EXHAUST AFTERTREATEMENT SYSTEM INJECTOR AND CONTROL

Abstract

In one aspect, an injector assembly for injecting a fluid into an exhaust aftertreatment system is provided. The injector assembly includes a housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice and configured to supply the fluid to the outlet orifice, and a valve having a longitudinal axis. The valve is oriented within the passageway and configured to translate along the longitudinal axis between a first position and a second position. A downstream end of the valve includes a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing inner wall is oriented at a second angle with respect to the longitudinal axis.

Description

FIELD OF THE INVENTION

This disclosure generally relates to exhaust treatment systems for internal combustion engines and, more particularly, to injectors for injecting fluid into an exhaust gas flow in an exhaust treatment system.

BACKGROUND

A diesel engine offers good fuel economy and low emission of hydrocarbons (HCs) and carbon monoxide (CO). A mixture of air and fuel in a combustion chamber is compressed to an extremely high pressure, causing a temperature to increase until an auto-ignition temperature of the fuel is reached. A ratio of the air to fuel for the diesel engine is much leaner (more air per unit of the fuel) than it is for a gasoline engine, and a larger amount of air promotes more complete combustion and better efficiency of the fuel. As a result, emission of HCs and CO is lower for the diesel engine than it is for a gasoline engine. However, with higher pressures and temperatures in the diesel engine, emission of nitrogen oxides (NOx) tend to be higher because high temperatures of the combustion chamber cause oxygen and nitrogen in intake air to combine and form the NOx.

Lower emission of NOx can be accomplished by manipulating operating characteristics of the engine. For example, along with reducing output of power and/or temperature of intake, coolant, and/or combustion, electronic controls and the injector are designed to deliver the fuel at a combination of pressure and timing of the injector and location of spray that allows the engine to burn the fuel efficiently without causing spikes in the temperature that increase the emission of the NOx.

Additionally, exhaust aftertreatment systems may be implemented downstream of the engine. Typically, catalysts are used to treat exhaust gas from the engine and convert pollutants such as carbon monoxide, HCs, and NOx into harmless gases. In particular, to reduce NOx emissions, the aftertreatment systems may employ a selective catalytic reduction (SCR) device that reduces NOx through reduction of chemicals. SCR catalysts are currently used in diesel aftertreatment systems. The SCR device is typically fluidly connected to and positioned downstream of a diesel oxidation catalyst (DOC) with a particulate filter (PF) (e.g., a diesel PF) provided between the SCR and DOC. However, the aftertreatment systems may have other relative arrangements of the DOC, PF, and SCR. For example, the aftertreatment system may have a DOC/SCR/PF arrangement, or a single-catalyst combined PF and SCR. The systems may include a reductant dosing system such as a diesel exhaust fluid (DEF) dosing system provided upstream of the SCR. The dosing system injects a reductant such as anhydrous ammonia (NH₃), aqueous NH₃, and/or a precursor that is convertible to NH₃ (e.g., urea ammonia or urea CO(NH₂)₂) into a flow of exhaust gas from the engine.

The DEF is a percentage of urea in water by weight and stored in a container such as a tank or removable and/or refillable cartridge. The DEF is pumped from the container and sprayed through an atomizing nozzle of the injector into the exhaust stream. Complete mixing of the urea or ammonia with the exhaust and uniform distribution of the flow facilitate high NOx reductions.

A urea-based SCR system may utilize gaseous NH₃ to reduce the NOx. During thermolysis, heat of the gas breaks the urea down into NH₃ and hydrocyanic acid (HCNO). The NH₃ and HCNO then enter the SCR, where the NH₃ is absorbed and the HCNO is further decomposed through hydrolysis into NH₃. When the NH₃ is absorbed, it reacts with the NOx and O₂ to produce water and nitrogen. An amount of NH₃ injected into the exhaust stream is an operating parameter. A required ratio of ammonia to NOx is typically stoichiometric and must be maintained to assure high levels of NOx reduction.

