Exhaust brake and pressure modulation valve

Abstract: An exhaust brake valve comprises a body (10) having a butterfly (13) pivotable therein. The butterfly spindle (14) is offset from the axis of symmetry of the body (10). A radially inwardly directed internal projection (31) provides a seat for the butterfly in the closed condition, and for small butterfly openings gives a gas escape path of larger area than if the butterfly (13) were pivotable in a plain bore.

Description: This invention relates to exhaust brakes and exhaust modulation (EPM) valves, and particularly to an EPM valve for the exhaust system of a diesel engined vehicle. Devices known as 'exhaust brakes' can be fitted into the vehicle exhaust system and which, by generating a back pressure, can assist the vehicle in braking. Similar devices, termed 'warm-up' valves, can also assist in cab heating and in reducing the emission of unburnt hydrocarbons by reducing the time for the engine to reach normal operating temperature. In general the greater the back pressure generated by an exhaust brake, the more effective the braking effect becomes. But the level of back pressure generated (typically 2-8 bar) must not usually exceed the maximum design back pressure of the engine, which is limited, for example, by the load of the engine exhaust valve springs. To limit the back pressure, the exhaust brake, normally a butterfly valve or a sliding gate, must either be locked into a position which is almost but not quite closed, allowing some exhaust gas to escape around the edge, or have one or more bleed passages formed in or around the butterfly or sliding gate. It is usual in practice to drill a hole or holes, and allow the butterfly or gate to sit in a fully closed position. The leakage rate is determined by the size and number of bleed passages and is dictated by the maximum allowable back pressure when the engine is running at the highest speed, and the flow of exhaust has through the exhaust brake is thus at its maximum. Typically, the bleed passages may total 200mm 2 for a butterfly diameter of 100mm, about 2.5% of total butterfly area. It follows that at lower engine speeds, especially in the normal driving range, the bleed holes apertures are larger than is necessary to reach the maximum back pressure at these lower speeds. Accordingly such exhaust brakes are rather ineffective at moderate gas flow rates because the back pressure generated is much less than the maximum permissible. Warm-up valves are desirable because the time for the engine to reach operating temperature is substantially reduced, typically from 80 to 40 minutes - this leads to a substantial reduction in the emission of unburnt hydrocarbons and a considerable improvement in fuel economy. The design requirements of a warm-up valve, when used to reduce the time for the engine to reach normal temperature, are not however the same as those applicable to exhaust brakes. In general only a very small bleed passage is required, typically not more than 5 mm in diameter for a butterfly diameter of 100 mm, because the engine is running at tickover speed, and the vehicle is stationary; back pressure generated is in the range 0.5-2.0 bar, depending on the engine application. In a typical application, the back pressure generated by a warm-up valve will be only about 25% of the maximum back pressure generated during operation of an exhaust brake. A conventional exhaust brake would thus be useless as an aid to reducing engine warm-up time because the bleed apertures are too large to generate significant back pressure at these low engine speeds and gas flow rates. Warm-up valves may be driver operated, to minimise the time for the cab heater to begin the work, or may be operated automatically in response to emission control apparatus, to reduce the quantity of unburnt hydrocarbons exhausted whilst the engine is warming up. A particular danger with warm-up valves is that the driver may drive off with the warm-up valve closed; because the bleed passage is very small, the back pressure generated may quickly exceed the design limit of the engine, and engine damage may follow. FR-A-1472588 describes an exhaust brake valve including a circular butterfly mounted pivotally in an exhaust duct on an offset pivot axis, and an abutment projecting from the sidewall of the duct which is arranged so that the circular face of the butterfly seals against the abutment when the valve is in the closed state. WO92/08887 relates to combining exhaust braking and engine warm-up functions in a single valve. The valve includes an offset butterfly, and a pneumatic actuator for operation of the butterfly. The invention provides an improved reactive exhaust brake which has significantly better performance than previous proposals. The present invention also provides an improved exhaust pressure modulation valve, which combines the functions of an exhaust brake and warm-up valve, has a single butterfly