GB2309052A - Joining exhaust pipes of i.c. engines in vehicles - Google Patents

Abstract

Exhaust pipes 22,24 from respective banks of a transversely mounted vee-type engine are joined together by a joint in which one of the pipes, 22, is straight and forms an inner pipe while a connector 28 turns the other pipe through 180 degrees and leads it into a common connector tube 26 which surrounds the inner pipe. The effective lengths L1,L2 of the respective pipes 22,24 are equal. The area D3 is twice D1 which is equal to D2. The free end of the inner pipe may be mitred (fig.7) or perforated (fig.8) or extend inside a flexible portion (18) of the outer pipe (26d) (fig.9). Portions of the pipes (22f, 24f, 26f, figs. 11,12) may be double-walled to provide thermal insulation. The arrangements of figs. 11 and 12 are for transversely mounted in-line engines.

Description

Vehicle Exhaust Svstems The invention relates to an exhaust system primarily for a vee type or a flat engine. In particular, but not exclusively, the invention relates to an exhaust system for such an engine wherein the banks of cylinders are arranged transversely within the engine compartment of a vehicle.

Vehicle engines of the vee type configuration comprise a first bank of cylinders aligned on one side of an engine block and a second bank of cylinders aligned on the other, and the banks are often arranged transversely in the vehicle engine compartment. It is known to provide an exhaust system for a vehicle having such a transversely arranged engine block with a forwardmost and rearmost cylinder arrangement wherein a forward exhaust downpipe is taken beneath the engine block and joined with the exhaust downpipe from the rearmost bank of cylinders at a common point along the exhaust pipe. In such a configuration, the length of the forwardmost downpipe from the point in the engine where the exhaust gases are emitted, to the point where it joins with the rearmost downpipe is much shorter than the equivalent length for the rearmost downpipe.

The uneven length of pipes generate unevenly timed pulses of gas pressure at and downstream of the point at which the downpipes join. This leads to an increase in the noise level of the engine for example.

It is known to attempt to overcome this problem by providing downpipes of equal lengths between the cylinder banks and the point where they join a common connector tube.

However, in certain crowded vehicle engine bays it is difficult to increase the length of the rearmost downpipe to match that of the forwardmost downpipe. Additionally, the increase in length of the rearmost downpipe is relatively expensive to manufacture. Also, it is not possible in such an arrangement to optimise the power output from the engine by tuning the exhaust to provide optimum back pressure at the cylinder exhaust outlet since the lengths are fixed.

Accordingly, an object of the invention is to avoid or at least mitigate these problems of the prior art. One aspect of the invention provides an exhaust system for an internal combustion engine having first and second cylinder banks each comprising at least one cylinder, the banks being arranged to drive a common crankshaft, and being angularly offset from each about the axis of the crankshaft, the exhaust system comprising first and second exhaust conduits for conducting exhaust gases from the first and second banks respectively, the two exhaust conduits converging into a single exhaust pipe at a joint having an inner wall and an outer wall, the inner and outer walls having a space therebetween forming the end of the first conduit and the inner wall forming the end of the second conduit and having a free end defining the end points of the first and second conduits, wherein the effective lengths of the first and second conduits measured from the respective cylinder banks to the free end of the inner wall are substantially the same.

The reason for keeping the effective lengths of the conduits the same is to ensure that the time taken for pulses of exhaust gas to travel from the cylinder to the point where the conduits join is the same for each bank of cylinders. This can be achieved by making the lengths of the two conduits the same if their cross sectional areas are the same. However, by varying the cross sectional areas of the conduits it is possible for them to have slightly different actual lengths, whilst keeping the effective lengths the same i.e. whilst keeping the time taken for pulses of exhaust gas to travel from the cylinder to the point where the conduits join the same. This is because the pulses will generally travel faster in a narrower pipe.

