Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02217959 1997-10-09
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates to a new or improved fuel
supply system for an internal combustion engine, to a manifold
specifically designed for use in such system, to a kit of
parts to enable retrofitting of such system in an existing
engine and to a method of controlling the fuel supply to
achieve the improved response to a number of environmental
conditions.
B. Description of the Prior Art
In internal combustion engines having carburettor
controlled fuel supplies, as is typical of engines used in
vehicles such as snowmobiles and personal watercraft, it is
well known that the rate of fuel flow in a fixed or variable
venturi carburettor is dependent upon the pressure
differential existing in the fuel system between the venturi
and e.g. a fuel bowl (otherwise called a float bowl or a float
chamber). In a conventional float bowl carburettor the
pressure differential is measured between the pressure in the
fluid float chamber (which is normally atmospheric pressure)
and the pressure at the discharge orifice of the fuel metering
system which is normally located in or adjacent the venturi in
the induction passage.
For optimum combustion, the relationship between the
mass air flow and the mass fuel flow delivered to the engine
by the carburettor should be kept constant, and to achieve
this the carburettor employs either a fixed or a variable
venturi (or some equivalent structure) such that when air
velocity in the induction passage is increased a pressure
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reduction (often called a vacuum) is created in the venturi
zone. This pressure reduction creates a pressure differential
between the induction passage and the fuel in the float
chamber, causing fuel to be drawn into the induction passage
at a flow rate that is proportional to the pressure
differential.
The amount or level of the venturi underpressure or
vacuum is mainly a function of air velocity through the
induction passage, but as is well understood, at a given
velocity, the mass air flow rate is affected by air density
which in turn is mainly a function of barometric pressure and
air temperature.
For example for a snowmobile operating at an
altitude of 2000 meters, a given air velocity in the
carburettor induction passage will deliver a very much reduced
mass air flow to the engine as compared to the same air
velocity when the snowmobile is operating at sea level, this
being due to the reduced barometric pressure and air density
at altitude. However since fuel flow is mostly a function of
the venturi underpressure or vacuum, the engine when operating
at altitude would tend to be supplied with a mixture that is
over rich in fuel. This phenomenon is well understood. For
example U.S. Patent 5,021,198 Bostelmann discloses a
carburettor system that is designed to adjust the fuel flow to
maintain the mass air fuel mixture ratio constant despite
changes in altitude.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a fuel
supply control system and method which without the use of a
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choke or the like is adapted to adjust the air fuel mass flow
ratio to provide a fuel enriched mixture in certain
situations, e.g. for starting a cold engine.
The invention provides a method for modifying the
air/fuel mixture ratio supplied to an internal combustion
engine of a vehicle to achieve a constant mass flow ratio in
spite of changes in atmospheric temperature conditions, said
fuel being drawn from a float chamber into a venturi in a
carburettor, wherein it is mixed with air before being
delivered into the engine, said method comprising: (a)
sensing the atmospheric temperature in the vicinity of said
vehicle and generating a signal indicative of said sensed
temperature; (b) supplying said signal to a control unit; (c)
operating said control unit to modify pressure within said
float chamber thus varying the pressure differential between
the venturi and said float chamber so that the mass flow ratio
of said mixture remains substantially constant.
The engine preferably also includes an air pressure
sensor and an engine temperature sensor both of which feed
signals to the electronic control unit which signals are also
used in modifying the fuel/air ratio of the mixture.
From another aspect the invention provides a method
for reducing the air/fuel mixture ratio supplied to an
internal combustion engine in cold start situations, said fuel
being drawn from a float chamber into a venturi in a
carburettor where it is mixed with air and delivered into the
engine, said method comprising: (a) sensing the temperature
of the engine and generating a signal when said temperature is
below a normal operating temperature range; (b) supplying
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said signal to a control unit; (c) operating said control
unit to elevate the pressure within said float chamber to
increase fuel flow into the venturi and thus increase the fuel
content of said mixture during periods when said signal is
received.
