Urban transport is essential to social and economic development. It enables movement of goods to markets and provides mobility to people for economic and personal activities. But motorized transportation also imposes social and economic costs; one of the most important being found in Hong Kong is the deterioration of urban air quality.
.
Improvements in gasoline and diesel engines aftertreatment technology are needed to meet upcoming environment quality goals in Hong Kong local. Different treatment devices to vehicle emissions have been increasingly helping with people to work out different emission problems. Devices in the past and current times are catalyst converts, two way and three way converters, NOx absorber and some electronic control components by using closed loop monitoring system. As for Systems using recently developed compact plasmatron fuel converters in conjunction with aftertreatment catalysts could also provide new opportunities for obtaining significant pollutant reduction.
.
This final written report will provide informative knowledge of exhaust aftertreatment devices in vehicle engines, and it will make readers to understand more about the working principle of emission treatment devices.
.
2.Abbreviations and Acronyms
CO : Carbon Monoxide
CO2 : Carbon Dioxide
NO : Nitric Oxide
NO2 : Nitrous Dioxide
NOx : Oxides of Nitrogen
SO2 : Sulfur Dioxide
HC : Hydrocarbon
VOCs : Volatile Organic Compounds
PM : Particular Matter
PM2.5 : Particulate matter of size 2.5 microns or smaller in aerodynamic diameter.
3.Definitions
-
Small Particles:
Particles that are larger than approx. 2μm will mostly be deposited in the respiratory passages. They may trigger allergic reactions and impair wellbeing. Allergy-sufferers, in particular, suffer from the small particles. Streaming eyes, dripping noses, shortness of breath or sneezing attacks reduce concentration levels and increases the risk of accidents. Sneezing at a speed of 60 miles an hour means you’re driving blind for 30m.
-
Fine Dust:
Particles that are smaller than approx. 2μm will predominantly be deposited in the lower parts of the respiratory passengers and in the lungs. They are the ones most commonly encountered in the air. Particles of this size cannot all be retained by the mucous membranes in the nasal and pharyngeal cavities. Most of them are deposited in the lower respiratory passages and in the lungs. These respirable particles include pollutants like bacteria and fungi spores, industrial dusts, fine abraded particles and soot or diesel soot. This is particularly injurious to human health, because it may trigger infections in the lung, and also transports toxic substances. Diesel soot is regarded as a cause for diseases of the respiratory tract, cardiovascular complaints and asthma. A carcinogenic effect is also ascribed to it.
-
Lead (Pb):
The dispersion of lead particles via combustion of fuel with additives, or from the lead contaminants in water, soil or dust. Lead is one of the heavy metals appeared in atmospheric air and it can be accumulated in human and animal bodies.
-
Carbon Monoxide:
Incomplete combustion of hydrocarbon fuels and tobacco smoking are two significant sources of CO. CO is a toxic gas, with a level near 15 ppm can significantly affect body chemistry. The reaction of human to CO level varies significantly, and the effects can be cumulative. Headaches and nausea are common symptoms for those who have been exposed to CO quantity above their tolerance.
-
Carbon Dioxide:
CO2 is an exhaled by-product of human and animal metabolisms, and therefore CO2 levels are typically higher in occupied spaces than for outdoor air. In heavily occupied spaces such as auditorium, CO2 levels will often be a major concern.
-
Sulfur Oxides:
Sox are the result of combustion of fuels containing sulfur and may enter a building through outdoor air intakes or from leakages in combustion systems within the building. When hydrolyzed with water, sulfur oxides can form sulfuric acid, creating problems in the moist mucous membranes that may cause upper respiratory tract irritation and induce episodic attacks in individuals with asthmatic tendencies.
-
Volatile Organic Compounds :
A variety of organic chemical species occur in a typical modern indoor environment, resulting from combustion sources, pesticides, building materials and finishes, cleaning agents and solvents, and plants and animals.
-
Particulate Matters:
A typical sample of outdoor air might contain soot and smoke, silica, clay, decayed animal and vegetable matter, lint and plant fibers, metallic fragments, mold spores, bacteria, plant pollens, and other living material. The sizes of these particles may widely range from less than 0.01 μm to the dimensions of leaves and insects.
4.Introduction
4.1.Air Pollutant from Geographical Factors & Vehicle Emissions
Air pollutant is a serious problem in Hong Kong. Its environmental efforts of course leave much to be desired. But one must note that its air quality is also affected by its location in the regional geographical setting. Hong Kong and the Pearl River Delta are situated in the same air-flow zone. Since the Pearl River Delta is surrounded by mountains on three sides, it is virtually like an enclosed area.
