, bp. The modified relationship and
engine speed are used in selecting one of the plurality of
compression ratio operating modes for the engine.
[0040] Thus, different environmental conditions and also engine
warmness are considered. The air charge temperature (act) and
engine warmness (i.e., engine coolant temperature, ect) are the
factors that contribute to the selection of engine compression
ratio mostly due to their impact on engine knock tendency. In
addition, barometric pressure (bp) is also a factor in the engine
compression ratio selection due to its direct impact on engine
load. All those factors are mapped as the 2D individual look-up
tables to compensate for the Base Engine Load Threshold for
Compression Ratio as composite multiplier. To obtain each of the
look up tables for act, ect and bp, ranges of air charge
temperature (act), engine coolant temperature (ect), and barometric
pressure (bp) are swept over normal expected ranges for these
engine operating variables.
[0041] That is, to generate the correction factor for act, i.e.,
corr_act, the engine is tested again at the nominal ect and bp and
a nominal engine speed. The engine load threshold Th_Load is
established at each of a plurality of act's. During the engine
test, with such engine operating at a nominal coolant temperature
(ect), a nominal barometric pressure (bp), and a nominal engine
speed, for each of a plurality of act's, the spark advance is
varied at each of the pluralities of compression ratio until a
minimal fuel consumption is achieved, or the borderline knock is
encountered. Thus, a two dimensional (2D) look up table is
generated under this nominal operating condition having as the
independent variable, act, and the dependent ACT Load Threshold
(ACT_Th_Load). Thus, a table is obtained of ACT Engine Load
Threshold (ACT_Th_Load) vs. air change temperature. For each of the
plurality of act's, a ratio (act_corr) is calculated of
ACT_Th_Load/Base Th_Load, where Base Th_Load is obtained from the
table in FIG. 2A. The calculated act_corr as a function of act is
2D table as shown in FIG. 2B.
[0042] In like manner, to generate the correction factor for ect,
i.e., corr_ect, the engine is tested again at the nominal act and
bp and a nominal engine speed. The engine load threshold is
established at each of a plurality of ect's. That is, during engine
test with such engine operating at a nominal air coolant
temperature (act), a nominal barometric pressure (bp), and a
nominal engine speed, for each of a plurality of ect's, the spark
advance is varied at each of the pluralities of compression ratio
until a minimal fuel consumption is achieved or the knock level is
reached. Thus, a two dimensional (2D) lookup table is generated
under this nominal operating condition having as the independent
variable, ect, and the dependent ECT Load Threshold (ECT_Th_Load).
Thus, a table is obtained of ECT Engine Load Threshold
(ECT_Th_Load) vs. engine coolant temperature. For each of the
plurality of ect's, a ratio (ect_corr) is calculated of
ECT_Th_Load/BASE Th_Load, where Base Th_Load is the Base Th_Load at
the nominal speed obtained from the table in FIG. 2A. The
calculated ect_corr as a function of ect is 2D table as shown in
FIG. 2C.
[0043] Finally, to generate the correction factor for bp, i.e.,
corr_bp, the engine is tested again at the nominal act and ect and
a nominal engine speed. The engine load threshold is established at
each of a plurality of BP's. That is, during engine test with such
engine operating at a nominal air coolant temperature (act), a
nominal engine coolant temperature (ect), and a nominal engine
speed, for each of a plurality of bp's, the spark advance is varied
at each of the pluralities of compression ratio until a minimal
fuel consumption is achieved which is the MBT timing which
corresponds to the highest engine torque or the knock is
encountered. Thus, a two dimensional (2D) lookup table is generated
under this nominal operating condition having as the independent
variable, bp, and the dependent BP Load Threshold (BP_Th_Load).
Thus, a table is obtained of BP Engine Load Threshold (BP_Th_Load)
vs. barometric pressure. For each of the plurality of bp's, a ratio
(bp_corr) is calculated of BP_Th_act/BASE Th_Load, where Base
Th_Load is the Th_Load at the nominal speed obtained from the table
in FIG. 2A. The calculated bp_corr as a function of BP is 2D table
as shown in FIG. 2D.
[0044] Having obtained as a result of testing the engine: Base
Th_Load; corr_act; corr_ect; and corr_bp, the determined nominal
relationship Base Engine Load Threshold (Base Th_Load) vs. Engine
Speed Table shown in FIG. 2A is, during normal engine operation,
modified by a factor, such factor being a function of a condition
under which such engine is operating to obtain a modified
relationship Th_Load as a function of engine speed. The condition
is one that has an effect on knock generation in such engine. Here,
for example, wherein the factor is a function of at least one of
engine coolant temperature, ect, engine air charge temperature,
act, and barometric pressure, bp. Preferably, the factor is a
function of all three; i.e., engine coolant temperature, ect,
engine air charge temperature, act, and barometric pressure, bp.
