Turbine and turbocharger

A turbine includes: a turbine impeller; a housing disposed so as to enclose the turbine impeller and including a scroll passage positioned on an outer circumferential side of the turbine impeller; a nozzle vane disposed inside an intermediate flow passage which is positioned, in an exhaust gas flow direction, on a downstream side of the scroll passage and on an upstream side of the turbine impeller; a plate disposed on a side of the intermediate flow passage with respect to an inner circumferential wall part of the housing, defining an inner circumferential boundary of the scroll passage, so as to face the intermediate flow passage such that a gap is formed between the plate and the inner circumferential wall part in an axial direction; and at least one guide vane disposed in the gap between the inner circumferential wall part and the plate in the axial direction. The at least one guide vane includes: a first end; and a second end disposed radially on an outer side of the first end and circumferentially downstream of the first end in the exhaust gas flow direction.

TECHNICAL FIELD

The present disclosure relates to a turbine and a turbocharger.

BACKGROUND

A turbocharger including nozzle vanes for adjusting flow of exhaust gas flowing into a turbine impeller has been used.

For example, Patent Document 1 discloses a turbocharger using a radial turbine including a plurality of nozzle vanes arranged in a circumferential direction inside a scroll through which a working gas (exhaust gas) passes. The nozzle vane used in this turbocharger has opposite nozzle-vane widthwise ends which protrude toward a pressure surface side more prominently at a leading edge and a trailing edge than at a middle portion. With such a shape of the nozzle vane, collision loss of the working gas is reduced on the leading edge side, and the flow of working gas flowing out from the nozzle is uniformized on the trailing edge side, so as to reduce the secondary flow loss at the nozzle vane and the blade.

CITATION LIST

Patent Literature

SUMMARY

Problems to be Solved

As a result of intensive studies by the present inventors, it has been found that, during operation of a turbocharger including nozzle vanes, turbulence of flow increases in a gap formed between a housing wall surface forming a scroll passage and a plate forming an intermediate flow passage in which the nozzle vanes are arranged, resulting in an increase in heat transfer rate between a fluid and the housing or a decrease in total pressure. Such increase in heat transfer rate and decrease in total pressure indicate the occurrence of heat loss and pressure loss in the turbine. It is thus desired to reduce such heat loss and pressure loss.

In view of the above, an object of at least one embodiment of the present invention is to provide a turbine and a turbocharger whereby it is possible to reduce heat loss and pressure loss due to turbulence of flow.

Solution to the Problems

(1) A turbine according to at least one embodiment of the present invention comprises: a turbine impeller; a housing disposed so as to enclose the turbine impeller and including a scroll passage positioned on an outer circumferential side of the turbine impeller; a nozzle vane disposed inside an intermediate flow passage which is positioned, in an exhaust gas flow direction, on a downstream side of the scroll passage and on an upstream side of the turbine impeller; a plate disposed on a side of the intermediate flow passage with respect to an inner circumferential wall part of the housing, which defines an inner circumferential boundary of the scroll passage, so as to face the intermediate flow passage such that a gap is formed between the plate and the inner circumferential wall part in an axial direction; and at least one guide vane disposed in the gap between the inner circumferential wall part and the plate in the axial direction. The at least one guide vane includes: a first end; and a second end disposed radially on an outer side of the first end and circumferentially downstream of the first end in the exhaust gas flow direction.

With the above configuration (1), since the guide vane including the first end and the second end disposed radially on the outer side of the first end and circumferentially downstream of the first end in the exhaust gas flow direction is disposed in the gap between the inner circumferential wall part and the plate, the flow from the scroll passage into the gap is guided by the guide vane radially outward and circumferentially downstream in the exhaust gas flow direction. Thus, exhaust gas flowing through the scroll passage is prevented from flowing into the gap, so that it is possible to prevent turbulence of flow which may occur by inflow of exhaust gas to the gap. Consequently, it is possible to reduce heat loss or pressure loss due to turbulence of flow in the turbine.

(2) In some embodiments, in the above configuration (1), the at least one guide vane is provided so as to protrude in the axial direction from a surface of at least one of the inner circumferential wall part or the plate toward the other of the inner circumferential wall part or the plate.

With the above configuration (2), since the guide vane is provided with an axial height between the plate and the inner circumferential wall part forming the gap in the axial direction, it is possible to effectively suppress inflow of exhaust gas from the scroll passage to the gap.