In some known DEF dosing systems, the size of a nozzle opening of the injector is generally constant, which may result in limited volumetric distribution of the reductant. Further, a mass flow of the exhaust gas may impact the spray of the urea and thus its distribution within the exhaust aftertreatment system.

Accordingly, it is desirable to provide improved SCR efficiency and reduce tailpipe NO_x. More specifically, it is desirable to provide a DEF injector and dosing control to generate uniform area distribution and improved reductant-exhaust mixing.

SUMMARY OF THE INVENTION

In one aspect, an injector assembly for injecting a fluid into an exhaust aftertreatment system is provided. The injector assembly includes a housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice and configured to supply the fluid to the outlet orifice, and a valve having a longitudinal axis. The valve is oriented within the passageway and configured to translate along the longitudinal axis between a first position and a second position. A downstream end of the valve includes a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing inner wall is oriented at a second angle with respect to the longitudinal axis.

In another aspect, an exhaust aftertreatment system for an internal combustion engine is provided. The system includes an exhaust gas conduit configured to receive exhaust gas from the internal combustion engine, a selective catalytic reduction device disposed within the exhaust gas conduit and configured to receive the exhaust gas, and an injector assembly coupled to the exhaust conduit and configured to inject a fluid into the exhaust gas. The injector assembly includes a housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice and configured to supply the fluid to the outlet orifice, and a valve having a longitudinal axis. The valve is oriented within the passageway and configured to translate along the longitudinal axis between a first position and a second position. A downstream end of the valve includes a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing inner wall is oriented at a second angle with respect to the longitudinal axis.

In yet another aspect, a method of dosing a fluid into an exhaust aftertreatment system configured to receive exhaust gas from an internal combustion engine is provided. The method includes providing an injector assembly having a housing and a valve translatable along a longitudinal axis within the housing, the valve having a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice, and the inner wall oriented at a second angle with respect to the longitudinal axis, providing a controller in signal communication with the injector assembly, the controller programmed to translate the valve between a closed position, a first open position, and a second open position, and selectively injecting the fluid through the outlet orifice and into the exhaust gas.

The above features and advantages and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, advantages, and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which:

FIG. 1 is a cross-sectional view of a prior art DEF injection assembly;

FIG. 2A is a cross-sectional schematic view of a portion of the prior art DEF injection assembly shown in FIG. 1 and in a closed position;

FIG. 2B is a cross-sectional schematic view of the prior art DEF injection assembly shown in FIG. 2A and in an open position;

FIG. 3A is a cross-sectional schematic view of an exemplary injection assembly according to the invention and in a closed position;

FIG. 3B is a cross-sectional schematic view of the injection assembly shown in FIG. 3A and in a first open position;

FIG. 3C is a cross-sectional schematic view of the injection assembly shown in FIG. 3A and in a second open position;

FIG. 4A is a bottom view of the injection assembly shown in FIG. 3A;

FIG. 4B is a cross-sectional view of the injection assembly shown in FIG. 3B and taken along line 4B-4B;

FIG. 4C is a cross-sectional view of the injection assembly shown in FIG. 3C and taken along line 4C-4C;

FIG. 5 is a cross-sectional view of an alternative injection assembly; and

FIG. 6 is a flow diagram of a method of operating the injection assembly shown in FIGS. 3A-5.

DESCRIPTION OF THE EMBODIMENTS

The following description is merely exemplary in nature and is not intended to limit the present disclosure or its application or uses. It should be understood that, throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

FIGS. 1, 2A, and 2B illustrate a known diesel exhaust fluid (DEF) injector assembly 10 that generally includes an injector 11 having a tip 12 that extends along a longitudinal axis ‘A’. Injector assembly 10 further includes a nozzle 14 having a body 16 that defines an inlet 18 (FIGS. 2A and 2B), a passageway 20, and a lower annular bottom wall 22. The bottom wall 22 defines an outlet orifice 24 at a central area of the bottom wall 22. The injector tip 12, inlet 18, passageway 20, bottom wall 22, and outlet orifice 24 are concentric with respect to each other, and the passageway 20 is adapted to deliver an atomizing fluid 26 such as urea or ammonia (FIG. 2B).