The engine crankcase chamber is subject to pressure
fluctuations during operation of the engine, and this chamber
can be utilized as the pressure generator by including a line
communicating the crankcase chamber to the control unit. At
low speeds of rotation of the engine corresponding to cranking
thereof this line will provide a sufficient flow of
pressurized air. However at higher engine speeds and throttle
openings the pressure will be insufficient so that an external
pump may be required. Preferably such pump is a mechanical
pump constructed to be driven by pressure pulses in the
crankcase chamber. The pump is provided for delivering the
flow of pressurized air at higher speeds of operation of the
engine, i.e. at speeds of idling and above. Alternatively,
the pressure generator may be a separate pump, for example
electrically driven from a vehicle battery.
DESCRIPTION OF THE DRAWINGS
The invention will further be described, by way of
example only, with reference to the accompanying drawings
wherein:
Figure 1 is a schematic view of a first portion of a
fuel supply control system in a snowmobile engine;
Figure 2 is a graph showing the carburettor float
chamber pressure as it varies with operating conditions;
Figure 3 is a schematic view showing a second part
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of the fuel supply control system;
Figure 4 is a schematic view of the overall fuel
control system of a three cylinder two-stroke engine;
Figures 5a and 5b are perspective views showing two
states of a manifold arrangement as included in the Figure 4
embodiment of the fuel supply control system; and
Figure 5c is a perspective view from the opposite
side showing a portion of the manifold of Figures 5a and 5b.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The fuel flow control system incorporates an
electronic control unit which is coupled to receive inputs
from a series of sensors and provide output signals to control
the fuel flow from the carburettor or carburettors. The
invention as described concerns a fuel supply system in a
snowmobile engine, but obviously is susceptible of many other
applications.
Referring to Figure 1, an electronic control unit
(ECU) 10 is connected to receive input signals from a
barometric pressure sensor 11 and an air temperature sensor
12, these sensors being mounted at locations on the snowmobile
where they are exposed to atmospheric conditions. The signals
from the sensors 11 and 12 are processed by the ECU which
produces an output signal that is sent to a solenoid 13 by
means of which the fuel flow from a carburettor 14 is adjusted
to compensate for air density at the location where a
snowmobile is operating. As mentioned above, air density is
mainly a function both of barometric pressure and of air
temperature, and by measuring these parameters by means of the
sensors 11 and 12 the ECU produces an output signal which
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modifies fuel flow to the snowmobile engine to compensate for
changes in the measured parameters.
The engine has a carburettor 14 that is of a well
known type having an induction passage 15 controlled by a
spring-loaded sliding piston 16 which carries a needle 17
slidably inserted in a fuel orifice 18 connected to draw fuel
from a float chamber 19. The induction passage comprises a
venturi which creates an underpressure or vacuum in the air
flowing therethrough, the pressure differential between the
venturi and the float chamber 19 resulting in fuel being drawn
into the induction passage through the orifice 18 and
thereafter delivered to the engine in mixture with the air
flow.
The solenoid valve 13 is designed to create a
controlled reduction of the pressure in the float chamber so
that the flow of fuel from the orifice 18 is modified in
accordance with the atmospheric air density with the result
that the mass air/fuel flow ratio is held substantially
constant.
The solenoid valve has a valve closure 21 mounted in
a manifold chamber 22 to which is coupled a first conduit 23
which is in communication with the venturi of the induction
passage 15 adjacent the orifice 18, and a second conduit 24
which is in communication with the carburettor float chamber
19, this second conduit including an atmospheric vent 25.
In operation, the above described system acts to
compensate for the mass air flow diminution (which occurs when
the snowmobile is operating at high altitudes) by reducing the
pressure within the float chamber 19 which in turn reduces the
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pressure differential acting on the fuel thus reducing the
fuel flow. To achieve the necessary reduction in pressure,
the system utilizes the underpressure or vacuum in the
induction passage venturi and applies this through the conduit
23, the manifold 22, and the conduit 24 to the float chamber.
The extent to which the float chamber is exposed to this
underpressure is determined by the solenoid valve 23 the
closure 21 of which cooperates with the end of the conduit 23
to open this to a greater or lesser extent in accordance with
the prevailing atmospheric conditions. For example the ECU 10
would be calibrated so that at some standard condition of
temperature and barometric pressure, the closure 21 would
completely seal the conduit 23 so that the float chamber would
be exposed to only atmospheric pressure via the vent 25 and
the conduit 24.