(3) In some embodiments, in the above configuration (1) or (2), the first end is positioned in an inner circumferential end portion of the gap, and the second end is positioned in an outer circumferential end portion of the gap.

With the above configuration (3), since the first end of the guide vane is positioned in the inner circumferential end portion of the gap, when exhaust gas flows into an inner circumferential region of the gap, the exhaust gas is easily guided to the outer circumferential side along the guide vane. Further, since the second end of the guide vane is positioned in the outer circumferential end portion of the gap, the exhaust gas is guided so as to flow on the outer circumferential side with respect to the gap. Thus, it is possible to effectively suppress inflow of exhaust gas from the scroll passage to the gap.

(4) In some embodiments, in any one of the above configurations (1) to (3), the at least one guide vane has a curved shape convex toward the scroll passage in a cross-section perpendicular to the axial direction.

With the above configuration (4), since the guide vane has a curved shape convex toward the scroll passage in a cross-section perpendicular to the axial direction, exhaust gas flowing from the scroll passage is prevented from remaining in the gap, and the exhaust gas is smoothly guided radially outward and downstream along the guide vane. Thus, it is possible to effectively suppress inflow of exhaust gas from the scroll passage to the gap.

(5) In some embodiments, in any one of the above configurations (1) to (4), the at least one guide vane includes a plurality of guide vanes arranged in a circumferential direction, and at least one of the plurality of guide vanes has a length in the circumferential direction which is larger than a length in the circumferential direction of a guide vane disposed on a circumferentially upstream side in the exhaust gas flow direction with respect to the at least one guide vane.

In a typical turbine, the length of the above-described clearance in the radial direction increases with distance circumferentially downstream in the exhaust gas flow direction. With the above configuration (5), since the guide vanes on more circumferentially downstream side in the exhaust gas flow direction have larger length in the circumferential direction in accordance with the increase in length of the clearance in the radial direction, it is possible to effectively suppress inflow of exhaust gas from the scroll passage to the gap by the guide vanes disposed in respective circumferential-directional regions.

(6) In some embodiments, in any one of the above configurations (1) to (5), the at least one guide vane has an axial height of not less than 30% of an axial height of the gap.

With the above configuration (6), since the guide vane has an axial height of not less than 30% of an axial height of the gap, it is possible to effectively suppress inflow of exhaust gas from the scroll passage to the gap.

(7) In some embodiments, in any one of the above configurations (1) to (6), in a cross-section perpendicular to the axial direction, when a rotational axis of the turbine is taken as a center, an angle at a position of a tongue of the scroll passage is defined as 0 degree, and the exhaust gas flow direction in a circumferential direction is taken as a positive angular direction, the at least one guide vane is positioned within a range of at least 220 degrees and at most 360 degrees.

According to findings of the present inventors, it has been found that in the vicinity of the outlet of the scroll passage, turbulence of flow particularly increases, so that the heat transfer rate between the fluid and the housing tends to increase, and the total pressure in the housing tends to decrease.

In this regard, with the above configuration (7), since the guide vane is provided within the range in which the above-described angle in the circumferential direction is at least 220 degrees and at most 360 degrees (i.e., in the vicinity of the outlet of the scroll passage), in this circumferential region, the exhaust gas is guided by the guide vane so as to flow radially outward and downstream. Thus, in this circumferential region, exhaust gas flowing through the scroll passage is prevented from entering the gap. Consequently, it is possible to effectively reduce heat loss or pressure loss in the turbine.

(8) A turbocharger according to at least one embodiment of the present invention comprises a turbine described in any one of the above (1) to (7) and a compressor configured to be driven by the turbine.

With the above configuration (8), since the guide vane including the first end and the second end disposed radially on the outer side of the first end and circumferentially downstream of the first end in the exhaust gas flow direction is disposed in the gap between the inner circumferential wall part and the nozzle plate, the flow from the scroll passage into the gap is guided by the guide vane radially outward and circumferentially downstream in the exhaust gas flow direction. Thus, exhaust gas flowing through the scroll passage is prevented from flowing into the gap, so that it is possible to prevent turbulence of flow which may occur by inflow of exhaust gas to the gap. Consequently, it is possible to reduce heat loss or pressure loss due to turbulence of flow in the turbine.

