GB2584473A

GB2584473A - Fuel rail assembly

Info

Publication number: GB2584473A
Application number: GB1908029.0A
Authority: GB
Inventors: Mehmet Tansug Onur; Akin Levent
Original assignee: Delphi Technologies IP Ltd
Current assignee: Delphi Technologies IP Ltd
Priority date: 2019-06-05
Filing date: 2019-06-05
Publication date: 2020-12-09
Anticipated expiration: 2039-06-05
Also published as: GB201908029D0; GB2584473B

Abstract

A common rail assembly 2 for a high pressure fuel system comprises a rail housing 6 having a bore 20 defining a rail volume for receiving high pressure fuel, and comprises a first region 22, having a first diameter 24, and a second region 26, having a second diameter 28, which defines a chamber for receiving at least a portion of a rail device 8, eg a pressure sensor or plug. An insert 40 is received within the bore 20 between the first and second regions 22, 26, wherein a seat 32 for the rail device 8 is defined on one surface of the insert 40 and another surface of the insert 40 defines a sealing surface 50 for engagement with the bore 20 to seal against high pressure fuel within the rail volume. The diameter of the seat 32 may be less than the first diameter 24. The seat 32 may be flat and engaged by the device 8 at a biting edge seal. The sealing surface 50 may be frusto-conical. A method of assembling the fuel rail is also disclosed and claimed.

Description

FUEL RAIL ASSEMBLY

FIELD OF INVENTION

The invention relates to the field of fuel rail assemblies for internal combustion engines. In particular, the invention relates to a fuel rail assembly and a method of making a fuel rail assembly.

BACKGROUND

Fuel rails are used as a reservoir for storing high pressure fuel in fuel injection systems. Fuel rails are fed by fuel pump assemblies that pressurise the fuel for storage in the fuel rail. From the fuel rail, fuel flows into injectors for injection into the cylinders of the engine.

Global emissions standards require an increase in the pressure at which the fuel is held within the fuel rail. This requires the fuel rail to withstand higher compressive stresses and greater loads at sealing interfaces, such as those between the main body of the fuel rail and a high pressure sensor. Since fuel rails are intended to be infinite life components, such sealing interfaces within the fuel rail must remain functional throughout the life of the fuel rail and consequently throughout the whole life of the internal combustion engine. This provides motivation for reducing the loads on sealing interfaces.

One way to reduce the loads on sealing interfaces is to reduce the size of the interface. This has the added benefit of allowing the use of smaller components, which allows for mass and cost decrease of the engine.

It is against this background that the invention has been devised.

STATEMENTS OF INVENTION

According to an aspect of the invention, there is provided a rail assembly for a high pressure fuel system. The rail assembly comprises a rail housing, provided with a bore, the bore defining a rail volume for receiving high pressure fuel, in use. The rail assembly additionally comprises a first region, having a first diameter, and a second region, having a second diameter, the second region defining a chamber for receiving at least a portion of a rail device. The rail assembly further comprises an insert received within the bore between the first and second diameter regions. A seat for the rail device is defined on one surface of the insert and another surface of the insert defines a sealing surface for engagement with the bore to seal against high pressure fuel within the rail volume.

The insert serves to decouple the relationship between the diameter of the portion of the rail device received within the bore and the first diameter.

Previously, the first diameter defined a minimum size for the sealing interface between the rail device and the bore and therefore necessitated a minimum size of the rail device. The addition of the insert allows smaller rail devices to be used for no reduction in the first diameter, which would otherwise disadvantageously reduce the capacity of the rail assembly to store fuel. A smaller rail device also creates a smaller sealing interface, which reduces the load on the sealing interface and extends its life.

A diameter of the seat for the rail device may be less than the first diameter. The sealing surface may define an annular seating line of the insert, which may have a diameter which is greater than the diameter of the seat for the rail device. The sealing surface may be a frusto-conical surface and the seat may be a substantially flat surface. The rail device may engage the seat via a biting edge seal.

In an embodiment, the rail device may be configured to engage the second region via a screw thread engagement, the axial force of said engagement serving to create the biting edge seal at the seat. The screw thread engagement may also serve to generate the seal at the sealing surface.