By arranging that the conduit 23 opens into the
induction passage 15 at a location very close to the fuel
orifice 18 it is ensured that the compensation is essentially
linear at any throttle opening condition, as illustrated in
Figure 2 which shows the float chamber pressure as a
percentage of the pressure at the fuel orifice 18 throughout
the duty cycle activation of the solenoid valve 13. In other
words the float chamber pressure is directly related to the
underpressure or vacuum around the discharge fuel orifice 18
in the induction passage.
Thus if the snowmobile is operating at high
altitude, the ECU will respond to the signals received from
the sensors 11 and 12 to activate the solenoid valve 13 in
such a duty cycle that the float chamber pressure is reduced
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to ensure that a constant mass air/fuel mixture is delivered
by the carburettor to the engine.
By "duty cycle" is meant the percentage of the
opening time of the solenoid valve 13 in relation to its fixed
cycle time. For example if the cycle time of the solenoid
valve 13 is 0.1 seconds, and the duty cycle is 500, then the
opening time of the solenoid valve 13 during each cycle will
be 0.05 seconds.
Referring to Figure 2, at standard atmospheric
pressure and temperature conditions, the ECU does not deliver
any signal~to the solenoid valve 13 which therefore remains
closed and the float chamber 19 is at atmospheric pressure,
this corresponding to a duty cycle percentage of 0 at the
solenoid valve 13. At increasing altitude, the air density is
reduced so that the ECU 10 in response to signals received
from the sensors 11 and 12 will deliver a signal to the
solenoid valve 13 opening it for a percentage of its duty
cycle corresponding to the specific atmospheric conditions of
pressure and temperature that have been sensed so that the
float chamber through the conduit 24 is exposed to an under
pressure or vacuum as indicated by the graph in Figure 2.
This system is calibrated such that at a 100% duty cycle of
the solenoid valve 13 (corresponding to the minimum air
density atmospheric conditions which will be encountered) the
float chamber under pressure will as shown be approximately
40% of the vacuum in the induction passage 15 at the location
of the fuel orifice 18. Between these two extremes the change
is essentially linear.
It will be understood that it is at all times
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possible to alter the mass air/fuel ratio in response to the
above discussed or other parameters by feeding appropriate
signals to the ECU 10.
In some circumstances it is desirable to provide a
fuel-enriched air/fuel mixture to the engine, e.g. during cold
starting of the engine. Traditionally this has been done by
use of a manual or automatic choke or primer. In the present
invention however this function is also included in the fuel
supply control system, and is also monitored by the electronic
control unit which acts to increase the pressure within the
float chamber 19 to provide the mixture enrichment required
during engine start-up and during warming up of the engine
from a cold start.
Referring to Figure 3 there is shown a snowmobile
engine 30 which is a two-stroke internal combustion engine
having a crankcase 31 in which pre-compression of the air/fuel
charge is carried out prior to the latter being delivered into
the engine cylinders. During low speed rotation of the engine
crankshaft (e. g. between 200 and 900 rpm) the pressure changes
that occur during pre-compression of the charge in the
crankcase can be utilized, and to this end a pressure line 26
communicates with the crankcase interior and through a check
valve 20 and a pressure line 34 supplies crankcase gases to a
pressure regulator 35, the latter supplying a regulated
pressure flow to a pressure line 36.
This first pressure source as mentioned is useful
only at low engine rpm because for a given throttle opening
the available pressure decreases with increasing rpm, as is
well understood in the technology of two-stroke engine
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applications. In effect, this pressure source is only useful
during the cranking stage of operation of the snowmobile
engine under consideration, the cranking speed being of the
order of 500 rpm. The idle speed of the engine is about 1,700
rpm which is well above the range when any useful pressure
output can be obtained from the above described pressure
source. Therefore at higher speeds, the second pressure
source is provided by utilizing the pressure pulsations
occurring in the crankcase to drive a diaphragm air pump 33.
