Fuel supply control system

A fuel supply control system including a major area diaphragm and a minor area diaphragm defining therebetween a reference pressure chamber. A control pressure chamber is located adjacent the major area diaphragm on a side thereof opposite to the reference pressure chamber and a controlled pressure chamber is located adjacent the minor area diaphragm on a side thereof opposite to the reference pressure chamber. A valve is connected to the diaphragms for regulating the opening of an orifice of a fuel passage and the controlled pressure chamber serves as a chamber of the fuel passage. The control pressure chamber is connected to a combustion air supply passage system, and the reference pressure chamber is connected to a portion of an air supply and exhaust passage system, in which the pressure is lower than the pressure in the combustion air supply passage system.

BACKGROUND OF THE INVENTION 
This invention relates to fuel supply control systems suitable for use with 
combustors of water heaters, etc., and more particularly it is concerned 
with a fuel supply control system for effecting proportional combustion. 
To enable combustion to take place in good condition, it is necessary to 
keep constant the fuel-air ratio of fuel-air mixtures. Particularly in 
combustors for effecting proportional combustion, it is essential that the 
fuel-air ratio be kept constant through the entire range of changes in the 
amount of heat of combustion which show variations in a wide range. 
This type of combustion apparatus is shown, for example, in Japanese 
Laid-Open Utility Model Publication No. 89537/1979. The fuel supply 
control system disclosed in this Publication comprises a diaphragm of a 
minor area and a diaphragm of a major area, three chambers separated from 
one another by the two diaphragms, and a valve connected to the two 
diaphragms for controlling the opening of a fuel passage, the chamber 
adjacent the diaphragm of the minor area serving as a chamber of the fuel 
passage upstream of the valve, the chamber between the two diaphragms 
opening in a chamber downstream of the valve and the chamber adjacent the 
diaphragm of the major diameter being connected to a combustion air supply 
passage system. 
The control system of the aforesaid construction has the disadvantage that 
the fuel-air ratio of the mixtures is easily influenced by errors in 
fabrication and assembly of the system and the parts located in its 
vicinity. 
SUMMARY OF THE INVENTION 
This invention has been developed for the purpose of obviating the 
aforesaid disadvantage of the prior art. Accordingly, an object of the 
invention is to provide a fuel supply control system capable of 
maintaining with high accuracy the fuel-air ratio of the fuel-air mixture 
at a predetermined desirable level. 
One of the characteristic features of the invention resides in that the 
chamber defined between the two diaphragms or large and small diaphragms 
is connected to a portion of an air supply and exhaust passage system of 
the pressure lower than that of the chamber adjacent the diaphragm of a 
major or large area.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the invention in the form of a liquid fuel supply control 
system will now be described by referring to FIGS. 1 and 2. 
In FIG. 1, numeral 1 designates a burner capable of carrying out 
proportional combustion. The burner 1 which extends straightly comprises a 
secondary air passage 2 located in the central portion and having a 
plurality of secondary air ports 3 in the form of slits disposed in 
side-by-side relation at its bottom end. A plurality of slit-like flame 
ports 4 are located on opposite sides of the secondary air passage 2 in 
side-by-side relation, and a fuel-air premix passage 6 is defined between 
an outer side of the burner 1 and a wall 5. As a fuel-air mixture flows 
out of the flame ports 4, primary combustion takes place in a combustion 
chamber 7, and complete combustion takes place with secondary air being 
supplied through the secondary air ports 3. The combustion gas is 
discharged through an exhaust cylinder 9 after giving off heat in a heat 
exchanger 8. Numeral 10 designates an air supply and exhaust cylinder. 
Water introduced into the heat exchanger 8 through a water supply port 11 
is heated by the heat given off by the combustion gas and flows out 
through a hot water dispensing port 12 as hot water. The temperature of 
the hot water is sensed by a temperature sensor 13, and the number of 
revolutions of a blower 15 for supplying combustion air is controlled by 
an RPM controller 14. That is, when the temperature of the hot water is 
higher than a predetermined value, the number of revolutions is reduced. 
Air for combustion is supplied through an air supply passage 16. A portion 
of the air is supplied through a secondary air passage 17 to the secondary 
air passage 2 of the burner 1, and the rest of the air is supplied to a 
venturi-like evaporator 18 which is heated to a predetermined temperature 
by a heater 19. A fuel supply nozzle 20 opens in a throat of the venturi. 
