Lichtgewicht uitlaatsysteem van Bosal

P. Steenackers, Bosal Research


Nederlandse samenvatting

Bosal is in de afsluitende fase van een omvangrijk researchprogramma dat moet resulteren in een totaal nieuw concept voor uitlaatsystemen. Deze Bosal "lichtgewicht uitlaten" maken het mogelijk een gewichtsbesparing te realiseren van 50 % ten opzichte van de huidige uitlaatsystemen. Kern van het concept is dat ieder afzonderlijk onderdeel van het uitlaatsysteem wordt bevestigd aan de carrosserie op een zodanige wijze dat slechts zeer geringe speling mogelijk is tussen de bodemplaat en de uitlaatdelen.
Een dergelijk systeem hoeft daarom niet langer zelfdragend te zijn en kan dus uit dunner materiaal worden vervaardigd. De mechanische stijfheid van het systeem wordt met dit nieuwe systeem gerealiseerd door de bodemplaat, terwijl de levensduur door verminderde trillingen aanzienlijk beter wordt. Het nieuwe van deze techniek is verder, dat door gebruik te maken van flexibele koppelingen het uitlaatsysteem niet beďnvloed wordt door motortrillingen. Ook het door hoogfrequente motortrillingen veroorzaakte geluid wordt op deze wijze met circa 10 dB verminderd, hetgeen leidt tot minimale uitlaatgeluiden in het interieur van een auto.

Omdat de uitlaat nauwelijks beweegt, kan er ruimte bespaard worden. Voor een deel kan die worden gebruikt om grotere dempers toe te passen. Dat heeft weer een positieve invloed op de geluiddemping. Daarmee kunnen autofabrikanten tegemoet komen aan toekomstige strengere wettelijke eisen ten aanzien van de geluidsproductie door auto's.

Bosal Light Weight Exhaust System


A new exhaust system concept is presented, the BOSAL Light Weight Exhaust system, which allows for exhaust weight reductions ranging from 30% to 50% (10 - 15 kg; 20 - 30 lbs.), as compared with more conventional designs. The weight savings are achieved essentially by reducing the material gauges (0.7 mm for tubes, 0.5 mm for silencer envelopes). This is made possible by the use of a new method for fixing the exhaust system under the vehicle. This fixing method reduces the system vibrations by almost an order of magnitude so that the fatigue life of the system is substantially improved even after the dramatic reduction of the material gauges. The concept has been extensively tested and various validation programs are now running in collaboration with car manufacturers.


Modern cars have a tendency to become heavier as a result of various efforts to increase comfort and to comply with ever more demanding environmental standards. In order to neutralize or even reverse this tendency many car manufacturers are involved in important weight saving programs. As a worldwide supplier of exhaust systems to the automotive industry BOSAL have developed a new Light Weight Exhaust concept which may contribute in a very substantial way to these efforts. Weight savings of the order of 10 - 15 kg (20 - 30 lbs.) can be achieved at marginal cost penalties, if any.

1. General concept

1.1. Some features of a conventional exhaust system

To highlight in a general way the advantages of the Light Weight concept we first recall two major characteristics of current conventional exhaust systems. These characteristics have major consequences in terms of system design and weight.

1.1.1. Hangers

A conventional exhaust system is fixed to the vehicle underfloor by means of rubber hangers. These hangers allow relatively important movements of the exhaust.

The movements are the result of various driving conditions (accelerations along X, Y and Z-axis) and of thermal expansions of the exhaust. Typical values for these movements are in the order of 2 or 3 cm. The total number of rubber mountings is rather limited (sometimes not more than 2 or 3 on small cars).

1.1.2. Downpipe

In the downpipe, current exhaust systems have in general a flex-joint. The flex-joint has a double function:

1.1.3. Strength

The conventional way of fixing an exhaust system under a car has important consequences in terms of system strength and durability. Because the system is fixed in a limited number of locations where it hangs rather loose under the car, it needs to assure its own mechanical strength. This requires primarily stiffness and resistance against bending and torsion moments. Because of the length of the system natural vibrational modes at low frequencies can be easily excited by driving conditions and are a potential danger for system durability. The designer will be forced to foresee sufficient material thickness for the pipework, the silencer endcaps and envelopes, to assure the necessary mechanical strength for the system.

One can conclude that the conventional way of mounting an exhaust system determines to a large extend the necessary material gauges and, thus, the weight of the system.

1.2. The essentials of a BOSAL Light Weight exhaust system

With the BOSAL Light Weight exhaust system a breakthrough in weight saving could be achieved by taking advantage of a totally new method for fitting the exhaust under the car.

1.2.1. Mountings

Each silencer box or catalyst is fixed to the underfloor by means of two brackets and 4 relatively stiff rubber mountings (see fig. 1). This type of support limits the movements of the exhaust system to a few milimeters (as compared to the conventional few centimeters). This results in bending and torsion moments which are reduced by allmost an order of magnitude, opening the possibility of using much thinner material gauges.

Figure 1.