The bore may additionally comprise a radially outwardly extending double angle chamfer between the seat and the sealing surface. In certain embodiments, the first region may define a transition volume and the bore may further comprise a recessed groove and a further region which defines the rail volume, wherein the recessed groove lies between the transition volume and the further region.

The insert may have a bore configured to allow fluidic communication of fuel between the rail volume and the rail device, in use. The rail device may take the form of a pressure sensor.

According to another aspect of the invention, there is provided a method of assembling a fuel rail assembly for a high pressure fuel system. The method comprises; providing a rail housing, a rail device and an insert, wherein the rail housing is provided with a bore comprising a first region and a second region. The method additionally comprises; inserting the insert into the bore at a first end of the rail housing, positioning the insert between the first region and the second region of the bore and inserting a first portion of a rail device into the second region via a screw thread engagement. The screw thread engagement serves to create a seal between the first portion of the rail device and a first surface of the insert and a seal between the bore and a sealing surface of the insert, as the first portion of the rail device is screwed into the bore.

It will be appreciated that preferred and/or alternative features of the first aspect of the invention may be incorporated alone or in appropriate combination into the method of the invention also.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which; Figure 1 is a perspective view of a fuel rail assembly comprising a rail housing and a rail device in the form of a high pressure sensor; Figure 2a is a perspective cross sectional view of a rail assembly as in the state of the art; Figure 2b is a perspective cross sectional view of a rail housing as in the state of the art; Figure 3 is a cross sectional view of a sealing interface between a rail housing and a high pressure sensor as in the state of the art; Figure 4 is a perspective cross sectional view of a rail assembly as according to an embodiment of the invention; Figure 5 is a cross-sectional view of a section of the bore of the rail assembly of Figure 4; Figure 6 is a cross sectional view of an interface between a high pressure sensor, an insert and a rail housing in the rail assembly of Figure 4; Figure 7 is a cross-sectional view of a rail housing for a rail assembly, as according to an alternative embodiment of the invention, and Figure 8 is a cross-sectional view of a rail housing for a rail assembly, as according to another alternative embodiment of the invention.

SPECIFIC DESCRIPTION

Figure 1 shows an example of a fuel rail assembly 2, designed particularly for diesel fuel, comprising a tubular rail housing 6, and a rail device 8 in the form of a high pressure sensor 10, engaged with a first end 4 of the rail assembly 2. Shapes and forms of rail assemblies are well known in the art and will not be discussed at length here unless specifically relevant to the details of the examples and embodiments presented herein. Similarly, a wide range of high pressure sensors are known in the art and consequently only features specifically relevant to the examples and embodiments presented herein will be discussed in detail.

Figure 2a shows an example of how the rail housing 6, best seen in Figure 2b, and high pressure sensor 10 are engaged in the state of the art. As can be seen in Figure 2b, the rail housing 6 is provided with a bore 20 that runs through the rail housing 6. It is to be understood that the use of the term 'bore' within this description is not intended to provide any description of how the bore itself is formed. The bore 20 comprises a substantially cylindrical first region 22, having a first diameter 24. The bore 20 additionally comprises a substantially cylindrical second region 26, having a second diameter 28 that is larger than the first diameter 24, the second region 26 being in fluidic communication with the outside environment at its first end 27a and also with the first region 22 at its second end 27b.

The second region 26 defines a chamber, configured to receive the high pressure sensor 10 via a female screw thread 30 running along the sides of the second region 26. At the second end 27b of the second region 26 is defined a flat, annular seat 32, created by the difference between the first and second diameters 24, 28.

At a first end 22a of the first region 22, spanning between the first region 22 and second region 26, a chamfer 34 is located that increases the diameter of the bore 20 from the first diameter 24. The chamfer 34 creates an increase in the diameter of the bore 20 of between approximately lmm and 2.5 mm. The function of the chamfer 34 is to avoid abrupt, sharp corners to alleviate stress on the rail housing.

