This article was first published in 2003.
One area of car technology that has changed relatively little in the last hundred years is exhaust muffling. If you could bring back a car enthusiast from the turn of last century and have them cut open a current high performance muffler they might see more packing and less expansion chambers - but they'd still instantly know what they were looking that. Compare that situation with carbies and electronic fuel injection, for example...
However, the pace of original equipment muffler change is now increasing - variable flow mufflers are being fitted to lots of new cars and some very tricky designs are emerging. But with the exception of just a handful of systems (for example the Variflow valve covered at "Pure Pipe Perfection 2 - Introducing the Secret Weapon..."), high performance aftermarket systems are stuck in a revolving door - fashion causes some external alterations but the insides of the mufflers don't change much from year to year.
So we've decided to take a look at some of the very latest muffler designs. Each of these has recently been patented - so you can't go into production making 'em. However, the designs give a really good insight into where contemporary muffler technology is headed. But are they any good? That's harder to tell. Because of the way in which patents are written - no evidence needs to be provided that the invention actually works - it's hard to judge how effective the design really is. Also, patented inventions need to be clearly new and different, so some designs avoid taking what seems to be the obvious next step because such an approach would bring them too close to something else that's already patented. But it's still worthwhile making an assessment of the on-screen technical merits of each design, something we've done in the breakout boxes.
A patent application lodged with the US Patents Office in December 2002, shows that Korea's Young Tae Kim has designed a muffler that uses an eddy-generating inner corkscrew. The muffler - perhaps it's better described as a resonator - is placed between the cat converter and a main, rear muffler.
This view shows the primary elements of the muffler. A perforated tube (green) is placed within a larger diameter unperforated pipe (light blue), with the pair of concentric tubes housed within a normal - although small in diameter - muffler body (brown). Surrounding the tubes is packing (purple), which could be fibreglass or (according to the application) asbestos. This purpose of the packing material is to "prevent the conduction of heat of the exhaust gas travelling through the pressure-reducing pipe to the outside of the housing, and from transmitting noise to the outside of the housing". That is, the exhaust gas energy isn't dissipated into the packing as it is in a conventional resonator or straight-through muffler.
Within the perforated central tube is a device (dark blue) that causes the exhaust gases to be accelerated as they pass through the muffler. This corkscrew centre strip "causes the exhaust gas to swirl which increases the exhausting ratio of the exhaust gas which reduces the pressure of the exhaust pipe".
When the pressure in the pipe is high, the exhaust gases are forced through the perforations into the space between the perforated tube and the solid tube that surrounds it. When the gas pressure is lower, the movement of the gas through these holes occurs in the opposite direction, with the 'stored' high pressure gases being fed back into the main flow.
The muffler is claimed to give advantages over conventional mufflers in both power and economy.
The eddy-generating resonator would need to have a very carefully designed central swirler if the backpressure wasn't to become excessive. Also, as mentioned above, it's likely that more perforations (though perhaps smaller in size or number) in the outer tube would aid silencing even more. In effect this design is a variation on the resonators used on many OE cars where an expansion chamber is accessible through a slot at one end of the inner tube, a tube which often isn't perforated at all. Yes, the design would probably work - but the sizing of the perforations, the volume of the chamber formed between the inner and outer tubes, and the size and shape of the central swirler would all be critical.
Variable Flow Exhaust System
Herbert Shumacher and Volker Geis of Germany have lodged a US patent application for an 'exhaust silencer system with variable damping characteristics'. The patent applicants mention current variable flow muffler designs, where the valve is placed within the muffler itself. They see two disadvantages with this system: the presence of the valve in the muffler inhibits high volume flows, and with high power engines where twin exhausts are needed, two such valve-equipped mufflers are required. Their design overcomes these problems - completely conventional mufflers can be used, and more than one muffler can be placed in a 'Y' configuration. Furthermore, a central muffler can be dispensed with.
This diagram shows the general arrangement. Two conventional mufflers (green) are connected to a common inlet pipe by means of a 'Y' junction (yellow). At the entrance to a Y-junction is placed a variable opening poppet valve (ie like a normal cylinder head exhaust valve) which here is shown in red. Its opening is controlled by a sprung vacuum canister (blue) and the valve and its seat are contoured to provide good flow when the valve is open.
In its most basic application the valve is held shut by a spring within the actuator, with the pressure of exhaust gases forcing it open. The development of a constant backpressure improves silencing while still allowing high flows as required by the engine. The spring that is used can be either linear or progressive (ie a rising or falling rate) in design, and can be externally adjusted in preload.
