Fuel interruption means of fuel tank

In a fuel interruption device for a fuel tank, a main valve is prevented from opening as a result of the action of an auxiliary valve during closure of the main valve; and more specifically, an auxiliary valve which is caused to move up and down by a float and a main valve which moves up and down in linkage with the up and down movement of the auxiliary valve are accommodated inside a casing which allows the inflow and outflow of liquid fuel and is provided at the end of a passage which connects the fuel tank and canister; a linking projection is provided on one of the valves, that is, either the auxiliary valve or the main valve, so that the main valve moves in linkage with the auxiliary valve in an action that is either advanced or retarded within a predetermined range, and a projection regulating groove into which the linking projection is inserted so that the projection can change its position within the range is formed in the other valve; and auxiliary valve seat and main valve seat open independently in the upper portion of the casing or in the end of the passage.

BACKGROUND OF THE INVENTION 
The present invention relates to a fuel interruption means which is 
installed in a passage that allows vaporized fuel to escape from a fuel 
tank into a canister and which prevents the inflow of liquid fuel into the 
canister. 
Fuel interruption means are installed in the passages which connects fuel 
tanks and canisters and ordinarily at the connecting point of the fuel 
tank and passage; and in addition, such means utilize the buoyancy of 
residual fuel inside the fuel tank in order to open and close a float 
valve, thus preventing the outflow of liquid fuel into the canister. In 
conventional fuel interruption means, the float valve is divided into a 
main valve which has a large opening area and an auxiliary valve which has 
a small opening area in order to insure reliable operation of the float 
valve even in cases where the difference between the internal pressure of 
the fuel tank and the pressure on the canister is large; and such fuel 
interruption means have become increasingly common (U.S. Pat. No. 
5,261,439, U.S. Pat. No. 5,313,978). The relationship between the main 
valve and the auxiliary valve in the present invention is based on such a 
difference in the size of the opening area. 
For example, the means disclosed in U.S. Pat. No. 5,313,978 operates so 
that when the vehicle tilts and the level of the liquid fuel reaches the 
fuel interruption means, an auxiliary valve which is formed integral with 
a float pushes the main valve upward so as to close the passage; and when 
the vehicle is back to its normal attitude and the fuel surface is 
lowered, even if there is a large pressure difference between the fuel 
tank and the canister so that the main valve tends not to open, the 
auxiliary valve first opens so that the pressure difference is alleviated, 
and then the main valve is opened. Thus, the means has an improved 
reliability of the valve opening and closing operation. 
SUMMARY OF THE INVENTION 
The fuel interruption means described in U.S. Pat. No. 5,313,978, the 
auxiliary valve, rather than functioning as an ordinary valve, functions 
as a driving force which pushes the main valve upward through the buoyancy 
of the float when the main valve is to be closed, and it functions as a 
so-called "pressure relief hole" when the main valve is to be opened. 
Accordingly, the auxiliary valve seat is installed in the main valve; and 
since the sealing characteristics of the auxiliary valve must be insured, 
the structure is naturally designed so that the auxiliary valve and main 
valve are in tight contact. From this fact, it is understood that the main 
valve and the auxiliary valve are in such a relationship that the behavior 
of each valve influences the other via the respective valve materials at 
least when the passage is closed. 
Accordingly, in cases where the vehicle jolts violently so that a violent 
wave motion is generated in the liquid fuel inside the fuel tank, the 
fluctuation of the liquid fuel would cause the auxiliary valve to move 
even if the main valve is deliberately close; and as a result, the 
behavior of the auxiliary valve causes the main valve to move as well, 
thus opening the main valve and allowing the flow of liquid fuel into the 
canister. 
Originally, the reason for installing both a main valve and an auxiliary 
valve is to insure an accurate and reliable function of the main valve, 
which has an opening area large enough to allow vaporized fuel to pass 
through in conformity with fluctuations in the liquid fuel by installing 
two types of large and small valves in parallel in the passage that 
connects the fuel tank and the canister. Accordingly, in view of the above 
problem, it was decided to re-examine the relationship between the main 
valve and the auxiliary valve from the standpoint of causing the auxiliary 
valve to function not as a mere driving force or pressure relief hole for 
the main valve but rather as a small-diameter valve in its own right, so 
that the main valve and auxiliary valve respectively prevent the inflow of 
the liquid fuel into the canister. 