Embodiments of the prior art and the invention, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a known vehicle engine and exhaust system; Figure 2 is a pulse train over time for the gas pressure produced using the system shown in Figure 1; Figure 3 is a schematic drawing of a second known engine similar to that shown in Figure 1 except that the downpipes are of equal length; Figure 4 is a pulse sequence over time for the gas pressure of a system shown in Figure 3; Figure 5 is a schematic sectional drawing of exhaust piping according to the invention; Figure 6 is an enlarged view of part of the piping shown in Figure 5; Figure 7 is a view of part of a second embodiment of exhaust piping according to the invention;; Figure 8 is a schematic view of part of a third embodiment of exhaust piping according to the invention; Figure 9 is a schematic view of a fourth embodiment of exhaust piping according to the invention; Figure 10 is a plan view of a fifth embodiment of part of exhaust piping according to the invention; Figure 11 is a sectional side-elevation view of a sixth embodiment of exhaust piping according to the invention; and Figure 12 is a sectional plan view of the sixth embodiment of exhaust piping according to the invention in which the downpipes are shown in an unrolled configuration, compared to Figure 11, for clarity.

Figures 1 to 4 provide details of known engine exhaust systems. Engine E shown in Figure 1 is a six cylinder engine in the vee configuration fitted transversely in a vehicle (not shown). Accordingly, the length Ll of the forwardmost downpipe 12 from the front cylinder block is longer than the length L2 of the rearmost downpipe 14. The downpipes 12 and 14 are joined at a common connector tube 16 wherein the flow of exhaust gases from the downpipes is mixed together and a single outlet path is provided between tube 16 and outlet 0. The exhaust passes from common connector tube 16 through a flexible connector 18 to a series of small inter-connecting pipes and boxes CB and B, to an outlet O typically provided at the rear of a vehicle.The first box CB after flexible connector 18 can comprise a catalyst for removing toxic gases whilst two known exhaust boxes B can also be provided. The exhaust system is typically hung resiliently on hangers H from the vehicle body or chassis C.

Figure 2 shows a single cycle over time of the uneven pulses of gas pressure observed downstream ofthejunction at tube 16. The uneven distribution over time is created due to the uneven length of pipes 12 and 14, in spite of the regular time interval between the exhaust stroke for each of the six cylinders. This uneven timing distribution gives rise to a decrease in the silencing effect of the exhaust system amongst other things.

Figure 3 shows a second embodiment of known exhaust piping wherein downpipes 12 and 14 are of equal length between the engine block and common connector tube 16. This configuration gives rise to the even sequence of gas pressure pulses shown in Figure 4.

However, the space in a vehicle engine bay with a vee type engine is commonly very restricted and it has proved difficult to accommodate this arrangement in some engine bays whilst maintaining a satisfactory gas flow. Also the length of the pipes 12 and 14 are dictated by the position of the common connector tube 16 within the engine which prevents optimisation of power requirements through adjustment of the lengths of the downpipes. Furthermore, it has been found that an uneven distribution of back-pressure at the upstream ends of pipes 12 and 14 is created thereby reducing the performance of the engine E.

Figures 5 and 6 show schematically exhaust piping 20a according to the invention wherein a vee configuration engine E to be fitted transversely in a vehicle is connected to a forwardmost downpipe 22 and rearmost downpipe 24. The downpipes 22, 24 each conduct exhaust gases from their respective bank of cylinders, and join at a common connector tube 26 wherein downpipe 22 protrudes axially into the common connector tube 26 to from an inner tube, whilst pipe 24 is attached peripherally to the common connector tube 26, which forms an outer tube, using an end-connector 28. The end connector 28 is U-shaped, having two parallel arms 28a, 28b, one of which is connected to the downpipe 24, and the other of which is connected to the common connector tube 26.

As can be seen from Figure 6, exhaust gases flow through pipe 22 into common connector tube 26 via an aperture 23 at the end of the inner tube, and similarly exhaust gases flow down pipe 24 through end-connector 28 which turns it though an angle of 1800 into the gap between the inner tube 22 and the common connector tube 26 at aperture 25. The exhaust gases from pipe 24 therefore flow over the end part of pipe 22, and substantially parallel to it, within common connector tube 26 before a common flow of exhaust gases then flows down through the rest of the exhaust system, such as flexible connector 18 piping and exhaust boxes B shown in Figure 1.

