GB2221252A - I.C. engine exhaust pipe - Google Patents

Abstract

A tubular member 11 connected between an exhaust manifold and a catalytic convertor (3, Fig. 1) comprises an inner tube 13 and an outer tube 14 with a gap between at least a part of the inner tube and the outer tube such that exhaust gas is able to flow within the inner tube and in the gap between the inner tube and the outer tube. The inner tube 13 serves to prevent intermixing between exhaust gases flowing within the inner tube and exhaust gases flowing in the gap between the inner tube and the outer tube 14. The outer tube 14 may be insulated or double walled. The exhaust manifold may duct exhaust gases from some engine cylinders to the inner tube 13 and from the remainder of the cylinders to the gap. The inner tube (23, Fig. 6) may be spaced from the outer tube (24) by a spacer (26). <IMAGE>

Description

EXHAUST PIPE FOR AN INTERNAL COMBUSTION ENGINE The present invention relates to an exhaust pipe for an internal combustion engine and, more particularly but not exclusively, relates to an exhaust pipe which conveys hot exhaust gases from the engine to a catalytic convertor.

There is an increasing trend towards the use of catalytic convertors in order to comply with stricter emissions regulations. However, it has been found that current catalytic convertors do not start to function efficiently for some time after engine start up from cold. This is because the catalyst used has to be heated to a so-called "light-off" temperature before it functions correctly. It is a problem that during the start up period the amount of emissions from an engine is at its worst. Previously, the only practical solution to this delay was to move the catalytic convertor closer to the engine, but the reduced length of the exhaust pipe between the engine and the convertor leads to reduced engine performance and inconvenient positioning of the convertor.

It is an object of the present invention to minimise or at least reduce the time to catalyst light off, without reducing engine performance, in a simple and economic manner.

According to the present invention there is provided an exhaust pipe for an internal combustion engine which includes a pipe including a manifold for connection to the engine and a tubular member connected to a catalytic convertor, wherein the tubular member comprises an inner tube and an outer tube, the inner tube being arranged within the outer tube and defining a gap between at least a part of the inner tube and the outer tube such that exhaust gases are able to flow within the inner tube and in the gap between the inner tube and the outer tube.

This has the advantage that greater insulation is obtained between a part of the exhaust gases and the external surface of the tubular member thereby reducing the loss of heat from the exhaust gases.

Preferably, the inner tube serves to prevent intermixing between exhaust gases flowing within the inner tube and exhaust gases flowing in the gap between the inner tube and the outer tube.

In one embodiment of the invention, the inner tube is welded to the outer tube at least at the ends thereof. The inner tube may be welded to the outer tube substantially along the length thereof.

In another embodiment of the invention, the exhaust pipe includes a spacing element positioned in the gap between the inner tube and the outer tube for maintaining the spacing therebetween. Preferably the spacing element is substantially star-shaped.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of an automobile incorporating an internal combustion engine and a catalytic exhaust system; Figure 2 is a schematic view of one embodiment of a tubular member for incorporation in an exhaust pipe according to the present invention; Figure 3 is a view in the direction arrow 'A' shown in Figure 2; Figure 4 is a cross-section on the line IV-IV shown in Figure 1; Figure 5 is a cross-section on the line V-V shown in Figure 1; and Figure 6 is a diagrammatic cross-sectional view similar to the view of Figure 2 of another embodiment of a tubular member for incorporation in an exhaust pipe according to the present invention.

Figures 1 to 5 show a tubular member 11 forming part of an exhaust system of a vehicle generally indicated 1. The exhaust system comprises a downpipe 2 including an exhaust manifold (not shown in detail) and the tubular member 11, a catalytic convertor 3, an exhaust pipe 4 and a silencer 5.

The exhaust manifold connects the downpipe 2 to an internal combustion engine 6 and has, as is well known in the art, a number of tubular elements corresponding to the number of outlet ports in the engine. In a four cylinder engine, for example, the exhaust gases from the outlet ports from cylinders 1 and 4 and from cylinders 2 and 3 are combined in order to obtain an optimum spacing of pulsations in the exhaust gases, such that the exhaust gases flow into a relatively low pressure in the exhaust manifold. This assists the exhaust phase of combustion, improves performance of the engine and reduces noise. For six cylinder, eight cylinder or other configurations of engines other sequences of exhaust gases may be combined or multiple manifolds and down pipes may be provided.

The tubular member 11 is provided with a flange 12 at one end for connection to the exhaust manifold, and at the other end is adapted for connection to the catalytic convertor 3.

The tubular member 11 comprises an outer tube 14 and an inner tube 13, the inner tube 13 being located within the outer tube 14 with a clearance therebetween to define a gap through which exhaust gases can flow.