More sophisticated versions of the system control the movement of the valve by means of the application of a vacuum to one side of the diaphragm in the canister, allowing the valve to be opened as directed by the engine management system. If a solenoid valve is pulsed in the vacuum line (a bit like electronic boost control on a turbo wastegate actuator), the opening can be made infinitely variable.
This is a really neat design. As we've covered in our stories on the Variflow exhaust butterfly, devices that give an exhaust a variable capacity can be brilliant at allowing effective silencing while still giving a great performance potential. By integrating the valve into a 'Y' junction, this design lends itself to space-effective applications (it doesn't take up any more room than would be required by the 'Y' anyway) and by using a poppet valve instead of a butterfly valve the need for a crank on the shaft is avoided and the technology already developed for engine valves can be applied. For example, the valve stem and valve bush materials can make use of well-proven technology - even if the assembly is placed close to the engine and so has to live with extreme heat. The ability to have a passive system that varies in opening with exhaust pressure or alternatively to have it electronically varied gives the device great flexibility, allowing the same basic design to be used in a number of applications, resulting in good economies of scale. And a final advantage - unlike a butterfly or sleeve valve, the system can be designed so that when the valve is fully open, there is no flow restriction at all.
High Performance Muffler
Carson Matherne of the US has lodged a patent application for a uniquely designed high performance muffler.
This diagram shows the design, as viewed from above with the upper part of the muffler removed. Exhaust gases enter through the single inlet at the top, and spread in a chamber (brown) which is about 150mm long. This chamber is connected to a tapered chamber (the latter about 250mm long and shown here in purple) by means of three perforated tubes (green). The first of these tubes is straight while those to each side have a slight bend to allow them to conform to the tapered exit section of the muffler. At 2 inches, the central perf tube is smaller in diameter than the muffler inlet and outlet tubes, which are 2.5 - 3 inches in diameter. The two outer perforated tubes are each 1.75 inches in diameter, and each of these is 175mm long. The muffler, although wide and long, is shallow in depth.
"The combination of a tapered geometry and free-flowing baffling provides a muffler which significantly reduces the transmission of heat back to the engine, increases engine performance, and produces a pleasant, deep sound while the engine is operating," says the application.
No mention is made of the packing material used or the size of the perforations.
This design uses the idea of jamming as much perforated tube into the space as possible. By broadening the muffler body, three - rather than the usual one - perf tubes can be used in this straight-through design. In addition, the cross-sectional area of the three tubes is much larger than a typical single-tube muffler, reducing restriction. The tapered exit is designed to bring the three flows back together with as little turbulence as possible, and the angle looks about right to achieve this. However, it's not clear how much of the inside of the muffler is filled with packing and in addition, without bellmouths (or at least flared entrances) it's likely that there would be substantial turbulence at the entrance to the three tubes, harming flow. However, it certainly doesn't look a bad design - although in production it would need some finessing.
The Bumper Muffler
George A Konstantakopoulos of the DaimlerChrysler Corporation, USA, has patented this muffler which is built into the rear bumper.
"By integrating the muffler into the rear bumper system of a vehicle there is a reduction in the amount of parts needed for vehicle assembly which reduces production time. Vehicle packaging constraints are also improved by moving the muffler up away from the lower portion of the vehicle into the bumper region."
Says the patent, "There continues to be a need... to provide a simple, cost effective, and compact way of providing a truly integrated bumper and exhaust system."
The muffler/bumper uses an extruded skeleton, both for structural support and to form the channels of the muffler. This extrusion "allows for greater wall thickness, improving the wearability and corrosion characteristics of the muffler."
"This method of producing the muffler system allows for a more rigid structure than that of traditional muffler systems. This added rigidity can be utilized in several ways including crash resistance and/or improving the rigidity of the overall frame structure as the skeleton can be attached directly to the frame members."
Exhaust gas enters one chamber and is directed along its length, being then forced through perforations located at the other end. From here it flows back along another chamber, before passing through perforations into another chamber. Finally, after travelling along yet another chamber, it exits the bumper/muffler through a tail pipe. A number of the chambers are packed with stainless steel fibres.
The plastic bumper cover fits into projections which are part of the extrusion, holding the face of the bumper some distance from the muffler/bumper itself. The tail pipe exits through the plastic face of the bumper. The implications of this design for the temperature of the bumper are not discussed.
While at first this design looks kind of irrelevant to performance cars, there are two important ingredients to look at. One is the sheer size of the muffler that it is possible to fit into the car - stretched out in a straight line we're talking a muffler silencing path that might be nearly 5 metres long! Secondly, the use of an extruded material allows for easy, cheap construction which is also strong and widely available (extruded sections can be bought in a huge variety of sizes and materials). So while a bumper/muffler is a bit hard to do aftermarket, a long, narrow, reverse flow extruded muffler is certainly a possibility.