to close the exhaust tract and has improved performance over valves previously proposed. According to the present invention, there is provided an exhaust brake valve comprising a body having an inlet, an outlet, and a passageway between said inlet and outlet, and a butterfly pivotable in the body to close communication between said inlet and outlet, said butterfly having a spindle and the spindle pivot axis being offset from an axis of symmetry of said passageway such that a resultant torque generated in response to increasing pressure at said inlet tends to open the butterfly said valve further comprising operating means for closing said butterfly and adapted to apply to said spindle a closing torque of the same magnitude as said resultant torque generated at a pre-determined pressure at said inlet, the body having a continuous radially inwardly directed projection, a peripheral radially outwardly facing edge of the butterfly sealing against a radially inwardly facing edge of said projection in the closed condition of said valve. Such a valve has a butterfly which reacts to inlet pressure so that the butterfly opens when the predetermined inlet pressure is exceeded. The predetermined inlet pressure is set at or just less than the safe maximum design back pressure of the engine thus ensuring that the butterfly will open if excessive back pressures are generated. In reactive mode the butterfly opens just enough to allow upstream pressure to drop to the predetermined level. It is important to ensure that movement of the butterfly is small to minimise mechanical and frictional losses. Excessive hysteresis results in slow closing of the butterfly which in turn reduces exhaust brake effectiveness since upstream pressure is permitted to drop well below the predetermined level. The present invention gives a more rapid gas escape path and thus it is easy to achieve a more constant regulation of back pressure. Preferably, the valve includes a bleed aperture from one side of the butterfly to the other said bleed aperture being sized to impose significant back pressure on an engine at tickover speed. Such an aperture enables the valve to be used as an EPM valve for reducing engine warm-up time. The radially inwardly directed projection ensures that marginal angular movement of the butterfly from the closed condition gives a much larger opening than would be achieved with a butterfly moving through the same angle in a plain bore. The effect is to reduce the extent of butterfly movement required to control engine back pressure close to the desired maximum. This permits better gas flow control characteristics and enables the valve to re-close at close to maximum back pressure with very low flow rates and consequently at correspondingly low engine speeds. Preferably the inwardly directed projection is circular, the closed butterfly being substantially at right angles to the longitudinal axis of the valve body. This arrangement requires slight clearance around the butterfly in the closed condition, but is relatively inexpensive to manufacture and suitable for use in applications where the required maximum back pressure is not too high. This projection may however be substantially elliptical where the butterfly is required to sit in the closed position at an angle to the longitudinal axis of the valve body. This arrangement ensures a tight seal around the butterfly and enables back pressure of over 5 bar to be sustained. Such an arrangement also permits the valve to be used as a diverter valve in twin stack exhaust systems, where a good gas seal is essential. The radial extent of the projection need only be slight, typically 3-4 mm for a valve body bore of about 100 mm. In high back pressure applications as mentioned above, it is usual for the butterfly to seat on the valve body, and therefore be substantially elliptical. In a preferred embodiment of the invention, the inwardly directed projection has a progressively increasing width in the direction parallel to the longitudinal axis of the valve body from the spindle axis to a point at 90° therefrom. Such an arrangement permits different butterfly seating angles depending on the direction of gas flow through the valve body. Preferably the projection defines one circular and one elliptical edge, thus providing a sealing edge for a substantially circular butterfly in one direction of gas flow, and another sealing edge for a substantially elliptical butterfly in the other direction of gas flow. This arrangement can lead to substantial cost savings since a single valve body may be used for different valves. The inwardly directed projection also leads to very substantial savings in cost and manufacturing time since only the bore of the projection need be machined, the reminder of the valve body being cast with opposed shallow tapers. Prior art valve bodies required the entire valve body bore to be machined to size. Hardened butterfly spindles are preferred