The flange 12 is adapted to connect the tubular member 11 to the exhaust manifold in such a way that one set of exhaust gases, such as the gases from cylinders 2 and 3 pass through the inner tube 13 and the other set of exhaust gases, such as the gases from cylinders 1 and 4 pass through the outer tube 14.

The inner and outer tubes 13 and 14 are joined together adjacent the ends of each tubular member by welding as indicated by the fillets 15 and 16. Although the inner tube 13 is shown as a D-shaped tube in Figures 2 to 5, it need not necessarily be so. Indeed the inner tube 13 could be any suitable shape, for example it could be of the same cross-sectional shape as the outer tube, but merely smaller in transverse dimension. One example of this is illustrated in Figure 6 where the inner tube 23 is circular in cross-section and is arranged within a circular outer tube 24. In order to minimise heat loss, particularly from the inner tube, the inner and outer are separated by a star-shaped element 26.

Alternatively, although it is not shown in detail in the drawings, the inner tube may be in contact with and secured to the outer tube along substantially its entire length.

This arrangement results in increased heat loss, but can reduce noises such as rattles from the tubular member.

If desired, the outside of the tubular member 11 can be insulated. Alternatively, the outer tube 14 could be made as a double skin pipe. That is to say the outer tube could be made from an outer skin and an inner skin which may be welded, if desired, at the flange 12. The inner skin may be separated from the outer skin by about lmm providing an air gap which absorbs noise in addition to providing insulation.

In use of the tubular member 11 in an exhaust system in combination with an internal combustion engine, exhaust gases flow out of the engine and through the manifold and enter the tubular member 11 at a temperature of about 600 C. As explained above, the exhaust gases are preferably combined into two streams for improving performance, although the present invention works equally well if the exhaust gases are combined into a single stream. The exhaust gases pass into both the inner tube 13 and the outer tube 14 and flow to the catalytic convertor 3. On reaching the catalytic convertor, the gases in the outer tube 14 have fallen to a temperature of about 2000C. However, the inner tube 13 prevents intermixing of the two streams of gas and the gases in the inner tube 13 remain at a temperature of about 6000C.

Because the gases in the inner tube reach the catalytic convertor 3 at a temperature of about 600 0C instead of about 2000C in the case of an exhaust system according to the prior art with the catalytic convertor similarly positioned, the catalyst is heated more rapidly and attains light-off more quickly. Afte. light-off the catalytic reaction is exothermic and is self-sustaining although it generates considerable heat which requires to be dissipated.

Quicker catalyst light-off enables a number of advantages to be achieved. For example, because initial emissions from a cold engine start-up are reduced, it is possible to reduce the amount of catalyst required. Alternatively, instead of using a three way catalyst and a relatively rich air-fuel mixture, it is possible to use an oxidation catalyst in combination with a lean-burn engine. As a further alternative, it is possible to employ a less costly catalyst.

All these alternatives enable economies to be made in the construction of motor vehicles without compromising engine performance. Indeed, according to the prior art the only way of obtaining quicker catalyst light-off was to position the catalytic convertor adjacent to the engine. This resulted in reduced engine performance because it was not possible to tune the pipe lengths to accord with the exhaust gas pulses, and also resulted in increased under bonnet temperatures because of the exothermic nature of the catalytic reaction. According to the present invention, it is possible not only to tune the pipe lengths, but it is also possible to position the catalytic convertor outside the engine compartment, for example in the transmission tunnel as shown in Figure 1, because of the double tube arrangement which maintains the temperature of part of the exhaust gases and results in faster catalyst light-off.

Claims (7)

1. An exhaust pipe for an internal combustion engine which includes a pipe including a manifold for connection to the engine and a tubular member connected to a catalytic convertor, wherein the tubular member comprises an inner tube and an outer tube, the inner tube being arranged within the outer tube and defining a gap between at least a part of the inner tube and the outer tube such that the exhaust gases are able to flow within the inner tube and in the gap between the inner tube and the outer tube.

2. An exhaust pipe as claimed in claim 1, in which the inner tube serves to prevent intermixing between exhaust gases flowing within the inner tube and exhaust gases flowing in the gap between the inner tube and the outer tube.

3. An exhaust pipe as claimed in claim 1 or 2, in which the inner tube is welded to the outer tube at least at the ends thereof.

4. An exhaust pipe as claimed in claim 3, in which the inner tube is welded to the outer tube substantially along the length thereof.

5. An exhaust pipe as claimed in claim 1 or 2, in which a spacing element is positioned in the gap between the inner tube and the outer tube for maintaining the spacing therebetween.

6. An exhaust pipe as claimed in claim 5, in which the spacing element is substantially star-shaped.

7. An exhaust pipe substantially as disclosed herein with reference to the accompanying drawings.