The present invention relates to a vehicle comprising an exhaust system for improving the torque characteristics of an engine, in particular of a single cylinder four stroke spark ignition engine.

The invention further relates to improvement in an exhaust system of a vehicle, in particular in vehicles using a single cylinder four stroke spark ignition engine, which results in enhanced performance.

More particularly the invention enables an improvement in the torque production of the engine at lower engine speeds. For example, the applicant has designed engines utilizing the invention in which torque improvements have been made at engine speeds ranging from 3000 to 5000 RPM, which is typically a normal vehicle driving zone, while still achieving high power outputs at higher engine speeds. This invention is especially useful to engines used as prime movers for two and three wheeled vehicles. For simplicity of explanation, a reference to "vehicles" in this specification should be taken to be a reference to vehicles using single cylinder, four stroke, 'spark ignition engines and a reference to 'engines' in this specification is meant for single cylinder, four stroke, spark ignition engines.

Vehicles are generally equipped with an exhaust system in order to achieve desired performance characteristics of the engine, reduce noise of combustion from of the engine and to allow products of combustion of air-fuel mixture to escape to the atmosphere in a defined direction. For example, a simple motorcycle exhaust system consists of header pipe, a muffler and if necessary, a guard or heat shield. The header pipe may be of single diameter or with one or more steps of different diameters. One end of the exhaust system is attached to the exhaust port of the engine and other end is at the rear of vehicle. The exhaust system is neatly packaged within the space available in the motorcycle.

One of the difficulties experienced with conventional exhaust systems for the above mentioned engines is that when the exhaust system is designed to achieve desired performance levels, adequate compromise between available torque at lower speeds and performance at higher speeds is difficult. Hence, to achieve better performance, particularly maximum net power at higher engine speeds, it is often necessary to sacrifice the better torque availability at lower engine speeds.

Yet another difficulty is that the improvement in performance, usually, is associated with increase in noise level which is undesirable and may also exceed statutory limits.

In order to overcome at least some of the above problems, the applicants have invented an exhaust system wherein higher torque is produced at lower engine speeds whilst still retaining the desired engine performance and torque at higher engine speeds and also maintaining desirable noise levels. This is achieved by connecting a chamber of predetermined volume, externally to the header pipe (stepped or otherwise) of the exhaust system at a point preferably between 40% to 60% of developed length of the header pipe from the face of exhaust flange along the length of said header pipe either directly or through a connecting tube. Conveniently, the location of the chamber is selected to accommodate it within available space in motorcycle and without sacrificing the ground clearance.

Usually, if a typical known engine is tuned to have a maximum net power at 8000 RPM, then a good engine can achieve a peak torque at 6000 RPM. In this case, the torque at 3500 RPM would be approximately 15% lower than the peak torque. This leads to poor driveability of vehicle at low engine speeds, while giving good performance at higher engine speeds. On the contrary, if the engines were to be tuned to achieve peak torque at lower speeds, for instance 5000 RPM, the peak power would occur at approximately 7000 RPM but would normally be of lesser value. Based on a comparison using the same vehicle gear ratios, this would mean that the vehicles with such engines would have better low speed drivability but poorer high-speed performance.

In order to achieve better performance of this engine, the applicants' improved exhaust system according to this invention enables at least two torque peaks to be produced in the torque vs. engine speed relationship.

In a certain embodiment of the invention, one of the torque peaks occurs at approximately 4500 engine RPM and second at approximately 6000 engine RPM. In such an application, it was found that the exhaust system enables availability of torque at 3500 RPM to be within 5% of the peak torque at 6000 RPM.

The improved exhaust system according to the present invention works on the principles of pulse tuning of the exhaust system. The chamber of a pre-determined volume is connected directly or by a tube of pre-determined diameter and length to header pipe at a developed distance preferably ranging from 40% to 60% of the total developed length of header pipe from the face of the exhaust flange. The two torque peaks are produced by the combination of reflection of pressure waves at the end of header pipe and that at the mouth of chamber.

In the past, various inventions were carried out in the field of exhaust systems. However, their teachings are very different to the present invention, for an example:

1. 1) US4779415: Illustrates use of a Helmholtz resonator attached to the exhaust manifold of a multicylinder engine for the purposes of noise suppression. Multicylinder engines are subject to very different exhaust gas dynamics as compared to single cylinder engines. The overlapping exhaust pulsations of a multicylinder engine create opportunities for exhaust tuning effects, which are not available to single cylinder engines.
2. 2) JP11-062547: Illustrates the use of Helmholtz resonators in an engine exhaust system which teaches to create specific frequency of waves by resonator effect by means of Helmholtz resonator, which silences the system resonance. As for the above case, the teachings are also illustrated with reference to multi-cylinder engine applications.
3. 3) US 3,254,484 describes means for manipulating sound pressure waves in relation to the intake and exhaust systems of internal combustion engines. This document further describes means for manually or automatically adjusting the effective acoustical length of the exhaust and intake systems of an engine while the engine is operating. As a consequence, this invention requires an exhaust system of variable effective acoustical length which is co-ordinated to match the engine speed.

Therefore, there are no teachings in these patents directed to torque enhancement and the special requirements for single cylinder engines, in clear contradiction to the applicants' teachings.

