Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02193813 2004-09-16
1
IDLE SPEED CONTROL FOR INTERNAL COMBUSTION ENGINES
This invention relates to a method for controlling the idle speed of an
internal combustion engine of a vehicle. In particular, the invention will be
described with reference to vehicles having automatic transmissions. It is
however to be appreciated that an application of the invention is also
applicable
to vehicles having manual transmissions.
A desirable feature of vehicles having an automatic transmission
containing a fluid torque converter between the engine and the driving wheels
is
"creep" this being a motion, normally experienced on flat ground, either
forward
or backward, of the vehicle when the brake thereof is released, the vehicle is
"in
gear", and the engine speed is at idle. Such vehicle creep is considered a
desirable feature in a vehicle as it allows the driver to achieve a slow,
steady
movement of the vehicle by merely releasing the brake and without having to
depress the accelerator or throttle of the vehicle. Thus, by operation of the
brake
alone, the driver can selectively achieve a slow forward or rearward motion of
the
vehicle or maintain the vehicle stationary when it is in gear. This nature of
operation of a vehicle is convenient and desirable in various traffic
conditions and
particularly in congested, slow moving traffic wherein the ability to achieve
small
movements of the vehicle without the need for the driver to move their foot
away
from the brake or to actuate the accelerator pedal is very desirable. It also
provides the vehicle with a positive feel during maneuvering in traffic and
when
parking and prevents vehicle roll back to a certain degree during hill starts.
Vehicle creep is generated by the torque transmitted through the
torque converter when the engine is idling in, for example, "drive". The main
criteria used to select a torque converter for an engine having an automatic
transmission are the engine torque characteristic. If the engine has
relatively
high low-speed torque, the torque converter will have a low stall speed
therefore
providing a "stiff" coupling. However, if the engine torque is generated at a
higher
speed, the torque converter will have a higher stall speed providing a "loose"
coupling. This ensures that the engine speed is kept close to the speed that
gives the highest torque during the commencement of movement of the
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vehicle and during general driving so as to achieve the best vehicle
performance. Therefore, the level of creep is governed by the torque converter
"stiffness" and the engine idle speed.
A number of criteria affect the selected engine idle speed for a vehicle
and engine combination. These include body vibration due to any resonances '
caused by engine vibrations, combustion stability at low engine speeds, the
accommodation of loads such as power steering, air conditioning and neutral
drive transitions, and the amount of vehicle creep desired.
In a vehicle having an automatic transmission, when the vehicle is
stationary and in "drive", the engine is loaded by the torque converter as it
tries
to rotate the input side of the torque converter with the output side of the
torque
converter being held stationary. Therefore, the higher the engine idle speed,
the higher the loading on the engine, and the more energy, and therefore fuel,
that is wasted primarily in the form of heat into the gearbox oil. Therefore,
idle
speed on an automatic vehicle has a significant effect on fuel economy.
However, where a low idle speed is used in order to limit fuel consumption and
exhaust emissions, that idle speed may be too low to achieve creep, or a
desired level of creep.
In addition to not providing the desired level of creep, such a low idle
speed may lead to undesirable shudders or jolts in the vehicle drive train if
the
driver depresses the accelerator harshly to rapidly increase engine speed from
this low idle speed. This is due to the fact that at such low idle speeds,
there is
very little or no load appearing in the vehicle drive train including the
torque
converter. Accordingly, under hard acceleration, the unloaded engine is able
to
rapidly gain momentum until the speed thereof approaches the stall speed of
the torque converter, at which point the engine suddenly takes up the torque
converter load and subsequently transfers a shock through the drive train.
This
may be further accentuated as the drive train may become unloaded to the point
,
that there no longer exists a pre-load between the various drive train
components. In such a case, clearances may need to be taken up during the '
initial increase in idle speed which may lead to a noticeable physical shudder
and audible impact.
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In the case of vehicles having manual transmissions, it is also desirable
to set the engine idle speed as low as possible. This however presents the
problem that engagement of the clutch by the driver may stall the engine if
the
engine idling speed is too low.
It is therefore an object of the present invention to provide a method of
controlling the idle speed of a vehicle engine.
With this in mind, there is provided according to one aspect of the present
invention a method of controlling the idle speed of an engine of a vehicle
having
an automatic transmission, including increasing the engine speed from a first
idle speed to a second idle speed higher than said first idle speed in
response
to a driver originating indication independent of an acceleration control of
the
vehicle that vehicle motion is desired, the higher second idle speed being set
to
achieve a desired level of vehicle creep.
The acceleration control of the vehicle may typically be an accelerator
pedal which is depressed by the driver to accelerate the vehicle. The engine
idle speed may be returned to the first idle speed when the driver originating
indication indicates that vehicle motion is no longer desired.
According to another aspect of the present invention, there is provided a
method of controlling the idle speed of an engine of a vehicle, including
increasing the engine speed from a first idle speed to a second idle speed
higher than said first idle speed in response to the release of a brake of the
vehicle brake system. The engine idle speed is preferably returned to the
first
idle speed when the brake is re-engaged.