Turning back to Figure 2a, the high pressure sensor 10 comprises a substantially cylindrical first portion 12 having a diameter that substantially matches that of the second region 26. A male screw thread 16 on the first portion 12 of the high pressure sensor 10 engages the female thread 30 on the sides of the second region 26, allowing the high pressure sensor 10 to be received within the second region 26 by engagement of the two screw threads 16, 30. At a first end 12a of the first portion 12 of the high pressure sensor 10 is positioned an annular edged protrusion 14 and a sensing opening 18, located within the edged protrusion 14. The axial force created by the engagement of the two screw threads 16, 30 causes the edged protrusion 14 to engage and plastically deform the seat 32, creating a so-called 'biting edge' seal therebetween, seen in better detail in Figure 3, with fluidic communication between the bore 20 and the sensing surface 18 of the high pressure sensor 10 maintained by the annular shape of the seal. The plastic deformation is limited to a depth of approximately between 10 and 150 pm. The precise extent of plastic deformation is dependent upon a number of parameters, such as the material used for the rail body and the size of the high pressure sensor. For example, for M18 sensors, a range of approximately between 10 and 150 pm may be suitable. For rail bodies formed from high grade materials to withstand higher rail pressures, the biting edge depth will be much lower. For low grade materials, used in lower rail pressure applications of 1600bar to 2000bar system pressures, the biting edge depth may reach up to 150 pm for M12 sensors.

In use, the bore 20 defines a rail volume for receiving high pressure fuel. The biting edge seal between the high pressure sensor 10 and the seat 32 prevents fuel held within the rail volume from escaping the rail assembly 2, although the pressure of the fuel stored within the rail volume may still be monitored by entering the sensing opening 18 of the high pressure sensor 10 thanks to the annular shape of the seal.

To create the biting edge seal the diameter of the edged protrusion 14, and by association the second diameter 28 and diameter of the first portion 12 of the high pressure sensor 10, must exceed both the first diameter 24 and the diameter of the bore 20 at the widest point of the chamfer 34. In fact, to create an effective biting edge seal, the diameter of the edged protrusion 14 must exceed the diameter of the bore 20 at the widest point of the chamfer 34 by at least 0.2 mm.

The choice of high pressure sensor 10 is therefore determined, at least in part, by the first diameter 24. As already discussed, there is a desire to reduce the size of sealing interfaces for mass and cost benefits -this would therefore necessitate a reduction in the first diameter 24, thus reducing the volume of the first region 22 of the bore 20 of the rail assembly 2. However, this is undesirable as it reduces the capacity of the rail assembly 2 to store pressurised fuel and fulfil its primary function.

This problem is solved by the current invention. The invention shares several features with the example of Figures 1, 2 and 3: these features will be referred to by the same reference numbers for the sake of clarity. Figure 4 shows an embodiment of the invention of a fuel rail assembly 2 for a high pressure fuel system, particularly for a high pressure diesel fuel system, having a first end 4 and a second end (not shown) and comprising a rail housing 6, provided with a bore 20. The bore 20 has a first region 22 having a first diameter 24, which defines a rail volume for receiving high pressure fuel, in use, and a second region 26 having a second diameter 28, which defines a chamber.

The second region 26 has a first end 27a in fluidic communication with the environment external to the rail assembly 2 at a first end 4 of the rail assembly 2 and a second end 27b in fluidic communication with the first region 22 of the bore 20. As with the example shown in Figure 2, the second region 26 has a female screw thread 30 running around its internal surface. Between the first region 22 and the second end 27b of the second region 26 is a radially outwardly extending annular chamfer 34 that, as seen best in Figure 5, transitions the bore 20 between the first diameter 24 and the second diameter 28. The chamfer 34 is a double angle chamfer, comprising a first zone 36, angled at a first angle to the first region 22 of the bore 20, and a second zone 38, angled at a second angle to the first region 22 of the bore 20. The second angle is steeper than the first angle. The first angle may lie between approximately 15° and 30°, preferably 30°, while the second angle may lie between approximately 30° and 45°, preferably 45°. In the embodiment shown in Figure 4, the first angle is approximately 30° and the second angle is approximately 45°.

