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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas recirculation system for an
internal combustion engine of an automotive vehicle. More particularly,
the invention relates to an exhaust gas recirculation system in which the
amount of recycled exhaust gas may be varied in response to operating
conditions including engine speed, load conditions, warm-up conditions,
vehicle speed, and temperature of intake air.
A known technique for reduction of emission of pollutants, particularly
nitrogen oxides, in the exhaust gases of an internal combustion engine
which are discharged to the atmosphere is to recycle a portion of the
exhaust gases to a stage preceding the combustion stage, usually to the
carburetor.
In a means according to one conventional approach, the air-fuel mixture is
made leaner or richer than the theoretical air-fuel ratio of 15, at which
nitrogen oxide emission is a maximum, and a comparatively small amount of
exhaust gas is recirculated. However, there are definite limits to the
amount of reduction of NO.sub.x emission that can be achieved by such a
means.
To provide increased control of NO.sub.x emission in order to meet the
requirements of government or other regulations without having an
excessively adverse effect on average engine operating conditions, it has
therefore been proposed to set the air-fuel ratio at the theoretical ratio
and to greatly increase the amount of recirculated exhaust gas.
To achieve recirculation of the desired large amounts of exhaust gas it has
been proposed to introduce the recirculated gas into a carburetor via
separate ducts which are upstream and downstream of the throttle valve in
the carburetor, i.e., upstream and downstream in terms of air flowing
through the carburetor, and, in order to maintain the ratio of
recirculated exhaust gas to the air-fuel intake more or less constant over
the range of moderately low to moderately high load conditions for the
engine to make the upstream supply of recirculated exhaust gas
proportional to the air intake and the downstream supply proportional to
the pressure downstream of the venturi section of the carburetor. In
conventional means, control of the flow rates of recirculated exhaust gas
is effected simply by orifices, and the large amount of exhaust gas
recirculated by conventional means is very disadvantageous in certain
operating conditions. In particular,
(1) Recirculation of a large amount of exhaust gas when the engine is
rotating at high speed or is operating under a high load, or when vehicle
speed is high, inevitably leads to reduced engine output and/or increased
fuel consumption rates;
(2) Depending on temperature conditions within and around the carburetor,
the contribution to a temperature increase made by exhaust gas
recirculated to the upstream portion of the carburetor can be the cause of
undesirable heating of the fuel float chamber, with consequent percolation
and escape of fuel. Alternatively, depending on ambient temperature
conditions and relative humidity of the intake air, recirculated exhaust
gas, which has a high moisture content, may be the cause of icing in the
carburetor.
To summarize, conventional means do not really consider relating the main
problem to exhaust gas recirculation, namely how to recirculate the gas
when definite advantages are achieved thereby, but stop recirculation of
the gas the advantages are largely or completely outweighed by
disadvantages relating to other aspects of engine operation.
SUMMARY OF THE INVENTION
The present invention solves this problem by providing an exhaust gas
recirculation means in which exhaust gas may be recirculated into the
carburetor of an internal combustion engine via two ports, one upstream of
and the other downstream of a throttle valve in the carburetor, and in
which recirculation lines leading to these ports are closed by the action
of valve elements controlled by a control unit, for example, an electrical
or electronic control unit, which receives input indicative of engine
speed, vehicle speed or engine load and which acts through the valve
elements to close only the recirculation line leading to the upstream
portion of the carburetor when the engine or vehicle speed and/or engine
load exceeds a set value, whereby the rates of exhaust gas recirculation
are automatically adjusted to optimum values for the complete range of
engine operating conditions. In a preferred embodiment, the control unit
receives input indicative of the air intake temperature and/or input
indicating when the engine is warmed-up, and acts through the valve
elements to close only the recirculation line leading to the upstream
portion of the carburetor when the intake air temperature falls below a
set value, and to close both recirculation lines during warm-up of the
engine.