Preferably, in the case of a vehicle having an automatic transmission, the
firsf idle speed can be set quite low, particularly when using multicylinder
piston-ported two-stroke engines, whilst the higher second idle speed can be
set to achieve a desired level of vehicle creep. In the case of a vehicle
having a
manual transmission, the higher second idle speed can be set to achieve
desirable launch characteristics to avoid vehicle "shunt" or engine stalling.
The
3~ use of the lower idle speed reduces torque converter load in the case of an
automatic vehicle and hence minimises fuel consumption.
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It should be noted that a combination of other inputs together with the
input from the vehicle brake system could be used to indicate that the vehicle
is
at rest or that the engine is disengaged from the transmission of the engine
and
that it is desirable to set the engine idle speed to the lower first idle
speed.
Such inputs may include but are not limited to vehicle road speed, a signal
indicating actuation of the accelerator pedal or, in the case of a manual
transmission, the clutch pedal and signals indicating the selected gear of the
vehicle gearbox. Obviously, these signals or any combination thereof may also
be used in conjunction with that derived from the brake system to indicate an
impending launch of the vehicle or that vehicle creep is desired in response
to
which the first idle speed can be increased to the higher second idle speed.
In a vehicle having an automatic transmission, the method may include
initially determining whether the automatic transmission is in a drive mode,
including reverse and different gear selections, before increasing the engine
idle speed to the higher second idle speed. The method may further include
returning the engine idle speed to the first idle speed when the automatic
transmission is shifted out of said drive mode.
In a vehicle having a manual transmission, the method may include
initially determining whether any forward or rearward gears have been selected
before increasing the engine idle speed to the higher second idle speed. The
second idle speed may be maintained at all times that the manual transmission
is in-gear. The method may further include returning the engine idle speed to
the first idle speed when "neutral" is selected. The method may further
include
determining the depression of the clutch pedal of the vehicle to indicate when
the clutch of the vehicle is disengaged and decreasing the engine idle speed
from the second idle speed to the first idle speed in response thereto.
The method may generally include determining when the vehicle is at
rest and thereby maintaining or returning the engine idle speed to the first
idle
speed. Furthermore, the idle speed may be rapidly changed in a stepping or a
vamping fashion between the first idle speed and the higher second idle speed.
It is also preferable that the ramp rate for increasing the idle speed from
the first
idle speed to the higher second idle speed decreases as the higher idle speed
2i9381~
W'O 96/D0844 PC'TIAU95I00391
is approached. This will reduce any undesirable drive train shudders or jolts
as
any drive train backlash is taken up in a vehicle having an automatic
transmission. Optimised ramp rates and second idle speed levels can be
derived for a specific type of vehicle by experimentation, trial and error or
a
' 5 combination of both.
The idle speed may be increased from the first idle speed to the higher
second idle speed by increasing the fuel supply rate to the engine.
Implementation of this method provides significant fuel economy gains
during the engine idle condition. As a significant portion of urban driving
normally requires the engine to be at idling speeds, and because 25% of the
USA Federal Test Procedure drive cycle is at idle, such gains in fuel economy
can have a very significant effect on overall fuel economy.
The brake light circuit of the vehicle may conveniently provide an
indication of the release and engagement of the brake from a brake applied
position thereof. Alternatively, a movement sensor may be used to sense the
movement of a brake pedal. The movement sensor may be a switch actuated
by the brake pedal. to indicate displacement of the brake pedal. The point of
switch actuation may be chosen to allow a degree of vehicle speed or creep
control via brake application prior to the switch being actuated and the idle
speed,being reduced. However, the relationship between brake pedal position
and stopping force from the brakes may vary significantly making pedal
position
unreliable for repeatable control of the idle speed. Therefore, a pressure
actuated switch may alternatively be connected to the hydraulic brake line as
the relationship between hydraulic pressure and brake force is likely to be
more
consistent. Furthermore, such pressure actuated switches may also have a
degree of in-built pressure hysteresis which would be beneficial in preventing
rapid cycling between engine speed levels close to the selected actuation
pressure.
Alternatively, a strain gauge or other load type sensor on the brake pedal
' 30 could be used. This would provide a variable analog signal representing
the
force applied to the brake. This signal could be provided to the engine
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management system which in tum could be programmed to allow the thresholds
for increases in idle speed and hysteresis to be set to desired values.
A movement sensor could in addition be used for sensing the movement
of the clutch pedal, the movement sensor preferably being a switch actuated by
the clutch pedal to indicate displacement of the clutch pedal. Preferably, the
'
switch would indicate the onset of depression of the clutch pedal. This would
thus allow the entire time taken for full clutch pedal depression and gear
selection for the engine idle speed to be increased.
Thus, preferably, the engine idle speed in a vehicle having an automatic
transmission varies in a predetermined fashion from the first idle speed to
the
higher second idle speed as the load on the brake pedal is released from the
engaged condition to the released condition. Alternatively, the engine idle
speed in a vehicle having an automatic transmission may vary in a
predetermined fashion from the first idle speed to the higher second idle
speed
as the position of the brake pedal moves from an engaged position to the
released position.