Advantageous Effects

At least one embodiment of the present invention provides a turbine and a turbocharger whereby it is possible to reduce heat loss of pressure loss due to turbulence of flow.

DETAILED DESCRIPTION

First, an overall configuration of a turbocharger according to some embodiments will be described.

FIG. 1is a schematic cross-sectional view of a turbocharger according to an embodiment, taken along the rotational axis O. As shown inFIG. 1, the turbocharger100includes a turbine1having a turbine impeller4configured to be rotationally driven by exhaust gas from an engine (not shown) and a compressor (not shown) connected to the turbine1via a rotational shaft2rotatably supported by a bearing3. The compressor is configured to be coaxially driven by rotation of the turbine impeller4to compress intake air flowing into the engine.

The turbine1shown inFIG. 1is a radial turbine in which exhaust gas as a working fluid enters in the radial direction. However, the operation system of the turbine1is not limited thereto. For example, in some embodiments, the turbine1may be a mixed flow turbine in which an entering working fluid has velocity components in the radial direction and the axial direction.

The turbine impeller4is housed in a housing6disposed so as to enclose the turbine impeller4, and includes a hub17connected to the rotational shaft2and a plurality of blades5arranged in the circumferential direction on an outer circumferential surface of the hub17.

The housing6includes a scroll passage8positioned on an outer circumferential side of the turbine impeller4and an inner circumferential wall part22defining an inner circumferential boundary9of the scroll passage8. As shown inFIG. 1, the housing6may include a turbine housing6awhich is a portion housing the turbine impeller4and a bearing housing6bwhich is a portion housing the bearing3.

On the outer circumferential side of the turbine impeller4, an intermediate flow passage10through which exhaust gas flows from the scroll passage8into the turbine impeller4is formed. In other words, the intermediate flow passage10is positioned, in the exhaust gas flow direction, downstream of the scroll passage8and upstream of the turbine impeller4.

Inside the intermediate flow passage10, a plurality of nozzle vanes14for adjusting exhaust gas flow entering the turbine impeller4is arranged in the circumferential direction.

The intermediate flow passage10is formed between a nozzle mount16to which the nozzle vanes14are mounted and a nozzle plate12(plate in the present invention) disposed on the opposite side across the nozzle vanes14in the axial direction of the turbine1(hereinafter also simply referred to as “axial direction”). The nozzle mount16is fixed to the bearing housing6bwith a bolt (not shown) or the like. Between the nozzle mount16and the nozzle plate12, for example, a pillar material (not shown) extending in the axial direction is disposed. The pillar material supports the nozzle plate12spaced from the nozzle mount16in the axial direction. Between the nozzle plate12and the inner circumferential wall part22of the housing6, an annular seal member26is disposed so as to suppress leakage of exhaust gas from the scroll passage8to a space downstream of the turbine impeller4(i.e., leakage of exhaust gas not via the turbine impeller4).

The nozzle vane14includes an airfoil portion extending between the nozzle mount16and the nozzle plate12.

Each of the plurality of nozzle vanes14is connected to one end of a lever plate18via a nozzle shaft20. Further, the other end of the lever plate18is connected to a disc-shaped drive ring19.

The drive ring19is driven by an actuator (not shown) so as to be rotatable around the rotational axis O. When the drive ring19rotates, each lever plate18rotates. Accordingly, the nozzle shaft20rotates around a rotation axis Q along the axial direction, so that the opening degree (blade angle) of the nozzle vane14is changed via the nozzle shaft20.

In the turbine1of the turbocharger100having this configuration, exhaust gas entering from an inlet flow passage30(seeFIG. 2) into the scroll passage8(see arrow G ofFIGS. 1 and 2) flows into the intermediate flow passage10between the nozzle mount16and the nozzle plate12, in which the nozzle vanes14control the flow direction of the gas so as to flow into a central portion of the housing6. Then, after acting on the turbine impeller4, the exhaust gas is discharged to the outside from an exhaust outlet7.

Further, the exhaust gas passage area inside the housing6may be changed by appropriately changing the opening degree of the nozzle vanes14in accordance with exhaust gas amount entering the turbine1to adjust the flow velocity of exhaust gas into the turbine impeller4. Thus, it is possible to obtain excellent turbine efficiency.