Thus as seen in Figure 3 the air pump 33 is divided by a
movable diaphragm 37, the chamber 38 on the upper side of the
diaphragm being in communication with a branch 27 of the
pressure line 26. On the underside of the diaphragm there is
a pumping chamber 37 designed to draw air from a line 32
(connected to the interior of the crankcase) through a plenum
40 and a non-return valve 41 and to deliver air under pressure
past a second non-return valve 42 into an output chamber 43
which communicates with the pressure line 34.
In operation, at low engine rpm as during cranking,
as described above a supply pressurized air is delivered
through the line 26 and through the check valve 20 and the
pressure line 34 to the regulator 35.
At higher engine speeds, e.g. at the idling speed of
1,500 rpm, as explained, the line 26 no longer deliver an
adequate flow of pressurized air. However in these
circumstances the pulsations from the crankcase through the
line 26, 27 produce a rapid fluctuation in the position of the
diaphragm 37 against the force of its return spring 44. These
fluctuations of the diaphragm cause small amounts of air from
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the line 32 to be drawn in past the one way valve 41 when the
diaphragm moves upwards, and then to be driven out of the
pumping chamber 39 past the one way valve 42 when the
diaphragm is moved downwards thus supplying a pressurized air
flow to the line 34 and the regulator 35.
Figure 4 shows a fuel flow control system which
incorporates elements from both Figures 1 and 3, and where
possible like reference numerals are used to illustrate like
parts.
The electronic control unit 10 is coupled to receive
signals from the barometric pressure sensor 11, the air
temperature sensor 12 and an engine temperature sensor 50 and
utilizes signals received from these sensors to control the
fuel supply to the engine 30 in the desired manner. As
described in relation to Figure 1, the ECU 10 delivers a
control signal to a solenoid 13 which is mounted in a manifold
122 the interior of which communicates with the float chambers
19 of each of a pair of carburettors 14 through conduits 124
and which communicates with atmosphere through a vent orifice
125. A vacuum conduit system 123 is exposed to the pressure
within the induction passage venturi of each of the
carburettors and communicates this pressure to the manifold
122 under control of the closure 21 of the solenoid 13. The
manifold 122 also carries a second solenoid 51 which is
connected to the ECU 10 and controls the supply of pressurized
air from the line 36 to the manifold 122 in accordance with
signals received from the engine temperature sensor 50.
Although not shown in Figure 4, the system for generating
pressure from the engine crankcase as shown in Figure 3 is
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included, and the output pressure line 36 therefrom is
connected to the interior of the manifold 122, this connection
being regulated by the solenoid 51.
From the foregoing it will be appreciated that the
pressure in the carburettor float chambers 19 is regulated
under the control of the ECU 10 in response to signals
received from the sensors 11, 12 and 50 to provide a mass
air/fuel flow mixture having the desired characteristics in
relation to various operating conditions of the engine.
Referring to Figures 4 and 5a and 5b, the manifold
122 is shown as constituting a pair of closed end tubes 120,
121, access to the interior of which is controlled through a
number of tubular connectors. The manifold 122 is designed
for use with the three cylinder engine 30. Specifically, on
the lower tube 121 at opposite ends thereof and in the middle
are three tubular connectors 123a for communication with the
vacuum conduits 123 that connect to the venturi of the
respective carburettors 14. Three further pairs of tubular
connectors 124 provide communication between the interior of
the manifold upper tube 120 and the float chambers of the
carburettors 14.
In an intermediate position in its length the
manifold 122 carries a block 130 in which are received the
solenoid valves 13 and 51 and the associated valve structure
(not shown in Figure 4). The block 130 also carries the
atmospheric vent 125 and a further tubular connector 136 (Fig.
5C) to receive the pressure line 134.
As will be understood, within the block 130 the
solenoid 13 controls communication of vacuum from the lower
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tube 121 to the upper tube 120 of the manifold, and thus
application of pressure reduction to the carburettor float
chambers. This is the condition represented by the arrows in
Figure 5b.
The solenoid 51 on the other hand controls
communication of pressurized air flow from the connector 136
to the interior of the upper tube 120, and thus controls
application of overpressure to the carburettor float bowls.
