GB2101205A - An exhaust system for internal combustion engines - Google Patents

Abstract

The system comprises an exhaust pipe 6 having an inlet end which surrounds the exhaust port, and a stub pipe 3 having an outwardly projecting flange plate 4 which is disposed between the inlet end of the exhaust pipe 6 and the engine surface, the stub pipe 3 projecting into the inlet end of the exhaust pipe by a distance equal to between 5% and 120% of the cross-dimension of the axis or major axis of the exhaust port, and the outer surface of the stub pipe 3 being spaced inwardly from the inner surface of the exhaust pipe 6. The stub pipe 3 reduces exhaust contamination of the fresh charge in the cylinder by reducing back flow through the exhaust port. <IMAGE>

Description

SPECIFICATION An exhaust system for internal combustion engines The invention relates to exhaust systems for internal combustion engines and provides a simple means of encouraging uni-directional flow in modern, high performance and race engines.

It has been common practice for a great number of years to design camshafts for fourstroke engines having extended opening periods both at TDC and BDC. The intention here has been to promote better cylinder filling in the high RPM ranges. At the low RPM ranges, however, the increased timing beyond the top and bottom dead centre points can cause a reversal of the flow of the gases, both in the intake and exhaust systems.

Tests have indicated that exhaust contamination due to reverse flow in the exhaust system is the biggest single factor reducing the low RPM torque output of an engine.

As the demand for higher specific power outputs has made itself felt, so the difficulty of maintaining low speed torque at satisfactory levels for road usage has increased. This has led to cylinder head designs with increased sophistication i.e. inclined valve designs rather than vertical valve designs, and cam profiles with increasingly rapid acceleration rates. However, the former incur added manufacturing costs and the latter tend to suffer reliability problems if the harshness of the cam profile is taken too far.

The present invention has been developed using data from airflow studies, and the behaviour of gases in the exhaust system, and provides a simple method of encouraging uni-directional flow in the exhaust system.

According to the invention there is provided an exhaust system for an internal combustion engine having a passage for exhaust gases leading from an exhaust valve to an exhaust port at a surface of the engine, the exhaust system comprising an exhaust pipe having an inlet end which, in use, surrounds said exhaust port, and a parallel-sided stub pipe having an outwardly projecting flange plate which, in use, is disposed between the inlet end of the exhaust pipe and said engine surface, the stub pipe projecting into the inlet end of the exhaust pipe by a distance equal to between 5% and 120% of the cross-dimension of the axis or major axis of the exhaust port, and the outer surface of the stub pipe being spaced inwardly from the inner surface of the exhaust pipe.

The inlet end of the exhaust pipe may be provided with an outwardly projecting peripheral flange which overlies part of the flange plate on the stub pipe, securing means, such as screws or studs, being provided to clamp the flange plate on the stub pipe between the flange on the exhaust pipe and the surface surrounding the exhaust port.

Alternatively, the inlet end of the exhaust pipe may be directly secured, for example by welding, to the flange plate on the stub pipe so that the stub pipe and exhaust pipe form a unitary assembly.

The present invention provides the ability to extend the power curve substantially in the lower RPM range and thus allows a longer period cam to be used, which is more conducive to the reliable production of high RPM power. Since an exhaust system according to the invention has a marked tendency to reduce reverse flow, it does, under part throttle conditions, considerably aid the running of the engine. At part throttle, the absolute pressure of the intake manifold is going to be substantially below that of the exhaust, hence during the overlap period, gases can be easily encouraged to flow from the exhaust port into the intake port, just prior to the induction stroke starting, due to the downward movement of the piston. By reducing back flow, exhaust contamination of the fresh charge at part throttle is reduced.As a result, leaner fuel/air mixtures can be burnt at part throttle and mileage gains are thus brought about.

The device can be applied to any internal combustion engine, either two-stroke or fourstroke, or any non-continuous burning cycle engine, such as a Wankel engine.

Apart from being sensitive to flow direction, the arrangement according to the invention also appears to damp the adverse effect of positive pressure waves arriving at the port without similarly reacting to negative pressure waves.

Negative pressure waves are, of course, needed to assist cylinder scavenging. In simple terms, therefore, it would appear that the device is pressure wave selective.

The following is a detailed description of embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is a schematic section through an exhaust port valve and an exhaust system according to the invention.

Figures 2 and 3 are similar views to Figure 1, the exhaust port valve being omitted, and Figures 4 and 5 are views into two different shapes of exhaust port.

Referring to Figure 1, exhaust gases passing out through the open exhaust valve 1 flow in a relatively unimpeded fashion through the exhaust port 2 and out through a stub pipe 3. The stub pipe 3 is of the same cross-sectional shape as the exhaust port when viewed axially, and it has welded to it an outer flange plate 4 of relatively thin section. The flange plate 4 is sandwiched between a flange 5 welded to the inlet end of the exhaust pipe 6 and the cylinder head. Both the exhaust pipe assembly 5, 6 and the stub pipe and flange plate assembly 3, 4 are held in place on the cylinder head in a conventional manner by fixing screws or studs and nuts (7 and 8).