Hereinafter, characteristics of the turbine1according to some embodiments will be described.

FIG. 2is a partial enlarged view ofFIG. 1.FIG. 3is a schematic cross-sectional view of the turbine1shown inFIG. 1, perpendicular to the rotational axis O.FIG. 3is a diagram of the turbine1viewed in the direction of the arrow B shown inFIG. 1, and shows a cross-section of a portion including the scroll passage8of the housing6, a nozzle plate12, nozzle vanes14, and guide vanes42(42A to42E) described later, but some components such as the turbine impeller4are not depicted for simplification of description.

As shown inFIG. 2, the nozzle plate12(plate) is disposed on a side of the intermediate flow passage10with respect to the inner circumferential wall part22of the housing6so as to face the intermediate flow passage10such that a gap24is formed between the nozzle plate12and the inner circumferential wall part22in the axial direction. Further, the turbine1include at least one guide vane disposed in the gap24. In the illustrated embodiment, as shown inFIG. 3, a plurality of guide vanes42(42A to42E) is arranged along the circumferential direction in the gap24of the turbine1. Hereinafter, the plurality of guide vanes42A to42E is also collectively referred to as the guide vane42.

As shown inFIGS. 2 and 3, each guide vane42includes a first end44and a second end46disposed radially on an outer side of the first end44and circumferentially downstream of the first end44in the exhaust gas flow direction.

According to the above embodiment, since the guide vane42is disposed in the gap24between the inner circumferential wall part22and the nozzle plate12, the flow of exhaust gas from the scroll passage8into the gap24is guided by the guide vane42radially outward and circumferentially downstream in the exhaust gas flow direction. Thus, exhaust gas flowing through the scroll passage8is prevented from flowing into the gap24, so that it is possible to prevent turbulence of flow which may occur by inflow of exhaust gas to the gap24. Consequently, it is possible to reduce heat loss or pressure loss due to turbulence of flow in the turbine1.

In some embodiments, the guide vane42is provided so as to protrude in the axial direction from a surface of at least one of the inner circumferential wall part22or the nozzle plate12toward the other of the inner circumferential wall part22or the nozzle plate12.

For example, in the exemplary embodiment shown inFIG. 2, the gap24is formed between a surface23of the inner circumferential wall part22along a direction perpendicular to the rotational axis O and one of opposite surfaces11,13of the nozzle plate12, namely, the surface11facing the surface23of the inner circumferential wall part22. Further, the guide vane42is provided so as to protrude in the axial direction from the surface11of the nozzle plate12toward the inner circumferential wall part22.

In some embodiments, the guide vane42may be provided so as to protrude in the axial direction from the surface23of the inner circumferential wall part22toward the nozzle plate12. Alternatively, in some embodiments, the guide vane42may be provided so as to protrude in the axial direction from the surface23of the inner circumferential wall part22and the surface11of the nozzle plate12.

In this case, since the guide vane42is provided with an axial height between the nozzle plate12and the inner circumferential wall part22forming the gap24in the axial direction, it is possible to effectively suppress inflow of exhaust gas from the scroll passage8to the gap24.

The guide vane42may have an axial height H (seeFIG. 2) of not less than 30% of an axial height D of the gap24.

Thus, since the guide vane42has an axial height H of not less than 30% of an axial height D of the gap24, it is possible to effectively suppress inflow of exhaust gas from the scroll passage8to the gap24.

In some embodiments, the first end44of the guide vane42is positioned in an inner circumferential end portion24aof the gap24, and the second end46of the guide vane42is positioned in an outer circumferential end portion24bof the gap24.

For example, in the case of the illustrated embodiment, as shown inFIG. 2, the gap24is formed in a radial-directional region corresponding to the range of extension of the inner circumferential wall part22in the radial direction. Accordingly, as shown inFIGS. 2 and 3, the inner circumferential end portion24aof the gap24is a portion of the gap24including the position of an inner circumferential end23aof the surface23of the inner circumferential wall part22in the radial direction, and the outer circumferential end portion24bof the gap24is a portion of the gap24including the position of an outer circumferential end23bof the surface23of the inner circumferential wall part22in the radial direction.