Referring to FIGS. 2A and 2B, a periphery 27 of the passageway 20 is defined between the injector tip 12 and an inner wall 28 of the body 16 and is filled with the atomizing fluid 26. The injector tip 12 is adapted to translate along the axis ‘A’ within the passageway 20 between a closed position (FIG. 2A) and an open position (FIG. 2B). In the closed position, the injector tip 12 obstructs the outlet orifice 24 to prevent fluid flow therethrough. In the open position, the injector tip 12 allows fluid flow through the outlet orifice 24. As such, the fluid 26 is sprayed out the nozzle 14 of the injector assembly 10 into a stream of exhaust gas (not shown), where the fluid 26 is subsequently mixed with the exhaust gas.

As shown in FIGS. 2A and 2B, an exterior of the body 16 is cylindrical such that passageway 20 has a constant diameter along axis ‘A’. Similarly, the injector tip 12 is cylindrical and has a constant diameter within passageway 20. As such, periphery 27 between injector tip 12 and inner wall 28 is constant and only provides a constant area for fluid flow therethrough.

FIGS. 3A, 3B, and 3C illustrate an exemplary injector assembly 100 that generally includes a housing 102 and a valve 104. In the exemplary embodiment, valve 104 translates within housing 102 along a longitudinal axis ‘B’ to selectively inject a fluid (e.g., urea, ammonia, hydrocarbon, etc.) into an exhaust gas stream (not shown) for mixing therewith. As shown, valve 104 translates from a closed position (FIG. 3A) to a first open position (FIG. 3B) and a second open position (FIG. 3C), as is described herein in more detail.

Housing 102 generally includes an outer wall 106, an inner wall 108, and a bottom wall 110. Inner wall 108 defines a passageway 112 extending through housing 102 between a housing inlet end 114 and a housing outlet end 116, and bottom wall 110 includes an outlet orifice 118 formed therethrough. Housing inlet end 114 is fluidly coupled to a reservoir (not shown) configured to supply a fluid (e.g., ammonia, urea, hydrocarbon, etc.) to passageway 112. Inner wall 108 includes a straight-walled portion 120 having walls 122 and a tapered wall portion 124 having tapered walls 126. As illustrated, walls 122 are substantially parallel, and walls 126 are tapered or converge as they extend from housing inlet end 114 to housing outlet end 116. Tapered walls 126 are oriented at an angle ‘α’ (FIG. 3A) with respect to axis ‘B’. As such, passageway 112 is substantially cylindrical within straight-walled portion 120 and is substantially frusto-conical within tapered wall portion 124.

Valve 104 includes an upstream end 128 and a downstream end 130 having an end surface 132. Downstream end 130 includes a tapered portion 134 having tapered walls 136 oriented at an angle ‘β’ with respect to axis ‘B’. As such, valve tapered portion 134 is substantially frusto-conical. In the exemplary embodiment, angle ‘β’ is less than angle ‘α’ such that a gap 138 is defined between housing tapered walls 126 and valve tapered walls 136.

In operation, in the closed position (FIG. 3A), valve 104 is seated within housing 102 such that valve end surface 132 blocks or obstructs outlet orifice 118. In the closed position, end surface 132 is substantially coplanar with housing bottom wall 110 to facilitate preventing fluid flow through passageway 112 out through injector assembly outlet 118 and into the exhaust gas (not shown). As illustrated in FIG. 4A, a perimeter or outer diameter 140 of valve end surface 132 is sealed against a perimeter or inner diameter 142 of housing bottom wall 110 (i.e., the outer diameter of outlet orifice 118).