.
And, owing to its position in such region, Hong Kong is basically susceptible to pollution because nitrogen dioxides, sulphur dioxide and respirable suspended particulates are blown towards it by air currents. As a result of the undesirable effects of both internal and external factors, air quality in Hong Kong has become a worrying problem.
.
One of the main causes of air pollutant in Hong Kong is vehicle emission. Hong Kong’s roads are the most crowded in the world, with almost 280 vehicles for every kilometer of road and over 11 million passenger journeys on public transportation every day. The city’s vehicle fleet is dominated by heavily polluting, aging goods vehicles, most of which run between the city and the Pearl River Delta. Diesel commercial vehicles are responsible 90% of RSPs and 80% of NO2 emissions from the entire road transport sector, despite making fleet of private cars have also added to congestion and pollution.
.
Vehicle Pollutant caused by heavy air pollutant is clearly visible in the following photos:
.
4.2.Health Implications
The mortality rate from vehicular pollutant can be twice as high near heavily traveled roads, based on a study conducted in Holland at residences 50 meters from a main road and 100 meters from a freeway. Since millions of people in Hong Kong live and work in close proximity to busy roads, this presents a major health risk to city residents. The Hong Kong Medical Association estimates that air pollutant can exacerbate asthma, impair lung function and raise the risk of cardio-respiratory death by 2 to 3 percent for every increase of 10 micrograms per cubic meterof pollutants. Studies by local public health experts have found that these roadside pollutant levels are responsible for 90,000 hospital admissions and 2,800 premature dealths every year.
.
4.3.Toxic Substances from Vehicle Pollutants
Toxic species such as carbon monoxide (CO2), hydrocarbon (HC), nitrogen oxides (NOx) and respirable suspended particulates (RSP) from the vehicle exhaust emissions are dominant contributors to the total air pollutants in Hong Kong and other major urban cities. Vehicle exhaust and dust particles from polluted roadside infiltrating into the vehicle compartment may make passengers feel unpleasant, or sometimes sick, also since the roads in Hong Kong are narrow and surrounded by numerous high rises, the air has turned extremely stuffy. Pedestrians are thus reduced to “human vacuum cleaners”.
5.Exhaust Aftertreatment Devices in HK Vehicles
In order to improve air quality, Hong Kong Government has introduced hybrid vehicles and many experts have already made lots of suggestion in this connection such as a mandatory conversion to Euro III and Euro Vi engines, and bio-fuel, for the purpose of effectively controlling vehicle emissions. In this written report, the topic I chose is “Aftertreatment Devices in Vehicle Engines”, I won’t expect myself to describe the things other than the aftertreatment devices and nor to say those bio fuels to the full. As a hard work via the study of lecture notes and the on-line searching, many ideas and relevant information can there be found in talking about the installation of emissions control devices for the exhaust pipes of vehicles. With a view to reducing the emission of sulphur dioxide, nitrogen oxides and toxic pollutants, different exhaust aftertreatment devices including thermal reactors, catalysts treatment devices (two-way and three-way catalysts), NOx absorber, Exhaust Sensor and feedback control.etc, are in full described in the following:
.
5.1.Thermal ReactorsIf the major exhaust emission products of concern were only CO and UHC, it is possible to oxidize them directly to CO2 and H2O in the exhaust system of the engine as long as the appropriate conditions can be obtained there.
One method of achieving this is the use of thermal reactor in the exhaust system. For the appropriate oxidation reactions to take place; the reactor needs to be at a certain minimum temperature and there must be sufficient oxygen available to complete the oxidation process.
.
To obtain 40% CO oxidation, the temperature must now reach about 580 degree at which value nearly 80% of the HC is converted. To achieve a similarly high level of CO conversion (i.e. 80%), the temperature must be around 700 degree to 750 degree. It is well above red heat for steel and could cause problem in design, construction, durability, operation, and safety. In addition, the higher the temperature of the reactor, the lower the residence time required for the gas to achieve good conversion values
.
A mean temperature range for the combustion product gases leaving the exhaust port is about 350 degree to 700 degree but this may reach as high as 900 degree under a full-power, high-speed operation. The exhaust temperatures vary throughout an engine cycle by about +/- 200 degree and that the exhaust cools rapidly in a normal exhaust pipe as it moves downstream from the exhaust port, dropping by around 50 degree to 100 degree per meter.
.