BRIEF DESCRIPTION OF THE DRAWING
A better understanding of the invention may be had from the following full
description thereof when read in reference to the attached drawings, in
which like numbers, refer to like parts, and in which;
FIG. 1 is a schematic cross-sectional view showing the main features of an
exhaust gas recirculation means according to a first embodiment of the
invention;
FIG. 2 is a graph showing the relation of exhaust gas recirculation to
vehicle speed in the means of FIG. 1;
FIG. 3 is a graph showing the relation of specific fuel consumption to the
exhaust gas recirculation ratio in the means of FIG. 1;
FIG. 4 is a view similar to FIG. 1 and showing another embodiment of the
invention;
FIG. 5 is a graph showing the relation of icing in a carburetor to intake
air temperature and relative humidity of the air in a carburetor; and
FIG. 6 is a view similar to FIG. 1 and showing another embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is shown a carburetor 1 comprising an air intake
circuit 1a which leads to a venturi section 5, a main nozzle 3a providing
communication between a fuel float system 3b and the venturi section 5,
and a throttle valve 4 downstream of the venturi section 5, i.e., on the
opposite side of venturi section 5 from the air intake circuit 1a, and
which produces an air-fuel mixture in a conventionally known manner and
supplies the mixture to be burned in one or more combustion chambers of an
engine, indicated schematically at E. The carburetor 1 may include other
conventionally known elements such as a choke valve, an idle port, and a
low speed port, not shown. In terms of air flow into the carburetor 1, the
air intake circuit 1a is preferably preceded by an air cleaner 2
comprising a filter 2a. Gases produced by combustion of the mixture are
exhausted from engine E through an exhaust pipe 8 and a portion thereof is
taken off from the exhaust pipe 8 by take-off line schematically indicated
at 9 and supplied by line 9 into the intake ends 6c and 7c of a first
recirculation line 6 and a second recirculation line 7, respectively,
lines 6 and 7 being separate from each other and connected to separate
branch lines of take-off line 9.
The first recirculation line 6 forms part of a first exhaust gas
recirculation means and has a delivery end 6a which opens into a portion
of carburetor 1 which is upstream of throttle valve 4 and, in this
embodiment, is upstream of venturi section 5 also. Flow of exhaust gas
through the recirculation line 6 can be throttled or completely stopped by
a first flow control valve 10 which is seated on a valve seat 6b defined
by wall portions of the recirculation line 6 and the degree of opening of
which is controlled by a diaphragm 13 through a rod 12 having one end
attached to the first flow control valve 10 and the opposite end connected
to one side of the diaphragm 13 extending across a first diaphragm unit
11. The portion of the diaphragm unit 11 which is in part bounded by the
side of the diaphragm 13 to which the rod 12 is connected is sealed or
connected to a constant pressure source and constitutes a constant
pressure chamber 11c. The portion of the diaphragm unit 11 which is on the
opposite side of the diaphragm 13 constitutes a negative pressure chamber
11a which is connected through a first control fluid duct 14 to a suitable
negative pressure source, for example, a portion of the intake air flow to
carburetor 1 which is at reduced pressure. In the control fluid duct 14
there is provided a first stop valve 15 which is controlled in a manner
described below by a control unit 23. In the negative pressure chamber 11a
there is provided a coil spring 11b which acts on the flow control valve
10 via the diaphragm 13 and the rod 12 and constantly exerts a force on
the control valve 10 urging it towards the valve seat 6b. Assuming that
the stop valve 15 is open, the diaphragm 13 moves against the force of
spring 11b, due to the force exerted as a result of the difference of
pressures in chambers 11a and 11c, and the control valve 10 is opened to a
degree dependent on the amount of movement of the diaphragm 13, and
exhaust gas is allowed to flow through the recirculation line 6 into the
carburetor 1. By connecting the control fluid duct 14 to a source the
pressure of which is proportional to air flow in the carburetor 1, the
movement of the diaphragm 13, and, hence the opening of the flow control
valve 10 and the rate of flow of the exhaust gas into the carburetor 1,
are varied in accordance with conditions in the carburetor 1, i.e., in
accordance with engine operating conditions. When the stop valve 15 is
closed, the difference between the pressure in the chamber 11a and the
pressure in the chamber 11c of the diaphragm unit 11 becomes insufficient
to counter the force of the spring 11b, which therefore seats the flow
control valve 10 on the valve seat 6b, thereby interrupting the supply of
the exhaust gas to the delivery end 6a of the recirculation line 6.