As injector assembly 100 moves from the closed position (FIGS. 3A and 4A) to the first open position (FIGS. 3B and 4B), valve 104 is translated along axis ‘B’ toward housing inlet end 114. In the first open position, valve end surface outer diameter 140 is distanced from tapered walls 126 such that an annular opening 144 is formed therebetween. As such, fluid may flow from housing inlet end 114, through gap 138 and annular opening 144, and subsequently through outlet orifice 118 for mixing with the exhaust gas.

As injector assembly 100 moves from the first open position (FIGS. 3B and 4B) to the second open position (FIGS. 3C and 4C), valve 104 is further translated along axis ‘B’ toward housing inlet end 114. In the second open position, valve end surface outer diameter 138 is distanced from tapered walls 126 such that annular opening 144 is formed therebetween. However, in the second open position, annular opening 144 is enlarged compared to its size in the first open position. This is due to tapered walls 126, 136 being oriented at the different angles ‘α’ and ‘β’. More specifically, because angle ‘α’ is greater than ‘β’, housing tapered walls 126 and valve tapered walls 136 diverge as they extend from housing bottom wall 110 toward housing inlet end 114. As such, the distance between valve outer diameter 140 and housing tapered walls 126 increases the further valve 104 is translated along axis ‘B’ away from housing bottom wall 110, which increases the outer diameter of annular opening 144 and thus increases the area through which fluid may flow.

Due to the angular orientation of housing tapered walls 126 with respect to valve tapered walls 136, the outer diameter of annular opening 144 increases as valve 104 moves from the closed position toward housing inlet end 114. For example, in the closed position, annular opening 144 is not present (see FIG. 4A). As valve 104 moves to the first open position, annular opening 144 has an outer diameter ‘D1’ (see FIG. 4B). As valve 104 moves further toward inlet end 114 to the second open position, annular opening 144 has increased in size to have an outer diameter ‘D2’.

As illustrated, ‘D2’ is greater than ‘D1’ such that the size of annular opening 144 in the second open position is greater than the size of annular opening 144 in the first open position. As such, the spray pattern (e.g., fluid droplet-size distribution) may change with the different opening areas of annular opening 144. Although injector assembly 100 is only shown in the first and second open positions, valve 104 may be translated or opened to any desired position from the closed position toward the housing inlet 114 to a maximum open position (not shown). Accordingly, the diameter of annular opening 144, and thus the volumetric flow of fluid, may be selectively controlled or varied for injection of a desired spray pattern (droplet size distribution) into the exhaust gas. Moreover, angles ‘α’ and ‘β’ may be varied to provide a desired size of annular opening 144 at a given location of valve 104 along axis ‘B’. In the exemplary embodiment, angles ‘α’ and ‘β’ are acute angles. Alternatively, angles ‘α’ and ‘β’ may be obtuse, which facilitates adjusting annular opening 144 with a relatively smaller change in vertical position of valve 104.

A controller 150 may control operation of injector assembly 100. In the exemplary embodiment, controller 150 is a dedicated control for injector assembly 100. However, controller 150 may be any suitable controller that enables injector assembly 100 to function as described herein. For example, controller 150 may be a vehicle controller. As used herein, the term controller refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

Controller 150 is programmed to translate valve 104 along axis ‘B’ to vary the size of annular opening 144, and to supply the fluid to injector assembly 100 at a desired pressure. In one embodiment, controller 150 is programmed to size the nozzle open area 144 directly proportional to an amount of pressure of the fluid that injector assembly 100 injects into the exhaust gas. Alternatively, controller 150 may supply the fluid to injector assembly 100 at a constant pressure.

Because nozzle open area 144 and injection pressure are variable, a length of penetration of the spray of the atomizing fluid out of outlet orifice 118 is variable. Additionally, the injection pressure and nozzle open area 144 are variable to provide different sized droplets of the fluid into the exhaust gas. For example, controller 150 may control urea-dosing based upon a flow rate of the exhaust. In a low exhaust flow, fluid may mix more uniformly with the exhaust gas when the fluid droplet size is relatively small. As such, controller 150 may be programmed to reduce the size of nozzle open area 144 and the injection pressure to reduce the spray-penetration distance, which improves SCR efficiency in the low exhaust flow. Conversely, in a high exhaust flow, fluid may mix more uniformly with the exhaust gas when the fluid droplet size is relatively large. As such, controller 150 may be programmed to increase the size of nozzle open area 144 to increase the spray-penetration distance, which improves SCR efficiency in the high exhaust flow.

