The present invention generally relates to a common rail assembly for an internal combustion engine. The invention particularly relates to a common rail assembly comprising a common rail and a pressure sensor. BACKGROUND OF THE INVENTION

In fuel injection systems of automotive engines, a common rail typically comprises an elongated cylindrical body with a longitudinal bore along its length axis defining an inner volume configured to receive high pressure fuel. The rail further comprises an inlet port and a plurality of tubular connection pieces forming outlet ports protruding from the cylindrical surface of the body.

In order to monitor and control the pressure inside the rail, a pressure control valve and a pressure sensor are assembled to the common rail body. The pressure control valve (also referred to as pressure regulator device) may be passive or electromagnetically controlled.

Conventionally, the pressure control valve and the pressure sensor close the opposite extremities of the high pressure chamber of the common rail. They are mounted by screwing inside respective end cavities that are located at both extremities of the longitudinal bore of the common rail body. Accordingly, both components have a generally cylindrical outer shape and are provided with an external thread that engages a corresponding thread in the lateral cavity wall. During screwing, the front side of the pressure sensor (same applies to the pressure control valve) comes into abutment against the cavity bottom face that generally extends in a plane perpendicular to the common rail’s length axis. In fact, the contact between the front end of the pressure sensor and the bottom face of the cavity will form a fluid-tight annular sealing contact. It is thus of importance to properly control the screwing process of the pressure control valve into the cavity to ensure that a proper pressure force exists at the interface between both parts, able to provide a sealed barrier even at high fuel pressures (up to 3000 bars). The screwing force is generally checked by means of a torque wrench.

As it is well known, the manufacturing process of the common rail conventionally starts with forging a steel blank of the common rail, the steel blank being then machined to form the cylindrical end cavities and the longitudinal bore is formed by gun-drilling from one of the cavities. After the machining is completed, the obtained common rail is subjected to an autofrettage process in order to improve its durability.

During the autofrettage, a high pressure nozzle is inserted in the end cavity and positioned in a bore in the bottom face of the end cavity that communicates with the longitudinal bore. The nozzle is subject to great forces due to the inner pressure. Good positioning of the nozzles in order to provide optimum sealing relies on uniform and predictable surfaces of the bore. Unfortunately, possible axial deviation (coaxiality) during gun drilling operation may create centring problems during autofrettage. This creates a non-uniform deformation of the bottom surface of the end cavity, i.e. the bottom cavity surface intended to cooperate with the front side of the pressure sensor to form a metal-to-metal annular seal is damaged.

Nevertheless, in the conventional approach, the issue of bottom face deformation is overcome by work hardening / high plastic deformation thereof, by applying a high screwing torque in order to achieve the required axial load for sealing. Although this conventional approach is widely used, it may lead to issues during service: difficulty to achieve proper sealing when replacing the pressure sensor, or risks of cracks or release of small particles due to tightening process.

OBJECT OF THE INVENTION

An object of the present invention is to provide an improved common rail assembly that does not exhibit the shortcomings of known systems. GENERAL DESCRIPTION OF THE INVENTION

The present invention concerns a common rail assembly of a common rail of a fuel injection system and a pressure sensor for monitoring the pressure sensor inside the common rail.

The common rail has a tubular body extending along a longitudinal axis, an inner longitudinal bore for, in use, containing pressurized fuel, the body comprising a plurality of transversally protruding fuel outlet pieces. Each fuel outlet piece comprises a branch bore extending along a direction transversal to the longitudinal axis and in communication with the longitudinal bore. Each fuel outlet piece also has a first annular sealing surface surrounding the branch bore as well as an outer threaded section.

It will be appreciated that the pressure sensor is fixedly assembled by screwing on one of the fuel outlet pieces of the common rail body, the pressure sensor comprising:

- a sensor housing with a pressure sensing arrangement located therein;

- a female connector portion comprising cylindrical connector cavity, in which the fuel outlet piece is engaged, wherein the connector cavity has an inner threaded section screwed on the outer threaded section of the fuel outlet piece;

- a sensing channel to allow flow of pressurized fuel from the branch bore towards the pressure sensing arrangement;

- a tubular nipple protruding in the connector cavity in axial continuation of the sensing channel, the nipple comprising a second annular sealing surface that comes into sealing engagement with the first sealing surface when screwing the pressure sensor onto the fuel outlet piece.