1.2.2. Downpipe

The conventional flex-joint in the downpipe is replaced by a system of two flexes, at about 0.5 m distance from each other (see fig. 2). Such an arrangement allows for:

Figure 2.

1.2.3. Thermal expansion

The thermal expansion of the exhaust system will be allowed for by a special design of the rubber mountings and an appropriate selection of stainless steel.

1.3. Weight reduction

The reductions in weight which have until now been achieved are in the order of 30 to 50 %. This means reductions in weight in the order of 10 - 15 kg (or even more on large cars). This can be realized by using 0.7 mm gauges for the tubing and 0.5 mm gauges for the silencers. The weight savings of course must not be limited to silencers and tubes. Manifold designs, often now combined with close-coupled converters, can strongly contribute to even further weight reductions.

1.4. Opportunities for increased acoustical attenuation without space penalty

The new concept makes it possible to mount larger silencer volumes without altering the vehicle underfloor space availability. This is possible because the static behaviour of the exhaust (limited movements relative to the car body) enables to reduce the required gap between the silencers and the vehicle underfloor. A gain in volume of about 20 % is easy to obtain, but in some cases it is even possible to gain 40 %. The enormous advantage of this in terms of acoustical attenuation in low frequencies is obvious. It is also possible not to enlarge the silencer volume but use the extra space which becomes available for other purposes. Another possibility is to reduce the silencer volume at the rear and to enlarge the front silencer, which is also beneficial for low frequency acoustics. In some cases, with small engines, it was achieved to delete completely the rear silencer.


2.1. Brackets and rubber hangers.

The basic principle is to support every silencer or converter (or any other important “heavy” element in the system) separately. The way of fitting must be such that typical driving accelerations in the X, Y and Z direction will not result in movements of the element exceeding a few mm. In fact the silencers and converters will be “carried along” as if they where part of the vehicle underbody.

A good way of realizing this is as follows (see fig. 1). A bracket A is welded to the inlet and outlet tube, as close as possible to this silencer (converters etc.). Every bracket is then supported by 2 rubber plots which are fixed to another metal bracket B that can be bolted directly onto the underfloor. In total a silencer is thus supported by 4 rubbers. These rubbers will be so designed as to have a dynamical stiffness of about 30 N/mm into the Y and Z direction (at an acceleration of 3 g a silencer of 4 kg. will undergo a force of ~ 30 m/s2 x 4 kg = 120 N; since there are 4 hangers the force per hanger will be about 30N resulting in a displacement of about 1 mm). The thermal expansion of the system, will result in elongations along the X axis in the order of 25 mm. These have to be allowed for by the rubbers. In this respect it is necessary to mount the rubbers in such a way that they have a pre-deformation in the direction opposite to the thermal expansion (see fig. 3) In this way the total deformation of the rubbers may be limited to typically 6 mm.

Figure 3.

Constructional details.

Bracket A is represented in fig. 4, material thickness is 1 mm, the bracket has a U-profile. A hole (H) is foreseen in the middle to prevent collection of water and dust in the U-profile. The U-profile is welded to the tube as indicated. This way of mounting the brackets minimizes heat transfer from the tube. This is necessary to limit the rubber temperatures. Around the tube a ring is foreseen to reinforce the tube at the fixing point and to allow for a safe welding joint.

Bracket B is also represented in figure 4. It has two bolt holes (K) for direct fixing to the car body. The schematic representations here given is typical for the mounting of a front silencer in the “tunnel” of the floor. At the rear some special hanger provisions may have to be taken at the car body side. Bracket B may have to be given a special shape. For fixing of the rubbers to the brackets, various methods can be used (vulcanization, clipping, … etc.).

The main features which have been taken into account are:

  1. easy mounting of the total system on the car assembly line
  2. stiffness of the bracketry (no vibrational modes lower than 300 Hz)
  3. maximum rubber temperatures (different types of rubbers are on the market)
  4. thermal expansions

An advantage of this construction is that the exhaust system is not hanging but laying on the rubbers (obviously better for rubber durability).

Figure 4.

2.2. Decoupling the exhaust system from engine vibrations and engine movements.

There are three important aspects here:

  1. The design of the decoupling system is in principle much simpler: only the engine is moving and the exhaust is fixed. In a conventional system, the movements of the exhaust are difficult to assess. Here they can simply be neglected.
  2. All the movements of the engine must be taken into account and the system of the two flexes must allow for these movements. In other words, the relative position of the engine and the two flexes, and the design of the flexes must be such that the movements of the engine can be supported by the flexes without danger for fatigue.
  3. The flexes must be chosen soft enough to assure the necessary vibrational decoupling. This requirement is not difficult to satisfy. The arrangement with two flexes provides in principle much more decoupling than one flex with the same stiffness. But, one can go further and reduce the stiffness of the flex because the flex has no carrying function. Only the small piece of tube between the two flexes has to be supported.


More than 15 different vehicles have now been equipped with Light Weight exhaust systems. Functional performances have been extensively measured and fine tuned. We review here the most important aspects.

