The present invention relates to a butterfly valve, and particularly but not exclusively to a butterfly of a pressure modulation valve for an exhaust brake of a vehicle. Pressure modulation valves in general, and exhaust pressure modulation valves (EPM valves) in particular, are designed to open when the fluid pressure applied to the valve reaches a predetermined level. In order for this to occur, a butterfly is typically pivotal ly mounted in a flow pipe on a spindle offset from the axis of the pipe. Accordingly, the fluid pressure generates a torque about the spindle, and this torque may be regulated by a spring or an actuator applying restraining torque to the spindle. In such a device, hysteresis is encountered due to the action of friction on the butterfly. This hysteresis is undesirable in an exhaust brake because precise control of backpressure is consequently hindered.

Friction at the spindle has been found to be roughly proportional to the spindle diameter. Since in practice the spindle diameter is selected in accordance with the bore diameter of the flow pipe, friction may be said to be proportional to the bore diameter. Torque applied to a butterfly by a fluid pressure is proportional to the offset of the spindle from the axis of the flow pipe, and so for a predetermined bore diameter it can be seen that the ratio of torque to friction may be increased by increasing the offset. This in turn will decrease the hysteresis inherent

to the valve .

In the case where the butterfly and pipe are circular, an offset pivot axis causes problems of clearance, particularly when the butterfly is fully open. In order to allow the butterfly to open fully, segments of the butterfly in the region of the spindle must be cut away. In order to prevent leakage of fluid around the butterfly at the cut away segments, clearance bushes must be provided to fill the gaps between the side wall of the flow pipe and the sealing edge of the butterfly. This leads to a loss of effective butterfly area and increased manufacturing expense.

If offset is increased (to reduce friction) larger segments must be cut-away to permit the butterfly to fully open. This reduces the effective area of the valve and increases manufacturing expense. The larger offset also requires larger clearance bushes which tend to increase the effective leak area defined by the bush/butterfly gap. The butterfly can be arranged to seal on the bore of the flow pipe by closing at an angle of less than 90°, but the leak area adjacent the clearance bushes cannot be sealed. Moreover this leak area can be rather large since an allowance for thermal expansion of the butterfly is required.

It is an object of this invention to provide increased offset without consequent loss of effective butterfly area.

According to a first aspect of the invention there is provided a butterfly valve comprising a cylindrical tubular

housing having an interior wall defining a fluid flow path, a butterfly pivotal ly mounted in the housing on an axis coplanar with the butterfly and perpendicular to and offset from the longitudinal axis of the cylindrical housing, the butterfly defining a sealing edge, and the valve including stop means to limit the maximum opening angle of the butterfly in use, wherein when the butterfly is in a fully closed state, the sealing edge thereof is sealingly contiguous with the interior wall, and when the valve is in a fully open state the butterfly engages said stop means such that the sealing edge is contained in an envelope defined by the projection of the open butterfly in the direction of the axis of the housing, and the plane defined by the sealing edge defines an oblique angle with the axis of the housing.

Preferably the butterfly comprises a spindle housing and a spindle mounted therein, wherein the envelope is defined by the projection of the spindle housing.

Preferably, in the fully open state, the sealing edge is contiguous with the envelope.

Hitherto it has always been assumed that a butterfly should be fully open (i.e. perpendicular to the flow duct) in order to give maximum throughput. In some butterfly valves however, the spindle diameter of the butterfly is significant, and is in fact the most important factor in determining the minimum cross-sectional area of the flow duct. It is the realisation of this fact that has resulted

in the present invention.

The practical effect of the invention is that a greater offset can be provided (and thus reduced hysteresis) for a given size of clearance bush. As a consequence, the manufacturing cost does not increase with offset as in the prior arrangement and the leakage area in the gap between the butterfly and clearance bush is not increased.

Preferably the oblique angle is greater than 2° and less than 10°. In a preferred embodiment, the oblique angle is 5°.