In the inventive common rail assembly the pressure sensor is no longer designed with a male interface received in an end cavity of the common rail body, but is mounted on a transversally protruding connecting piece, which may be a conventional fuel outlet piece. In the conventional manufacture of the common rail body, the fuel outlet pieces are not engaged by a nozzle during autofrettage. The pressure sensor is thus mounted on a port of the common rail that is not damaged by the autofrettage process, which will permit to ensure a high quality seal. Sealing issues -known from the prior art- due to surface deformation at the bottom of the pressure sensor cavity are thus avoided.

The present invention thus overcomes the technical prejudice, by which conventional pressure sensors are designed with a male interface accommodated in a large diameter cavity.

Advantageously, the branch bore in the outlet piece includes a diverging end section with the first annular sealing surface; and the second sealing surface is provided at the periphery of the nipple. The nipple tip thus fits into the diverging end section and the first and second annular sealing surfaces are in engagement in the assembled state of the pressure sensor, i.e. when the pressure sensor is screwed on the fuel outlet piece. This provides a metal to metal seal that no longer relies on flat surfaces, since at least the branch bore end section has a diverging profile. It is thus possible to achieve a high quality seal with a lower screwing torque.

Preferably, the first and/or the second sealing surface is/are frustoconical surface. In alternatives, the first sealing surface is frustoconical and the second sealing surface is a rounded annular surface.

In embodiments, the cylindrical cavity of the female connector comprises a bottom wall attached to the sensor housing; and the tubular nipple is formed by a metallic hollow cylinder (or sleeve) protruding from the bottom wall of the female connector toward the fuel outlet pipe.

The female connector and the tubular nipple may be formed in one piece. Alternatively, the nipple is formed by a separate tubular piece inserted through the bottom wall of the female connector.

Since the pressure sensor is no longer mounted in an axial cavity, the longitudinal bore of the common rail may be simply drilled as a blind bore, i.e. the common rail body only has one axial opening (with cylindrical end cavity) and it closed at the opposite end.

Preferably, a plurality of the fuel outlet pieces have the same design, namely same diameter, whereby the pressure sensor can be interchangeably screwed onto any one of these similar fuel outlet pieces. This is advantageous to adapt to different engine configurations.

In embodiments, the pressure sensor further comprises a terminal connector (or socket) for connection of the pressure sensor to an external control unit configured control and evaluate the sensor signal.

The present invention brings a number of advantages over the conventional approach described herein above:

- Serviceability is improved. The metal to metal seal provided by the cooperating first and second sealing surfaces of the nipple and branch bore does normally not damage the sealing surfaces, in particular due to lower screwing force. It is thus easier to dismount or replace the pressure sensor.

- Cleanliness level is improved. The standard approach leads to mechanical hardening of the cavity bottom surface, which may generate, when improperly handled during servicing, some metallic particles.

- Packaging is improved. The ability to mount the pressure sensor onto any of the fuel outlet pieces allows modifying the position of the pressure sensor depending on the engine configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawings, wherein:

Fig. 1 is a perspective view a common rail assembly according to an embodiment of the invention;

Fig. 2 is a longitudinal cross-section view of the assembly of Fig.1 , where the cutting plane contains longitudinal axis A and passes through the centre of the pressure sensor;

Fig. 3 is a view of detail 3 of Fig. 2 showing the pressure sensor;

Fig. 4 is a cross-section view of an alternative design of the pressure sensor. DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of the common rail assembly 10 will be described with regard to Figs. 1 and 2 and essentially comprises a common rail 12 (also known as fuel rail or fuel accumulator) and a pressure sensor 14 fixedly mounted to the common rail 12. As it is known in the art, other elements such as a pressure control valve and inlet or outlet ducts, not shown, may also be coupled to the common rail12. This common rail assembly 10 has been designed for use in diesel engines but can be adapted to be integrated in fuel injection systems using other fuels.

