Glossary of Terms

Air Gap
Air gap pipes, comprise a tube within a tube with a gap between them. Used to reduce thermal inertia for faster catalyst warm up and/or to improve heat and noise insulation.

Ash
The incombustible particulate matter in the exhaust gas, mainly residues of components of the lubricating oil.

Attenuate
To reduce the strength of a signal. Referring to exhaust system noise: to lessen or decrease the amount of noise emitted from the exhaust system.

Attenuation
A measure of the reduction in strength of a signal. Referring to exhaust system noise: a measure of the noise reduction achieved by a given exhaust design.

Backpressure
The pressure above atmospheric within an exhaust system, caused by flow resistance. Typically, the lower the backpressure, the better the engine performance.

Bellows
A thin metal cylinder with convolutions. It is flexible and is used as part of flexible tube components.

Bobbins
Used as a way of eliminating vibration between the muffler hanger brackets and under-body hanger brackets.

Brick
See Substrate.

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

Carbon Dioxide (CO₂)
A compound of carbon and oxygen, naturally occurring in the atmosphere.

Carbon Monoxide (CO)
A compound of carbon and oxygen. Produced by the combustion process of the engine. It combines with haemoglobin in the bloodstream preventing the absorption of oxygen. In extreme cases, inhalation can cause death through asphyxiation. It can be easily removed from the exhaust gas by an oxidation catalyst.

Catalyst
A substance that lowers the amount of energy required for a chemical reaction to occur. In practise a catalyst speeds up the rate of a reaction. An automotive catalyst comprises a high specific surface area substrate, typically ceramic or metal, onto which an active (catalytic) wash-coat is added. The wash-coat is designed to further increase the specific surface area and make as much of the active material as possible available to the exhaust gas to increase the reaction rate.

Catalyst Module
The metal housing containing the catalyst.

Catalyst Wall Thickness and Cell Density
Describes the properties of the substrate onto which the catalyst wash-coat is deposited. High cell density increases surface area and therefore increases reaction rate. Low wall thickness can increase surface area and reduces pressure drop and heat capacity (thereby aiding ‘light off’). Usually expressed in the form of cells per square inch/wall thickness(1/1000th inch), ie 600/3.

Catalytic Converter
The complete unit, including catalyst wash-coat, catalyst substrate, holding medium and outer can. See Catalyst.

CCRT^® System
Catalysed Continuously Regenerating Trap. A CRT^® system with a catalyst coating on the filter element. This can improve system performance in demanding applications.

Cell Density
Refers to the number of cells per unit area in the honeycomb cross section of the catalytic substrate. Typically expressed in units of cells/inch2, ie 600 cpsi.

Ceramic Catalyst
Uses a ceramic extrusion for the substrate of the catalytic converter. Ceramic substrates are used in most high volume applications and come in a variety of wall thickness and cell densities. Extrusions are available in various cross sectional profiles, including, round, oval, racetrack and irregular.

Ceramic Substrate
A ceramic extrusion usually in the form of a cylinder with channels running its full length, used as the basis of both the catalyst and the filter. The channels in the catalyst are open at both ends. In the filter every other channel is blocked forming a chequer board effect. This forces the gas to flow through tiny pores in the walls of the channels preventing the passage of soot.

Close-Coupled Catalyst
A catalyst converter mounted in close proximity to the engine. In some cases there may be a second converter to complete the exhaust emissions conversion.

Component Durability Testing
A test of a component that replicates real-life operating conditions, thereby giving an indication of expected life in-service.

Computational Fluid Dynamics (CFD)
The computer simulation of fluid flow fields and properties that can be used to predict the behaviour of exhaust gases in the different components of the exhaust system. CFD is frequently used to provide the optimal shape and path of the exhaust system.

Computer Assisted Engineering (CAE)
The use of computers to simulate real–world conditions to predict operating results. CAE allows the developer to build fewer prototypes, thus streamlining the time to production.

Conduction
The process of energy transfer through a solid. In an exhaust system this is typically heat.

Convection
The process of energy transfer through fluids or between fluids and solid bodies.

Conversion Ability
A measure of the substrate’s capacity of converting pollutants. The higher the number the greater the conversion ability.

Converter
See Catalytic Converter.

Corrosion
Reaction of metal with atmospheric elements, such as oxygen or water, to cause oxidation.

CRT^® System
Continuously Regenerating Trap. A unique JM patented process for the removal of particulate matter (PM) from the exhaust gas stream using a special filter. The system operates passively and is self cleaning. It achieves this by using a catalyst upstream of the filter to produce exhaust gas conditions that enable the carbon fraction of the PM to be burnt off at typical diesel exhaust gas temperatures.

Design for Manufacturing (DFM)
A product development philosophy that takes into account the manufacturing process. The goal is to design a product that meets the user’s requirements, but is also efficient to manufacture with quality and repeatability. The philosophy is also referred to as ‘simultaneous engineering’ and ‘concurrent engineering’.