As seen in Figure 4, the rail assembly 2 further comprises an insert 40. The insert 40 is substantially frusto-conical in shape and has a central bore 42, which has a diameter less than the first diameter 24. The insert 40 comprises first and second opposing faces 44, 46, the first opposing face 44 and second opposing face 46 being flat and parallel to each other. The first opposing face 44 has a diameter larger than the first diameter 24 but smaller than the second diameter 28; the second opposing face 48 has a diameter that is smaller than the first diameter 24. The insert 40 further comprises an external frusto-conical surface 48, angled to the first and second opposing faces 44, 46 by an angle that is shallower than both the first and second angles of the chamfer 34 of the bore 20. In the embodiment shown in Figure 4, the angle of the frusto-conical surface 48 is preferably between 10° and 20°.

The central bore 42 of the insert 40 is configured to allow fuel to pass therethrough, in use. The insert 40 is positioned at the boundary between the first and second regions 22, 26 of the bore 20, with the frusto-conical surface 48 engaging the bore 20 at the boundary between the first zone 36 of the chamfer 34 and the first region 22 of the bore 20.

As in the example shown in Figures 1 and 2, a rail device 8, exemplified here as a high pressure sensor 10 comprises a first portion 12 having a male screw thread 16, configured to engage the female screw thread 30 of the second region 26 of the bore 20. At a first end 12a of the first portion 12 is located an annular edged protrusion 14 and a sensing opening 18, located within the edged protrusion 14. In engaging the male screw thread 16 of the first portion 12 of the high pressure sensor 10 with the female screw thread 30 of the second region 26 of the bore 20, an axial force is generated in the direction of the longitudinal axis of the rail assembly 2. This axial force is sufficient to create a biting edge seal between the edged protrusion 14 and a seat 32 for the high pressure sensor 10, defined on the first opposing face 44 of the insert 40. Again, this biting-edge seal causes some plastic deformation of the material of the insert 40 on the first opposing face 44; this plastic deformation is preferably limited to a depth of approximately between 5 and 30 pm, but may lie outside this range depending on assembly parameters.

The axial force of the engagement of the male and female screw threads 16, 30 also serves to create a conical seal between the frusto-conical surface 48 and the chamfer 34. The frusto-conical surface 48 of the insert 40 therefore defines a sealing surface 50 for engagement of the insert 40 with the bore 20. The sealing surface 50 defines an annular seating line of the insert 40 for engagement with the bore 20. The conical seal is specifically at the boundary between the first zone 36 of the chamfer 34 and the first region 22 of the bore 20. The angle of the first zone 36 of the chamfer 34 and that of the frusto-conical surface 48 allows the insert 40 to create an effective conical seal under the axial force of the screw thread engagement. The insert 40 itself and the sealing of the insert 40 within the bore 20 are shown in more detail in Figure 6.

In use, the rail volume, defined by the first region 26 of the bore 20, stores high pressure fuel. The high pressure fuel is prevented from escaping the bore 20 by the biting edge seal at the seat 32 and the conical seal at the sealing surface 50. The central bore 42 within the insert 40 maintains fluidic communication between the sensing opening 18 of the high pressure sensor 10 and the fuel, allowing fuel to enter the sensing opening 18 and consequently allowing the high pressure sensor 10 to measure the pressure of fuel within the bore 20.

The insert 40 allows decoupling of the relationship between the size of the first region 22 and a required size for the first portion 12 of the high pressure sensor 10. It removes the requirement that the diameter of the edged protrusion 14 must exceed the first diameter 24 in order to create an effective biting edge seal. This allows the use of high pressure sensors 10, or other rail devices 8, with smaller first portions 12, which leads to reductions in the mass, as well as cost of the component. Smaller sealing interfaces will also reduce process parameters such as assembly moments applied on the high pressure sensor 10, thereby extending the lifetime of the sealing interface.

