Pollution Management
� What are we managing, and why?
� Regulatory controls & progress in Europe
� Technology for conventional powertrains
� The direction of engine development
� Electric and hydrogen-fueled propulsion
� Short and long-term solutions
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Regulated Pollutants
� Carbon Monoxide (CO): − Readily displaces oxygen in the blood and also has cumulative effects
− Caused by incomplete oxidation of carbon in the fuel
− Easy to control – needs only further oxidation in the exhaust stream
� Hydrocarbons (THC, HC, NMHC): − Total and non-methane are separately regulated since methane is not considered harmful
− Caused by incomplete combustion of fuel and lubricating oil
− Easy to control with oxidation catalyst
� Nitrous Oxides (NOx): − Predominantly NO and NO2, responsible for respiratory complaints and acid rain formation
− Caused by dissociation and recombination of nitrogen and oxygen from the induction air, under high combustion temperatures and pressures.
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HC and NOx combine to generate photochemical smog
Regulated Pollutants
� Particulates:− A complex blend of carbon, organic compounds and incombustible matter
which varies according to operating conditions− Ash from inorganic matter, soot, sulphates and unburnt/partially burnt hydrocarbons
− Heavily influenced by fuel quality and mixture formation
− Composition changes in the exhaust as volatile fractions condense around a solid nucleus
− Typically, 90% of the mass is from particles >10nm, yet 90% of the quantityare <2.5nm− Different measurement techniques for particulate mass and particulate number are
sensitive to different size distribution of particles.
− Accurate direct measurement of particulate mass by weighing becomes increasingly difficult as the limits and sample mass get smaller
� Ammonia (NH3)− As a pollutant: A by-product of 3-way catalysts, particularly at low temperature
− Deliberate addition to enable aftertreatment systems to function
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Carbon Dioxide
� Unregulated pollutant, but the effects of CO2 on global warming are a significant driver of new technology.
� European Commission reports that 12% of CO2 generation in the Union is from vehicles
� Most Member States employ some kind of taxation principle based either on CO2 emission or fuel consumption− Used as a driver to motivate the population in to newer, more efficient vehicles
� European fleet average in 2017 was 118.5g/km in 2017, against a target of 130g/km− 22% reduction since 2010
� 2021 fleet average target is reduced to 95g/km
� Increasing complexity of engine and emissions management systems tend to have an adverse effect on CO2 emissions
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EU Regulatory Progress – Passenger Cars
Euro 1 Euro 2 Euro 3 Euro 4 Euro 5 Euro 6 Euro 6d
1990 1995 2000 2005 2010 2015 2020
SI CI SI CI SI CI SI CI SI CI SI CI SI CI
Euro 1 2.72 2.72 0.97 0.97 0.14
Euro 2 2.2 1 0.5 0.7 0.08
Euro 3 2.3 0.66 0.2 0.15 0.5 0.56 0.05
Euro 4 1 0.5 0.1 0.08 0.25 0.3 0.025
Euro 5 1 0.5 0.1 0.068 0.06 0.18 0.23 0.0045 0.0045 6E^11
Euro 6 1 0.5 0.1 0.068 0.06 0.08 0.17 0.0045 0.0045 6E^12 6E^11
Euro 6d 1 0.5 0.1 0.068 0.06 0.08 0.17 0.0045 0.0045 6E^11 6E^11
SI = Spark Ignition
CI = Compression Ignition
PNCO THC NMHC NOx THC+NOx PM
THAILAND
Limits in g/km
!
EU regulatory progress – Heavy Duty Engines
� Ammonia limit introduced at Euro 6
� Testing and enforcement has been adapted and improved for Euro 6:− Transient test cycles for certification values− ‘Not to Exceed’ limits on randomly generated cycle− Real Driving Emission (PEMS) testing on public
highway.− In-Service conformity testing− On-board diagnostic testing
� The forces for change in the EU are different to those in Asia− NOx emission is critical in the EU− PM/PN dominates in SE Asia
Thailand
How to tackle exhaust pollution?
In order of complexity……
1. Less vehicle usage = less pollution and less CO2 emission.
2. More efficient drivetrains
3. Alternative or better refined fuels
4. Cleaner combustion
5. Better aftertreatment
6. No combustion –> switch to electric vehicles
The most appealing for our environment
Attractive, but not the most practical option
� Focus on items 2 to 5
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2. Drivetrain Improvements – Engine Development
� Benefits are principally lower CO2 emissions, but lighter loads also impact on other pollutants.
