Glossary Complex Exhaust System

8/4/2019 Glossary Complex Exhaust System

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A Glossary of Complete Exhaust System Terms

Absorption: The mechanism by whichacoustic energy is converted into heat byfriction in absorptive media. See Pack.

Acoustics: A science that deals with theproduction, control, transmission, reception andeffects of sound.

Airborne: A noise that is transmitted by theatmosphere and can be heard outside the car.

Air Cleaner Housing: The housing thatretains the air filter and forces the route ofcombustion air through the filter medium.

Air Filter: The medium that eliminates themajor particulate from the air used in theengine combustion process. Purolator is an airfilter supplier. Also called an air cleaner.

Air Gap: Air gap pipes, including manifolds,comprise a tube within a tube and a gap ofgenerally at least 4 mm in between. Used toreduce thermal inertia for faster catalyst warmup and/or to improve heat and noise insulation.

Air Induction: See Intake System.

Air Meter: A sensor that measures the amount(volume) of air that passes through the intake.Often mounted in the air intake duct.

Air2Air: A customized term referring to thedesign of the whole air management system,including the induction system, the cold-endexhaust and hot-end exhaust.

The concept is that, if the air managementsystem is designed as a whole, the outcome

will be a more efficient (performance,complexity and cost) air management solution.

ArvinMeritor Gas Noise Simulation(AGNES): A method of predicting exhaust gasflow noise, developed internally byArvinMeritor.

Attenuate: To lessen or decrease the amountof noise. Primarily used in reference to low-end periods.

Attenuation: A measure of the noisereduction achieved by a given exhaust design.

Automatic Dynamic Analysis of MechanicalSystems (ADAMS Modeling): See Multi-BodyDynamic Modeling.

Backpressure: The total resistance to flow inthe exhaust system. Typically, the lower thebackpressure, the better the engineperformance.

Barrelly: An exhaust system noise thatsounds as if the system was dumping into atank or barrel.

Bean Can: A typical bean can consists of aperforated tube, with or without insulation,enclosed by a concentric outer shell. Used for

muffler tuning.

Bellows: A thin metal cylinder withconvolutions. It is very flexible and is used aspart of flexible tube components.

Blowback: Noise, predominantly high-frequency, that is reflected back to the


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automobile from pavement or from automobilesor buildings that are being passed.

Blub-Blub: Usually an intermittent low noteoccurring at deceleration while driving.Sometimes occurs under acceleration

conditions.

Boundary Conditions: The loading andconstraints on a component or system. Theparameters that can quantify boundaryconditions include forces, moments,displacements (or restraints thereof), rotations(or restraints thereof), accelerations, andtemperatures, as well as the locations of any ofthese parameters.

Bracket Creep: Materials that stretch when

subjected to a constant load for long periods oftime are said to creep. The effect significantlyincreases with temperature.

Braid Liner: Flex tubes for petrol systemshave a liner fitted inside to protect the bellowsfrom the effect of very hot gas. Diesel systemsoperate at a much lower temperature and so donot require a liner.

Brick: See Substrate.

Bright Tip Garnish: Added to the tip of thetailpipe to create a more aesthetically pleasingvisual element. Typically chrome-plated orpolished stainless steel.

Canning: The process of enclosing a ceramiccatalyst or filter into a metal housing.

Cap Ring: Similar to pipe noise, but occurringat muffler caps. See Pipe Noise.

Catalyst: A substance that speeds up the rate

of a chemical reaction. An automotive catalystcomprises an inert substrate onto which anactive wash coat is added. The wash coat isusually comprised of an alumina base withvarious additives such as ceria, and theprecious metals (platinum, palladium andrhodium) in different ratios according to theapplication and current metal price. The typicalcatalyst speeds up the oxidation of

unconverted hydrocarbons and carbonmonoxide to water and CO2. The widely usedthree-way catalyst also reduces nitrogen oxides(NOx) to nitrogen.

Catalyst Wall Thickness and Cell Density:

Describes the properties of the substrate ontowhich the catalyst wash coat is deposited.High cell density increases surface area andthereby increases reaction rate. Low wallthickness also increases surface area andreduces pressure drop and heat capacity(thereby aiding light off). Usually expressedas 600/3, which is to say, 600 cells per squareinch/3 thousandth of an inch wall thickness.