Towards the end of the exhaust stroke, the exhaust passing out through the port 2 will tend to slow down. At low engine speed, there may be sufficient time for the exhaust in the exhaust pipe 6 to actually cease motion in the correct direction i.e. towards the atmospheric end of the exhaust pipe. If both the inlet and exhaust valves contained within the cylinder head are open, due to the fact the engine cycle is in the overlap position, then there is a tendency for the exhaust, being at a higher pressure than the intake, to reverse its flow direction and travel back past the exhaust valve 1.

The illustrated design of the stub pipe 3 and the exhaust pipe 6 are such that the flow efficiency in the forward direction is far higher than in the reverse direction, hence under these conditions the combined forms of the stub pipe 3 and the exhaust pipe 6 will serve to reduce the tendency of the exhaust gases to reverse flow back into the combustion chamber and pollute the incoming charge.

Depending upon the speed range at which the engine is to operate, the design and flow characteristics of the exhaust port, the exhaust pipe 6 may vary in form. High speed operation tends to favour large diameter exhaust pipes, whereas low speed operation tends to favour smaller exhaust pipes. To accommodate this situation, the exhaust pipe may have to vary in form. Referring to Figure 2, the exhaust pipe may have to reduce in section by virtue of a tapered diameter 9 so as to be able to produce the desired size of exhaust pipe diameter at a point 10 reasonably close to the exhaust port. This, of course, must be achieved without incurring any major restriction to flow.Use of the arrangement shown in Figures 1 and 2, where the stub pipe and flange plate form a separate unit, allows simplified production of the device where complex exhaust port shapes have been developed for maximum performance by means of airflow studies. Where a simple port shape, i.e. round or rectangular, is used, a simpler method of construction, as shown in Figure 3, may be employed. In the arrangement of Figure 3, the device uses a single plate 11 to which are secured both the exhaust pipe 12 and the stub pipe 13. Still referring to Figure 3, the form of the exhaust pipe 12 beyond that shown may take the form of a cone or a parallel pipe, depending upon the size of exhaust pipe which is required to produce the best results.

Referring to Figures 4 and 5, when simple exhaust port shapes are used at the cylinder head exit face, i.e. round as shown in Figure 4, or rectangular as shown in Figure 5, the inner stub pipes 14 or 1 5 may or may not have their centrelines co-axial with the centre-line of the exhaust pipes 16 or 17.

In any of the above arrangements, the stub pipe is a simple parallel pipe, and the extent to which it projects from its mounting plate 4 or 11 may be equal to between 5% and 120% of the crossdimension of the axis or major axis of the exhaust port at the exhaust manifold face. The stub pipe then projects into a conventional exhaust pipe 6 as shown in Figure 1, which can be up to twice the diameter of the axis or major axis of the exhaust port 2, or it can, as in Figure 2, project into a conventional exhaust pipe embodying a taper 9 to bring it down to the desired diameter.

The arrangements described above represent a simple means of manufacturing a device which will give marked reduction in reverse flow of exhaust gases, leading to a superior power curve and increased fuel economy. The invention allows the use of a longer period of valve opening for more high RPM power output. It will also allow the use of large diameter exhaust pipes, thus helping peak HP without sacrifice at lower RPM, and it allows more accurate carburettor calibration due to reduced reverse flow pulses. Carbon deposits in the intake tract and around the back of the intake valve may also be markedly reduced.

The invention also commonly produces a reduction in exhaust valve temperature, which in some cases will allow higher compression ratios to be used without incurring problems with detonation.

Claims (7)

1. An exhaust system for an internal combustion engine having a passage for exhaust gases leading from an exhaust valve to an exhaust port at a surface of the engine, the exhaust system comprising an exhaust pipe having an inlet end which, in use, surrounds said exhaust port, and a parallel-sided stub pipe having an outwardly projecting flange plate which, in use, is disposed between the inlet end of the exhaust pipe and said engine surface, the stub pipe projecting into the inlet end of the exhaust pipe by a distance equal to between 5% and 120% of the cross-dimension of the axis or major axis of the exhaust port, and the outer surface of the stub pipe being spaced inwardly from the inner surface of the exhaust pipe.

2. An exhaust system according to claim 1, wherein the inlet end of the exhaust pipe is provided with an outwardly projecting peripheral flange which overlies part of the flange plate on the stub pipe, securing means being provided to clamp the flange plate on the stub pipe between the flange on the exhaust pipe and the surface surrounding the exhaust port.

3. An exhaust system according to claim 1, wherein the inlet end of the exhaust pipe is directly secured to the flange plate on the stub pipe so that the stub pipe and exhaust pipe form a unitary assembly.

4. An exhaust system according to any of ciaims 1 to 3, wherein the internal cross-section of the stub pipe is substantially the same as the cross-section of the exhaust port.

5. An exhaust system according to any of claims 1 to 4, wherein the exhaust pipe is reduced in cross-section as it extends away from the exhaust port.

6. An exhaust system according to claim 5, wherein the exhaust pipe comprises a parallelsided portion extending away from tne exhaust port and leading to an inwardly tapered portion, the aforesaid stub pipe being wholly located within the parallel-sided portion of the exhaust pipe.

7. An exhaust system for an internal combustion engine, substantially as hereinbefore described with reference to the accompanying drawings.