FIG. 5 illustrates an alternative injection assembly 160 that is similar to injection assembly 10 shown in FIGS. 3A-3C, except housing 102 includes a second straight-walled portion 162 and valve 104 includes downstream walls 164. Straight-walled portion 162 includes walls 166 downstream of tapered wall portion 124 and proximate outlet orifice 118, and downstream walls 164 are oriented downstream of tapered walls 136 and proximate valve end surface 132. In the exemplary embodiment, downstream walls 164 and walls 166 are substantially parallel to axis ‘B’. However, walls 164, 166 may be oriented at any suitable angle that enables injection assembly 160 to function as described herein.

FIG. 6 illustrates a urea-dosing-control method 200 for improving urea-exhaust mixing. The operation of injector assembly 100 (involving nozzle open area 144 and fluid injection pressure) may be varied based on the exhaust gas flow rate, and the exemplary method compensates for impact of such rate on the spray to allow for a more uniform distribution of the urea spray in a greater range of the rate. As such, the method controls the operation of injector assembly 100 based upon an engine operation condition.

Method 200 includes, at step 202, determining the exhaust gas flow rate. In the embodiment shown, the rate is determined by any suitable sensor such as an electronic-control unit (ECU) or a mass flow sensor (not shown). At step 204, controller 150 determines if the measured mass flow rate of the exhaust gas is a low measured flow rate or a high measured flow rate based on the measured flow rate.

If controller 150 determines the measured flow rate is a low exhaust gas flow rate, at step 206, controller 150 adjusts injector assembly 100 to have a relatively smaller nozzle open area 144. For example, controller 150 may position valve 104 in the first open position (FIG. 3B). However, if controller 150 determines the measured flow rate is a high exhaust gas flow rate, at step 208, controller 150 adjusts injector assembly 100 to have a relatively larger nozzle open area 144. For example, controller 150 may position valve 104 in the second open position (FIG. 3C).

As such, at step 206 and/or step 208, controller 150 adjusts injector assembly 100 to vary the fluid droplet size as a function of at least the injection pressure and/or and nozzle open area 144. Alternatively or additionally, at step 210, the desired nozzle open area and injection pressure can be calculated by use of a calibration table.

Described herein is an injector assembly and method for varying fluid injection into an exhaust gas stream. The injector assembly includes housing inner walls and valve walls oriented at different angles such that the inner walls and the valve walls diverge. The position of the valve may be varied to vary the size of an annular opening formed between the valve and housing inner walls. As such, the amount of fluid flow through an outlet orifice may be varied. In addition, the fluid injection pressure may be varied. As such, the fluid droplet size and spray penetration into the exhaust gas may be varied depending on conditions in an exhaust gas aftertreatment system. The system thus compensates for the impact of the exhaust-flow rate on the urea spray distribution and allows for more uniform spray distribution in a greater exhaust flow rate range. This results in reduced tailpipe NO_x emission with improved urea mixing at varying exhaust gas flow conditions.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the application.

Claims

1. An injector assembly for injecting a fluid into an exhaust aftertreatment system, the injector assembly comprising:

a housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice and configured to supply the fluid to the outlet orifice; and

a valve having a longitudinal axis, the valve oriented within the passageway and configured to translate along the longitudinal axis between a first position and a second position, wherein a downstream end of the valve includes a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing inner wall is oriented at a second angle with respect to the longitudinal axis.

2. The injector assembly of claim 1, wherein the second angle is larger than the first angle.

3. The injector assembly of claim 1, wherein the first and second angles are acute angles.

4. The injector assembly of claim 1, wherein the valve further includes an end surface and the housing further includes a bottom wall, and wherein in the first position the valve is oriented such that end surface is positioned within the outlet orifice and the valve end surface is substantially coplanar with the housing bottom surface.