A vehicle tilt indication means may also be used for providing a signal
indicating the angle of inclination of the vehicle. The tilt indication means
can
provide both an indication of the degree of inclination, as well as whether
the
inclination is upward or downward (ie: positive or negative).
The tilt indication means may preferably be a tilt switch. Alternatively, the
tilt indication means may be a fuel tank level indicator. The signal from the
fuel
tank level indicator may be "damped" by comparing the current level indication
signal with the average of the level indication signals over a predetermined
period, for example, over the last 30 minutes.
The level indication signal, together with other signals indicating the
vehicle operating status as noted above, can be used to allow the controlled
amount of creep to be achieved under different conditions. ,
The control method may also adjust the first and/or second idle speeds in
response to changes in engine operating parameters. These parameters may, '
for example, include engine coolant temperature, changes in engine load or
vehicle speed.
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Tln this regard, the control method may adjust the second idle speed to
provide a controlled rate of increase of the amount of creep. For example, the
control method may provide a controlled amount of creep to allow a
predetermined speed of forward or rearward movement regardless of whether,
' 5 for example, the vehicle is on an upward or downward decline, is towing a
trailer or the engine is cold. The control method may also provide a
controlled
amount of creep to maintain the vehicle stationary when the vehicle is on an
upward incline such that there is no rearward roll-back of the vehicle.
Alternatively the control method may provide no creep when the vehicle is on a
downward incline and the vehicle is in a drive mode. Creep may however be
provided by the control method when the vehicle is on a downward incline and
a reverse or rearward motion is desired.
According to yet another aspect of the present invention, there is
provided a vehicle engine control system for a vehicle having an automatic
transmission, including control means for increasing the engine speed from a
first idle speed to a second idle speed higher than said first idle speed in
response to a driver originating indication independent of an acceleration
control of the vehicle that vehicle motion is desired, the higher second idle
speed being set to achieve a desired level of vehicle creep.
According to a further aspect of the present invention, there is provided a
vehicle engine control system for a vehicle, including control means for
increasing the engine speed from a first idle speed to a second idle speed
higher than said first idle speed in response to the release of a brake of the
vehicle brake system. The means is also capable of returning the engine speed
to the first idle speed upon re-engagement of the brake.
The method of the present invention is particularly applicable to direct
injected two stroke engines because of the low idle speeds that can be
achieved due to the firing frequency and the speed of response to fuelling
changes. That is, the low idle speeds can be achieved and very rapid changes
' 30 in speeds can be achieved such that the utilisation of this control
method would
not be apparent to the driver. However, this control method is also applicable
for four stroke and other engines.
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The above described management of the engine fuelling rate in the
transition from the first idle speed to the second higher idle speed can be
incorporated in a conventional electronic management system by those skilled
in the art. It will be appreciated that the fuelling rate will be influenced
in the
known manner by other conditions of operation of the engine such as engine '
coolant temperature, and other loads on the engine such as from air
conditioners, heaters and lights. Those experienced in the art can readily
program the electronic management system to accommodate these factors.
To facilitate an understanding of the present invention, reference is made
to the accompanying drawing which illustrates an example embodiment of the
method of controlling the idle speed of a vehicle engine according to the
present invention. It is to be appreciated that this is only one example
embodiment of the present invention and that there are alternative ways of
implementing the control method of the present invention.
Figure 1 shows a flow diagram of the control strategy of the electronic
control unit (ECU) of the engine management system of a vehicle having an
automatic transmission according to the method of the present invention.
It is common practice in engine management systems to perform "closed
loop idle" control of the idle speed of the engine. Under such control, the
actual
engine idle speed is compared with a desired target idle speed at regular
intervals. If the engine idle speed is higher or lower than the target valve,
then
the idle speed is returned to the desired valve. At step 1 of the flow
diagram, the
ECU is providing closed loop idle control of the engine idle speed.
At step 2, the ECU periodically checks whether the gearbox is in a drive
mode which includes any forward or rearward gear selections. If the gearbox is
not in a drive mode, then the desired target engine idle speed is maintained
or
set to a low normal or "base" idle speed at step 5. However, if the gearbox is
in
"a drive mode", then the ECU checks to see if the foot or parking brake is -
applied in step 3. If the foot or parking brake is applied, then the desired
target
engine idle speed is maintained or set to the low normal or "base" idle speed
at
step 5. If however the foot or parking brake is not applied, then the desired
target engine idle speed is set at a higher or "creep" engine idle speed at
step 4.
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This higher idle speed is maintained until the speed of the engine is
increased
in response to an increase in the driver load demand, the gearbox is returned
to
"neutral", or the foot or parking brake is applied at which time the engine
idle
speed is reset to the low normal idle speed.
For simplicity, this flow diagram does not show the step or ramping
function step that may occur as the target engine idle speed is changed from
one value to another, however, it is to be appreciated that such functions can
be
incorporated into the control strategy as required.