� Evolution, not revolution− Does not require radical re-design or costly additional parts -> Often a win-win situation
� Can be applied to any engine type
� Examples− Idle stop & cylinder deactivation > Savings depend upon traffic condition
− Low friction coatings on piston rings and skirts > 2-5%
− Honing patterns on cylinder walls > 1%
− Reduced tension on shaft seals > 0.3%
− Lower viscosity lubricants > 0.5%
− Electrically driven oil / coolant / auxiliary pumps with operation matched to actual demand > Variable gains
− Intelligent transmission – Optimises engine performance to road conditions and driver demand >1.5%
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2. The Cost of Technology (Petrol engine car - per vehicle)
Minimizing engine friction $50
Stop-start $500
Mild hybrid $1500
Lean-burn GDI engine$2500
Plug-in hybrid $5000
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2. Turbo Compounding
� Electrical− Secondary turbine connected to a generator to provide
additional electrical energy for vehicle systems
− Used in Formula 1 - in conjunction with regenerative braking can give power unit efficiency in excess of 50%
− Turbo can also be electrically driven to eliminate lag
� Mechanical− Secondary exhaust turbine is mechanically connected to the
engine crankshaft
− Direct impact on engine power output
− Better suited to commercial vehicles
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3. Fuel Quality
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� Global availability of low-sulphur fuel in 2018
� Thailand has made better progress than her neighbours, but more improvement is desirable
Euro IV 50ppm
3. Alternative Fuels & Fuel Additives
� CNG:
− Widely used in commercial vehicles
− Abundant as a fossil fuel, easy to obtain from refinery by-products or bio sources
� LPG:
− Commonly used in taxis and light-duty applications, including industrial
− Can be derived from a greater number of sources than petrol/diesel
� Low refinement cost
� Energy density is lower than liquid fuels
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Smaller, more volatile fraction
hydrocarbons, are inherently cleaner
burning and contain less sulphur
3. Alternative Fuels & Fuel Additives
� Biodiesel/Bioethanol: − Either as a percentage in fossil-based fuels, or as a whole fuel
− Generally has lower sulphur content than fossil fuels, but otherwise has not been shown to significantly affect emission performance in real-world situations
− Only viable in regions where using biomass for fuel does not impact on food resource.
� Fuel ether additives:− Oxygenated fuel additive, assists in the oxidation of CO and HC
− Raises fuel octane rating, allowing higher compression ratios
� Fuel-borne catalysts:− Compounds of iron, cerium or platinum
− Do not affect combustion, but reduce the temperatures required for soot regeneration in exhaust particulate filter systems
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4. Fuel Management & Mixture Formation
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� Port injection (Spark ignition)− Emission related variables are injector positioning and timing with respect to valve opening− Mechanisms are well understood and unlikely to change significantly− Significant advances in inlet port design to improve gas flow within the cylinder− Bore/stroke ratios optimized for fastest flame propagation− Economical to manufacture!
� Direct injection (Compression and spark ignition)− Particulate and HC emission are heavily influenced by injector nozzle design− Small injector orifices less reliable in service− Practical limitations to maximum fuel pressure
4. Air Management:
� Turbocharging:− Energy recovery from waste exhaust gas raises the overall efficiency of the powertrain
− Allows for excess air conditions which can be utilized either for optimizing power output or for emission control
− Smaller power unit may be used
� Supercharging:− Provides excess air for emissions and power, but does not recover waste heat energy
− Electric supercharging – variable boost at any engine speed (needs >12v supply)
− Turbo/supercharger combinations
� Ozone Seeding:− Ozone generator located in the air manifold/ports
− Useful in assisting the low-load operation of homogenous charge compression ignition (HCCI) engines
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4. Exhaust Gas Recirculation (EGR)
� Used as a NOx control measure in all diesel engines and some gasoline engines
� Exhaust gas acts as a diluent and restricts the amount of free oxygen available for combustion, lowering peak combustion pressure
� EGR reduces peak combustion temperatures as exhaust gas has a higher specific heat capacity than air.
� Typically, 5-30% EGR is used, depending upon the engine load.