Numeral 21 designates constant fuel-air ratio means for controlling fuel 
supplied through a fuel inlet 22 as by a fuel pump to keep the fuel volume 
at a level commensurate with the air volume for combustion. The constant 
fuel-air ratio means 21 is maintained in communication with the fuel 
nozzle 20 through an orifice 26 and a fuel outlet 25. To provide a fuel 
volume commensurate with the air volume, the pressure in the air supply 
passage 16 is supplied through a pressure transmitting pipe 23 to the 
constant fuel-air ratio means 21 which also receives through another 
pressure transmitting pipe 24 a supply of pressure from a portion of an 
air supply and exhaust passage system including the throat of the venturi, 
an exhaust accumulating chamber 9a and exhaust cylinder 9 that has a lower 
pressure than the air supply passage 16. 
FIG. 2 shows in detail the construction of the constant fuel-air ratio 
means 21, in which g indicates the direction of the gravitational force. 
The constant fuel-air ratio means 21 comprises a main body 31, 32 having a 
diaphragm 33 of a major or large diameter and a diaphragm 34 of a minor or 
small diameter formed of flexible material hermetically fixed at their 
outer peripheries in the main body 31, 32, to define therein a reference 
pressure chamber 35 defined between the two diaphragms 33 and 34, a 
control pressure chamber 36 located adjacent the major diameter diaphragm 
33 on a side thereof opposite the reference pressure chamber 35 and a 
controlled pressure chamber 37 located adjacent the minor diameter 
diaphragm 34 on a side thereof opposite the reference pressure chamber 35. 
The reference pressure chamber 35 is connected through a riser pipe 38 and 
the pressure transmitting pipe 24 to the low pressure section of the air 
supply and exhaust passage system, with the riser pipe 38 rising a 
predetermined amount h as subsequently to be described. The control 
pressure chamber 36 is connected to the high pressure section through the 
pressure transmitting pipe 23, and the controlled pressure chamber 37 is 
connected through an inlet orifice 39 to the fuel inlet 22 and through the 
fuel output 25 and orifice 26 to the fuel nozzle 20. The fuel nozzle 20 
opens in a position which is higher than the liquid level of the 
controlled pressure chamber 37 by a distance H. A portion of the 
controlled pressure chamber 37 above the fuel outlet 25 is full of air. A 
portion of the controlled pressure chamber 37 downstream of the inlet 
orifice 26 is a secondary pressure chamber for the fuel, and a portion 
upstream of the inlet orifice 26 is a primary pressure chamber. 
The two diaphragms 33 and 34 are hermetically interconnected by a 
connecting rod 40. Numerals 41, 42, 43, 44 and 45 respectively designate a 
spacer, a seat, a coil spring, a bolster and a pin which are provided for 
connecting the two diaphragms 33 and 34 to the connecting rod 40 and a 
seat 46. Numeral 47 designates a fixed seat of the diaphragm 34. 
The connecting rod 40 is connected through a link 50 to a valve 51 for 
opening and closing the inlet orifice 39. Numeral 52 designates a pivot, 
and numerals 53 and 54 are connecting pins. The distance between the 
connecting pin 53 and pivot 52 is equal to or larger than the distance 
between the pivot 52 and pin 54. The diameter of the inlet orifice 39 is 
sufficiently smaller than the diameter of the minor diameter diaphragm 34. 
In operation, air for combustion is supplied from the air supply and 
exhaust cylinder 10 through operation of the blower 15. This causes the 
pressure Pa of the air for combustion in the air supply passage 16 to be 
applied to the control pressure chamber 36 and a pressure Po lower than 
the pressure Pa to be applied to the reference pressure chamber 35, so 
that the diaphragm 33 is moved downwardly to open the valve 51. This 
allows fuel to flow into the controlled pressure chamber 37 through the 
fuel inlet 22 to raise the pressure P2 of the controlled pressure chamber 
37. A rise in the pressure P2 of the controlled pressure chamber 37 moves 
the diaphragm 34 upwardly to close the valve 51. Thus the pressure P2 of 
the controlled pressure chamber 37 is kept at a level at which the force 
exerted on the diaphragm 33 by the pressure differential Pa-Po and the 
force exerted on the diaphragm 34 by the pressure differential P2-Po 
balance. At this time, since the diaphragm 33 has an area A1 larger than 
an area A2 of the diaphragm 34, the pressure P2 becomes higher than the 
pressure Pa. The fuel is supplied by this pressure P2 and the flow rate is 
controlled by the orifice 26, so that the fuel of the controlled flow rate 
is supplied through the fuel nozzle 20 to the evaporator 18. At this time, 
primary air is supplied simultaneously from the blower 15, to atomize the 
fuel supplied to the evaporator 18. 