The results of the examination led to the creation of a fuel interruption 
means for a fuel tank in which: an auxiliary valve which is caused to move 
upward and downward by a float and a main valve which moves upward and 
downward in linkage with the upward and downward movement of the auxiliary 
valve are accommodated inside a casing which is provided at the end of a 
passage connecting the fuel tank and canister and allows the inflow and 
outflow of liquid fuel; a linking projection is provided on one of the 
valves, that is, either on the auxiliary valve or on the main valve, so 
that the main valve moves in linkage with the auxiliary valve in such an 
action that is either advanced or retarded within a predetermined range, 
and a projection regulating groove into which the linking projection is 
inserted so that the projection can change the position within such a 
predetermined range is formed in the other valve; and an auxiliary valve 
seat and a main valve seat are independently opened in the upper portion 
of the casing or in the end of the passage. 
In the present invention, the auxiliary valve and the main valve can make 
relative action changes within a predetermined range, ordinarily in the 
vertical direction, and also the auxiliary valve seat and main valve seat 
are independently provided, so that even if one valve is opened, the other 
valve will not open; more specifically, even if the auxiliary valve is 
opened, the main valve will not open. When it is necessary to avoid the 
relative action changes of the main valve and auxiliary valve, it is not 
very effective to push the main valve upward by the auxiliary valve that 
is caused to rise by a float as in conventional means. 
Accordingly, so as to drive the main valve upward by a spring and to push 
the main down under ordinary conditions, it is preferable to design so 
that a linking projection or projection regulating groove provided on the 
main valve is engaged with a projection regulating groove or linking 
projection provided on the auxiliary valve, the float causes the auxiliary 
valve to rise as a result of an increase in the amount of liquid fuel 
inside the fuel tank or as a result of a wave action generated in the 
liquid level inside the fuel tank so as to release the engagement between 
the linking projection and projection regulating groove, thus raising the 
main valve by the spring. 
For example, in one fuel interruption means: the main valve is a flattened 
hollow cylinder that has an annular upper end surface and an opened lower 
end surface (in other words, the main valve is formed as a doughnut-shaped 
valve which has a circular gap at the center), and a projection regulating 
groove which runs in the opening and closing direction is formed on the 
side surface of the main valve; the auxiliary valve has a valve which is 
formed on the upper surface of a solid cylinder and projects out loosely 
from the center of the upper surface of the main valve, and it also has a 
linking projection which is formed on the side surface of the auxiliary 
valve so as to be inserted into the projection regulating groove of the 
main valve; and an annular main valve seat and a circular auxiliary valve 
are formed concentrically at the end of a passage which opens into a 
casing that accommodates the main valve and auxiliary valve. 
The main valve is driven upward by a spring which is installed between the 
upper surface of the valve body and the casing. Under ordinary conditions, 
in which the amount of liquid fuel inside the fuel tank is small, the 
linking projection of the auxiliary valve engages with the lower end of 
the projection regulating groove formed in the side surface of the valve 
body of the main valve, so that the main valve is pushed downward by the 
load of the auxiliary valve. The auxiliary valve is lifted by the buoyancy 
of the float so that the load is lessened or eliminated, and the main 
valve itself is caused to rise so as to close the main valve seat, and the 
auxiliary valve closes the auxiliary valve seat by way of the buoyancy of 
the float. Each of the main valve and auxiliary valve may be provided with 
a sealing if necessary. 
The main valve and the auxiliary valve are linked together by inserting the 
linking projection into the projection regulating groove so that a 
variation in the relative attitudes of the valves is permitted within 
certain limitations. More specifically, in cases where the auxiliary valve 
moves upward or downward or to the left or right within the range defined 
by the projection regulating groove, the linking projection does not press 
against the inside edge of the projection regulating groove, and 
therefore, the main valve is not affected by the behavior of the auxiliary 
valve Furthermore, the main valve is driven upward by the spring, and the 
load of the auxiliary valve is applied to the projection regulating groove 
by the linking projection; and when, in such a case, the main valve and 
auxiliary valve are both raised, the linking projection is positioned in 
the middle of the projection regulating groove, thus allowing the 
auxiliary valve to act independently of the main valve within the range 
defined by the projection regulating groove. 