As seen best in Figure 6, in certain embodiments, the insert may additionally comprise a so-called self-alignment region 52, a region of constant diameter that extends from the insert 40, with the second opposing face 46 defined at the end of the self-alignment region 52. The diameter of the self-alignment region 52 is less than first diameter 24; this allows it to fit within the first region 22. The purpose of the self-alignment region 52 is to allow easy positioning of the insert 40 within the bore 20 as the diameter of the self-alignment region 52 means that positioning the self-alignment region 52 within the first region 22 of the bore 20 naturally causes the insert 40 to be positioned correctly such that effective seals with the high pressure sensor 10 at the seat 32 and the bore 20 at the sealing surface 50 may be created.

In alternative embodiments of the invention, the angle of the frusto-conical surface 48 may instead lie between the first and second angles of the chamfer 34. This therefore leads to the sealing surface 50 of the insert 40 engaging the bore 20 at the boundary between the first and second zones 36, 38 of the chamfer 34. This boundary is best seen in Figure 5. In further embodiments, the angle of the frusto-conical surface 48 may exceed that of the second angle of the chamfer 34. In this case, the sealing surface 50 of the insert 40 therefore engages the bore 20 at the boundary between the second zone 38 of the chamfer 34 and the second region 26 of the bore 20.

In further alternative embodiments of the invention, the chamfer may only comprise a first zone 36 with a first angle. In these embodiments, the angle of the frusto-conical surface 48 may either be shallower than the first angle, in which case the sealing surface 50 of the insert 40 engages the bore 20 at the boundary between the first zone 36 and the first region 22, or the angle of the frusto-conical surface 48 may be steeper than the first angle. In this case, the sealing surface 50 engages the bore 20 at the boundary between the first zone 36 and the second region 26 of the bore 20.

In the embodiments described above, the first region 22 of the bore 20 may be considered to be a gundrill; that is, the main volume for storage of high pressure fuel within the rail assembly 2. However, in certain embodiments, as shown in Figure 7, the gundrill 60 may be spaced from the insert 40 (not shown in Figure 7) and chamfer 34 by a transition volume 62 and a recessed groove 64, with the transition volume 62 located adjacent the second region 26 and the recessed groove 64, and the recessed groove 64 located adjacent the transition volume 62 and the gundrill 60. In such embodiments, the transition volume 62 may be considered to define the first region 22 of the bore 20. As with embodiments described above, in these embodiments the insert 40 is positioned at the boundary of the first region 22 and second region 26.

The recessed groove 64 is a localised widening of the diameter of the bore 20 to a third diameter 66. It is necessary in rail assemblies for heavy-duty applications, such as truck engines, where the bore 20 can be very long. This can create concentricity faults within the bore 20, where the gundrill 60 extends at a slight angle through the rail housing 6. The tolerance for concentricity when machining the gundrill 60 is known: therefore, at any given length into the gundrill 60, a circular locus of material that might be machined for the gundrill 60 can be determined. The third diameter 66 corresponds to the diameter of this circular locus at the determined position of the recessed groove 64 such that it is known that the cross-section of the gundrill 60 will completely lie within that of the recessed groove 64.

The transition volume 62 is a region of the bore 20 with a reduced diameter compared to the recessed groove 64. In certain embodiments, the diameter of the transition volume 62 may be similar to that of the gundrill 60, although this is not the case in all embodiments. At first sight the smaller diameter transition volume may enable use of a smaller size high pressure sensor 10. However, in embodiments where a recessed groove 64 is present, this is not the case as it is necessary to machine the recessed groove 64 from a first end 6a of the rail housing 6, and the diameter of the transition volume 62 should be greater than that of the gundrill 60 from a machining feasibility point of view.

The absolute diameter of the transition volume 62 is determined by the required diameter of the recessed groove 64, since the recessed groove 64 must be machined from the first end 4 of the rail assembly 2 to allow it to mitigate for concentricity faults within the gundrill 60, which is machined from the second end of the rail assembly. The diameter of the transition volume 62 is, however, smaller than that of the second region 26.

In further embodiments of the invention, used in light-duty applications, a recessed groove 64 is not necessary, as illustrated in Figure 8. This is because the reduced length of the bore 20 in comparison to heavy-duty applications means the potential for concentricity faults is minimised. However, a transition volume 62 may still be present, which in this embodiment has a smaller diameter than the gundrill 60. In this case, the transition volume 62 is also considered to be a first region 22 of the bore 20. As with other embodiments, the insert 40 (not shown in Figure 8) is positioned at the boundary between the first region 22 and the second region 26.