The effective lengths L 1 and L2 respectively of the downpipes 22 and 24 are measured from equivalent points on opposite banks of cylinders to the positions indicated in Figure 6 at the downstream end in the common connector tube 26, i.e. at aperture 23 of pipe 22 in tube 26. The end connector 28 can be positioned under the sump of engine E. The lengths L1 and L2 of downpipes 22 and 24 can be adjusted by adjusting the length by which tube 22 protrudes into the common connector tube 26, whilst keeping them substantially equal. Such adjustment is found to improve considerably noise and power output in design trials of the exhaust system.

A mean diameter Dl and D2 for pipes 22 and 24 respectively is also indicated in Figure 6.

The radial cross-sectional area of common connector tube 26 is determined by its mean diameter D3 shown in Figure 6, and is preferably equal to the total area of the radial cross sectional areas of pipes 22 and 24. Accordingly, the radial cross-sectional area of the region between pipe 22 and tube 26 through which the exhaust gases of pipe 24 flow is preferably substantially equal to the cross-sectional area of pipe 24. This ensures that the velocity of the exhaust gas flow from the pipes 22 and 24 is substantially equal at the mixing point adjacent aperture 23 of pipe 22.

However, in designing the exhaust piping 20a for a given engine, it is possible to vary the relative diameters and lengths of pipes 22 and 24 and tube 26, in order to optimise the power performance from the engine and also the noise and vibration characteristics of the exhaust system. Additionally, the radial position of pipe 22 in common connector tube 26 can be varied but preferably this is positioned concentrically within tube 26.

Figure 7 shows part of a second embodiment of exhaust piping 20b according to the invention wherein the end 30 of the forwardmost downpipe 22b is mitred. The longer side of pipe 22b is positioned adjacent to outlet aperture 25 from downpipe 24. The effective lengths of the downpipes 22b and 24 are measured to a point approximately half way between the longer and shorter sides of the mitred end 30.

In another form, as shown in Figure 8, downpipe 22c a regular array of apertures or perforations 32 is provided through the end of the pipe 22c within common connector tube 26. Beneficially, the mitre and perforations can be varied in designing the piping to fine tune out particular vibrations or resonances, to optimise performance of the engine, and to reduce disparities in exhaust back pressure.

Figure 9 shows a further embodiment of exhaust piping 20d according to the invention wherein downpipe 22d extends through an initial region of common connector tube 26d into a flexible connector 18 which links to the catalyst box CB.

A benefit of piping design according to the invention is that catalyst box CB can be brought closer to the point where the downpipes 22 and 24 join so that the catalyst experiences higher exhaust gas temperatures compared for example to the configuration shown in Figure 1. In the example shown in Figure 9, the downpipe 22d extends into flexible connector 18. This configuration has the benefit that the temperature of the catalyst is quickly raised after starting the engine, to an operational temperature at which it is effective in reducing toxic gases from the exhaust gas.

Figure 10 shows a plan view of part of exhaust piping 20e according to the invention wherein part of a forwardmost downpipe 22e is shown leading into common connector tube 26e. Rearmost downpipe 24e joins into the common connector tube 26e at an endconnector 28e, such that exhaust gases emitted from downpipe 24e enter common connector tube 26e radially in the region of the mitred end 30e of downpipe 22e, which enters common connector tube 26e axially. Also shown in figure 10 is a flange connector 34 for connecting together downpipe 22e with an upper part thereof which is substantially similar to the three-part downpipe construction 36 leading away from manifold flange 38 which connects to a cylinder block of an engine.

The length L I of downpipe 22e is slightly less than the length L2 of downpipe 24e. This is to compensate for the mean diameter of pipe 24e being slightly larger than that of the pipe 22e, the diameters being different because this has been found to equalise the exhaust back-pressures better. The ratios of diameter to length for the two pipes are approximately equal.

Figures 11 and 12 show a sectional side-elevation views of a further embodiment of the invention wherein piping 20f can be fitted to an in-line cylinder engine design such as a four cylinder engine which might be transversely mounted in a vehicle, i.e. not to a veetype engine. In this configuration the lengths, L1 and L2, of the downpipes 22f and 24f can be made substantially equal and adjusted together in design trials by adjusting the length of pipe 22f which protrudes into tube 26f.