The common rail 12 typically comprises an elongated tubular body 16 extending along a main axis A. The body 16 comprises an internal longitudinal bore 18 shown in Fig. 2, which extends parallel to the longitudinal axis A forming an internal chamber receiving, in use, pressurized fuel. The body 16 typically has a relatively thick wall thickness for improved pressure resistance. The longitudinal bore 18 is a blind bore extending from an open end 20 of the body 16. The open end 20 shown in Figs. 1 and 2 comprises a cylindrical end cavity 22 wider than the longitudinal bore 18 that, in use, receives a pressure control valve, not shown. The end cavity has a bottom face extending perpendicularly to axis A.

The common rail body 16 is, e.g., a forged metallic body, in particular made from steel, stainless steel or micro-alloyed steel. Forming of the common rail body 16 by forging is common in the art and conventional for common rails for diesel engines. Alternatively, the rail body may be manufactured according to other appropriate techniques, in particular by assembly of preformed sections.

Reference sign 24 designates two mounting pads 24 provided to fix the rail 12 in the engine. The pads 24 have a centre bore 26, through which a fixing screw (not shown) passes and further engages into a threaded bore in an engine component (e.g. cylinder head or engine block). This is only an example and those skilled in the art may use other fixing means to mount the common rail 12 in the engine. The common rail 12 is provided with a plurality of tubular-shaped protrusions, here six. One protrusion is a fuel outlet piece 28 connected in use to a low pressure fuel return duct. As shown in Fig. 2, the fuel outlet piece 28 comprises an internal passage 30, which is in communication with the end cavity 22 receiving the pressure control valve. The end cavity is often provided with an M18 thread.

Three protrusions serve as fuel outlet pieces 32 for, in use, providing fuel to respective fuel injectors that allow fuel injection into the combustion chambers of the gasoline engine, not shown. Depending on the engine configuration the rail may comprise more or less fuel outlet pieces 32.

The fuel outlet pieces 32 are formed as cylindrical sleeves 34 integral with the common rail body 16 and extending transversally to the longitudinal axis A, in particular radially, protruding from the lateral surface of the body 16. Each cylindrical sleeve 34 comprises an internal passage or branch bore 36 in communication with the longitudinal bore 18 and opening at an end face 38 of the fuel outlet piece 32. The branch bore 36 is typically centred in the fuel outlet piece 32 and extends radially. The end of the branch bore 36, away from the rail body 16, comprises a diverging end section 40 with an annular sealing surface 42 surrounding the branch bore 36. The diverging end section 40 may e.g. be formed as a frustoconical surface, widening up toward the end of the piece 32. Each cylindrical sleeve 34 also includes an outer threaded section 44, adjacent end face 38. Conventionally, the outer threaded section 44 is used for fixing the fuel pipes or ducts for the supply to and from the common rail 12.

Protrusion 32’ is similar in design to the fuel outlet pieces 32, but is used as inlet port. It is thus connected, in use, to a high pressure line to supply pressurized fuel to the common rail 12. This is only an example and in fact any protrusion 32 could serve as fuel inlet port.lt will be appreciated that the pressure sensor 14 is assembled by screwing onto one other fuel outlet pieces 32, as further described below. This contrasts with the common practice by which common rails have a specific cavity for the pressure sensor that is machined at one axial end of the common rail body. It may be further noted that the pressure sensor 14 may be fixed to any one of the fuel outlet pieces 32, which is convenient to adapt to the engine configuration.

The pressure sensor 14 comprises a roughly cylindrical sensor housing 46 between a socket-forming terminal connector 58 and a female connector portion 66. The sensor housing 46 defines an internal sensor chamber 54 delimited peripherally by a cylindrical wall 52 that extends between an outer face 48 of the female connector portion 66 and an opposite upper wall 50. The cylindrical wall 52 of the housing 46 has here a faceted external shape. A pressure sensor arrangement 56 is located in the sensor chamber 54.