It is necessary to run the engine rich so that the extra fuel can be burned in the reactor in order to obtain the required reactor temperatures. However, thermal reactors are untenable as a long-term solution.
.
5.2.Catalytic treatment of exhaust systems (Two-way Catalysts)
Catalytic converters have the advantage that exhaust temperature can be kept moderately low; they use metallic catalysts to promote the desired reactions at low temperatures than would otherwise be needed. They could be a base metal (e.g copper, Cu, or chromium, Cr) but are more usually noble metal (platinum, Pt, palladium,Pd, rhodium, Rh). In spite of their expense, the noble metal types are commonly used as these have been found to be better catalysts.
.
Early converters, called "two-way" converters, burned a percentage of the unused hydrocarbons (HC) and carbon monoxide (CO) produced by the relatively inefficient, low compression engines of the day.
Two-way (oxidizing) converters burn HC and CO molecules with the assistance of a precious-metals catalyst. This process "converts" these harmful gasses into water vapor and carbon dioxide (CO2). It's important to understand that two-way converters are most effective when used with engines that have a lean air/fuel mix because this condition provides ample oxygen to "burn" the pollutants.
A two-way catalytic converter has two simultaneous tasks:
-Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
-Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: CxH2x+2 + 2xO2 → xCO2 + 2xH2OThis type of catalytic converter is widely used on
diesel engines to reduce
hydrocarbon and
carbon monoxide emissions. They were also used on spark ignition (
gasoline) engines in USA market automobiles through 1981, when the two-way converter's inability to control
NOx led to its supersession by three-way converters.
.
5.3.Catalytic treatment of exhaust systems (Three-way Catalysts)Three-way converters use two catalyst processes — reduction and oxidation —- and a sophisticated oxygen storage/engine control system to convert three harmful gasses - HC, CO and oxides of nitrogen (NOx). This is not an easy task: the catalyst chemistry required to clean up NOx is most effective with a rich air/fuel mix, whereas HC and CO reduction are most effective with a lean air/fuel bias. To operate properly, therefore, a three-way converter first must convert NOx (with a rich air/fuel bias), then HC and CO (with a lean bias).
Older three-way converters, called "three-way with air" or "three-way plus oxidation," perform this process by introducing additional oxygen between the reduction and oxidation stages to create a lean condition for the oxidation catalyst. (These converters are easily identified by their air tube.)
.
Modern three-way units, found on most vehicles manufactured since the late 1980s, rely on an advanced catalyst chemistry that stores and releases oxygen on a single substrate, and an oxygen monitoring and control system (utilizing one or more O2 sensors) that causes the engine to oscillate between lean and rich conditions. This oscillation, combined with the oxygen storage and release on the catalyst surface, enables the unit to convert all three harmful gasses with the same catalyst brick.
Today's "three-way" OBD II converters are just the last step in a highly sophisticated emissions control process. The chief component of this system is the engine itself, which, when operating properly, is significantly more efficient — and environmentally friendly — than its 1970s and 80s counterparts. Other leading components and systems playing important roles in reducing emissions in today's vehicles are engine sensors/controls, improved combustion chamber design and electronic fuel injection technology, each of which enhance the efficiency of the combustion process, resulting in fewer unburned pollutants.
A three-way catalytic converter has three simultaneous tasks:
-Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
-Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
-Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + 2xO2 → xCO2 + 2xH2O
5.4.Common Causes of Failure in Catalyst Systems
Most failures are caused by conditions or evens other than converter malfunction:
-Melted or ratting brick: Usually caused by raw fuel entering the converter and superheating the substrate.
-Contaminated brick: Oil, antifreeze, or fuel additives coat and plug up the substrate, preventing the converter’s chemical reaction from taking place.
-Road damage: caused from road debris striking the converter and causing internal damage.
.
5.5.NOx Absorber
The NOx Absorber Catalyst (NAC) was first demonstrated by Toyota in the late 1990s as a means to remove NOx from gasoline direct injection (GDI) and light-duty diesel engines. The excess exhaust oxygen associated with these combustion systems prevents the effective use of 3-way catalyst systems (TWC) so prevalent in stoichiometric gasoline engine applications.
3-way catalyst systems are operated at or close to, the stoichiometric ratio, such that the very low oxygen concentration in the exhaust allows a set of 3-way reactions to take place, over a precious metal catalyst, between the hydrocarbons (HC), the carbon monoxide (CO) and the oxides of nitrogen (NOx) producing nitrogen, carbon dioxide and water vapor as by-products.