In certain embodiments, the second region 26 of the bore 20 may comprise an undercut feature 68 towards its second end 27b, best seen in Figures 4 and 6. The undercut feature 68 creates a region of increased diameter in the second region 26. This region does not have a uniform diameter; rather, its diameter increases from an end closest to the first end 27a of the second region 26 to an end closest to the second end 27b of the second region 26. The purpose of the undercut feature 68 is to ensure that the insert 40 can be fitted into the bore 20 in worst-case scenarios of stacking of manufacturing tolerances for the bore 20, chamfer 34 and insert 40.

The invention also relates to a method for assembling a fuel rail assembly 2 for a high pressure fuel system, particularly for a high pressure diesel fuel system. The method comprises providing a rail housing 6, which is provided with a bore 20, a rail device 8 and an insert 40. The insert 40 is inserted into the bore 20 at a first end 6a of the rail housing 6 and positioned at a boundary between a first region 22 and a second region 26 of the bore 20, a sealing surface 50 of the insert 40, defined by a frusto-conical surface 48, engaging a double angle chamfer 34 located between the first and second regions 22, 26. A first portion of the rail device 8 is subsequently inserted into the bore 20 at the first end 6a of the rail housing 6, with a male screw thread 16 on a first portion of the rail device 8 engaging a female screw thread 30 on an internal surface of the second region 26 of the bore 20, located closer to the first end 6a of the rail housing 6 than the first region 22 of the bore 20. The method further comprises screwing the first portion of the rail device 8 into the second region 26 to create a biting edge seal between an annular edged protrusion 14 located at a first end of the first portion of the rail device 8 and a seat 32, defined on a first opposing face 44 of the insert 40, and also to create a conical seal between the frusto-conical surface 48 of the insert 40 and the chamfer 34.

The invention also relates to a method of making a bore 20 for a rail assembly 2 for a high pressure fuel system, particularly for a high pressure diesel fuel system, the bore 20 comprising a first region 22 and a second region 26, having first and second diameters 24, 28 respectively. The method comprises machining the first region 22 from either a first end or a second end of the rail assembly 2 and machining the second region 26 from the first end of the rail assembly 2. In certain embodiments of the invention, the first region 22 forms a gundrill and is machined from the second end of the rail assembly. In other embodiments of the invention, the first region 22 forms a transition volume 62 and is machined from the first end of the rail assembly 2.

The method additionally comprises machining a chamfer 34 at the boundary between the first and second regions 22, 26 of the bore 20. The chamfer 34 is machined first at the boundary of the first and second regions 22, 26 of the bore 20. The chamfer 34 is machined at an angle of approximately between 30° and 45°, preferably at 45°. An autofrettage operation is then applied to the bore 20.

The autofrettage nozzle may have an angle of approximately between 15° and 30°, preferably 30° which divides the chamfer 34 into a first zone 36, having a first angle mirroring that of the autofrettage nozzle and a second zone 38 having a second angle mirroring that of the machined chamfer.

In certain embodiments, the initial machining of the chamfer may be made at an angle of between approximately 15° and 20°. The autofrettage operation is then applied with a nozzle at an angle of between approximately 15° and 30° but at a steeper angle than that of the machined chamfer. This subsumes the machined chamfer such that the chamfer 34 only comprises a first zone 36 having a first angle mirroring that of the autofrettage nozzle.

It is to be understood that the effects of the autofrettage operation on the chamfer 34 is not the primary function of the autofrettage operation, which is undertaken to place a region of a rail housing 6 of the rail assembly 2 closest to the bore 20 into compression, thus increasing the durability of the rail assembly 2. Instead, the creation of the double angle chamfer 34, or the subsuming of the machined chamfer is a secondary effect of performing the autofrettage operation.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims. For instance, the exact shapes of the bore and the insert are not of critical importance to the invention and it will be appreciated that, although exemplified here as a high pressure sensor, the rail device may take the form of any device that may be positioned at the end of a fuel rail, such as a plug.