Since the evaporator 18 is heated by the electric heater to a temperature 
of about 250.degree. C., the fuel in atomized particles is vaporized and 
mixed with the primary air. The fuel-air mixture is passed through the 
fuel-air mixture passage 6 and issued through the flame ports 4 of the 
burner 1, so that primary combustion takes place. Secondary air is 
supplied through the secondary air passages 17 and 2 and the secondary air 
ports 3 to enable complete combustion to take place. The combustion gas is 
passed through the heat exchanger 8, and vented as exhaust gas through the 
exhaust cylinder 9 and air supply and exhaust cylinder 10. 
Water flows through the heat exchanger 8. When the hot water flowing out of 
the hot water dispensing port 12 rises above a predetermined value in 
temperature, the temperature sensor 13 senses this temperature and issues 
a signal which is supplied to the RPM controller 14 to reduce the voltage 
impressed on the motor of the blower 15, to thereby reduce the number of 
revolutions thereof to reduce the volume of air for combustion. A drop in 
the volume of air for combustion causes a drop in the pressure Pa applied 
to the control pressure chamber 36, so that the valve 51 closes and the 
pressure P2 of the controlled pressure chamber 37 also drops. This results 
in a reduction in the flow rate of fuel flowing through the fuel nozzle 
20. Thus the fuel volume can be varied while the air and fuel can be kept 
at constant proportions in volume and the fuel-air mixture can be burnt in 
good condition. At the same time, the temperature of the hot water flowing 
through the hot water dispensing port 12 can be kept constant at all 
times. 
The relation between the fuel volume and the air volume will be described 
hereunder by using formulae. 
The relationship between the upwardly directed force exerted on the 
constant fuel-air ratio means 21 and the downwardly directed force exerted 
thereon are expressed by equation (1). Consequently the pressure P2 of the 
controlled pressure chamber 37 can be indicated by equation (2), and the 
fuel volume Qf can be indicated by formula (3). 
##EQU1## 
where A.sub.1 : effective pressure receiving area of diaphragm 33. 
A.sub.2 : effective pressure receiving area of diaphragm 34. 
A.sub.o : area of inlet orifice 39. 
Pa: pressure of control pressure chamer 36. 
Po: pressure of reference pressure chamber 35. 
P1: pressure of primary pressure chamber. 
P2: pressure of controlled pressure chamber 37. 
C.sub.1 : Po/Pa . . . constant sufficiently smaller than 1. 
C.sub.2 : Pn/Pa . . . constant sufficiently smaller than 1. 
Pn: pressure in the vicinity of fuel nozzle 20. 
Wo: weight of the movable parts. 
r.sub.1 : distance from pivot 52 to pin 54/distance from pivot 52 to pin 53 
. . . which is constant, r.sub.1 .ltoreq.1. 
.gamma.: specific weight of fuel 
H: height from the liquid level of controlled pressure chamber 37 to fuel 
nozzle 20. 
The air volume Qa can be expressed by the following formula: 
##EQU2## 
Thus, from formulae (3) and (4), the ratio of the fuel volume supplied to 
the burner 1 to the air volume supplied thereto can be expressed by the 
following formula: 
##EQU3## 
Thus if the value of H is set such that .DELTA.P=0, the fuel volume will 
change with variations in the air volume while the proportions of the fuel 
and the air are kept constant, so that combustion can be sustained in good 
condition from a high combustion state to a low combustion state. 
No problem would arise if .DELTA.P=0 in equation (5). However, there is the 
case where the relation .DELTA.P=0 cannot be established because of 
fabrication errors and the condition in which the system is installed. In 
this case, it is necessary to enable combustion to take place 
satisfactorily by minimizing changes in the ratio Qf/Qa. To this end, one 
would have to increase A1/A2 and reduce C1 and C2 in equation (4). Thus in 
the invention, the diaphragm 33 has a larger area than the diaphragm 34 
and Po is the pressure prevailing in a section (throat of the evaporator 
20, exhaust accumulating chamber 9a, exhaust cylinder 9, etc.) which is 
sufficiently lower than Pa. This enables changes in the ratio Qf/Qa to be 
minimized at low cost. 