The modified relationship and engine speed are used in selecting
one of the plurality of compression ratio operating modes for the
engine.
[0045] More particularly, during normal engine operation, and using
ect, act and bp as independent variables, and measured or estimated
act, ect and bp, the process obtains correction factors by using 2D
look-up tables for each variable ect, act and bp; i.e., corr_ect as
a function of ect (FIG. 2C), corr_act as a function of act (FIG.
2B) and corr_bp as a function of bp (FIG. 2D), respectively. Then
the obtained correction factors corr_ect, corr_act and corr_bp are
multiplied together to produce a total correction multiplier,
corr_total=[corr_ect][corr act] [corr_bp].
[0046] Then, all the correction factors (corr_total) to compensate
engine coolant temperature (corr_etc) and environmental conditions
(air charge temperature (corr_act) and barometric pressure
(corr_bp)), which are modeled as 2D tables as described above
during engine testing, are multiplied to the Base load threshold
(Base Th_Load) table (FIG. 2A) to calculate the switching load
threshold (CR_SW_Load) of compression ratio. In this way, the base
load threshold (Base Th_Load) can be compensated at different
environmental conditions and engine temperatures to create a new
switching threshold (CR_SW_Load).
[0047] The method includes determining a hysteresis load as a
function of engine speed and applying such determined hysteresis
load to the modified relationship to inhibit switching oscillations
between a pair of the plurality of compression ratio operating
modes. More particularly, hysteresis of engine load (Hyst_Load)
(FIG. 2E) is then applied to (CR_SW_Load) to avoid switching
oscillation of the compression ratio, as shown in FIG. 2F. This
hysteresis bias is determined subjectively by a test driver
operating a test vehicle.
[0048] Referring now to FIG. 3, the method used for selection of
the compression ratio for the engine during normal operation, i.e.,
after engine testing, is described.
[0049] It Step 300 the compression ratio selection algorithm
commences.
[0050] In Step 302, the engine speed, load (i.e., cylinder air
charge), engine coolant temperature (ect), air charge temperature
(act), and barometric pressure (bp) are read or computed.
[0051] In Step 304, using ect, act and bp as independent variables,
the process obtains correction factors by using 2D look-up tables
for each variable ect, act and bp; i.e., corr_ect as a function of
ect, corr_act as a function of act, and corr_bp as a function of
bp, respectively. Then the obtained correction factors corr_ect,
corr_act and corr_bp are multiplied together to produce a total
correction multiplier, corr_total=[corr_ect] [corr
act][corr_bp].
[0052] In Step 306, a Base load threshold, Base Th_Load, is
calculated for the compression ratio selection using engine speed
as an independent variable, FIG. 2A.
[0053] In Step 308, the calculated load threshold (Th_Load) and the
total correction multiplier (corr_total) are multiplied together to
thereby obtain the load threshold for compression ratio switching
CR_SW_Load, i.e., CR_SW_Load=[corr_total][Th_Load].
[0054] In Step 310, a Load hysteresis (Hyst_Load) obtained from a
2D look-up table using engine speed an independent variable is
applied to the Load threshold (Th_Load) for providing a compression
ratio switching (CR_SW_Load) in order to prevent switching
oscillation. Thus, the Load hysteresis provides a bias between a
curve CR_SW_Load I and a curve CR_SW_Load II, as shown in FIG.
2E.
[0055] Thus, in Step 312, if, for a measured engine speed, the
determined engine load is less than CR_SW_Load I, the CPU 66 will
select the high compression ratio operating mode. If, for a
measured engine speed, the determined engine load is greater than
CR_SW_Load II, the CPU 66 will select the low compression ratio
operating mode. If the engine is operating in the high compression
ratio mode, the CPU 66 will not switch to the low compression ratio
mode unless the engine load rises above CR_SW_Load II. Likewise, if
the engine is operating in the low compression ratio mode, the CPU
66 will not switch to the high compression ratio mode unless the
engine load falls below CR_SW_Load II.
[0056] A number of embodiments of the invention have been
described. For example, while the factor described above was a
function of engine coolant temperature, engine air charge
temperature and barometric pressure, such factor may also be a
function of one or more of valve events, humidity, exhaust gas
recirculation (EGR), and fuel type, i.e., octane level of the fuel.
Thus, it will be understood that various modifications may be made
without departing from the spirit and scope of the invention.
Accordingly, other embodiments are within the scope of the
following claims.
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