This invention advantageously reduces the problems associated with providing clearance for an offset butterfly as it approaches the fully open condition. It is a further advantage of the invention that the working angular travel of the butterfly is reduced, without loss of effective cross-sectional area; this reduces the required stroke of an actuator connected to the spindle or other means of opposing torque generated by the butterfly.

The invention also allows the clearance bushes to be dispensed with in the case of a small offset.

In the case where a butterfly valve is in its open position, it is desirable to present little or no resistance to the passage of fluid past the butterfly, in order to cause minimal disruption to the flow pattern. Preferably in the first aspect of the invention, the butterfly is shaped so as to be streamlined in the flow direction. The butterfly may comprise side projections of

aerofoil section.

According to a second aspect of the invention there is provided a butterfly valve comprising a tubular housing having an interior wall defining a fluid flow path, a butterfly pivotally mounted in the housing on an axis coplanar with the butterfly and perpendicular to the axis of the housing, wherein the butterfly is shaped so as to be streamlined in the flow direction. Preferably the butterfly comprises side projections of aerofoil section. The side projections are preferably shaped so as to define a boundary layer about the butterfly in use which is not susceptible to the formation of separation zones between said butterfly and said boundary layer. In particular the external profile of the butterfly may present a substantially smooth non-discontinuous surface from the maximum spindle diameter to the region of the edge thereof. The butterfly may have such a smooth surface on one or both sides thereof, and or on one or both faces of one limb thereof, or any practicable and effective combination. According to a third aspect of the invention there is provided a method of opening a butterfly valve from a fully closed state to a fully open state, the butterfly valve comprising a cylindrical tubular housing having an interior wall defining a third flow path, a butterfly pivotally mounted in the housing on an axis coplanar with the butterfly and perpendicular to and offset from the longitudinal axis of the cylindrical housing, the butterfly

defining a sealing edge which in the fully closed state is sealingly contiguous with the interior wall, the method comprising the step of rotating the butterfly to a stop position at which the sealing edge is contained in an envelope defined by the projection of the open butterfly and the plane defined by the sealing edge defines an oblique angle with the axis of the housing.

A particular embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which:

Figure 1 is an elevation in the flow direction of a prior art example butterfly valve with the butterfly closed.

Figure 2 corresponds to Figure 1 and shows the butterfly open;

Figure 3 is a partial section through the valve of Figure 1 showing the butterfly in its fully open condition;

Figure 4 is an elevation in the flow direction of a butterfly valve in accordance with the invention, with the butterfly closed;

Figure 5 corresponds to Figure 4 and shows the butterfly open;

Figure 6 is a partial section of the inventive valve and

corresponding to Figure 2; and

Figure 7 corresponds to Figure 6 and is an elevation showing a schematic external travel stop.

Referring to Figure 1, a prior art butterfly valve 10 comprises a right cylindrical tubular housing 12 and a substantially circular butterfly 14. Dimensions are exaggerated in order to illustrate features of the valve. The housing and butterfly typically of cast iron. The diameter of the butterfly 14 is substantially the same as the internal diameter of the housing 12, thereby enabling the butterfly 14 to be used to obstruct the flow of fluid through the housing 12 when in the closed condition. In Figure 1, the gap between the butterfly 14 and the housing 12 is enlarged for reasons of clarity. The butterfly 14 comprises a tubular spindle sleeve 16

(shown in dotted outline in Figure 1) extending in the plane of the butterfly 14 parallel to but offset from a diameter of the butterfly 14. The offset 15 shown in the drawings is exaggerated in order to demonstrate the invention; in reality the offset is usually much smaller.

The spindle sleeve 16 receives a spindle 17. A keying arrangement (not shown) is used to engage the sleeve 16 with spindle 17 for pivotal movement of the butterfly 14 with the spindle 17. The spindle 17 is journalled in the housing 12 perpendicular to the longitudinal axis of the housing 12.

On one side the spindle 17 passes through the housing 12 and can be connected to an actuator (not shown) for control of the butterfly 14.