Still in FIG. 1, the exhaust gas supplied into the second recirculation
line 7 is supplied via the delivery end 7a of the line 7 into a portion of
the carburetor 1 which is downstream of the throttle valve 4. The
recirculation line 7 forms part of a second recirculation system that has
basically the same construction and manner of functioning as the
abovedescribed first exhaust gas recirculation system and comprises a
second flow control valve 16 which is seatable on a valve seat 7b defined
by wall portions of the line 7 and which controls the flow of the exhaust
gas in the line 7, a second diaphragm unit 20 divided into a negative
pressure chamber 20a and a constant pressure chamber 20c by a diaphragm 19
which acts through a rod 18 to control the position of the flow control
valve 16, a coil spring 20b provided in the negative pressure chamber 20a
and exerting a constant force tending to cause the diaphragm 19 to seat
the flow control valve 16 on the valve seat 7b, whereby the flow of the
exhaust gas in the line 7 is stopped, a second control fluid duct 21 which
connects the negative pressure chamber 20a of the diaphragm unit 20 to a
suitable negative pressure source, and a second stop valve 22 which is
controlled by the control unit 23 and is actuable to open and close the
control fluid duct 21 selectively.
Needless to say, instead of connected to a negative pressure source, the
control fluid duct 14 and/or the control fluid duct 21 may be connected to
a positive pressure source, in which case the spring 11b and/or the spring
20b is provided on the other side of the diaphragm of the corresponding
diaphragm unit.
The control unit 23 is suitably an electrical or electronic unit, which is
not necessarily positioned adjacent to the carburetor 1 in the manner
shown in FIG. 1, and which receives input signals s, r, L, and v. The
input signal s indicates that engine E is warming up, and can be supplied,
for example, from a switch actuated by a device which detects the
temperature of cooling water in the engine E. The input signal r relates
to the engine speed and can be supplied by any suitable device responsive
to engine rotation. The input signal L indicates that the engine load is
above a certain set level, and is supplied, for example, from an element
which compares the pressure of intake air with the pressure of air ambient
to the engine or from a position of the throttle valve. The input signal v
indicates that the vehicle speed is above a certain set level, and is
suitably supplied from an element which measures or computes rotary or
peripheral speed of any one of the wheels of the vehicle driven by engine
E. Needless to say, the various elements providing such input signals to
control unit 23 can be associated with or provided on branch lines
connecting to elements which supply signals for display on a driving
dashboard, i.e., the invention does not necessarily require provision of
supplementary detection elements in an automotive vehicle.
In response to these input signals, the control unit 23 supplies as an
output a signal p causing the stop valve 15 and the stop valve 22 to close
when the warming up signal s is received, and a signal q to close only the
stop valve 15 when any one of the signals r, L, or v is received, the stop
valve 22 otherwise being left open. This action is achieved by providing
in the control unit 23 an output terminal which is connected directly to
an input terminal for the signal s and supplies an output to close the
valve 22, and an OR circuit the output terminal of which is connected to
close the valve 15 and which receives all the signals s, r, L and v as an
input, for example. The stop valves 15 and 22 may be any type of valve
that is actuable by electrical signals, for example, solenoid-controlled
valves.
By this action, therefore, recirculation of the exhaust gas is completely
stopped during warm-up of engine E and, when suitable running conditions
have been achieved, the exhaust gas is recirculated via both lines 6 and
7, but recirculation of excessive amounts of exhaust gas during high
engine speed conditions or at high vehicle speed is avoided by closure of
the recirculation line 6 when these conditions are attained.