Since A2&gt;Ao, and rl.ltoreq.1, the pressure P2 at which the fuel is supplied 
is substantially A1/A2 times as high as the pressure Pa at which the air 
is supplied, as can be clearly seen in equation (2). This increases the 
pressure differential between the fuel supply pressure P2 and the air 
pressure Pn, thereby reducing the rate of change in the proportions of the 
supplied fuel and supplied air. 
In gas governors, a spring is mounted to cancel out the weight Wo of 
movable parts, etc. However, when the fuel is kerosene, its specific 
gravity .gamma. is higher than that of gas. Thus, in equation (5), the 
term of .DELTA.P can be rendered .DELTA.P=0 with the mounting position H 
of the constant fuel-air ratio means 21 relative to the fuel nozzle 20, 
and hence the cost of production of the constant fuel-air ratio means 21 
can be minimized in relation to the position of mounting H. The mounting 
position H is related to other factors also, and is determined by taking 
all of the factors into consideration. 
This arrangement also offers the advantage that the fuel nozzle 20 is 
higher in position than the fuel outlet 25, thereby avoiding the risks of 
air being collected in the fuel supply pipe when no combustion takes 
place. 
As can be clearly seen in equation (5), the influence of the inlet pressure 
P1 can be minimized by reducing the ratio Ao/A2 anr r1. 
When combustion equipment is actually installed, the distance between the 
combustion equipment and air supply and exhaust cylinder 10 may vary 
depending on installation condition, and the condition of external air to 
which the air supply and exhaust cylinder 10 is exposed may also vary, 
resulting in a variation in the air volume (air pressure Pa). It is 
necessary to keep constant the value Qf/Qa, even if air pressure Pa is 
varied to external causes. In the invention, the atmospheric pressure is 
not used but the pressure in a portion of the air supply and exhaust 
passage system extending from the air supply port of the air supply and 
exhaust cylinder 10 to its exhaust port is used as the pressure Po of the 
reference pressure chamber 35. As a result, the pressure Po is varied in 
conjunction with a change in the pressure Pa, thereby rendering the value 
of C1 in equation (5) substantially constant. Also, since the pressure Pn 
of C2 is varied in conjunction with a variation in the pressure Pa, the 
value of C2 can be kept substantially constant. Consequently the value in 
the brackets [ ] in the .sqroot. in equation (5) is not largely 
susceptible to the aforesaid external causes. Thus the Qf will vary 
substantially in conjunction with a variation in the pressure Pa and the 
change in the ratio Qf/Qa can be minimized. 
When the pressure Po is the pressure in the throat of the venturi in the 
evaporator 18, the change in the ratio Qf/Qa in relation to .DELTA.P can 
be reduced. 
Since the pressure transmitting pipe 24 communicating with the reference 
pressure chamber 35 is partly constituted by the riser pipe 38 which has a 
height larger than 1.2 times the air pressure Pa (Pa/0.8.apprxeq.1.2 Pa 
with 0.8 being the specific gravity of kerosene), the liquid fuel would be 
collected in the riser pipe 38 to press the diaphragm 33 upwardly in the 
event of the rupture of the diaphragm 34, and hence the valve 51 would be 
moved downwardly to close the inlet orifice 39. Thus, the combustion is 
interrupted. Also fuel leak through the pressure transmitting pipe 24 
could be avoided. 
The presence of an air sump above the controlled pressure chamber 37 
prevents the diaphragm 34 from contacting with the liquid fuel, thereby 
prolonging the service life of the diaphragm 34 and reducing the cost of 
the entire system. The air sump can be reduced in size by providing the 
controlled pressure chamber 37 with an air venting valve at its side. 
The constant fuel-air ratio means 21 of the aforesaid embodiment may be 
placed upside down so that the gravitational force may be acted in the 
reversed direction. In this case, the constant fuel-air ratio means 21 is 
placed higher by a predetermined amount H above the fuel nozzle 20. Such 
placement becomes unnecessary if a spring for establishing .DELTA.P=0 is 
provided on the side of the diaphragm 33 of the major diameter. When it is 
desired to prevent fuel leaks which may be caused by the rupture of the 
diaphragm 34, a riser pipe extending upwardly may be used. 
It is possible to obtain a structure that the link 50 is dispensed with or 
the diaphragm 34 is exposed to the primary pressure. The air pressure in 
the control pressure chamber 36 may be an increased pressure produced by 
raising the pressure of air in the combustion air supply passage 16 by 
using an additional blower or other suitable means.