In the closed condition the butterfly is adapted to shut at an angle to the axis of the housing, thereby to provide effective seal; the edge of the butterfly may be machined accordingly. The closing angle may be rather large for example 25° to the axis of the housing where significant thermal expansion must be accommodated. The butterfly 14 is truncated by the removal of two segments thereof of equal area from opposite sides of the butterfly 14 adjacent the spindle 17. The segments are formed by parallel chords of the circular butterfly 14, the chords being perpendicular to the axis of the spindle 17. The resulting flats 18 are required to enable the butterfly to swing through a full right angle and attain the position illustrated in Figures 2 and 3: the butterfly 14 being substantially parallel to fluid flow through the butterfly valve housing 12, the flow direction being indicated by arrow A.

The flats 18 reduce the effective area of the butterfly. Bushes 19 are required in order to occlude the clearances created by the flats 18, but they reduce the effective cross sectional area of the housing, and due to their rather complicated shape they increase manufacturing expense. Large bosses 20 are required to accommodate the bushes 19.

Figure 2 illustrates the effective cross-sectional area of the butterfly valve in the open condition, comprising the areas 21A and 21B. Figure 3 shows the relatively massive construction of the butterfly which is necessary for strength and dimensional stability. The wings of the butterfly are of relatively large thickness, but are reduced at the edge to provide a narrow sealing land 22 for abutment with the housing 12. The large part of the circumference formed by flats 18 cannot be sealed by abutment, and since an operating clearance is required they constitute an inevitable leakage path. The larger the flats 18, the larger the leakage path.

Figure 4, a butterfly valve 100 according to a particular embodiment of the invention is constructed similarly to the butterfly valve of Figures 1-3. As shown in Figure 3, the butterfly 114 has an offset axis, but lacks flats. Accordingly, bushes to occlude gaps created by cut¬ outs are not required. The butterfly 114 is shown in the open condition in Figure 5. The butterfly 114 further comprises solid shaped portions 124 arranged to deflect fluid flowing through the housing 112 around the butterfly. The shaped portions 124 are arranged on either side of the butterfly downstream of the leading edge of the butterfly 114 and of the spindle sleeve 116, as illustrated, and are smoothly curved to present a somewhat spherical surface. In that way, by modifying the shape of the boundary layer which forms around the butterfly when fluid flows past, separation

of the boundary layer may be reduced or avoided and throughput thereby increased. In the open condition, the projected area of the butterfly 114 in the direction of the longitudinal axis of the housing 112 is at a minimum, being defined by the projection of the spindle sleeve 116 which is the thickest part of the butterfly. The butterfly 114 lies at an angle θ (approximately 5°) from the direction of the axis of the tubular housing 112. Such an angle allows the butterfly 114 to turn to the fully open position without the need for flats to prevent fouling of the butterfly 114 on the sidewall of the housing 112. Stagnation and separation in the flow around the butterfly is avoided by use of the shaped portions 124. A stop to limit travel of the butterfly beyond the desired open condition may be provided in any suitable manner. For example, Fig. 7 corresponds to Fig. 6 and shows an external boss 130 of the housing 112 engageable with a schematic operating arm 131 of the spindle 117; a simple screw threaded stop 132 is illustrated. The butterfly is closed by moving the arm 131 in the direction of arrow A by any suitable means, for example a pneumatic actuator.

The solid shaping of the butterfly provides the necessary strength and support for the sealing edge 125 thereof, and thus the sealing edge 125 can be somewhat narrower than the prior art.

The preferred embodiment permits an offset to be provided in a butterfly valve which does not have the cut-

outs hitherto necessary to enable the butterfly to lie parallel with the axis of the housing when in the open condition.

The butterfly can also have a lower drag coefficient when open than prior art butterflies, since stagnation and separation zones can be reduced by the provision of the shaped portions.

The butterfly could also include shaping of both sides of both limbs, thereby to improve flow in the duct, though as will be appreciated the rounded part of the spindle sleeve 116, as illustrated, does not provide significant resistance to flow.