Catalytic Converter: The complete unit,including catalyst wash coat, catalyst substrate,

holding medium and outer can. See Catalyst.

Cell Density: Refers to the number of cellsper inch in the honeycomb cross section of thecatalytic substrate.

Ceramic Catalyst: Uses a ceramic extrusionfor the substrate of the catalytic converter.Ceramic substrates are used in most high-volume applications and come in a variety ofwall thickness and cell densities. The extrusioncan have several cross-sectional profiles,

including, round, oval, racetrack and irregular.

Channel Shape: See Profile.

Chickery: A light exhaust noise that soundslike paper rattling.

Chirpy: See Chickery.

Clean: See Smooth.

Close-Coupled Catalyst: A catalytic

converter mounted in close proximity to theengine. In some cases there may be a secondconverter to complete the exhaust emissionsconversion.Coast: See Rundown.

Coke Bottle Hoot: See Hoot.


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Cold End: The back half of the exhaustsystem, comprised of the muffler, resonator,piping and garnish. Sometimes includes a flextube.

Component Durability Testing: Tests

performed on pieces of a system, ranging fromentire hot-end or cold-end assemblies to suchportions as converters, hangers and joints ofpipes to other system elements. The tests canbe single-axis or multi-axis, hot or cold, usingconstant-amplitude loads, block loads orproving ground-based time histories, with loadinputs at a single location or multiple locations.The chosen test configuration depends on testobjectives and the loading characterizationknown for the specific subsystem orcomponent.

Computational Fluid Dynamics (CFD): Thecomputer simulation of the movement of fluidsthat predicts the behavior of exhaust gases inthe different components of the exhaustsystem. The simulation shows speed andlocation of the flow and predicts pressurelosses. CFD is frequently used to provide theoptimal shape and path of the exhaust systemand to identify the ideal position of lambdasensors.

Computer Assisted Engineering (CAE): Theuse of computers to simulate real-worldconditions to predict operating results. CAEallows the developer to build fewer prototypes,thus streamlining the time to production.

Condensate: A liquid inside muffler boxes.When burned, fuel and air produce water,among other things. This can condense insidea cold exhaust system and become trapped.Other products of combustion can be dissolvedin the water. The water is converted to a weak

acid, which attacks the metal of the exhaustsystem. It can be removed from the system byvarious means.

Conduction: The method by which heat istransferred through solid objects.

Convection: The principal method by whichheat is transferred from hot exhaust gas to theexhaust system components.

Conversion Ability: A unit measure of thesubstrates capacity of converting pollutants.

Used for comparative purposes only. Thehigher the number, the greater the conversionability.

Converter: See Catalytic Converter.

Correlation of Full-System Simulation: Fullsystem durability drive files (i.e. the test inputsthat command the simulator) should becorrelated to actual proving ground conditionsin a number of comparison steps of simulatorresults to their proving ground counterparts.

These comparison steps include the chassisacceleration and driveline torque inputscompared in the frequency domain and timedomain, strains at key locations in the systemcompared in the frequency domain and timedomain, and ultimately calculated fatiguedamage at key locations in the system. A time-frequency plot is a tool that allowssimultaneous checking of correlation in thefrequency domain and time domain.

Corrosion: Reaction of metal with

atmospheric elements, such as oxygen orwater, which causes oxidation.

Crack: A sharp snapping noise that usuallyoccurs in exhaust systems with mufflerslocated in a rearward position. The noisenormally occurs on quick deceleration inneutral, though in some systems, it occursunder hard acceleration in gear. Sounds like asmall piece of sheet metal vibrating.

Crossover: A connecting link between the two

banks of a V-type engine exhaust system,either through the engine or between theexhaust components.

Damage-Based Editing: A method ofapplying fatigue calculations on strainmeasurements to determine which portions ofloading content contribute such minimalprojected fatigue damage that they can be


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removed from the test, thereby shortening thetest.

Decoupler: A much shorter form of flexibletube. An important difference is that flexibletubes are much softer than decouplers.

Decouplers can carry load, whereas flexibletubes need hangers very close to both ends.

Design for Manufacturing (DFM): A productdevelopment philosophy that takes intoaccount the manufacturing process expertiseand limitations. The goal is to design a productthat meets the users requirements, but is alsoefficient to manufacture with quality andrepeatability. The philosophy is also referred toas simultaneous engineering and concurrentengineering.