Results obtained by the means of the invention are illustrated in the graph
of FIG. 2 to which reference is now had, and in which the abscissa shows
values of vehicle speed, determined directly or on the basis of engine
speed, and the ordinate shows values of the exhaust gas recirculation
ratio, defined as the amount of recirculated exhaust gas divided by the
intake fuel-air mixture and multiplied by 100. The curve a shows the
overall recirculation ratio, the curve b the recirculation ratio of the
exhaust gas supplied to the upstream portion of the carburetor 1 by the
recirculation line 6, and the curve c the recirculation ratio of the
exhaust gas supplied to the downstream portion of the carburetor 1 by the
recirculation line 7.
Because of the respective locations of the delivery ends 6a and 7a of the
first and second recirculation line 6 and 7, the upstream supply of
recirculated exhaust gas tends to increase proportionally to the increased
air intake which accompanies increased vehicle or engine speed, whereas
the flow of the recirculated exhaust gas via the recirculation line 7 is
greatly influenced by the negative pressure downstream of the throttle
valve 4 and, therefore, tends to decrease as the vehicle speed increases.
The net result is that, in the range of speed of about 30-70 km/h the
recirculation ratio of the total amount of exhaust gas supplied into the
carburetor 1 via the recirculation lines 6 and 7 remains generally
constant at a value somewhat higher than 15%. When the vehicle speed
reaches about 70 km/h, the signal v is supplied to the control unit 23
and, consequently, the recirculation ratio drops rapidly to a value of
about 4 to 6%. This lowering of recirculation ratio is effected to prevent
engine output from falling and to reduce the fuel consumption ratio.
Reference is now had to FIG. 3, which shows the relation achieved by the
means of the invention between the exhaust gas recirculation ratio and the
specific fuel consumption when the air-fuel ratio of the mixture produced
in the carburetor 1 is 14, 15, and 16 and the engine speed and the mean
effective output pressure Pe are maintained constant at 1,500 rpm and 3
kg/cm.sup.2, respectively. It is seen that, for all three air-fuel ratios,
the fuel consumption is minimum when the recirculation ratio is in the
vicinity of 5%.
It is thought that the reason for minimum fuel consumption for the
recirculation ratio of about 5% is as follows. In an engine in which an
Otto cycle is repeatedly effected, the thermal efficiency is influenced by
the ratio of the specific heat of components of a mixed gas, in this case,
the air-fuel mixture and recirculated exhaust gas, and the pressure inside
the cylinder defining the combustion chamber at the start of the
compression stroke. However, since a change in the ratio of the specific
heat of the gas mixture components between times when the exhaust gas is
recirculated and the exhaust gas is not recirculated is very small, for
example, on the order of 1.38:1.40, which may be ignored for practical
purposes, the thermal efficiency is directly governed by the pressure in
the cylinder, that is, by the amount of recirculated gas and,
theoretically, from this point of view, the thermal efficiency should
improve as the amount of recirculated exhaust gas is increased. However,
since the recirculated exhaust gas is comparatively inert, it tends to
lower the speed of combustion, which off-sets the advantage of the
increased pressure in the cylinder due to the gas. The net result of these
mutually cancelling factors is that maximum thermal efficiency is achieved
when the exhaust gas recirculation ratio is in the range of 5 to 10%.
Comparing FIG. 2 and FIG. 3, it is seen that the system of the invention
offers the considerable advantage that a minimum specific fuel consumption
is achieved when high vehicle speeds are reached.
Reference is now had to FIG. 4 which shows another embodiment of the
invention in which a control unit 23 further receives, in addition to the
signals received in the embodiment of FIG. 1, a signal T indicating that
the temperature of air in the air intake air portion of the carburetor 1
is below a certain temperature, and other parts are same as in FIG. 1. The
signal T may be supplied, for example, by a bimetallic element or other
suitable means constituting a temperature detector 26 which is mounted on
the filter 2a adjacent to the inlet of the carburetor 1, and has
associated therewith a suitable circuit (not shown) for transmission of
signals to the control unit 23.