The full operating range of the system is calibrated
such that for fuel enrichment (corresponding to cold
start/warm up conditions) a 100% duty cycle for the solenoid
51 is provided at a predetermined ratio between atmospheric
pressure and the pressure provided by the air pump 33. For
reduction of the proportion of fuel in the air fuel mixture
ratio (compensation for low atmospheric pressure or altitude)
this system is calibrated to give 100% duty cycle operation of
the solenoid valve 13 at a predetermined maximum ratio of
negative (vacuum) pressure to atmospheric pressure. The
effects of the duty cycle operation of the two solenoid valves
13 and 51 can to some extent offset one another e.g. for high
altitude cold start situations.
Instead of the mechanical pump 33 described in
relation to Figure 3, it would of course be possible to
utilize various other pump arrangements, and in particular a
battery driven electric pump.
As is well understood, when an engine is cold it is
difficult to vaporize a sufficient amount of the liquid fuel
in the combustion chamber for the engine to operate properly.
Vaporization and atomization are adversely affected by low
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temperature, and therefore in cold start conditions it is
necessary to increase the quantity of fuel in order to
compensate for poor atomization, and this is typically done by
using a primer such as a choke or other enrichment system at
the carburettor. As the engine gradually warms up during
operation, the air fuel mixture atomizes and vaporizes more
readily, and if the enriched mixture ratio is maintained, the
engine performance will be reduced and the spark plugs may
become fouled. The control system described herein and
illustrated in the drawings overcomes this difficulty and will
operate to enrich the air fuel mixture in cold start
conditions, and automatically to reduce and remove the
enrichment when the engine warms up. This is done by the
electronic control unit 10 which receives signals from the
engine temperature sensor 50 and modifies the pressure in the
float bowls of the carburettors 14 as required to provide the
necessary degree of mixture enrichment. The sensor 50 can be
mounted at any convenient location on the engine 30, e.g. for
a liquid cooled engine, within the engine coolant jacket. As
described above in relation to Figure 2, the pump 33 is driven
by pressure pulses in the engine crankcase as communicated
through the line 27 to deliver a flow of pressurized air
through the line 34. This pressurized air is delivered to the
block 130 through the connection 136 and enters the upper tube
120 of the manifold under control of the solenoid 13 which is
driven by the ECU 10. Pressure from the tube 120 is
communicated to the float bowls of the carburettors 14 through
the tubes 124 to increase the pressure differential between
the float bowls and the carburettor venturi and thus increase
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fuel flow to the extent required. As the engine temperature
increases, the ECU 10 responds to signals from the sensor 50
to reduce the duty cycle of the solenoid 51, and thus reduce
the overpressure applied to the carburettor float bowls until
normal engine operating temperature is reached and this
overpressure is completely eliminated.
The ECU 10 also controls the solenoid 13 in
accordance with signals received from the air temperature
sensor 12 which is conveniently located in the engine air
filter (not shown) and from the barometric pressure sensor 11.
The duty cycle of the solenoid 13 is controlled such that
underpressure or vacuum from the lower tube 121 of the
manifold (which communicates with the venturis of the
carburettors through the lines 123) enters through the block
130 into the upper manifold tube 120 and hence acts to reduce
the float bowl pressure of the carburettors 14 producing a
leaner air fuel mixture corresponding to the reduced air
density that occurs for example at increased altitudes.
The use of the manifold 122 as shown particularly in
Figures 5a and 5b makes it possible to use a single pair of
solenoids 51, 13 to control the fuel flow in a number of
carburettors (three as shown in the three cylinder engine of
Figure 4). Without the manifold, individual pairs of
solenoids 13 and 51 would have to be provided for each
respective carburettor 14.
The fuel control system as described in the
foregoing can readily be provided as a retrofit on existing
engines, and conveniently is provided in kit form the kit
including
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a) the electronic control unit 10 together with the
atmospheric pressure and temperature sensors and the
engine temperature sensor;
b) the manifold 122 together with the block 130
including the connections for the various lines as
described above;
c) the pump 33;
d) modified carburettors 14 including connections
to the float bowls and venturis thereof; and
e) electrical connections and pneumatic connections
for coupling the various parts of the system.
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