Diesel Particulate Filter (DPF): A filter in theexhaust system that removes particulate matterfrom the exhaust. Used on diesel passengercars and trucks.

Diffusion Length: The distance from thecenter of a substrate to the active catalytic siteson the substrate walls.

Double Skinned: The skin of a component,usually a muffler, made from two layers of

metal. Friction between the two layers reducesvibration and thereby reduces noise.

Down Pipe: The pipe used to route theexhaust flow from the manifold or turbo to theunderbody of the vehicle.

Drain Holes: To reduce/eliminateaccumulation of condensate in an exhaustsystem muffler or resonator.

Drive-Away: A method of exhaust evaluation

in which one person stands behind thestationary automobile and listens, as anotherperson drives the car away from him.

Drumming: A low-end period that varies inintensity at a constant engine or road speed.Usually descriptive of body noise.

Dual-Mode: A design of muffler wherebymuffler-tuning volumes are adjusted internallyduring operation to optimize attenuation andbackpressure.

Duplicate PG Damage: See Correlation of

Full-System Simulation.

Emission Simulation Program (ESP): Asoftware program developed in-house byArvinMeritor that predicts emission levels.

Emissions: The gases generated by thecombustion process of the engine. Generallyused in reference to legislated pollutants.Typical emissions include Nitrogen Oxide(NOx), Carbon Monoxide (CO) andHydrocarbons (HC).

End Chambers: Typically, the internal portionof the muffler, between the baffle and cap.

Engine Vibration: An engine contains largenumbers of moving components that causevibration when in operation. Different types ofengines produce different vibration levels.

Exhaust: See Emissions.

Exhaust Noise Levels: The total noise

generated by the exhaust system.

Exhaust System: The system provides ameans to discharge and treat the exhaust gasfrom the engine. The system encompasses theengine manifold to the tailpipe and is generallycomprised of muffler(s), resonator(s),converter(s), tubing, sensors and sometimes aflex joint and/or garnish. The overall goal of thesystem is to reduce NVH (noise, vibration, andharshness), treat emissions, manage heat androute exhaust gas. In addition, sensors located

at key points are used to optimize engineperformance and emissions.

Exhaust System FSD Validation: See Full-System Durability Testing.

Exhaust System Response Correlation: SeeCorrelation of Full-System Simulation.


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External Noise Reduction: A predictedquantifier of the attenuation of an exhaustsystem, defined as the noise reduction fromsource (engine input) and measured tailpipenoise.

Fabricated Manifold: A generic term appliedto either tubular manifolds or pressed manifold.

Failure Mode: The way an exhaust systemfails to perform function or meet requirements,such as breakage, too noisy, leaks, etc.

Fatigue-Based Editing: See Damage-BasedEditing.

Finite Element Analysis (FEA): A computersimulation technique using 3D modeled parts

that show color mapping of stresses, based onexpected conditions in application.

Flexible Tube: A component comprised of abellows with a braid on the outside and aprotective liner on the inside. It is used onexhaust systems to reduce vibrationtransmitted to the exhaust system from theengine and to reduce the effect of engine rockon the exhaust system.

Flow Noise: A component of tailpipe noise.

Noise generated by gas flowing through theexhaust system. Turbulence related.

Free Kick: Quick acceleration anddeceleration at standstill with transmission inneutral.

Full-System, Full-Service Supplier (FSFSS):The philosophy of the supplier providing asolution based on a set of performancestandards supplied by the customer. Thesupplier would develop, test and validate the

design. This would include designing withcomputer-aided simulations, prototyping andvalidating the design with accelerated testingsimulating actual conditions.

Full-System Durability Drive File: Actualroad conditions used to program the full-systemdurability test fixture.

Full-System Durability Testing (FSD): Testsperformed on an entire system, usually in amulti-axial, inertially reacted simulation ofvehicle motion, usually for rough-roadconditions. These tests are typically run atoperating temperatures representative of a

proving ground or the field. ArvinMeritor hascreated a tested apparatus that allows anaccelerated test of the full exhaust system andair induction system to be testing for durability.The system is programmed using actual roadconditions and simulates 100,000 miles in fourweeks.

Full-System Rig: See Full-System DurabilityTesting.