Since the diesel engine operates with an excess of oxygen the TWC is ineffective, and as a result special measures have to be taken to achieve NOx control in the exhaust. The NOx absorber catalyst operates in a cycle; wherein NOx is stored for a period of time on the catalyst surface as a nitrate, and is later released from the catalyst surface under locally rich conditions, and reduced to nitrogen.
NOx absorber catalyst cycle shows the operation of the NAC as a repeating cycle of processes. The first process converts nitric oxide (NO) to nitrogen dioxide (NO2) over a precious metal catalyst. The conversion to NO2 is required for the subsequent process which is the storage of NOx onto the catalyst surface as a nitrate. The NO2 reacts with the base metal oxide components of the catalyst to form a stable nitrate compound on the surface- effectively storing the NOx.
5.6.Exhaust Sensors and Feedback Control
The currently used exhaust gas purification system consists of a catalytic converter and an electronically controlled air/fuel management system as shown in the following figure.
The oxygen sensor measures the net oxygen content that is proportional to stoichiometry in the exhaust gas. The air inlet and fuel injection are controlled to provide a stoichiometric ratio between oxygen (air) and fuel. The objective is to keep the air-to-fuel ratio (A/F-ratio) within the so-called lambda window (as presented in following Fig. 2).
In this narrow window, the high conversions (>80-90%) of CO, HC and NOX are achieved simultaneously. If the A/F-ratio is below 14.6, the exhaust gas contains more reducing reactants (CO, HC) than oxidizing reactants (O2, NOX) and the engine operates under rich conditions.
If the A/F-ratio exceeds 14.6, the engine operates under lean conditions. The reduction reactions of NOX are favored under rich conditions, whereas the lean conditions favor the catalytic oxidation reactions of CO and hydrocarbons. The refinement of the engine management system affects both the performance and the durability of the emission control system. Due to its performance in promoting the main reactions to reach completion and at the same time minimizing the extent of the secondary reactions, the closed-loop-controlled three-way catalyst has become the most widely applied technique for catalytic emission control.
6.Conclusion
Catalysts converter after-treatment technologies could play an important role in the effort to meet the urgent need to reduce pollutant from diesel and gasoline vehicles. Hong Kong Government, I suggest at this end, should actively consider the introductions of bio-fuel and the up-to-the-minute emission device to Hong Kong market as bio-fuel contains no sulphur, so the emission of sulphur dioxide is very low, also new device like compact plasmatron fuel converter catalyst system should help Hong Kong to be a Low-pollutant country in the coming future.
Many thanks to the HONDA web site that provided me with a tool for creating a 360-degree view of the car (say the red one: Accord V8, 09). Honestly, all the 3-D car pictures can definitely enhance the quality of my written report.
Best Regards
Ken Ngan Chi Keung
Urban transport is essential to social and economic development. It enables movement of goods to markets and provides mobility to people for economic and personal activities. But motorized transportation also imposes social and economic costs; one of the most important being found in Hong Kong is the deterioration of urban air quality.
The mortality rate from vehicular pollutant can be twice as high near heavily traveled roads, based on a study conducted in Holland at residences 50 meters from a main road and 100 meters from a freeway. Since millions of people in Hong Kong live and work in close proximity to busy roads, this presents a major health risk to city residents. The Hong Kong Medical Association estimates that air pollutant can exacerbate asthma, impair lung function and raise the risk of cardio-respiratory death by 2 to 3 percent for every increase of 10 micrograms per cubic meterof pollutants. Studies by local public health experts have found that these roadside pollutant levels are responsible for 90,000 hospital admissions and 2,800 premature dealths every year.
Catalytic converters have the advantage that exhaust temperature can be kept moderately low; they use metallic catalysts to promote the desired reactions at low temperatures than would otherwise be needed. They could be a base metal (e.g copper, Cu, or chromium, Cr) but are more usually noble metal (platinum, Pt, palladium,Pd, rhodium, Rh). In spite of their expense, the noble metal types are commonly used as these have been found to be better catalysts. 
Catalysts converter after-treatment technologies could play an important role in the effort to meet the urgent need to reduce pollutant from diesel and gasoline vehicles. Hong Kong Government, I suggest at this end, should actively consider the introductions of bio-fuel and the up-to-the-minute emission device to Hong Kong market as bio-fuel contains no sulphur, so the emission of sulphur dioxide is very low, also new device like compact plasmatron fuel converter catalyst system should help Hong Kong to be a Low-pollutant country in the coming future.