to reduce hysteresis. Typically said pre-determined pressure would be in the range 2-8 bar, depending on the application, and the bleed aperture would be sized to impose a back pressure of about 25% of maximum or about 0.5-2.0 bar at flow rates commensurate with an engine running at tickover speed. In one embodiment said bleed aperture has a total area of not more than 0.25% of the area of said butterfly. Thus the bleed aperture is in size an order of magnitude less than the bypass aperture in a conventional exhaust brake. In a preferred embodiment having a butterfly area of 7850 mm 2 , a single bleed aperture having an area of 12.5 mm 2 was found adequate to impose a back pressure of approximately 0.5 bar at engine tickover; the maximum permissible back pressure at maximum engine speed being about 4 bar. The valve is safe when used in 'warm-up' mode because the butterfly will react to excessive back pressures. The area of said bleed aperture is too small to significantly affect reactive operation of the butterfly and in any event the pre-determined inlet pressure can be set at a level which compensates for the effect of a small throughput of exhaust gas. Such an exhaust pressure modulation valve provides both functions with a single butterfly and additionally significantly improves the effectiveness of the exhaust brake at moderate engine speeds. The offset of the butterfly pivot axis is typically in the range 2 to 4 mm, for a valve having a circular exhaust tract and a diameter of 100-150 mm. In one embodiment, the butterfly spindle is loaded by a return spring and operably connected to actuator means via a control spring whereby the resultant torque tending to close the butterfly can be set by for example, appropriate selection of one or both of the springs so as to balance the opening torque generated at a predetermined level of back pressure. This arrangement of balanced torques acting on the valve is particularly valuable because the butterfly is sensitive to very small changes in back pressure at the predetermined level at which the butterfly is set to open. An important further advantage of the invention is that butterfly movement is reduced for a given degree of gas flow control. Accordingly the required operational travel of an actuator for the butterfly when operating in reactive mode is very small. The actuator is also used for moving the butterfly from the inactive to the active position. It is preferred that the said valve closing torque be adjustable. In the case of an actuator arranged to push or pull a torque arm pivotal with the valve, adjustment of the applied torque can be achieved by varying the length of the torque arm. This arrangement is suitable whether the actuator is mechanically, electrically, pneumatically or hydraulically operated. In a preferred embodiment, a butterfly actuator includes an external reaction spring co-extensive with the actuator body and reacting the body against a mounting frame. Preferably the spring is a coil spring and may have an adjustable abutment to permit the reactive pre-load to be changed. Pneumatic actuators are usually connected to a compressed air supply line providing line pressure to act against an internal return spring. The external reaction spring for the actuator body has the advantage that movement of the actuator piston against an internal spring is avoided - seal friction is obviated and hysteresis thus reduced. Actuation of the butterfly may be in response to driver action, for example in exhaust brake or cab heat mode, or may be under automatic control, for example to control exhaust emissions. The predetermined pressure at which the butterfly opens reactively may be varied by control means in accordance with engine management or other systems to suit particular operating requirements. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a graphical comparison of a conventional exhaust brake and an EPM valve according to the present invention; Figure 2 is an elevation of a pneumatically operated EPM valve; Figure 3 is a view on the EPM valve in Figure 2 from the outlet side; Figure 4 is a plan view of the EPM valve shown in Figure 2; Figure 5 is an axial section through a valve according to the invention and in one configuration; Figure 6 is an axial section through the valve illustrated in Figure 6 and in another configuration; and Figure 7 is a plan view of a valve similar to that of Figure 4. Figure 1 illustrates the difference between a conventional exhaust brake and a reactive exhaust brake. The dotted line illustrates the effect of conventional exhaust brake on back pressure (P) with increasing engine gas flow (F); gas flow of course increases with increasing engine speed. A conventional exhaust brake includes one or more relatively large holes in the butterfly so that at maximum gas flow (maximum engine speed) the back pressure developed by the exhaust brake does not exceed the maximum design back pressure