Gap Bulk Density (GBD): The density of the

support mat after catalytic converter canning.An indicator of the pressure exerted by the mat.

Gas Rush: See Flow Noise.

Genetic Algorithm: A prediction method usedby ArvinMeritor to automatically optimizeexhaust design for noise and backpressure.

H-Pipe: A junction where two pipes are joinedby a cross pipe. Used on dual exhaustsystems to further reduce backpressure.

Hanger: The supporting rod to which theexhaust isolator is attached.

Hanger Force: A measure of vibration input,due to exhaust vibration, to the vehicle body.This is a primary design target for somecustomers.

Harmonic: Whole number multiples of thefundamental frequency.

Harshness: Related to sharpness, with lowerfrequencies intermingled with the highfrequencies. Has an unpleasant rattling sound.

Heavy: Descriptive of low-end periods thatsound deep and resonant usually below100 cps.

Helmholtz Resonator: See Resonator.


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Helmholtz Theory: A volume that is attachedby a conduit of a given length and cross-sectional area will attenuate a given frequencyof noise.

High Frequency: A rapid, sharp cracking

noise that sounds like small pieces of sheetmetal popping and cracking. Also includeshiss and swish sounds.

Highs: See High Frequency.

Hiss: A high-frequency noise due to highvelocity of gas. Sounds like an air or gas leak.

Hoot: A low note, just below the intermediaterange, that sounds like a stream of air blownover a Coke bottle.

Hot End: The front part of the exhaust systemcomprised of the manifold, down pipes and thecatalytic converter. May include a flex joint.

Hydrocarbons: Pollutants that result fromincomplete combustion of fuel and engine oil.

Individual Noise: An intermediate frequencynoise (250-1500 cps) that follows the engineRPM. Sounds like an identifiable explosiongoing down the exhaust system every time an

exhaust valve opens.

Induction: See Intake System.

Induction Noise Levels: The total noisegenerated by the intake system.

Induction System: See Intake System.

Intake System: The portion of the air systemthat comprises the air inlet through the throttlebody. The air is cleaned of particulate matter

and used for engine combustion. Commonlyincludes the air cleaner body, air filter andductwork.

Interior Noise Levels: The noise detected bythe vehicle occupants. Typically,measurements are taken around the hearingareas of the driver and the front and rearpassengers.

Intermediate Pipe: The pipes used to routethe exhaust flow between resonators, mufflersand converters.

Intermediates: Noise frequencies above thelow-end range or 250-300 cps to 1500 cps.

These are usually not affected by volume(Helmholtz resonators) or spit chambers, butare quieted in reversing sections and otherdevices.

Intumescent Mat: A mat that expands withincreased temperature.

Irregular Profile: The shape of a substratethat uses an irregular cross section comprisedof radii and straight sides.

Isostatic Strength: A substrates ability towithstand compressive forces.

Light Off: The time from a cold engine start tothe point of 50-percent pollutant conversion.

Light-Off Converter: A converter close to themanifold that heats up quickly to convert theemissions, especially hydrocarbons at start up.

Listen to Interior Re-synthesized Acoustics(LIRA): A method of re-synthesis of vehicle

interior noise for theoretical changes to exhausttailpipe noise. Developed by ArvinMeritor.

Local Fatigue: There are many instanceswhere structural simulations can be performedat the component level. When fatigue damageaccumulates at a rather limited number ofplaces in a system, the analysis is targeted atkey areas in order to identify structuralcharacteristics. This analysis is quitecomputationally intensive and therefore shouldbe limited to the locations where necessary.

Local Load: See Local Fatigue.

Lows: Noise where a period or periods occurin the low-frequency range.

Manifold: The portion of the engine thatcombines the exhaust gases into one or twofinal paths from the pistons on the engine.


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After the exhaust gases are combined, they aretypically routed through catalytic convertersand then a combination of resonator(s) andmuffler(s). The typical types of manifolds arecast iron, cast stainless, stamped, fabricatedbent tubes, and more recently, hydro-formed

tubes.

Maniverter: An assembly in which themanifold and converter are one piece.

Mass Flow Sensor (MFS): See Air Meter.

Mat: The material between the brick andhousing in a tourniquet-type converter. Thematerial holds the brick or bricks in place andcreates a seal, forcing the exhaust to travelthrough the cells of the substrate.