Figure 12 shows piping 20f in a form where pipes 22f and 24f are 'unrolled' from the configuration shown in Figure 11. Also, piping 20f comprises a downpipe 22f having a double skin or air gap arrangement. Downpipe 24f similarly can comprise double skin region 40, which is not seen in the elevation view of Figure ii. Also, common connector tube 26f can comprise a double skin region 40. Accordingly, a flow of gases through downpipes 22f and 24f into common connector tube 26f experience enhanced thermal insulation provided by the double skin regions 40. The exhaust gases can then flow through a flexible connector 18 before entering a catalyst box and the rest of the exhaust system.Beneficially, the reduced heat loss from the downpipes and common connector tube gives rise to a more rapid heating of the catalyst thereby effecting increased efficiency in the reduction of toxic gases from the exhaust gas. Additionally, the double skin reduces noise from the exhaust system and acts as a shield ("grass-shield") to minimise the risk of fire, for example in off-road driving conditions. Also, the air gap pipes can beneficially be manufactured using separate components, i.e. a pipe and separate outer skin sleeve because of the relatively short lengths required in this embodiment compared to known arrangements where the downpipes extend separately a relatively long way to a junction. In the known art the air gap pipes are bent together and contain shot in the air gap region during bending to prevent fracture.

Further, because the effective lengths of the two pipes 22f, 24f is not determined solely by the position of the junction box 28, there is substantial flexibility in the positioning of the junction box, which can simplify packaging in the vehicle. Moreover the junction box 28 and upstream end of common connector tube 26 can be attached directly onto a flange 34f for connection onto a manifold (e.g. 36, 38) to an engine cylinder block, thereby omitting the air gap sections of pipes 22f and 24f shown in Figures 11 and 12. This has the benefit of bringing a catalyst further upstream in the exhaust system.

Claims (16)

1. An exhaust system for an internal combustion engine having first and second cylinder banks each comprising at least one cylinder, the banks being arranged to drive a common crankshaft, and being angularly offset from each about the axis of the crankshaft, the exhaust system comprising first and second exhaust conduits for conducting exhaust gases from the first and second banks respectively, the two exhaust conduits converging into a single exhaust pipe at a joint having an inner wall and an outer wall, the inner and outer walls having a space therebetween forming the end of the first conduit and the inner wall forming the end of the second conduit and having a free end defining the end points of the first and second conduits, wherein the effective lengths of the first and second conduits measured from the respective cylinder banks to the free end of the inner wall are substantially the same.

2. A system according to claim 1 wherein the joint includes a bend portion forming part of the second conduit and arranged to turn exhaust gas passing through the second conduit through an angle.

3. A system according to claim 2 wherein one end of the bend portion is formed by the outer wall.

4. A system according to claim 3 wherein the bend portion is substantially U-shaped having two arms one of which is formed by the outer wall.

5. A system according to claim 4 wherein the first conduit ends in a straight section of pipe extending into the bend portion through its base and forming the inner wall.

6. A system according to any foregoing claim wherein the cross-sectional area defined by the outer wall is twice that defined by the inner wall.

7. A system according to any preceding claim wherein the inner and outer walls are substantially concentric.

8. A system according to any preceding claim wherein the free end of the inner wall is mitred.

9. A system according to claim 8 when dependent on claim 4 wherein the longest side of the inner wall is positioned nearest to the other of said arms.

10. A system according to any foregoing claim wherein the end of the inner wall has an array of apertures through it.

11. A system according to any preceding claim wherein part of the outer wall comprises a flexible connector for connecting to further exhaust piping.

12. A system according to any foregoing claim wherein at least part of the outer wall comprises an outermost layer and an innermost layer with a gap therebetween.

13. A vehicle including an internal combustion engine and an exhaust system according to any foregoing claim, the engine having first and second cylinder banks each comprising at least one cylinder, the banks being arranged to drive a common crankshaft, and being angularly offset from each about the axis of the crankshaft, wherein the engine is transversely mounted in the vehicle.

14. A vehicle according to claim 13 wherein the joint is positioned beneath the engine.

15. An exhaust system substantially as hereinbefore described with reference to the accompanying drawings.

16. A vehicle including an exhaust system according to any foregoing claim, the vehicle being substantially as hereinbefore described with reference to the accompanying drawings.