Diesel Particulate Filter (DPF)
A filter in the exhaust system that removes particulate matter from the exhaust. The CRT^® is the market leading diesel particulate filter system.

Down Pipe
The pipe used to route the exhaust flow from the manifold or turbo to the underbody of the vehicle.

EEV
Environmentally Enhanced Vehicles – A European standard with specific limit values for extra low emission vehicles, that recognises the contribution such vehicles can make to reducing transport related pollution.

Electronic Service Indicator (ESI)
A device fitted to the vehicle, used to measure the backpressure within the CRT^® or SCRT^® exhaust system and indicate when filter service is necessary.

Emissions
The products generated by the combustion process of the engine. Generally used in reference to legislated pollutants. Typical emissions include, particulate matter (PM), nitrogen oxides (NO_X), carbon monoxide (CO) and hydrocarbons (HC).

Engine Vibration
An engine contains large numbers of moving components that cause vibration when in operation. Different types of engines produce different vibration levels.

ETC
European Transient Cycle. The test cycle used for emissions certification of heavy duty diesel engines in Europe. The ETC is based on real road cycle measurements and consists of 3 parts; city, rural and motorway driving.

Euro Standards
Euro 1 though Euro 5, refer to the emissions performance required by commercial diesel vehicles as per European legislation. The standards for heavy duty diesel engines regulate the NO_x, particulate matter, hydrocarbons, carbon dioxide and smoke. Particulate matter, NO_x, HC and CO emission levels are measured in g/kWh.

Exhaust
See Emissions.

Exhaust Noise Levels
The total noise generated by the exhaust system.

Exhaust System
The system provides a means to discharge and treat the exhaust gas from the engine. It should do this without exhibiting vibrations that are transmitted to the vehicle structure and manifest themselves as NVH (noise, vibration, and harshness) issues.

External Noise Reduction
A predicted quantifier of the attenuation of an exhaust system, defined as the noise reduction from source (engine input) and measured tailpipe noise.

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

Fatigue
A material failure caused by repeated application of loads that are individually too small to cause failure.

Filter
A device (typically ceramic but metallic variants are also available), used to trap particulate matter.

Filter Module
The metal housing containing the filter.

Finite Elements Analysis (FEA)
A multi-dimensional computer simulation technique for the prediction of material behaviour.

Flexible Tube
A component comprised of a bellows with a braid on the outside and a protective liner on the inside. It is used on exhaust systems to reduce vibration transmitted to the exhaust system from the engine and to reduce the effect of engine rock on the exhaust system.

Flow Noise
A component of tailpipe noise. Noise generated by gas flowing through the exhaust system. Turbulence related.

Harmonic
Whole number multiples of the fundamental frequency.

Hydrocarbons (HC)
An organic compound of hydrogen and carbon. HC emissions in exhaust gases are undesirable and are leglislated for. They are known to be carcinogenic but can be removed from the exhaust gas by use of a catalyst.

Inlet module
The metal housing into which the exhaust downpipe from the engine enters the CRT^® system.

Intermediate Pipe
The pipes used to route the exhaust flow between resonators, mufflers and converters.

LEZ – Low Emission Zone
A defined area, typically in city centres, where vehicles entering must meet a specific exhaust emissions standard. LEZ currently exist in Gothenburg, Malmo, Lund, Stockholm and Copenhagen, and are proposed for London and Amsterdam. The standard typically specifies a specific Euro standard or a Euro standard plus the fitment of a diesel particulate filter such as CRT^®.

Manifold
The device for feeding fresh charge into the combustion chamber or for collecting burned gases from it.

Mat
The material between the substrate (e.g ceramic or silicon carbide filter) and its housing. The material holds the substrate in place and creates a seal, forcing the exhaust to travel through the cells of the substrate. It also isolates the substrate from shock and vibration and helps limit thermally generated stress.

Metallic Catalyst
A metal corrugate for the substrate of the catalytic converter. Typically, metal catalysts result in lower backpressure.

Modulation
The variation in amplitude of a signal with time. The presence of modulated noise has a significant effect on sound quality of exhaust tailpipe noise.

Muffler
A device used in the exhaust system to alter or eliminate objectionable frequencies. The muffler disrupts the exhaust flow with a combination of baffles, tubes and passages. The sound is changed with a combination of slots, perforations and fillers, such as glass wool.

Nitrogen Dioxide (NO₂)
A compound of nitrogen and oxygen. NO₂ is a major contributor to photochemical smog and acid rain, and is irritating to the eyes, respiratory system and skin. It can be removed from the exhaust gas by SCR, such as that found in the SCRT^® system.