Mechanical: See Vibration.

Metal Fatigue: Cracks in metal caused byrepeated application of loads that areindividually too small to cause failure. Thesusceptibility of steels to metal fatigue variesconsiderably with chemical composition.

Metallic Catalyst: A metal corrugate for thesubstrate of the catalytic converter. Metalsubstrates are better suited for low-volume

applications since they have lower capitaltooling costs and higher material cost thanceramic substrates. They can only beproduced in round and racetrack cross-sectional profiles. Typically, metal catalystsresult in lower backpressure.

Metallic Flutter: See Crack.

Mid-Bed Sensors: A sensor placed betweenthe bricks in a two-brick system.

Modal: Relative to the modes of a structure.Every structure exhibits modes that are definedby their shape and their natural frequency. Ifthe structure is excited at a natural frequency,then it responds by vibrating in itscorresponding mode shape at an amplifiedlevel.

Modulation: The variation in amplitude of anoise signal with time. The presence ofmodulated noise has a significant effect onsound quality of exhaust tailpipe noise.

Moo: Noise in which a low-end period is drawn

out.

Muffler: A device used in the exhaust systemto alter or eliminate objectionable frequencies.The muffler disrupts the exhaust flow with acombination of baffles, tubes and passages.The sound is changed with a combination ofslots, perforations and fillers, such as glasswool.

Multi-Body Dynamic Modeling: Involvessolving the equations of motion for a vehicle,

system or component(s) that are assumed tobe rigid bodies, flexible bodies or acombination. Determines such behavior as thetransformation of inputs at certain locations onthe bodies (e.g. a vehicle hitting bumps) intothe reactions at other location(s) (e.g. forces atexhaust hangers or moments at an exhaustsystem joint).

Natural Frequency Dynamometer: Invehicles, the energy from the fuel is used toovercome air resistance and friction. In a test

cell, there is no air resistance or friction, so adynamometer is used to absorb the energyinstead. There are several different types. Themost common types are eddy current andwater brake. The energy absorbed is turned toheat.

Noise: Any sound that is undesired orinterferes with one's hearing. In an exhaustsystem, noise can come from the tailpipe, thesurface of the exhaust system (muffler shell, forexample), or from the structure of the system

transmitted through the hangers.

Noise, Vibration and Harshness (NVH):Terminology describing the soundcharacteristics of an exhaust system or vehicle.

Noise Transfer Function: A measure of thevehicle behavior, which quantifies therelationship between the noise or vibration of a


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source (i.e. exhaust system) and the perceivednoise in the vehicle cabin.

Non-Intumescent Mat: A mat that does notexpand during increased temperatures.

NOx Trap: A device for the absorption andcatalytic conversion of nitrogen oxides.Resembles a catalytic converter.

Off-Idle Boom: Same as off-idle period or off-idle low.

Off-Idle Lows: A period that is predominate ata point just above idle RPM. These periodshave a booming characteristic and are usuallybelow 60 cps.

Open Sound: See Barrelly.

Orders: The number of events (usually noiseor vibration pulsations) per revolution of theengine. This results in a frequency thatchanges with engine RPM.

Oval Profile: The shape of a substrate thatuses a cross section comprised of four radiiwith no straight sides.

Overall noise: The total noise energy of a

sound. That is, the sum of energy within allmeasured frequency bands.

Over-Run: A low note that resonates as theengine is decelerated with the car at astandstill.

Over-Run Fluff: See Pop Back.

Overtone: The second or higher harmonic of afundamental tone.

Oxidation: A specific form of corrosion inwhich the metal reacts with oxygen present inthe atmosphere to form a thin layer of metaloxide on the outer surface of the metal. Thisfilm is brittle and cracks easily under vibrationor thermal loads, exposing fresh metalunderneath.

Pack: A volume of absorptive material, usuallyfibrous, which is inserted into mufflers toabsorb high-frequency noise.

Particulate Matter: Material emitted from theexhaust that is in the solid phase, after cooling

to ambient temperature and diluting with air.Typically comprised of high carbon-contentmaterial (soot), a soluble oil fraction andsulphate.

Peak Mat Pressure: The maximum forcesustained on the perimeter of the mat from thetourniquet process/canning.

Period: An audible note, loud enough to bedistinguishable from background noise, usuallyin the lower frequency ranges up to 250 cps.