Thus, since the first end44of the guide vane42is positioned in the inner circumferential end portion24aof the gap24, when exhaust gas flows into an inner circumferential region of the gap24, the exhaust gas is easily guided to the outer circumferential side along the guide vane42. Further, since the second end46of the guide vane42is positioned in the outer circumferential end portion24bof the gap24, the exhaust gas is guided so as to flow on the outer circumferential side with respect to the gap24. Thus, it is possible to effectively suppress inflow of exhaust gas from the scroll passage8to the gap24.

In some embodiments, the guide vane42has a curved shape convex toward the scroll passage8in a cross-section perpendicular to the axial direction. In other words, in some embodiments, the guide vane42has a concave curved shape in a cross-section perpendicular to the axial direction, when viewed from the rotational axis O to the radially outer side.

For example, in the illustrated embodiment, as shown inFIG. 3, each of the guide vanes42A to42E has a curved shape convex toward the scroll passage8.

Thus, since the guide vane42has a curved shape convex toward the scroll passage8in a cross-section perpendicular to the axial direction, exhaust gas flowing from the scroll passage8is prevented from remaining in the gap24, and the exhaust gas is smoothly guided radially outward and downstream along the guide vane42. Thus, it is possible to effectively suppress inflow of exhaust gas from the scroll passage8to the gap24.

In some embodiments, at least one guide vane42of the plurality of guide vanes42has a length in the circumferential direction which is larger than a length in the circumferential direction of a guide vane42disposed on the circumferentially upstream side in the exhaust gas flow direction with respect to the at least one guide vane42.

For example, in the turbine1shown inFIG. 3, among the plurality of guide vanes42A to42E, the guide vane42E has a length LEin the circumferential direction which is larger than lengths LAto LDin the circumferential direction of the guide vanes42A to42D disposed upstream of the guide vane42E. In the turbine1shown inFIG. 3, the guide vanes42A to42E positioned more downstream in the exhaust gas flow direction have larger length in the circumferential direction. In other words, the lengths LAto LEof the guide vanes42A to42E in the circumferential direction satisfy LA<LB<LC<LD<LE.

In a typical turbine, the length of the gap24in the radial direction increases with distance circumferentially downstream in the exhaust gas flow direction.

For example, in the exemplary embodiment shown inFIG. 3, the position of the inner circumferential end23aof the surface23of the inner circumferential wall part22in the radial direction does not significantly change over the entire circumference, whereas the position of the outer circumferential end23bin the radial direction changes outward with distance circumferentially downstream in the exhaust gas flow direction. Accordingly, in the embodiment shown inFIG. 3, the length of the gap24in the radial direction increases with distance circumferentially downstream in the exhaust gas flow direction.

In this regard, according to the above embodiment, the guide vanes42on more circumferentially downstream side in the exhaust gas flow direction have larger length in the circumferential direction in accordance with the increase in length of the gap24in the radial direction. Thus, it is possible to effectively suppress inflow of exhaust gas from the scroll passage8to the gap24by the guide vanes42disposed in respective circumferential-directional regions.

In some embodiments, in a cross-section perpendicular to the axial direction, when the rotational axis O of the turbine1is taken as a center, an angle at a position of a scroll tongue32is defined as 0 degree (seeFIG. 3), and the exhaust gas flow direction in the circumferential direction is taken as a positive angular direction, at least one guide vane42is positioned within a range of at least 220 degrees and at most 360 degrees. The range R1shown by the hatched area inFIG. 3represents this angular range (at least 220 degrees and at most 360 degrees), and the angle Φ represents an example of angle within this range. The scroll tongue32is a connection portion between the start and end of a scroll part of the housing6forming the scroll passage8.

For example, in the exemplary embodiment shown inFIG. 3, among the plurality of guide vanes42A to42E, the guide vanes42D,42E are positioned within the angular range R1.

According to findings of the present inventors, it has been found that in the vicinity of the outlet of the scroll passage8, turbulence of flow particularly increases, so that the heat transfer rate between the fluid (exhaust gas) and the housing6tends to increase, and the total pressure in the housing6tends to decrease.

In this regard, according to the above embodiment, since at least one guide vane42is provided within the range R1in which the above-described angle in the circumferential direction is at least 220 degrees and at most 360 degrees (i.e., in the vicinity of the outlet of the scroll passage8), in this circumferential region, the exhaust gas is guided by the guide vane42so as to flow radially outward and downstream. Thus, in this circumferential region, exhaust gas flowing through the scroll passage8is prevented from entering the gap24. Consequently, it is possible to effectively reduce heat loss or pressure loss in the turbine1.