� Was used historically in lean-burn petrol engines and is now seen again in GDI applications
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4. Exhaust Gas Recirculation (EGR)
� Too much EGR− Limits soot oxidation, leading to high particulate emission
− Reduces thermal efficiency
− Recirculation of particulates can lead to engine wear
� Current designs incorporate high and low pressure circuits, EGR coolers and, in some applications, also a separate particulate filter
� EGR also used to control mixture temperature through selective use of cooled/uncooled gas flows− Hot EGR used at low to medium loads and during engine warm-up
− Cold EGR gives better NOx reduction
� EGR found to increase NO2/NOx ratio which assists both DPF and SCR performance
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4. Engine Cycles – By Tradition
� Spark ignition:− Mixture formation outside of the combustion chamber
− Spark initiates a single flame front which propogates rapidly through a homogenous fuel/air charge
− HC & CO emissions are generated by incomplete combustion or poor mixture formation
� Compression ignition:− Mixture formed inside the combustion chamber by high-
pressure direct injection
− Fuel/air charge ignites spontaneously leading to high peak pressure
− Combustion is inherently high pressure and temperature, which favours NOx formation
− PM generated by non-homogenous nature of the charge, can be overcome by very lean operation
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4. The New Generation – Somewhere in Between
� Homogenous charge compression ignition (HCCI)− (Volatile) fuel mixture is homogenous
− No ‘flame front’, no unburnt fuel > low HC and CO
− combustion temperature is significantly lowered > low NOx
− Lean operation > Low soot generation
− Almost perfect in terms of emission control
− Cylinder temperature is critical - only controllable over a very small range of operating conditions, so not well suited to on-road applications
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4. The New Generation – Somewhere in Between
� Premixed charge compression ignition (PCCI)− HCCI, but leaning towards diesel operation
− Fuel injection begins during the induction cycle, allowing for an almost homogenous mixture
− Phasing of the final injection ‘pip’ can be timed to control ignition of the whole mixture
− Small flame front rapidly propagates and increases compression sufficiently to ignite the homogenous mixture
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4. The New Generation – Somewhere in Between
� Spark controlled compression ignition− HCCI, but leaning towards gasoline operation
− Secondary injection creates a rich mixture zone around the spark plug
− Spark initiates the flame front creating an ‘air piston’, and pressure rise ignites homogenous charge
− Can operate in different modes, from normal spark ignition through to full HCCI
− Variable fuel/air mixtures: Lean (as low as 29:1) for light load fuel economy, stoichiometric for normal running and rich (or additional secondary injection) for power and aftertreatment regeneration
− Systems are complex and include new components such as in-cylinder temperature and pressure monitoring
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4. Modified Engine Cycles
� Variable compression− Modification of engine stroke or connecting rod effective length
− Compression ratio can be optimized whilst driving for efficiency or clean running
� Variable valve timing/lift− Atkinson/Miller/Budack cycles – Compression ratio not the same as expansion ratio, higher efficiency and
lower NOx
� Six-stroke cycle− Compression ignition – two combustion cycles in three revolutions
− Allows further oxidation of soot particles
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4. Gasoline Direct Injection –A case study of technology advancing faster than legislation
� GDI first introduced to the market in 1997
� Characterised by higher compression ratios and higher efficiency
� Current systems have varying levels of complexity− Direct injection only or direct + port injection
� GDI engines potentially generate similar PM and PN emissions to compression ignition engines, yet not regulated until Euro 5!
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12+ Years!
5. Exhaust Aftertreatment – 3-Way Catalyst
� Application: Spark ignition only
� Very little substantial design change since their introduction− Introduction of cold ambient emission testing forced a move towards
close-coupled cat’s, or a combination of close-coupled and under-floor
− Different monolith: Metal foil or ceramic honeycomb
� Must operate within a close margin of stoichiometric, so do not permit lean-burn engine operation
� Todays catalysts are designed using computational fluid dynamics techniques to improve flow distribution across the bed− Variations in cell density across the monolith allows for greater
efficiency and less volume
� Manufactures balance the precious metal (Pt, Pd, Rh) content in the washcoat according to global precious metal markets, with little change to efficiency.
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#1 Pt/Rh -reduces
NOx
#2 Pt/Pd -oxidisesCO & HC
5. Exhaust Aftertreatment - Lean NOx trap
� Application: All lean-burn engine types
� Similar in construction to a 3-way catalyst, but the function is store NOx emissions during lean-burn operation
� LNT’s can be effective from cold-start
� Regeneration is achieved by running the engine momentarily with a conventional stoichiometric and homogenous air/fuel mixture, then adding extra fuel to create a mixture rich in CO and HC.
� Not so efficient across the whole range of driving conditions
� Require periodical de-sulfurization cycles to remove SO3 contamination:
− Rich operation for several minutes
− Raises bed temperatures to 650-750ºC
− Degrades overall fuel efficiency
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5. Exhaust Aftertreatment – Oxidation Catalysts (DOC/POC)
� Application: Compression ignition (DOC), gasoline direct injection (POC)
� A flow-through type monolith which resembles the three-way catalytic converter
� Different catalyst composition (Pt/Pd) to optimize activity under lean conditions
� In diesels: − Acts on HC and CO, and also SOF component of particulate emissions which arise from unburnt fuel
− Oxidation of NO to NO2 is useful for the passive regeneration of particulate filters and improves performance of SCR systems further downstream.