5. The injector assembly of claim 4, wherein in the second position the valve is oriented such that an annular opening is defined between the valve downstream end and the housing inner wall.

6. The injector assembly of claim 5, wherein the valve is configured to translate to a third position between the first position and the second position, and wherein in the third position the valve is oriented such that a second annular opening is defined between the valve downstream end and the housing inner wall.

7. The injector assembly of claim 6, wherein the second annular opening is smaller than the annular opening in the second position.

8. The injector assembly of claim 1, wherein the fluid is at least one of ammonia, urea, and hydrocarbon.

9. The injector assembly of claim 1, wherein the fluid is urea, and further comprising a urea reservoir fluidly coupled to the passageway to supply the urea thereto.

10. An exhaust aftertreatment system for an internal combustion engine, the system comprising:

an exhaust gas conduit configured to receive exhaust gas from the internal combustion engine;

a selective catalytic reduction device disposed within the exhaust gas conduit and configured to receive the exhaust gas; and

an injector assembly coupled to the exhaust conduit and configured to inject a fluid into the exhaust gas, the injector assembly comprising: a housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice and configured to supply the fluid to the outlet orifice; and a valve having a longitudinal axis, the valve oriented within the passageway and configured to translate along the longitudinal axis between a first position and a second position, wherein a downstream end of the valve includes a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing inner wall is oriented at a second angle with respect to the longitudinal axis.

11. The system of claim 10, wherein the second angle is larger than the first angle.

12. The system of claim 10, wherein the first and second angles are acute angles.

13. The system of claim 10, wherein the valve further includes an end surface and the housing further includes a bottom wall, and wherein in the first position the valve is oriented such that end surface is positioned within the outlet orifice and the valve end surface is substantially coplanar with the housing bottom surface, and in the second position the valve is oriented such that an annular opening is defined between the valve downstream end and the housing inner wall.

14. The system of claim 10, wherein the fluid is urea, and further comprising a urea reservoir fluidly coupled to the passageway to supply the urea thereto.

15. A method of dosing a fluid into an exhaust aftertreatment system configured to receive exhaust gas from an internal combustion engine, the method comprising:

providing an injector assembly having a housing and a valve translatable along a longitudinal axis within the housing, the valve having a tapered wall oriented at a first angle with respect to the longitudinal axis, and the housing having an outlet orifice and an inner wall defining a passageway, the passageway fluidly coupled to the outlet orifice, and the inner wall oriented at a second angle with respect to the longitudinal axis;

providing a controller in signal communication with the injector assembly, the controller programmed to translate the valve between a closed position, a first open position, and a second open position; and

selectively injecting the fluid through the outlet orifice and into the exhaust gas.

16. The method of claim 15, further comprising providing the injector assembly with the second angle being larger than the first angle.

17. The method of claim 15, further comprising:

providing a selective catalyst reduction device in the exhaust aftertreatment system downstream of the injector assembly; and

supplying urea as the fluid to the injector assembly to selectively inject urea into the exhaust gas.

18. The method of claim 15, further comprising:

measuring a mass flow rate of the exhaust gas in the exhaust aftertreatment system; and

selectively injecting, with the injector assembly, a predetermined amount of fluid into the exhaust gas based on the measured mass flow rate of the exhaust gas.

19. The method of claim 15, further comprising:

measuring a mass flow rate of the exhaust gas in the exhaust aftertreatment system;

determining, with the controller, whether the measured mass flow rate is a low mass flow rate or a high mass flow rate;

moving the valve to the first open position if the measured mass flow rate is the low mass flow rate, the first open position defining a first annular opening between a downstream end of the valve tapered wall and the housing inner wall; and

moving the valve to the second open position if the measured mass flow rate is the high mass flow rate, the second open position defining a second annular opening between the downstream end of the valve tapered wall and the housing inner wall, wherein the second annular opening is larger than the first annular opening.