FIGS. 4A to 4Fare schematic views of the guide vane42according to an embodiment in a cross-section including the circumferential direction and the axial direction.

The shape of the guide vane42in a cross-section including the circumferential direction and the axial direction is not limited to a particular shape, and may have various shape, for example, as shown inFIGS. 4A to 4F.

For example, in the embodiment shown inFIG. 4A, one end of the guide vane42in the axial direction is connected to the surface23of the inner circumferential wall part22, and the other end is connected to the surface11of the nozzle plate12. In other words, the guide vane42is provided so as to protrude in the axial direction from the surface23of the inner circumferential wall part22and the surface11of the nozzle plate12. In this embodiment, the guide vane42has an axial height H equal to an axial height D of the gap24.

Alternatively, in the embodiments shown inFIGS. 4B to 4F, one end of the guide vane42in the axial direction is connected to the surface11of the nozzle plate12, and the guide vane42is provided so as to protrude in the axial direction from the surface11of the nozzle plate12toward the inner circumferential wall part22. In these embodiments, the guide vane42has an axial height H smaller than an axial height D of the gap24.

Further alternatively, although not depicted, in some embodiments, one end of the guide vane42in the axial direction may be connected to the surface23of the inner circumferential wall part22, and the guide vane42may be provided so as to protrude in the axial direction from the surface23of the inner circumferential wall part22toward the nozzle plate12. In this case, the guide vane42has an axial height H smaller than an axial height D of the gap24.

In the embodiments shown inFIGS. 4A to 4D, the thickness of the guide vane42in the circumferential direction is constant at t1in the axial direction.

In some embodiments, as shown inFIGS. 4E and 4F, the thickness of the guide vane42in the circumferential direction may vary with position in the axial direction. For example, in the embodiments shown inFIGS. 4E, 4F, the guide vane42has a thickness t2in the circumferential direction at the end adjacent to the nozzle plate12, and the thickness of the guide vane42decreases as the inner circumferential wall part22gets closer in the axial direction, and the thickness is minimum at the end adjacent to the inner circumferential wall part22. More specifically, the thickness of the guide vane42at the end adjacent to the inner circumferential wall part22is zero in the embodiment shown inFIG. 4E, and t3(t3<t2) in the embodiment shown inFIG. 4F.

The shape of a pair of side surfaces48,49(seeFIGS. 4A to 4F) of the guide vane42extending along the axial direction may include a straight line or may include a curved line in a cross-section including the circumferential direction and the axial direction.

In some embodiments, for example as shown inFIGS. 4A, 4B, and 4E, at least one of the pair of side surfaces48,49may include a straight line extending along the axial direction.

In some embodiments, for example as shown inFIGS. 4D and 4E, at least one of the pair of side surfaces48,49may include a straight line extending obliquely with respect to the axial direction. In the exemplary embodiments shown inFIGS. 4D and 4E, at least one of the pair of side surfaces48,49includes a straight line extending obliquely toward the circumferentially upstream side in the exhaust gas flow direction from the nozzle plate12to the inner circumferential wall part22in the axial direction. In other embodiments, at least one of the pair of side surfaces48,49may include straight line extending obliquely toward the circumferentially downstream side in the exhaust gas flow direction from the nozzle plate12to the inner circumferential wall part22in the axial direction.

In some embodiments, for example as shown inFIGS. 4A, 4B, and 4D, the pair of side surfaces48,49may include straight lines substantially parallel to each other.

In some embodiments, for example as shown inFIGS. 4C and 4F, at least one of the pair of side surfaces48,49may include a curved line convex toward the circumferentially upstream side or downstream side in the exhaust gas flow direction. In the exemplary embodiment shown inFIG. 4C, each of the side surfaces48,49includes a curved line convex toward the circumferentially upstream side in the exhaust gas flow direction. In the exemplary embodiment shown inFIG. 4F, among the pair of side surfaces48,49, the side surface48, which is positioned on the circumferentially downstream side, includes a curved line convex toward the circumferentially upstream side in the exhaust gas flow direction, and the side surface49includes a straight line along the axial direction.

Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented.

REFERENCE SIGNS LIST