� In GDI: − Acts to oxidise particulates, but with quite low efficiency (45%)
� Problems with sulphur:− Oxidation of sulphur dioxide forms sulphate particles, which contribute significantly to the overall amount
of particulate matter generated by an engine
− Like the LNT, Pt/Pd oxidation catalysts are poisoned by sulphur and lose efficiency
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5. Exhaust Aftertreatment –Particulate Filters (GPF/DPF)
� Application: Compression ignition and gasoline direct injection
� Wall-flow type is highly effective, >90% efficiency (including PM2.5).
� Flow through only 45-50% efficient
� Periodical (active) regeneration type…..− Additional fuel injected into exhaust /DOC to heat the
filter > 600ºC
− Post-injection or secondary exhaust injector
− Direct impact on fuel economy!
� Or continuous (passive) regeneration…..− Substrate is catalysed
− Oxidising agent is NO2 which is generated by an upstream oxidation catalyst
− Reaction temperature is much lower >260ºC
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5. Exhaust Aftertreatment - Particulate Filters
� Particulate filters are established technology used across many different industrial, transport and non-road applications
� Advances in design include:− Segmented construction: Prevents thermal cracking and permits varying
cell density across the filter
− Asymmetric casing, wider inlet than outlet improves efficiency
− No washcoat - catalyst incorporated within the cell matrix
− Different geometry for inlet/outlet cell to increase filtration area – increases soot loading by up to 50% for the same back-pressure
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5. Exhaust Aftertreatment – Filter Regeneration & Practical Issues
� Filter regeneration requires certain operating conditions which are not always achievable in the real world – excessive back-pressure and engine malfunctions are not uncommon!
� Both passive and actively regenerating filters require differential pressure monitoring and soot estimation algorithms in the aftertreatment management systems
� Not all PM can be burned off, oil combustion and inorganic matter in fuel creates ash which reduces filtration efficiency
� Sulphur poising of catalysed filters
� Issues with current technology using post-injection or secondary injection: − B20 does not burn as effectively over DOC, requiring more fuel to regenerate DPF
− Direct waste of fuel
− During regeneration, emissions often exceed limit values
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5. Exhaust Aftertreatment – Filter Regeneration
� Solutions?− Move the filter as close as possible to the engine to retain as much heat as possible
− Turbo by-pass − Exhaust stream is hotter and more energetic
− Electrically heated filter− Regeneration can be independent of engine running conditions
− Heater can be upstream or incorporated in the filter substrate
− Microwave heated filter
− Soot is highly absorbent of microwave energy
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5. Exhaust Aftertreatment – Retro-Fitting DPF’s on Commercial Vehicles
� Successful in Europe prior to the introduction of Euro 4 (heavy duty) as a short-term solution
� Reliant on low-sulphur fuels to prevent sulphate particle accumulation in the filter
� Government sponsored schemes (and tax benefits) reduced the cost burden on operators
� When combined with a DOC, catalysed ‘flow-through’ type is self cleaning and zero maintenance
� Wall-flow DPF can be used, but on-board regeneration mechanisms add cost and complexity− Stand-alone exhaust fuel injector
− Electrical heater
− Fuel-borne catalyst
� Most types can be removed for cleaning
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5. Exhaust Aftertreatment – Selective Catalytic Reduction (SCR)
� Application: Compression ignition engines and GDI
� ‘Selective’ in that it targets only NOx emission
� DEF (Adblue) 30-33% aqueous urea solution injected into mixing chamber ahead of the SCR, hydrolyses to ammonia gas in the hot exhaust stream
� Catalytic action is provided by vanadium, copper or zeolite depending upon the design temperature of the exhaust stream
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5. Exhaust Aftertreatment – Selective Catalytic Reduction (SCR)
� SCR technology alone can achieve NOx reduction of 90 percent, potentially rendering EGR and LNT technology redundant − In practice, EGR will remain as it has other attributes in controlling combustion
− SCR is robust enough to allow the engine calibration to be tuned towards maximum efficiency in the knowledge that NOx can be removed later
� To be most effective, urea dosing is quite aggressive:− Leads to excess ammonia in the exhaust stream.
