Exhaust silencer

Exhaust silencers reduce noise emissions from internal combustion engines by reflecting pressure pulses and absorbing acoustic energy. For MAN and Mercedes trucks, buses and vans, the silencing characteristics, exhaust gas flow rate and flow resistance must be matched to the vehicle configuration. A damaged silencer does more than cause excessive noise: leaks can direct hot exhaust gases towards brake lines, wiring or bodywork. Internal blockages can also increase exhaust back pressure and reduce engine power.

Available designs include reactive silencers with chambers and baffles, absorptive silencers with perforated tubes and heat-resistant fibrous material, and combination designs. Depending on the application, housings are made from materials such as aluminised steel or stainless steel. Material thickness, weld quality, chamber volume and internal bore diameter all contribute to resistance to corrosion, vibration and thermal stress. The permissible exhaust back pressure is a limit, not a minimum pressure to aim for.

On older MAN F2000 vehicles with D28 engines or Mercedes Atego variants with OM 904 engines, the construction can differ significantly from more recent emissions variants. Modern systems may combine noise reduction with a DPF (Diesel Particulate Filter) and SCR (Selective Catalytic Reduction) in an aftertreatment module. Such a module is not interchangeable with a conventional silencer. On MAN Lion’s City, Mercedes Citaro and Sprinter models, the available installation space, exhaust discharge direction and mountings also vary by configuration.

Older conventional exhaust silencers primarily reduce noise and do not incorporate exhaust gas aftertreatment. In exhaust silencers that carry out exhaust gas aftertreatment, the housing also contains catalysts and filter elements, depending on the emissions variant. A DPF traps soot particles, while SCR uses AdBlue to reduce nitrogen oxide emissions. With these designs, sensor connections, the dosing system configuration and the specified regeneration and maintenance procedures are important alongside the mechanical connections.

Exhaust silencers are available for side discharge and for vertical discharge behind the cab. The exhaust silencer and the connected exhaust pipes must suit the intended discharge direction. For side discharge, the position of the exhaust outlet and its clearance from surrounding vehicle components are critical; for vertical discharge behind the cab, the vertical pipe, its support and heat shielding also require attention. These designs are not interchangeable without checking the connections, pipe routing and mountings.

Always identify parts using the vehicle’s chassis number. Visual comparison or selection based solely on the model name is never sufficient. Then check the engine variant, emissions class, wheelbase, connection diameters, flange patterns and mounting point positions. For slip joints, insertion depth and fit determine sealing performance; for flanged joints, the sealing faces and gaskets must match the specified configuration.

Condensate and road salt cause rust perforation, particularly around folded seams, welds and mounting straps. Loose baffles produce a metallic rattle, while deteriorated sound-absorbing material causes increasing exhaust noise. Cracks around connection stubs often indicate installation under stress, defective rubber exhaust mounts or insufficient movement compensation by the exhaust flex pipe.

1. Allow the system to cool and inspect the shell, connections and supports. Look for soot marks, corrosion perforations, deformation and signs of contact. Check rubber exhaust mounts for cracks and hardening.
2. If a leak is suspected, carry out a smoke test using equipment suitable for exhaust systems, observing the permissible test pressure. Check joints and seams; do not pressurise the system with compressed air without controlling the pressure.
3. If there is a loss of power, measure the exhaust back pressure using a suitable pressure gauge or pressure transducer at the specified test point. Record the value in millibars at the specified engine speeds and loads, and compare it with the manufacturer’s specifications. A measurement at idle alone does not rule out a blockage.
4. Where possible, locate the restriction by measuring pressure upstream and downstream of the suspect component. Also check for collapsed exhaust flex pipes, dented pipes and the differential pressure across the diesel particulate filter before replacing the silencer.

Replace exhaust silencers with rust-perforated shells, loose internal components or permanent deformation. A localised repair will not resolve internal corrosion or altered silencing performance. support heavy units during removal and check whether mounting straps, gaskets and clamps can be reused in accordance with the installation instructions.

Install the new unit without mechanical stress, allow clearance for thermal expansion and restore the heat shields. Tighten fasteners to the specified torque. A new silencer fitted to worn mounts may crack again. Once the system has warmed up, check for leaks, resonance and contact with surrounding components. Braem can provide technical support with selection by chassis number, so that connections and the installation configuration are checked in advance.