Referring to the graph of FIG. 5, the abscissa represents values of
relative humidity of intake air and the ordinate represents values of the
temperature thereof. In the absence of the recirculated exhaust gas
admixed therewith, the intake air is practically never in a
super-saturated state in which the relative humidity is greater than 100%.
The region in which icing is liable to occur is that bounded by the
generally-paraboric curve (a) in the drawing, in which the relative
humidity is in the range 50-100% and temperature centers on a value of
3.degree.-5.degree. C. and extends to an upper limit of 20.degree. C. and
to a lower limit of close to -15.degree. C. These temperatures noted for
the intake air are, of course, influenced to a considerable extent by the
temperature of the engine as a whole, and are somewhat higher than the
external ambient air temperature. The reason for the upper limit of
20.degree. C. is that, even supposing a certain amount of lowering of
temperature of air subsequent to intake thereof due to the latent heat of
vaporization, temperatures reached are not such as to permit icing. On the
other hand, if the temperature of the intake air is in the vicinity of or
lower than about -15.degree. C., although air temperature is favourable to
icing, the absolute moisture content of the air is so low that the
quantity of ice which may form is insufficient to have any practical
effect on the functions of the carburetor. The system of the invention
makes no contribution to avoidance of icing in the region bounded by the
curve (a) of FIG. 5.
With the addition of the upstream recirculated exhaust gas, however, the
relative humidity of the air-fuel mixture in the intake portion of the
carburetor readily becomes higher than 100% and icing may occur over the
whole range of intake air temperature from below -20.degree. C. to
+20.degree. C. In this respect, the system of the invention offers a
definite advantage since the supply of the recirculated exhaust gas via
the recirculation line 6 is interrupted when the temperature of the intake
air is in the range in which admixture of the exhaust gas therewith is
liable to cause icing. For the above noted reason, 20.degree. C. is the
upper limit of this range, and the temperature detector is therefore
preferably made such that a signal T is supplied continuously to the
control unit 23 while the intake air temperature is lower than 20.degree.
C., but is not supplied when air intake temperature is 20.degree. C. or
higher.
Referring now to FIG. 6, there is shown another embodiment of the invention
in which the delivery end 6a of the recirculation line 6 opens into a
portion of the carburetor 1 which is intermediate the venturi section 5
and throttle valve 4 and is also in communication with the delivery end of
a supplementary air duct 25, the intake end of which opens into a portion
of the carburetor 1 upstream of the venturi section 5. In this embodiment
of the invention, the flow rate of air which is delivered via the duct 25
into the carburetor 1 is inversely proportional to the flow rate of the
exhaust gas delivered into the carburetor 1 via the recirculation line 6,
since the recirculated exhaust gas is at a generally higher pressure than
that of the air in the supplementary air duct 25, delivery of air from the
duct 25 into the carburetor 1 being completely or almost completely
stopped at high flow rates of the exhaust gas in the recirculation line 6,
whereby there is automatic enrichment of the air-fuel mixture produced in
the carburetor 1 as the exhaust gas recirculation ratio is increased. To
ensure a correct supply of the exhaust gas into the carburetor 1, there is
suitably provided in the supplementary air duct 25 an orifice element 25a
which has a smaller cross-sectional area than that of the delivery end 6a
of the recirculation line 6, whereby the duct 25 presents a greater
resistance to flow.
In all of the above described embodiments of the invention, the stop valves
15 and 22 may, of course, be directly positioned in the recirculation
lines 6 and 7, respectively. The diaphragm units 11 and 20 are preferably
included, however, since, as noted earlier, by the connection of the ducts
14 and 21 to a source in which the value of pressure is related to intake
air pressure in the carburetor 1, the rate of exhaust gas recirculation
can be varied in relation to operating conditions of the engine E.
Although the present invention has fully been described in connection with
the preferred embodiments thereof with reference to the accompanying
drawings, it is to be noted that various changes and modifications will be
apparent to those skilled in the art. Such changes and modifications are
to be construed being included within the true scope of the present
invention unless they depart therefrom.
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