− Heavy duty applications: DEF consumption is 4-8% of fuel consumption
− Passenger cars are typically 0.1-0.2% of fuel consumption
� Driver inducement for urea dosing system faults – 1st stage warning, 2nd stage power limitation, 3rd stage ‘creep mode’
� Unlike most aftertreatments, SCR is relatively unaffected by sulphur poisoning, however, it can get ‘plugged’ by ammonium sulphate deposits
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5. Exhaust Aftertreatment – Ammonia Slip Catalyst (ASC)
� Application: Clean-up stage for SCR and sometimes LNT
� SCR is most effective with ‘rich’ ammonia:NOx ratio of 1:2
� ASC Oxidises any ammonia remaining in the exhaust stream
� A balancing act – ASC’s must operate in a manner which is selective to the oxidation of ammonia without regenerating NO. − Pt catalyst, low temperature
� In some instances, ASC can be part of a dual-layer SCR, although this configuration is less efficient and more difficult to control
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5. Aftertreatment Packaging
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� Aftertreatment components can be packaged in a variety of ways− Available space
− Thermal management
� Integration of ATS’s
GDI example: Diesel example:
Aftertreatment Management & On-Board Diagnostics
� Complexity and balance of different pollution control mechanisms requires advanced control and monitoring:
� Example: Euro 6 Compression ignition with EGR+DOC+DPF+SCR+ASC
Additional equipment required (over and above Euro 4 vehicle);− Sensors for:
− O2, Combined NOX/O2 sensor (SCR upstream/downstream), particulates
− Exhaust temperature in 5 locations: manifold, DOC upstream, DPF upstream/downstream, SCR upstream
− Exhaust pressure in 3 locations: manifold, DPF upstream/downstream
− Exhaust fuel injector (DOC upstream)
− Urea injection system (Separate dosing control unit, injector, tank, line pressure & temperature monitoring, urea quality sensor, pump)
− EGR high/low pressure circuit with cooler, diverter valves, pressure and temperature sensing
� Failure or blockage in any system can result in high emissions, so all are monitored by OBD for continuity, plausibility and functionality
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Hybrid Drivetrains
� Mild hybrid:− Electric motor(s) used to supplement conventional drivetrain
� Full hybrid:− Vehicle may be propelled either by the engine, the electric traction motor or a varying combination of both
� In both cases, there is no direct contribution to emission reduction. The benefits come from regenerative braking and the opportunity to use more fuel-efficient modes of engine operation.
� Range extender:− Powertrain is essentially that of an electric vehicle with an on-board engine driven generator for re-
charging the traction battery.
− The generator engine can be optimized to operate under fixed conditions
− Alternative engines/fuels can be adopted (Free piston engines, Sterling engines, gas turbines)
− Subject to normal emission limits
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Pure Electric Vehicles
� Shifting the pollution issue− PM emissions from conventional power stations create the same issues as vehicle emissions
− Nobody wants nuclear power
− Green alternatives are not sufficient to replace nuclear and thermal power stations
� Energy density of batteries limits range and performance
� Availability of power supply for recharging:− Not everybody has access to overnight charging
− Fast charging points are still significantly slower than refilling a fuel tank
� Pollution is generated away from major urban areas, so effects on health are reduced
� The benefits of pure EV are more universally suited to 2-wheelers at this point in time
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Hydrogen and Fuel Cells
� Hydrogen infrastructure needed to support technology
� Energy density is lower than liquid fuels - storage is not so convenient
� Hydrogen as a fuel for spark-ignition engine:− Generates only water and NOx (and a trace of HC from engine
lubricants).
− Not suitable for all engines
� Fuel cells− Require high purity gas
− Zero pollution
− Performance is equivalent to conventional combustion engine or hybrid vehicle
− Range limited only by fuel storage capacity
− Expensive
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What direction to take?
� Short Term:− GDI, HCCI and its derivatives will become mainstream for light-duty
− DPF/SCR+EGR combinations will be the accepted norm for compression ignition, although LNT will be used for lighter duty vehicles
− Development of alternative catalyst washcoats which are less dependent on precious metals
− Greater dependency on electrically driven subsystems – maybe 48V?
− Euro 6 and currently available technology can go a long way to improving air quality
� Long Term:− Hybrids with automated electric-only modes for high pollution areas
− Telemetry linking on-board emission monitoring to enforcement authorities and/or service centres
− Adaptive powertrain operating modes to suit location and demand
− Drive-by roadside emission testing to identify the worst polluters (already in use in HK and EU)
− Light-duty emission limits for ammonia
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Closing Thoughts - Driving Changes
� In a growing market (Thailand: 5%/year), the growth of the vehicle pool will offset advances made in emission reduction− Technology must be adopted more rapidly to realize an actual reduction in air pollution levels
� Change requires the coming together of several factors:− Availability of technology
− Health concerns
− Political willpower
− Public pressure (air quality, global warming)
− Availability of suitable fuels – SULPHUR CONTENT
− Cost of implementation
� Economies of scale
− Developing nations should benefit more quickly from the advances made in other markets
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