Shaping the Future

Exhaust After Treatment

Systems

Exhaust After-Treatment

Reducing the exhaust pollution emitted from the vehicle by using “clean-up” technology within the exhaust pipe system.

Exhaust Catalysts are the primary technology used

Exhaust After-Treatment

Standard Spark-Ignition (petrol) engines have relied on a “3-Way” Catalyst to reduce CO, HC & NOx

Until recently Compression-Ignition (diesel) engines in passenger cars have not needed after treatment systems, recent legislation is changing this.

The introduction of Direct Petrol Injection (DPI or GDI) systems has further complicated the emissions after treatment requirements

SI Emissions After-Treatment

Conventional 3 – Way Catalysts

Require the engine to operate at or near stoichiometric air-fuel ratio (lambda = 1)

Platinum, Palladium and Rhodium in Aluminium Oxide washcoat

SI Emissions After-Treatment

Conventional 3 – Way Catalysts

Accelerate both oxidation and reduction reactions through the use of small quantities of Platinum, Palladium and Rhodium metals

Carbon Monoxide is oxidised to Carbon Dioxide

Hydrocarbons are oxidised to Carbon Monoxide and Water

Oxides of Nitrogen are reduced to Nitrogen and Oxygen

The presence of the catalytic material ensures that the above reactions occur at lower temperatures and quicker than they would have otherwise.

The reactions are exothermic. The exhaust temperature leaving the catalyst is higher than that entering the catalyst

SI Emissions After-Treatment

The 3-Way Catalyst requires the engine

to oscillate near lambda = 1

Catalyst Efficiency

SI Emissions After-Treatment

Catalyst “Light Off” Temperature The 3-Way Catalyst achieves acceptable

conversion efficiencies when it is above a

certain temperature

Catalysts take time to start to work!!

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CI Emissions After -Treatment

Carbon Monoxide is not a concern

HC is dealt with via Oxidation Catalysts (One Way)

Major emphasis is on NOx and particulate matter treatment NOx after-treatment options include;

Cooled EGR Selective Catalytic Reduction NOx Absorbers (NOx Storage Catalyst, Lean NOx Traps)

CI – HC After-Treatment

Oxidation Catalysts simply help to complete the ‘partial combustion’ that formed the HC in the first place.

Pre light off performance is critical (most of the HC is produced at cold start)

Fortunately hydrocarbons are stored on the catalyst at low temperatures and released at high (post light off) temperatures.

The release might not be fully completed before light-off is reached – leads to poorly understood phenomena

Storage and release regimes may be repeatedly crossed in light-duty driving

Pattern of storage/release/oxidation strongly depends on the species of hydrocarbon. Heavy molecules are much more easily stored than light molecules

Selective Catalytic Reduction (SCR) Systems

Ammonia-SCR systems react ammonia (NH3) with the NOx to form nitrogen (N2) and water (H2O).

There are three reaction pathways:4NH3 + 4NO + O2 = 4N2 + 6H2O2NH3 + NO + NO2 = 2N2 + 3H2O8NH3 + 6NO2 = 7N2 + 12H2O

Any source of ammonia can be used as the reductant but most commonly the source is an aqueous solution of urea (typically 30-35%)

This decomposes in the exhaust stream in two stages to form ammonia and carbon dioxide (CO2)

CI – NOx After-Treatment

NOx - Selective Catalyst Reduction

(CRT – Continuously Regenerating Trap – Particulates )

Diesel Exhaust Fluid (DEF) Tank

NOx - Selective Catalyst Reduction

Diesel Exhaust Fluid (DEF) Tank

Selective Catalytic Reduction (SCR) Systems

Hydrocarbon-SCR (lean NOx reduction) systems use hydrocarbons as the reductant.

The hydrocarbon may be that occurring in the exhaust gas or it may be added to the exhaust gas.

The advantage of this system is that no additional reductant source (e.g. urea) need be carried but these systems cannot yet offer the performance of ammonia-SCR systems.

The reaction pathways depend on the hydrocarbon used but the following describes the total reaction in the system:HC + NOx = N2 + CO2 + H2O

CI – NOx After-Treatment

CI – NOx After-TreatmentSelective Catalytic Reduction (SCR) Systems

Absorber Catalyst (Lean NOx Trap)

Under lean conditions NO2 is absorbed onto the catalyst and stored as a nitrate

Under fuel rich conditions the nitrate is released as NO and then reduced to N2

Absorber material also absorbs sulphur (from fuel) forming a stable sulphate so reducing the trap’s efficiency

CI – NOx After-Treatment

Two main methods of particulate reduction Oxidation Catalysts

Reduce gaseous compounds CO, HC and CO2 and the heavy (near liquid) hydrocarbons on the particulate. Do not affect the core carbon matter. Expect a reduction in particulate matter of ~ 25%

Particulate Filter

Basically a solid (carbon) particle trap with high temperature regeneration (oxidation) capability. Passive and active systems in use;

Passive – continuous oxidation without additional energy input

Active – oxidation with energy input (eg fuel)

Expect 90-95% clean up rates

CI – Particulates After-Treatment

Oxidation Catalysts The diesel oxidation catalyst is designed to oxidize carbon monoxide, gas phase hydrocarbons, and the SOF fraction of diesel particulate matter to CO2 and H2O:

At high temperatures oxidation of sulphur dioxide to sulphur trioxide occurs which combines with water forming sulphuric acid:

….hence the need for low sulphur fuels

CI – Particulates After-Treatment

Particulate Filters (DPF)

Filt

erin

g Effi

cien

cy (

%)

Particle “Diameter” (microns)

10-210-3 10-1 101100

Diff usionI nterception I nertia

Sedimentation

Total Effi ciency

Filt

erin

g Effi

cien

cy (

%)

Particle “Diameter” (microns)

10-210-3 10-1 101100

Diff usionI nterception I nertia

Sedimentation

Total Effi ciency

Some organics and sulphuric acid pass through the filter.

These can nucleate downstream to form new

particles (NB: smaller than the trapped ones)

Regeneration every 300 to 1000Km

(Ash disposal every 120,000 Km)

CI – Particulates After-Treatment

Particulate Filters (DPF) Regeneration

Passive RegenerationPassive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Many cars don't get this sort of use though so manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

Active RegenerationActive Regeneration occurs when the level of soot in the filter reaches around 45%. The ECU makes small adjustments to the fuel injection timing and increases the exhaust gas temperature. This increases the exhaust temperature which then initiates the regeneration process, burning away the soot trapped in the DPF.

CI – Particulates After-Treatment

Particulate Filters (DPF) Regeneration

Metallic catalysts are used to reduce the temperatures required to burn the particulates (regeneration)

Two systems: Additive (aDPF): the metallic catalyst is added to the fuel (Active)

Coated or Catalysed (cDPF): the metallic catalyst is imbedded in the washcoat of the filter (Passive)

cDPF present less of a control challenge and is suited to retro fit applications, aDPF is used where the conditions for cDPF cannot be assured

CI – Particulates After-Treatment

Example After Treatment System

Combination of Oxidation, Filter Systems and NOx trap systems (JM)

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