Periods can be caused by the exhaust system,air induction system or other automotivecomponents.

Pipe Noise: A pinging noise due to gasimpinging on the walls of pipe or from thetransmittal of high-frequency noise throughwalls of pipe before entry into muffler.

Pop Back: An intermittent low note that occursduring quick deceleration with transmission inneutral. Sounds like a period excited by rush of

gas back into the exhaust system.

Power Spectral Density (PSD): A plot of aninput or response variable squared, divided byfrequency (vs. frequency). Examples ofvariables of interest include acceleration andstrain/stress. PSD is one of several ways tooutput the results of spectral analysis. Otherways include ASD (amplitude spectral density),ESD (energy spectral density), and FFT (fastFourier transform). The objective of spectralanalysis is to determine the frequencies at

which the input or response is most significant.

Precious Metals: Used to coat the substrateduring the wash-coat process. Precious metalsare the key components that catalyze thechemical reaction to convert pollutants from theengine (nitrogen oxide, carbon monoxide andhydrocarbons). The typical metals that areused are palladium, platinum and rhodium.


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They are deposited within an alumina-basedwash coat.

Predictive Analysis: Testing through the useof computer simulation to predict the outcomein actual application. Predictive analysis is not

an exact science, but is continually improved tonarrow the gap between predictive behaviorand real-world results.

Predictive Model Improvement: Therefinement of a simulation model to predictreal-world conditions. Predictive modelingstrives to simulate real-world conditionsthrough computer simulations, with constantcomparisons to refine and improve theaccuracy of the simulation.

Pressed Manifold: A manifold made frompressed steel.

Profile: Refers to the cross-sectional shape ofthe substrate.

Pulsating Flow: The opening and closing ofthe valves in rapid succession creates a rhythmdue to the changes in the pressure of theexhaust flow. The closer the valves are to theengine, the more likely there will be a pulsatingflow in the exhaust. The further away from the

engine, the more the exhaust stream evens outinto a smooth, continuous flow.

Putt-Putt: A noise that sound like a muffled,small one-cylinder engine. Usually heard atlow engine speeds or idle. It may sound like aloose baffle rattling or individual rapping onacceleration or deceleration.

QED: A method developed by ArvinMeritor tooptimize exhaust designs using geneticalgorithms.

Quarter-Wave Tuner: Consists of a smalltube that is attached to an exhaust system. Ithas the pipe length of the quarter wave and isterminated at the other end. The attenuationvaries with the pipe diameter and length.Placing a drain hole at the terminated end maybroaden the tuning.

Quinkie Tube: Much like the quarter wavetuner, except it is attached at the problemfrequency quarter wave and terminatesdownstream at a length that will phase out theproblem frequency one-half wave longer orany odd multiple of the one-half wave.

Racetrack Profile: The shape of a substrateusing a cross section comprised of two straightsides connected by two 180-degree radii.

Radiated noise: Applied to exhaust systems,this is a measure of noise emanating from asurface of a component, such as a muffler orcatalyst, due to the vibration of its surface.Also called shell noise.

Radiation: The principal method whereby heat

is transferred from very hot exhaustcomponents to surrounding objects or othercomponents.

Rap: A modulation of several frequency bandsat approximately the same dB level at the samefrequency. This can emanate in lowerfrequency as what is called individual beat ormotor boat characteristic at the higherfrequency.

Rasp: A metallic noise consisting of

intermittent high-frequency noise. Oftenassociated with single-muffler exhaust systemsand caused by nonlinear acoustic behaviorresulting in pressure-wave steepening.

Raspy: A high-frequency noise similar to spit,but more frequent.

Real-Time Pressure Mapping Equipment: Apatented system developed by ArvinMeritorand used in the production of converters, usingthe tourniquet process. There is a very narrow

operating window between adequate holdingpressure at high temperature and the requiredpressure during assembly. The system utilizestwo hydraulically controlled actuators to ensurefull 360-degree application of pressure onto theconverter assembly.

Rebound: See Blowback.


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Resonance: Noise or vibration behavior at amore or less constant frequency, due toacoustic standing waves or mechanicalvibration modes.

Resonant Frequency: A frequency, usually

low, that excites the exhaust system, causing aperiod to stand out over surrounding noise.