Nitrogen Monoxide (NO)
A compound of nitrogen and oxygen. Toxic by inhalation and irritating to eyes and skin.

Nitrogen Oxides (NO_x)
Generic term for all compounds of nitrogen and oxygen.

Noise
Any sound that is undesired or interferes with one’s hearing. In an exhaust system, noise can come from the tailpipe, the surface of the exhaust system (muffler shell, for example), or from the structure of the system transmitted through the hangers.

Noise, Vibration and Harshness (NVH)
Generic term used to describe the study of subjective and objective noise and vibration.

Non-Intumescent Mat
A mat that does not expand during increased temperatures.

OEM’s - Original Equipment Manufacturers
The companies designing and manufacturing vehicles.

Orders
The number of events (usually noise or vibration pulsations) per revolution of the engine. This results in a frequency that changes with the engine RPM.

Outlet Module
The metal housing from which the exhaust exits your CRT^® system.

Overall Noise
The total noise energy of a sound. That is, the sum of energy within all measured frequency bands.

Oxicat
A diesel exhaust catalyst that oxidises carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO₂) and water (H₂O) in the exhaust gas. Certain catalysts can also have a capacity to oxidise NO to NO₂.

Oxidation
A specific form of corrosion in which the metal reacts with oxygen present in the atmosphere to form a thin layer of metal oxide on the outer surface of the metal. This film is brittle and cracks easily under vibration or thermal loads, exposing fresh metal underneath.

Particulate Matter (PM)
The solid content, primarily soot (carbon) and ash in the exhaust gas normally observed as dark or black smoke.

ppm
“Parts Per Million”. A measurement used to quantify very small amounts of a trace element within a larger overall substance.

Precious Metals
Used as catalytic material in automotive catalysts and coated on the substrate during the wash-coat process. Precious metals are key components that catalyse the chemical reaction to convert pollutants from the engine (nitrogen oxide, carbon monoxide and hydrocarbons). The typical metals that are used are palladium, platinum and rhodium and they are often deposited within an alumina-based wash-coat.

Radiated noise
Applied to exhaust systems, this measure of noise emanating from a surface of a component, such as a muffler or catalyst, due to the vibration of its surface. Also called ‘shell noise’.

Radiation
The process of energy transfer from a hot surface to surrounding surfaces that are not in direct contact. The energy transfer is due to the flow of electromagnetic radiation.

Resonance
Noise or vibration behaviour at a more or less constant frequency, due to acoustic standing waves or mechanical vibration modes.

Resonant Frequency
A frequency, usually low, that excites the exhaust system, causing a period to stand out over surrounding noise.

Retrofit
The fitment of an exhaust system, typically an emissions control system, to a vehicle already in service. Retrofit strategy is employed by fleet operators as an economical way to addressing transport related air pollution.

SCRT^® System
A combination of SCR and CRT^® in a single exhaust emissions reduction system. Capable of removing NO_X, PM, HC and CO, providing the best possible exhaust pollutants removal available. SCRT^® is a registered trademark of Johnson Matthey plc.

Selective Catalytic Reduction (SCR)
A process for removing NO_X, by reducing with a reductant such as ammonia over a catalyst. Developed for commercial vehicle exhaust treatment worldwide

Service Port
An access port usually built into the inlet module of the CRT^® System. This allows a smoke meter probe to be inserted into the exhaust stream so as to take an engine smoke reading measurement.

Smoke Reading
The visible and measurable opacity (blockage of light) of the exhaust gas. This is related to the amount of smoke emitted by the engine.

Soot
The combustible matter in the exhaust gas created as part of the normal engine combustion process typically observed as smoke. Mostly comprised of carbon.

Spark Arrestor
A device which removes incandescent particles from a gas stream. Fitted to vehicles and stationary engines which are operated in potentially flammable atmospheres.

Sound Quality
Describes the quality of noise rather than just level. Sound quality is an advancing science by which subjective perceptions of noise can be quantified and converted into objective levels.

Stainless Steel
Steel with chromium and nickel to improve resistance to corrosion. There are many different types of stainless steel available. Eminox typically uses 304 (316 for spark arrestors).

Substrate
Refers to the catalytic or filter structure, generally comprised of either a ceramic or metallic honeycomb.

Substrate Density
Refers to the number of cells per unit area in the honeycomb cross section of the catalytic substrate.

Substrate Strength
The strength or ability of a brick/substrate to withstand failure.

Tailpipe
The pipe exiting the rear of the muffler and typically visible from the rear of the vehicle.

Tailpipe Noise
Noise radiated from the tailpipe orifice, consisting of a combination of order noise (derived from engine pressure pulsations) and flow noise (from turbulence caused by gas flow through the system). This is a significant design target that drives the overall design of the exhaust system and determines the muffler volumes required.

Wash-Coat
Generic term for the combined support material and catalytic material deposited onto the substrate surface.