of the engine - this is illustrated by point C1. Accordingly at low gas flow rates, the butterfly holes are too large to generate an effective back pressure, and in the normal operating range of the engine, the back pressure generated may be only 20-50% of the maximum permissible - illustrated by points C2 and C3. Work done is illustrated by the area below this curve, and can be seen to be very low at low rates of gas flow. In contrast, the chain-dot of Figure 1 illustrates the operating curve of a reactive exhaust brake operating in a plain bore and when opening. The operating curve rises steeply at higher engine speeds. The difficulty with this valve is that butterfly movement is significant in order to give an opening sufficient to cause the desired drop in upstream pressure; frictional and mechanical losses result in hysteresis and cause a lag in butterfly closing movement; back pressure consequently drops well below the desired level before the butterfly re-closes, and the performance curve approaches that of the non-reactive valve as flow and pressure reduce. The solid line of Figure 1 illustrates the operating curve of an opening reactive exhaust brake incorporating the continuous internal projection of the present invention. In the normal operating range of the engine the increase of back pressure with increasing gas flow rate is very small, the consequence being that very high back pressure may be developed at low engine speeds. Butterfly movement is reduced for a given gas flow and accordingly mechanical and friction losses are also reduced, leading to a quicker reaction time and more rapid butterfly closing. As a result the performance curve on valve closing closely approaches the valve opening curve. Accordingly the effectiveness of the exhaust brake at low engine speeds is improved, as indicated by the work done. The exhaust brake of Figures 2-4 includes a cylindrical valve body 10 for connection by end flanges 11,12 to an exhaust tract and within which is a butterfly-type valve 13 mounted upon a spindle 14 to pivot about an axis displaced by a predetermined distance d (typically 4 mm for a valve of up to 125 mm diameter)from a diameter of the valve body 10. The direction of gas flow is indicated by arrow D. Rigidly mounted on the butterfly valve spindle 14 externally of the valve body 10 is a lever 15 connected to an adjustable piston rod 16 of a pneumatic actuator 17, the body of which is pivoted on a mounting plate 18 by a pivot pin 19. An internal return spring of the actuator urges the piston rod 16 rightwards as illustrated to hold the lever 15 against a return stop 21 and retain the butterfly in the open condition. The mounting plate 18 is attached to the valve body 10 by bolts 22 and a stud and nut of stop 21. The stop 21 acts on the other end of lever 15 in the closed condition of the butterfly. The butterfly 13 is closed by compressed air supplied to the actuator 17 by a pressure control valve 23 connected to a source S of compressed air, the air supplied acting against the actuator return spring to generate a closing torque on the butterfly 13 and urge the valve closed. The pressure of air supplied to the actuator is controlled by the valve 44 which is set to hold the valve closed against the actuator return spring and the torque generated by the exhaust gas at the prescribed maximum back pressure level. If the back pressure starts to rise under the action of increased exhaust gas glow an imbalance is created due to the offset spindle axis and the resulting torque combined with the actuator return spring torque overcomes the pneumatic actuator torque so that the butterfly opens allowing the back pressure to drop to the prescribed level at which butterfly recloses. The valve 12 is thus reactive and opens at a pre-set back pressure to prevent engine damage while ensuring that the valve is fully effective as an exhaust brake throughout the normal operating range of the engine. Figure 5 and 6 illustrates a preferred form of valve body 30 having an asymmetric internal projection 31. The projection 31 provides a first butterfly seat defined by semi-circular edges 32,33, which meet adjacent the spindle axis 34; the spindle axis is offset from the longitudinal axis of the bore by a small amount d. The butterfly 36 is shown in the closed condition, the reactive open condition being illustrated by dotted lines; the butterfly pivots clockwise to open. Gas flow is in the direction indicated by arrow F1. The projection 31 provides a second butterfly seat defined by edges 42,43 at about 70° to the longitudinal axis of the valve and each in the form of half an ellipse. In this case gas flow is in the direction F2. The butterfly 46 is shown closed, the reactive open condition being illustrated by dotted lines as for Figure 5; the butterfly pivots anticlockwise to open. A second travel stop is not required. In this way a single inwardly directed projection serves to provide