Resonator: Typically a Helmholz resonatorbuilt into a muffler. The design typically has alarger tube encompassing a smaller tube,creating a closed volume around the smallertube. The wall of the smaller tube hasperforations that allow frequencies into thecross-sectional area, which is usually filled withglass wool or stainless steel wrap. Also usedto describe a small muffler.

Reversing Section: The section of the mufflerin which the gas enters, reverses through atube or holes and then exits through anothertube or set of holes.

Rig Test: A test performed on a rig, usuallyusing a hydraulic ram to apply an oscillatingload to a component. Rig tests can be run hotor cold. Hot gas can be supplied by gas-firedburner or by a slave engine.

Rigid Body: A mode of the structure wherethe element moves rigidly. There is no relativemovement between the points of this element.Movement only occurs between the elementand the rest of the structure. For example, theexhaust system on a car moves rigidly as awhole.

Road Excitation: See Correlation of Full-System Simulation.

Road Load: Loads applied to the exhaust

system originating from the road surface.

Rosette: A special type of strain gauge fromwhich the data can be taken to calculateprincipal stresses/strains. The data can also beused to determine the extent to which thestress/strain state at a given location on thesystem is uni-axial or multi-axial.

Roughness: See Harshness.

Round Profile: The shape of a substrate thatuses a round cross section.

Rubber Isolators: Used as a way of

eliminating vibration between the mufflerhanger brackets and under-body hangerbrackets.

Rundown: A low note that resonates as theautomobile decelerates.

Selective Catalytic Reduction (SCR): Aprocess for removing Nox, by reducing with areductant such as ammonia over a catalyst.Being developed for commercial vehicleexhaust treatment in Europe.

Separation Rings: Used in two-brick systemsto separate the bricks. Usually, allows asensor (mid-bed sensors) to be placedbetween the two bricks separation rings.

Sharpness: A noise that is noticeable whenlistening to the tailpipe, with the characteristicof gas rushing over sharp edges. Frequencyrange would be above 250 cps.

Shell Noise: Similar to pipe noise, but

occurring in the muffler through the shell.

Shift Point: The point at which automatictransmissions shift and cause a rapid change inengine RPM and a resulting surge of noisethrough the exhaust system.

Silky: See Smooth.

Single-Skinned: Skin made of one layer.

Siphons: See Drain Holes.

Smooth: The description of an exhaustsystem whose overall noise level is low andhas no outstanding or distinguishable periodsor characteristics.

Snarly: Related to harshness, but morepronounced at only certain frequencies or


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when the accelerator is operated up and downin a certain range.

Snorty: Related to snarly, but morepronounced at certain frequencies than othersand usually at lower frequencies than snarly.

Sound Quality: Describes the quality of noiserather than just level. Sound quality is anadvancing science by which perceptions ofnoise can be quantified and converted intoobjective targets.

Spit: An intermittent, high-frequency noise.

Spit Chamber: A part of a muffler. Usually atubular resonator closed down at the endaround a perforated or slotted tube. This

attenuates high- and some intermediate-frequency noises.

Stainless Steel: Steel with chromium andnickel to improve resistance to corrosion.There are many different types of stainlesssteel available.

Standard Wall Catalyst: Catalyst substratewith less than .008 of wall thickness(.0055 to .008). Usually with low cell density(350 to 400 cells per square inch).

Substrate: Refers to the catalytic structurecomprised of either ceramic honeycomb ormetallic honeycomb. The substrate is coatedwith a solution of precious metals to provide achemical reaction.

Substrate Density: Refers to the number ofcells per inch in the honeycomb cross sectionof the catalytic substrate.

Substrate Strength: The strength or ability of

a brick/substrate to withstand failure whensubjected to the pressure of canning or atourniquet process.

Substrate Volume: The overall size of thesubstrate, measured in liters or in cubic inches.

Suck Back: See Pop Back.

Swish: See Hiss.

System Boundary Conditions: Theconditions at the boundaries of the system. Forthe full exhaust system, they are the isolatorsand the engine or the decoupler.

Tailpipe: The pipe exiting the rear of themuffler and typically visible from the rear of thevehicle.

Tailpipe Noise: Noise radiated from thetailpipe orifice, consisting of a combination oforder noise (derived from engine pressurepulsations) and flow noise (from turbulencecaused by gas flow through the system). Thisis a significant design target that drives theoverall design of the exhaust system and

determines the muffler volumes required.