The upper wall supports the terminal connector 58, which is configured to cooperate with a plug in order to connect the pressure sensor to an external control unit, not shown, with a control and evaluation circuit.

The pressure sensing arrangement 56 comprises sensing elements 60 adapted to determine the fuel pressure inside the rail from the deformation of a separation wall 62. The sensing elements 60 are mounted to one side of the separation wall 62 located inside the sensor chamber 54, whereas the opposite side of the separation wall 62 receives the fuel pressure. The pressure sensing arrangement 56 may be based on any appropriate pressure sensing technology known in the art like, for example strain gauges.

The pressure sensor 14 is mounted to the rail body by means of the female connector portion 66, which is screwed on a given outlet piece 32 (second from the left in Figs.1 and 2). The female connector portion 66 forms a female interface and includes a body 67 (generally made from steel) that defines an open cylindrical connector cavity 68 delimited radially by an annular wall or shell 78 and axially by a bottom wall 70 that defines a bottom surface 71 inside the cavity 68. The annular wall 78 is provided with an internal threaded section 80, which screws onto the outer thread 44 of the outlet piece 32. Threads 44 and 80 may e.g. be M14, or other suitable diameter. Reference sign 64 designates a sensing channel of the pressure sensor 14, which is here integrated in the body 67 of the female connector portion 66. This sensing channel 64 is generally cylindrical and oriented along a central axis B of the cavity 68. Sensing channel 64 opens into the cavity 68 to be in communication with the branch bore 36 of the outlet piece 32 and is closed at the other end by the separation wall 62 supporting the sensing elements 60.

Inside the cylindrical cavity 68 of the female connector portion 66, the pressure sensor 14 comprises a tubular nipple 72 formed as a metallic cylinder protruding from the bottom surface 71 toward the fuel outlet pipe 32, in axial continuation of the sensing channel 64. The nipple 72 forms a straight pipe connection from the sensing channel 64 to the branch bore 36. The nipple 72 is typically centrally located in the cavity, concentric with the annular wall 78.

The nipple 72 has a free end 74 that cooperates with the branch bore 36 of the sleeve 34. The free end of the nipple 74 comprises an end section with an annular sealing surface 76, preferably frustoconical. This sealing surface 76 has a cone angle that is lower than that of the first sealing surface 42 of the outlet piece 32. The nipple 72 is dimensioned to engage, when the pressure sensor 14 is screwed on the outlet piece 32, into the end section 40 of the branch bore 36 of the outlet piece 32 such that the sealing surfaces 42 and 76 engage with each other and thus achieve a fluid-tight annular sealing contact. In this context, it may be noted that the body 71 of the female connector portion 66, or at least the tubular nipple 72, is advantageously made from a material having a greater hardness than that of the outlet pieces of the common rail.

It shall be appreciated that the metal-to-metal annular seal with frustoconical surfaces provides a high quality sealing that requires less screwing torque than the conventional design of pressure sensors with male interface.

It may be further noted that the second sealing surface may be designed as frustoconical surface with different cone angles, or as any surface that provides a suitable sealing contact with the first sealing surface. In particular, the second sealing surface can be alternatively formed as rounded annular surface, e.g. sherical segment, or other shapes, but a flat annular surface (perpendicular to the cavity axis) is to be avoided.

In embodiments, not shown, the second annular sealing surface may be a conical peripheral surface of an annular bulge provided at the free end of the nipple.

Whereas in Figs.1 to 3 the nipple is manufactured in one piece with the female connector portion body 67, it is also possible to manufacture the nipple separately. This is the case in the embodiment of Fig. 4, where the nipple 72’ is thus a tubular piece separate from the body 67’. Here the nipple 72’ is formed by a protruding end section of a cylinder 73 having a closed end. The cylinder 73 is arranged in a passage 82 of matching diameter through the bottom wall 70' of the body 67, with its open end section protruding in the cavity 68'. Cylinder 73 may be assembled in the passage by screwing or welding, or any appropriate means. In the assembled state, the cylinder thus defines the sensing channel 64' and forms the separation wall 62' at the interface with the sensor chamber 54', on which the sensing elements 60' are mounted.