Furthermore, the main valve seat and the auxiliary valve seat are provided 
completely independently of each other, and the auxiliary valve is raised 
by a float and the main valve is raised by a spring; accordingly, there is 
no contact between the two valves, the behavior of the auxiliary valve has 
no effect on the main valve. In other words, when the main valve and the 
auxiliary valve are both closed, the linking projection stays at near the 
intermediate position of the projection regulating groove, and the main 
valve and the auxiliary valve completely independently close the main 
valve seat and the auxiliary valve seat with a space in between; 
accordingly, one does not affect the other. The range in which the action 
of the auxiliary valve does not effect the main valve is determined by the 
positional relationship between the main valve seat and auxiliary valve 
seat, the respective shapes and sizes of the valve seats, the relationship 
between the linking projection and the projection regulating groove, and 
the shape and size of the projection regulating groove, etc. 
In the fuel interruption means described above, the behavior of the 
auxiliary valve is prevented from influencing the main valve; and even if 
the auxiliary valve should open, the main valve remains closed. However, 
by forming the main valve or auxiliary valve with a more or less convex 
spherical surface shape, and by forming the main valve seat or auxiliary 
valve seat corresponding to the main valve or auxiliary valve so that the 
main valve seat or auxiliary valve seat has a more or less concave 
spherical surface shape which is in sliding contact with the main valve or 
auxiliary valve, it is possible, for example, to maintain the auxiliary 
valve in closed state even when the auxiliary valve has made its own 
behavior. Generally, the movement of main valves or auxiliary valves is a 
swinging movement which is caused by a liquid surface in which a wave 
action has been generated. Accordingly, by causing the main valve and the 
main valve seat or the auxiliary valve and the auxiliary valve seat to 
make sliding contact via the respective spherical surfaces, the main valve 
or auxiliary valve is allowed to pivot about the vicinity of each valve 
seat with the valves kept closed. 
In the fuel interruption means of the present invention, as described 
above, the main valve which has a large opening area and the auxiliary 
valve which has a small opening area are installed independently and in 
parallel. Accordingly, the auxiliary valve, which in conventional means 
functions only as a pressure relief hole for the main valve, is endowed 
with an intrinsic valve function as a fuel interruption means, that is, a 
function which prevents the inflow of liquid fuel into the canister, while 
at the same time any mutual influence resulting from mutual action of the 
main valve and auxiliary valve is eliminated. Such a blocking of mutual 
action is optimally accomplished by making the auxiliary be raised and 
lowered by a float and the main valve be raised and lowered by a spring so 
that the respective movement is independent from each other. However, even 
in the case wherein the auxiliary valve is in contact with the main valve 
and pushes the main valve upward, a blocking of mutual action can be 
accomplished by reducing the amount of the contact. 
Furthermore, by constructing the respective valves of the fuel interruption 
means of the present invention so that the valves have curved surfaces, it 
is possible to keep the valves closed even when the main valve or 
auxiliary valve behaves independently, thus preventing the inflow of 
liquid fuel into the canister with even greater reliability. Thus, the 
fuel interruption means that contains the main valve and auxiliary valve 
has a reliability of operation and can prevent the inflow of liquid fuel 
into the canister, which is the original object of the means, with greater 
assurance.

DETAILED DESCRIPTION OF THE INVENTION 
Embodiments of the present invention will be described below with reference 
to the attached drawings. FIG. 1 is a longitudinally sectional perspective 
view showing the fuel interruption means of the present invention. FIGS. 2 
through 4 are sectional views of the fuel interruption means of the 
present invention. In FIG. 1, a main valve 1 and an auxiliary valve 2 are 
both in an open state, and an end 3 of the passage leading to the canister 
(not shown in the figures) is opened so that vaporized fuel can escape 
into the canister (as indicated by the dotted arrows in FIG. 1). The 
casing 4 is provided with inlet/outlets 5 for liquid fuel in the side 
surfaces and bottom surface thereof so that the inflow and outflow (as 
indicated by the thick double-headed arrows in FIG. 1) of liquid fuel 
between the interior of the casing 4 and the interior of the fuel tank 6 
is assured. The fuel interruption means is constructed so that when liquid 
fuel fills the fuel tank or a wave action is generated in the surface of 
the liquid fuel 7 due to vibration of the vehicle, etc., the liquid fuel 7 
that flows into the casing 4 pushes the auxiliary valve 2 upward, thus 
reducing or eliminating the load of the auxiliary valve 2 applying onto 
the main valve 1, which is driven upward, so that the main valve 1 is 
allowed to rise. 