different types of butterfly seat which may be used to manufacture valves for conventional and high back pressure applications. The end flanges may be chosen to suit the exhaust system into which the valve is to fit; this is a matter of choice but for the purposes of illustration the flanges are identical to those shown in Figure 2-4. The open butterfly positions shown in Figures 5 and 6 illustrate typical maximum movement required to ensure effective reactive operation of the exhaust brake. Figure 7 illustrates an alternative exhaust brake similar to that shown in Figure 4; the same parts are given the same reference numerals. The actuator 17 has an external spring 51 operable between a collar 52 of the actuator body and an abutment 53 fixed to the frame 18. The actuator is guided by a pin 54 located in an open slot 55 of the frame. In use the external spring provides reaction for the actuator when pressurised to close the butterfly. As the gas pressure increases against the butterfly, the increasing torque developed due to the offset butterfly spindle is reacted through the spring 51. At the desired level the spring yields allowing the butterfly to open and thus reduce engine back pressure. Since butterfly movement in reactive mode is small, actuator articulation is also small and the clearance between the actuator body and abutment 53 can be minimal. A particular advantage of the external reaction spring 51 is that variations in air pressure to the actuator do not affect reactivity provided that net actuator force exceeds the reaction spring load; the actuator acts as a strut. Because reaction is against a spring rather than through movement of the actuator piston relative to the actuator body friction and therefore hysteresis is further reduced. In the preferred embodiment collar 52 comprises a cylindrical clamp secured by a nut and bolt 56 and which may be moved axially on the actuator body to vary the pre-load of the spring 51. The collar could alternatively be fixed or screw-threaded for movement. A variable spring pre-load is useful for adjusting the point at which the butterfly opens. Movement of the butterfly is small, as previously mentioned, and the rate of spring 51 is of less importance.

Claims: An exhaust brake valve comprising a body (10) having an inlet, an outlet and a passageway between said inlet and outlet, and a butterfly (13) pivotable in the body to close communication between said inlet and outlet, said butterfly having a spindle (14) and the spindle pivot axis being offset from an axis of symmetry of said passageway such that a resultant torque generated in response to increasing pressure at said inlet tends to open the butterfly (13), said valve further comprising operating means (15,16,17) for closing said butterfly and adapted to apply to said spindle a closing torque of the same magnitude as said resultant torque generated at a pre-determined pressure at said inlet, said body (10) having a continuous radially inwardly directed projection (31), characterized in that a peripheral radially outwardly facing edge of the butterfly (13) seals against a radially inwardly facing edge of said projection in the closed condition of the valve. A valve according to claim 1 wherein said projection 31 is substantially circular, the butterfly (13) being substantially at right angles to the longitudinal axis of the valve body in the closed condition. A valve according to claim 1 wherein said projection (31) is substantially elliptical, the butterfly (13) being at an angle of less than 90° to the longitudinal axis of the valve body in the closed condition. A valve according to claim 1 wherein the projection (31) has a progressively increasing width in a direction parallel to the longitudinal axis of the valve body (10) from the spindle axis to a point 90° therefrom. A valve according to claim 4 wherein the projection provides one substantially circular sealing edge (32,33) and one substantially elliptical sealing edge (42,43). A valve according to any preceding claim wherein the radial height of said projection (31) is less than 4% width of said passageway. A valve according to any preceding claim wherein the portions of said passageway on either side of said projection have a divergent cross-section. A valve according to any preceding claim and further including a gas by-pass for said butterfly. A valve according to claim 8 wherein said bypass comprises one or more bleed apertures in the butterfly (13). A valve according to claim 9 wherein said one or more bleed apertures comprise not more than 0.25% of the cross-sectional area of said butterfly (13). A valve according to any preceding claim and having a spindle offset less than 4% of the width of said passageway.

Inventors: FLETT MAGNUS EDWARD, THOMPSON DEREK, FLETT, MAGNUS EDWARD, THOMPSON, DEREK
Applicants: WABCO AUTOMOTIVE UK, WABCO AUTOMOTIVE U.K. LIMITED
Application year: 1993
Main classification: F02D9
IPC codes: F02D9