References used: 2 -rail assembly 4 -first end of the rail assembly 6 -rail housing 6a -first end of the rail housing 8 -rail device -high pressure sensor 12 -first portion of the high pressure sensor 12a -first end of the first portion of the high pressure sensor 14 -annular edged protrusion 16 -male screw thread 18 -sensing surface -bore 22 -first region of the bore 22a -first end of the first region 24 -first diameter 26 -second region of the bore 27a -first end of the second portion 27b -second end of the second region 28 -second diameter -female screw thread 32 -seat 34 -chamfer 36 -first zone 38 -second zone -insert 42 -bore of the insert 44 -first opposing face 46 -second opposing face 48 -frusto-conical surface -sealing surface -gundrill 62 -transition volume 64 -recessed groove 66 -third diameter 68 -undercut feature

Claims

CLAIMS: 1. A rail assembly (2) for a high pressure fuel system, the rail assembly (2) comprising a rail housing (6) provided with a bore (20), wherein the bore (20) defines a rail volume for receiving high pressure fuel, in use, and comprises a first region (22), having a first diameter (24), and a second region (26), having a second diameter (28), which defines a chamber for receiving at least a portion of a rail device (8), the rail assembly (2) further comprising an insert (40) received within the bore (20) between the first and second regions (22, 26), wherein a seat (32) for the rail device (8) is defined on one surface of the insert (40) and another surface of the insert (40) defines a sealing surface (50) for engagement with the bore (20) to seal against high pressure fuel within the rail volume.
2. The rail assembly (2) of claim 1, wherein a diameter of the seat (32) for the rail device (8) is less than the first diameter (24).
3. The rail assembly (2) of claim 2, wherein the sealing surface (50) defines an annular seating line of the insert (40) having a diameter which is greater than the diameter of the seat (32) for the rail device.
4. The rail assembly (2) of any preceding claim, wherein the sealing surface (50) is a frusto-conical surface (48).
5. The rail assembly (2) of any preceding claim, wherein the seat (32) is a substantially flat surface and the rail device (8) engages the seat (32) via a biting edge seal.
6. The rail assembly (2) of claim 5, wherein the rail device (8) is configured to engage the second region (26) via a screw thread engagement and wherein the axial force of said engagement serves to create the biting edge seal at the seat (32).
7. The rail assembly (2) of claim 6, wherein the screw thread engagement serves to generate the seal at the sealing surface (50).
8. The rail assembly (2) of claim 7, wherein the bore (20) additionally comprises a radially outwardly extending double angle chamfer (34) between the seat (32) and sealing surface (50).
9. The rail assembly (2) of any preceding claim, wherein the first region (22) defines a transition volume (62), the bore (20) further comprising a recessed groove and a further region which defines the rail volume, and wherein the recessed groove lies between the transition volume and the further region.
10. The rail assembly (2) of any preceding claim, wherein the insert (40) has a bore (42) configured to allow fluidic communication of fuel between the rail volume and the rail device (8), in use.
11. The rail assembly of any preceding claim, wherein the insert (40) comprises a region of constant diameter, configured to facilitate positioning of the insert (40) within the bore (20).
12. The rail assembly (2) of any preceding claim, comprising a rail device (8) in the form of a pressure sensor. 20
13. A method of assembling a fuel rail assembly (2) for a high pressure fuel system, the method comprising; providing a rail housing (6), a rail device (8) and an insert (40), wherein the rail housing (6) is provided with a bore (20) comprising a first region (22) and a second region (26), inserting the insert (40) into the bore (20) at a first end (6a) of the rail housing (6), positioning the insert (40) between the first region (22) and the second region (26) of the bore (20), inserting a first portion of a rail device (8) into the second region (26) via a screw thread engagement, wherein the screw thread engagement serves to create a seal between the first portion of the rail device (8) and a first surface of the insert (40) and a seal between the bore (20) and a sealing surface (50) of the insert (40) as the first portion of the rail device (8) is screwed into the second region (26) of the bore (20).