The main valve 1 has a valve body structure in which the center of the 
upper surface of a flattened hollow cylinder whose bottom surface is open 
is punched out in circular form, and an annular seal 8 is attached 
thereto; in addition, a projection regulating groove 10 which engages with 
a linking projection of the auxiliary valve 9 is formed on the side 
surface of the main valve 1. A projecting end 11 of the auxiliary valve 2 
protrudes in a loose state from the circular space located in the center 
of the seal 8. A main valve seat 12 located at the passage end 3 opens in 
circular form so as to correspond to the shape of the seal 8. In the main 
valve 1 of this example, a coil spring 14 (outside) is installed between 
the main valve 1 and the bottom surface of the casing 4 with the coil 
spring extending along a flange 13 formed on the outer circumference of 
the upper surface of the main valve 1, so that the main valve 1 is 
consistently urged upward. Ordinarily, however, the linking projection 9 
is engaged with the projection regulating groove 10 so that the load of 
the auxiliary valve 2 is applied to the main valve 1, thus pushing the 
main valve 1 downward and opening the valve. 
The auxiliary valve 2 is a thick cylinder which is obtained by hollowing 
out the interior of a float 16 which is an integral unit and accommodates 
a coil spring 15 inside thereof so that the coil spring 4 is installed 
between the auxiliary valve 2 and the bottom surface of the casing 4; and 
in addition, the auxiliary valve 2 has a valve having a smoothly curved 
projecting end 11 on the upper surface thereof. Furthermore, the auxiliary 
valve 2 is provided with the linking projection 9 which is formed on the 
side surface thereof so as to engage in sliding contact with the 
projection regulating groove 10. The coil spring 15 supplements the 
buoyancy of the float 16 against the load of the auxiliary valve 2 and 
acts to increase the response speed of the opening-and-closing action of 
the valve. In the present embodiment, the respective strengths of the coil 
springs 14 and 15 of the main valve 1 and of the auxiliary valve 2, 
respectively, are preferably selected and combined so that the main valve 
1 which is driven upward by the coil spring 14 is pushed downward by the 
auxiliary valve 2, thus opening the valve, separating the main valve 1 
from the auxiliary valve 2 in a floating state, so that the behaviors of 
the respective valves do not influence each other. Moreover, in order to 
insure the upward and downward motion which accompanies no shift in the 
horizontal direction, the auxiliary valve 2 is surrounded by a cylindrical 
guide 17 which is installed upright on the bottom surface of the casing 4. 
The operation of this embodiment will be described with reference to the 
state of the liquid fuel 7 inside the fuel tank 6. FIG. 2 is a 
perpendicular cross-sectional view of the fuel interruption means which 
shows the state in FIG. 1, that is, a state in which the amount of liquid 
fuel 7 in the fuel tank 6 is small so that there is no danger of the 
inflow of liquid fuel from the passage end 3 into the canister and in 
which both the main valve 1 and the auxiliary valve 2 are open. FIG. 3 is 
a perpendicular cross-sectional view showing a state in which the level of 
the liquid fuel 7 has risen from the state shown in FIG. 2, and the 
auxiliary valve 2 is lifted by the buoyancy of the float 16 with the main 
valve 1 opened by the downward pushing action of the linking projection 9 
so that only the main valve 1 is closed. FIG. 4 is a perpendicular 
cross-sectional view showing a state in which the auxiliary valve 2 has 
risen even further from the state shown in FIG. 3, so that the auxiliary 
valve 2 is closed also, thus closing off the passage end 3. FIG. 5 is a 
perpendicular cross-sectional view showing a state in which the auxiliary 
valve 2 in FIG. 4 has pivoted about the vicinity of the auxiliary valve 
seat 18 as a result of wave action generated in the surface of the liquid 
fuel 7. 