3.1. Acoustics

3.1.1. Tailpipe noise

In all cases the tailpipe noise levels were equal or better compared to conventional systems. More silencer volume in the front of the system benefits the tailpipe level at low frequencies. More volume overall is an obviously advantage for acoustics.

3.1.2. Radiated noise

The radiated noise levels are excellent. Best results are obtained with round silencer sections. Long front silencers (i.e. 1 meter length) with oval sections may need stiffening with extra baffles. Engine vibrations, turbocharger noise and other acoustical vibrations which propagate along the exhaust system may contribute significantly to radiated noise. A large amount of these vibrations is filtered away by the installation of the two flexibles.

3.1.3. Vibration levels

The high frequency vibration levels of the exhaust system (generated by the engine vibrations, and propagating down into the exhaust) are typically 10 dB lower in level with a Light Weight system then with a conventional exhaust (see example in fig. 5). This graph is measured in one location and on a certain vehicle. But the order of magnitude is representative of the general trend of which we have now many examples. Such low vibrational levels are necessary to assure vibration levels on the underfloor which are equal or identical. That lower system vibrations result in similar underfloor vibrations is due to the fact that we use a much larger number of plots for the Light Weight system than hangers in a conventional system.

The reason for the low vibration levels is essentially the better filtering of these vibrations by the double flex arrangement.

The stiffness of the rubber hangers is only a secondary element in this respect. The necessary stiffness of the rubber hangers is given by the requirement of strongly reduced static or quasi static movements of the exhaust and the strength of the rubbers to carry the system without allowing relative movements between the exhaust and the car body. As already indicated, the rubbers need for this purpose a stiffness of about 30 N/mm. Less stiffness results in unacceptable silencer movements.

Figure 5.

3.1.4. Interior noise levels

Levels are equal compared to conventional. The noise spectrum can be somewhat different, some harmonics being reduced, some others being increased. Overall levels are typically identical.

4. Durability requirements

4.1. Level of displacements amplitudes - stress levels in the system

Under the actions of driving accelerations in the X,Y and Z direction an exhaust system is subjected to bending moments and thus stress in various critical parts.

We cannot go here into much details. Tables 1 and 2 give comparative values of measured bending moments and stress measured on an exhaust when the car on which it was mounted was vibrated on a 4-post vibration rig. The movements of the vehicle were representative of a pavé test. The comparison shows bending moments of 4 to 6 times lower values for the Light Weight system. Stresses are still 2 to 3 times lower (corresponding to a 50% reduction of the material gauges). One can conclude that even thinner gauges could be envisaged but technological limitations prevent this so far.

An extremely severe durability test was conducted with a vehicle on the Millbrook Proving Grounds. A parcours was selected including pavé, high speed, water splash, gravel road etc. 7000 km were covered under these conditions. An absolutely faultless behavior of the exhaust system was reported.

More than 10 vehicles are now currently running with Light Weight systems and BOSAL is involved with various customers in predevelopment programs to further validate the concept both from a functional and durability point of view.


Maximum and minimum bending moment [Nm]
Vertical Inlet Front Muffler 19 -17 84 -72.5
Horizontal Inlet Front Muffler 16 -14 67 -60
Vertical Outlet Front Muffler 10 -9 59 -56
Horizontal Outlet Front Muffler 8 -12 46 -50.5
Vertical Inlet Rear Muffler 10 -9 38 -34
Horizontal Inlet Rear Muffler 5 -6 37 -18.5

Table 1.

Maximum and minimum stresses [MPa]








Vertical Inlet Front Muffler





Horizontal Inlet Front Muffler





Vertical Outlet Front Muffler





Horizontal Outlet Front Muffler





Vertical Inlet Rear Muffler





Horizontal Inlet Rear Muffler





Z-acceleration Body (g)





Table 2.


5.1. Thin gauge tubing

Light Weight exhaust systems have been manufactured with pipes of 0.7 mm wall thickness. This has been realised in 304 and also 409 and 441 stainless steel grades. so far never any breakage of systems in service has been reported including the severe Millbrook tests. damage by gravel impact on tubes has always been minimal. It might be good during development to take care not to locate thin wall tubing in the conical area’s behind wheels where gravel projections may be concentrated. Such gravel projections could result in minor cosmetical damage. Functional damage (perforations) have never been observed.

5.2. Thin gauge silencers

The silencers are designed with material gauges of 0.5 mm for the envelopes, the mantel and the baffles.

Lock seeming of such thin gauges is abandoned for lack of strength and lack of resistance against internal pressure levels. Silencers are welded, for the envelope a special fast welding technique was developed.


The BOSAL Light Weight Exhaust system is now a proven concept. Both functional and durability aspects have been extensively tested and validated. Development projects with various OEM’s are now being planned.

It must be empasized that the very important weight savings, in the order of 10 - 15 kg (20 to 30 lbs.) can be realized without any major cost penalty, in some cases cost savings could even be achieved.


Dr. ir. P. Steenackers
Director Bosal Advanced Research
E-mail :