The invention will now be further elaborated with reference to following figures enclosed with this specification: -

• Fig 1 - Illustrates a typical exhaust system of engines according to the prior art.
• Fig 1A - Illustrates top view of location of typical exhaust system in motorcycle according to prior art
• Fig 1B - Illustrates side view of the location of typical exhaust system in motorcycle according to prior art
• Fig 2 - Illustrates torque curve of a typical engine according to the prior art.
• Fig 3 - Illustrates an exhaust system incorporating a chamber according to the present invention.
• Fig 3A: - Illustrates top view of location of typical exhaust system in motorcycle according to the present invention.
• Fig 3B: - Illustrates side view of location of typical exhaust system in motorcycle according to the present invention
• Fig 4 - Illustrates details of joining the chamber to header pipe of exhaust system of engine according to the present invention.
• Fig 5 - Illustrates typical torque curve of engine fitted with improved exhaust system according to the present invention.

In one embodiment, an improved exhaust system according to the invention comprises of a header pipe small (11), an exhaust flange (12), a connecting tube (13), a chamber (14), a tube (16), a header pipe large (17), a nipple (19), a washer (20), a bolt (21) and bracket (18).

Fig.1, 1 A and 1B illustrate a typical exhaust system and its location according to the prior art. It consists of header pipe (1) one end of which is assembled with flange (2). The flange (2) is connected to cylinder head (not shown) of the engine with suitable bolts or studs and nuts. The other end of the header pipe is either welded or assembled to the muffler (3). Exhaust gases from the cylinder head are led to the said header pipe and then to the muffler and then released to the atmosphere. The heat shield (4) provided on the muffler guards the rider and pillion from the heat of the exhaust system. This exhaust system is well known and does not need any detailed explanation.

Fig 2 illustrates typical wide open throttle torque curve of a typical engine fitted with the exhaust system according to prior art. It is observed from the curve that the peak torque is occurring at around 6500 engine RPM.

Figs 3, 3A, 3B and 4, illustrate an improved exhaust system according to the invention having a chamber (14), a connecting pipe (13) connecting said chamber to tube (16), said tube (16) being positioned between a front header pipe (11) and rear header pipe (17), the header pipe (11) being provided with an exhaust flange (12) and said rear header pipe (17) is connected to the said muffler (15).

As an alternative to using the tube (16) as an intermediate connector between the front header pipe (11) and rear header pipe (17) the connecting pipe (13) could be welded directly to header pipe (1) as shown in fig (1).

As an alternative to connecting the said connecting tune (13) and said chamber (14) to the header pipe (1), the said chamber (14) could be welded directly to header pipe (1) as shown in fig (1).

As shown in Figs 3, 3A, 3B and 4 the header pipe (1) is split as a front header pipe (11) and a rear header pipe (17). The ends of said front and rear header pipes connecting to the engine exhaust and the said muffler are identical to the header pipe (1). The developed length of said front header pipe (11) is decided considering the parameters like engine power, torque requirement at various engine revolutions, diameter of the pipe etc. In order to strengthen the fitment of chamber (14)/ a tube (16) is welded joining the rear header pipe (17) and the said chamber (14). The muffler (15) is connected to rear header pipe (17). The said connecting tube (13) is provided at a point, preferably 40% to 60% of developed length of header pipe (that is the combined length of the front and rear header pipes and connecting tube) from the face of said exhaust flange (12) along the length of the said header pipe. The connecting tube (13) or tube (16) is provided with a nipple (19). The other end of nipple is threaded which is closed by means of a washer (20) and a bolt (21). The said washer and bolt can be removed to allow for the convenient measurement of exhaust gas emission values by a suitable instrument. Bracket (18) is provided between the chamber (14) and rear header pipe from strength considerations.

The chamber (14) is of a pre-determined volume and is made of two sheet metal halves welded together. Conveniently, reinforcing dimples (25) are added to the top surface (24) of the chamber (14) to provide rigidity and reduce noise caused by the vibration of this surface. An additional convenient feature of the present embodiment is the location of the bottom surface (26) of the chamber (14) to be slightly higher and sloped downwards towards the connecting pipe 13. This allows for any accumulated moisture to drain from the chamber (14) back into the exhaust system.

The length of connecting pipe (13) is so chosen that the desired result of improvement in torque characteristics is achieved and the said chamber is well located beneath the motorcycle in available space without sacrificing ground clearance. This is a very simple matter of design and in some cases can be readily determined with non-inventive trial and error.

Fig 3A and 3B illustrates a typical such location. The shape of the chamber and construction can vary with location on motorcycle.

Fig 5 illustrates typical wide open throttle torque performance curves of the engine fitted with the exhaust system according to depicted embodiment of the invention with respect to the engine speed in RPM. It can be observed that the torque of the engine between the speed range of 3000 to 5000 RPM which is the normal vehicle driving zone for a typical motorcycle has substantially improved, while maintaining substantially the original toque at engine speeds beyond 7000 RPM which is important for the performance of the engine. The applicants have carried out substantial testing and concluded that improvement in torque of a typical engine in the speed range between 3000 to 5000 RPM is of the order of 5 to 7%. Also, the vehicles do meet statutory requirements of noise level.

An improved exhaust system for vehicles as described herein and as illustrated in the drawings accompanying herewith and having variations and modifications well within the knowledge of a person skilled in the art without deviating from scope of the invention are within the scope of the present invention and its embodiments.