As described above, in a state in which the amount of liquid fuel 7 is 
small, the linking projection 9 applies the entire load of the auxiliary 
valve 2 to the lower end of the projection regulating groove 10 so that 
the main valve 1 is pushed downward though the main valve 1 is driven 
upward by the coil spring 14. As a result, both the main valve 1 and the 
auxiliary valve 2 are open as shown in FIG. 2. In this state, even if some 
wave action is generated in the surface of the liquid fuel 7, there is 
little danger that liquid fuel 7 will flow into the canister from the 
passage end 3. Accordingly, the valve is fully opened so that vaporized 
fuel inside the fuel tank 6 is sufficiently able to escape into the 
canister. 
When the amount of liquid fuel 7 is increased or when a wave action is 
generated in the surface of the liquid fuel 7 due to vibration, etc. of 
the vehicle, buoyancy constantly or instantaneously acts on the float 16 
of the auxiliary valve 2, so that the load of the auxiliary valve 2 
applied to the main valve 1 is either reduced or eliminated. As a result, 
the main valve 1 is pushed upward by the coil spring 14 so that the main 
valve seat 12 is closed as shown in FIG. 3. In order to close the main 
valve quickly in response to a wave action generated in the surface of the 
liquid fuel 7, it is necessary to appropriately set the relationship of 
the strengths and stroke values of the coil springs of the main valve and 
auxiliary valve, and the buoyancy of the float of the auxiliary valve, 
etc. In the present embodiment, the seal 8 of the main valve 1 completely 
closes off the main valve seat 12. 
When the amount of liquid fuel 7 is increased even further from the state 
described above, or when the wave action generated in the surface of the 
liquid fuel 7 becomes even more violent, the linking projection 9 of the 
auxiliary valve 2 rises in the projection regulating groove 10 as a result 
of the buoyancy of the float 16, thus the auxiliary valve seat 18 is 
closed. In this state, the passage end 3 is completely closed off, so that 
the fuel tank 6 and canister can be completely separated from each other. 
The smoothly curved projecting end 11 of the auxiliary valve 2 enters into 
the auxiliary valve seat 18, so that the auxiliary valve seat 18 is 
completely closed. 
Conventionally, there was such a danger that when wave action in the 
surface of the liquid fuel continues in the state shown in FIG. 5, though 
in this situation both the main valve and the auxiliary valve need to be 
closed, the float was moved by the motion of the liquid fuel, and the 
auxiliary valve pivoted about the vicinity of the auxiliary valve seat and 
opens, and furthermore, this action of the auxiliary valve was transmitted 
to the main valve so that even the main valve also opened. In the fuel 
interruption means of the present invention, on the other hand, as shown 
in FIG. 5, the main valve 1 and the auxiliary valve 2 are completely 
independent, so that the actions of the respective valves do not influence 
each other. Accordingly, even if the auxiliary valve 2 should pivot, the 
main valve 1 can be kept closed. In particular, the closure of the main 
valve 1 can be stably maintained by lifting the main valve 1 using the 
coil spring 14, thus pressing the seal 8 against the main valve seat 12, 
as in the present embodiment. 
In the present embodiment, the range in which the main valve can be 
separated from the pivoting auxiliary valve is determined by the size of 
the central space in the annular upper surface of the main valve and the 
engaging relationship of the projection regulating groove and linking 
projection, etc. If these values are large, the main valve can be isolated 
from a correspondingly large action of the auxiliary valve; in such a 
case, however, it is necessary to increase the size of the fuel 
interruption means or to reduce the area of the main valve seat or 
auxiliary valve seat. Accordingly, the dimensions above should be 
determined in accordance with the desired performance of the fuel 
interruption means and the expected motion of the liquid fuel. 
FIG. 6 is an enlarged view corresponding to the portion indicated by arrow 
A in FIG. 5 of the fuel interruption means in which the projecting end 11 
of the auxiliary valve is formed into substantially a convex spherical 
shape, and the corresponding auxiliary valve seat 18 is formed into 
substantially a concave spherical shape. As seen from FIG. 6, the 
formation of the projecting end 11 of the auxiliary valve into a roughly 
convex spherical shape and the formation of the auxiliary valve seat 18 
into a roughly concave spherical shape make it possible for the auxiliary 
valve 2 to pivot while the projecting end 11 is in a sliding contact with 
the auxiliary valve seat 18. As a result, the closure of the auxiliary 
valve 2 can be maintained so that the inflow of fuel into the canister is 
even more securely prevented. This is also a result of allowing a certain 
behavior of the auxiliary valve which is obtained by separating the main 
valve and auxiliary valve. Thus, in the fuel interruption means of the 
present invention, reliability is greatly improved compared to 
conventional means wherein some pivoting motion of auxiliary valves occur 
even though an attempt is made to suppress the action of the auxiliary 
valves by force, and, as a result, a slight gap is formed which leads to 
the danger of fuel flowing into the canister. 