Take Away: See Drive-Away.

Teary: See Raspy.

Thin-Wall Catalyst: A catalyst substrate withless than .005 of wall thickness (.0045).Usually with medium cell density (400 to 600cells per square inch).

Time-Frequency Plot: A 3D plot that allows

simultaneous visualization of the time domainand frequency domain contents of a signal.The two horizontal axes are time andfrequency, and the vertical axis is themagnitude of the observed signal. The plotmight be two-dimensional, in which case thetwo axes are time and frequency. A scale ofcolors represents the magnitude. See alsoCorrelation of Full-System Simulation.

Timet: Company that is the worlds largesttitanium producer and distributor. See also

Titanium.

Titanium: A metal used largely in theaerospace industry for its high strength-to-weight ratio and corrosion resistance.ArvinMeritor is the first manufacturer to usetitanium in a mass-produced exhaust system,which was introduced on the Corvette Z06.


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Torca Joint: An inexpensive way of attachingtwo parts of an exhaust together. It consists ofa clamp wrapped around the joint andtightened by a bolt.

Tourniquet Process: A technique for housing

a substrate/brick by wrapping sheet metalaround the outside of the brick. The brick isfirst wrapped with a mat, and then the sheetmetal is wrapped around the outside andclosed by rolling the leading edge with thetrailing edge. Benefits of this process are low-cost tooling and even, repeatable pressure toretain and seal brick(s). The process is idealfor thin and ultra-thin catalyst substrates withlow isostatic strength and for low volumeapplications. Suitable for two brick systems.

Tourniquet Wrap: See Tourniquet Process.

Transfer Path Analysis: An experimentalmethod available from ArvinMeritor thatquantifies the contribution of noise andvibration sources to vehicle interior noise. Thismethod relies on measurement of noise-transfer functions.

Transmission Loss: A measured or predictedquantifier of exhaust attenuation under certainideal conditions. Often used to characterize

muffler designs and to correlate predictionmethods.

Two-Brick Systems: A converter comprisedof two substrates of the same profile, mountedin line.

Ultra-Thin Wall Catalyst: A catalyst with lessthan 0.004 of wall thickness (.0025 to .0035).Usually with high cell density (600 to 900 cellsper square inch).

Under-Body Converter: A converter locatedunder the car and away from the manifold,typically after the down pipe.

Under-Body Hanger Brackets: Brackets offthe underside of the vehicle used to suspendthe muffler system.

Under-Body Heat Shield: A heat shieldattached to underside of the car to prevent hightemperatures from being radiated onto theunderbody.

Usage Profile: The way in which vehicles are

driven in the real world. Different vehicles willhave different usage profiles, as they will beused for different purposes. This can besignificant, as it affects the loads andenvironment to which the exhaust is subjected.

Vehicle Mapping: Acquisition of data on avehicle in the field or on a proving ground,using many types of sensors (accelerometers,strain gages, thermocouples, engine parametermeasurements, etc.), in order to characterizeour systems operating environment and

response to that environment.

Vibration: A vibration in the vehicle or exhaustsystem, which can be sensed physically and isairborne. It may resonate the exhaust systemperiod.

Vibration Heat-Affected Zone: The areaimmediately surrounding a weld that has beendamaged by the extreme heat of the weldingprocess. The damage usually affects theresistance to corrosion and fatigue.

Visco-Elastic: Support mat stiffness isdependent upon the rate of compression. Thefaster the compression, the greater thestiffness. The support mat continues to relax(reduce in pressure) after manufacture of theconverter assembly is complete.

Wash Coat: Catalytic material applied to thesubstrate, either prior to the precious metals orat the same time as the precious metals. Thewash coat has some catalytic activity and

increases the disersion (surface area) of theprecious metal components to achieve highcatalytic activity and stability.

Whistle: A very high-frequency continuousnote that occurs at quick deceleration and oracceleration with transmission in neutral.


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Usually caused by using too wide slots inslotted tubing or by breaks in gas passagetubing.

Wire-Mesh: Steel wire knitted together tomake seals and spacers to guarantee an airgap.

Y-Pipe: A junction where two pipes cometogether and join into one.

LGP01-07072301

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Glossary Complex Exhaust System