FIG. 7 is an enlarged view corresponding to the portion indicated by arrow 
A in FIG. 5 of a fuel interruption means that has the valve structure 
shown in FIG. 6, wherein a bowl-shaped curved supporting surface 19 is 
formed in the upper surface of the auxiliary valve 2, and a supporting 
projection 20 which projects downward from the main valve 1 makes sliding 
contact with this curved supporting surface 19. In this fuel interruption 
means, the main valve is not pushed upward by a coil spring; instead, the 
auxiliary valve 2 pushes the main valve 1 upward via the curved supporting 
surface 19 and supporting projection 20. Accordingly, contact between the 
auxiliary valve 2 and the main valve 1 is necessary. However, as shown in 
FIG. 7, such a contact is limited to the small area of the curved 
supporting surface 19 and supporting projection 20; and in addition, the 
auxiliary valve 2 pivots while the supporting projection 20 makes a 
sliding contact with the curved supporting surface 19 so that the action 
of the auxiliary valve 2 has no effect on the main valve 1. As a result, 
the danger that the main valve will open as a result of the action of the 
auxiliary valve 2 is reduced though not to the same extent as in the 
previously described fuel interruption means in which the main valve 1 is 
pushed upward by the spring 14 (see FIG. 1). The structure in which the 
main valve is pushed upward by a coil spring is most desirable from the 
standpoint of isolating the actions of the main valve and auxiliary valve 
from each other in the present invention; however, since coil springs are 
expensive, a structure in which the main valve is pushed upward by the 
auxiliary valve as in a conventional means may be used in order to reduce 
costs. 
In the example shown in FIGS. 8 and 9, a plurality of vertical ribs 21 
(eight ribs in this case) are provided so as to project from the inside 
circumferential surface of the casing 4 in a radially oriented 
configuration. These ribs are used as a guide for the float instead of the 
cylinder which is the guide 17 for the float 16 in the example shown in 
FIG. 2; and the bottom surface of a coil spring 14 is placed on the upper 
ends of the vertical ribs 21. This structure makes it possible to 
accommodate a larger float 16 in a casing 4 of the same size, so that the 
opening-and-closing sensitivity of the valve is correspondingly increased. 
The example shown in Figures 10 through 12 is a fuel interruption means in 
which a large float 16 can be accommodated, filling the entire interior of 
the casing, even though the casing 4 has a smaller diameter. More 
specifically, FIG. 10 is a perpendicular cross-sectional view of a fuel 
interruption means in which the diameter of the casing 4 is reduced by 
installing a coil spring 14 between the shoulder 22 of the float 16 and 
the undersurface of the main valve 1, showing a perpendicular 
cross-sectional view in which both the main valve 1 and the auxiliary 
valve 2 are open. FIG. 11 illustrates a state in which the main valve 1 is 
closed and the auxiliary valve 2 is open, and FIG. 12 illustrates a state 
in which both the main valve 1 and the auxiliary valve 2 are closed. The 
movements of the main valve 1 and auxiliary valve 2 are exactly the same 
as in the first example shown in FIG. 1. 
By way of the present invention, a main valve and an auxiliary valve, which 
act accurately and reliably in response to fluctuations in the liquid 
fuel, function as respective independent valves, and the effect of pivot 
motion of the auxiliary valve to the main valve is suppressed so that the 
inflow of liquid fuel into the canister can be more or less completely 
prevented, thus improving the reliability of operation in a fuel 
interruption means. 
In structural terms, the main valve seat and auxiliary valve seat are 
separated, and the area of surface contact between the main valve and 
auxiliary valve is merely eliminated. Accordingly, there are relatively 
few differences between the fuel interruption means of the present 
invention and a conventional fuel interruption means. As a result, a 
production line for the fuel interruption means of the present invention 
can be obtained by slightly modifying a conventional production line; and 
the present invention has such an advantage that a product having an 
improved performance can be manufactured with a small plant investment. 
Furthermore, for the same reasons, manufacturing costs are relatively low.