Pollutants Formation Mechanisms
Four main pollutants that should be minimized by optimizing combustion and improving aftertreatment of exhaust gases are hydrocarbons (HCs), nitrous oxide (NOx), sulfur dioxide (SO2), and carbon monoxide (CO). Carbon dioxide (CO2) is a greenhouse gas, and not a pollutant.
There are two main types of internal combustion engines: the spark-ignition (SI) gasoline engine and the compression-ignition (CI) diesel engine. The CI engine with its lean burning nature has the benefit of one-fifth the HC emissions of SI engines. Both yield an impressive combustion efficiency of 98% for CI and 95% to 98% for SI. This is surprising given the non-homogeneous mixture, which contains rich and lean spots during combustion.
The higher the compression ratio (rc = 16 to 20 for CI and rc = 6 to 10 for SI), the more fuel leaks past exhaust valves and into crevice volumes. Up to 3% of the fuel is trapped, given the gap size is larger when the engine is cold. Unfortunately, these fuel-rich zones cause soot to form, and due to the high temperature and pressure during combustion, large amounts of NOx are formed. The max temperature occurs at an expansion ratio (ER) of 1, but due to the short time available for each engine cycle, incomplete mixing occurs, and max NOx formation occurs at ER = 0.95. CI engines also operate with a lean ER, so there is plenty of oxygen available to form NOx.
The power of the diesel engine is controlled by the amount of fuel injected, not by the air supply, as is the case with an SI engine. An idling SI engine has a nearly closed throttle, and the air supply is restricted, which causes not enough oxygen to burn with the fuel, and emissions are poor. However, the CI engine is unthrottled and has enough oxygen to burn the fuel, causing fewer emissions at idle. However, when operating under high engine loads (wide open throttle – WOT), the CI engine operates with a rich mixture. This leads to poor fuel economy and significant emissions.
Fortunately, this black smoke has become much cleaner since 2000, and the exhaust odor is not as pungent due to the reduction in sulfur content. More than 90% of carbon particles are consumed during combustion. Due to the higher rc and temperatures, more lubricating oil is used and vaporized, which accounts for 25% of the carbon in soot. Soluble organic fraction (SOF) due to expansion cooling is up to 50% at low engine loads, but only 3% at high engine loads. Therefore, the benefits CI offers at light loads, due to not being air-limited and reduced HC emissions, are increased SOF due to high amounts of oil used and cool temperatures that cause more expansion cooling.
Exhaust Gas Recirculation
Exhaust gas recirculation (EGR) is a NOx reduction technology that recirculates a portion of the exhaust gas with the incoming air. This exhaust gas acts as a diluent to prevent the dissociation of nitrogen and oxygen in the air by decreasing peak combustion temperatures (high temperatures encountered in CI engines due to high compressive heating). The dissociation of nitrogen from diatomic to monatomic (N2 -> 2N) is highly dependent on temperature, with a much more significant amount of nitrogen generated in the range of 2,500 – 3,000 K. Other reactions that contribute to NOx formation are O2 -> 2O and H2O -> OH + H2.
NOx is one of the primary causes of photochemical smog, which has become a major problem in major cities in the world. Most modern CI engines use EGR, and fortunately, EGR can eliminate all but a fraction of a percent of NOx. Another harmful exhaust product that EGR can decrease is CO, which is produced when CO2 dissociates according to CO2 -> CO + O. Unfortunately, EGR still leaves significant amounts of particulate matter (PM) unburned black soot in the power stroke, which must be filtered.
Recirculating exhaust gases increase the specific heat capacity (T = Q / cp) of the incoming air and downstream air-fuel mixture (AFR). This lowers the adiabatic flame temperature and increases volumetric efficiency. Although the combustion temperature decreases, waste heat in the soot is recovered and less fuel is burned, with the net result of a small reduction in combustion efficiency. During high engine loads, the peak combustion temperature must be high; the opposite is true for low engine loads. By tailoring the EGR flow to the engine conditions, less EGR can be used for high load conditions.
There are both internal and external EGR types. A turbocharger (turbine-compressor) is almost always used in conjunction with EGR recirculation. After the compressor, there is an intercooler to combat compressive heating, and a separate amount of EGR passes through this EGR cooler. This mixture then flows into the combustion chamber for combustion at a lower peak combustion temperature.
Diesel Particulate Filter
A diesel particulate filter (DPF) is required in the exhaust system due to stricter emissions regulations, especially the high particulate matter (PM) levels that result from EGR. The soot in the filter is cleaned or regenerated by oxidizing it. Thermal regeneration can be achieved through either an active or passive system. Active systems spray air and fuel into the exhaust manifold to increase the temperature. This can reduce emissions, but leads to a fuel penalty and emissions from burning this added fuel. Fortunately, there are other forms of heating the exhaust gas, such as electrically-assisted diesel particulate filter (EADPF). When the back pressure reaches 150 mbar, the fuel is injected and combusted, so the exhaust reaches 550 °C; which allows the active DPF system to function as intended. The passive system uses materials that act as an oxygen catalyst. The catalyst lowers the required soot oxidation temperature, and regeneration of the filter is possible without a fuel penalty. In addition, thermal stresses on the system are avoided. As it is impractical to require the driver to clean the filter with compressed air or other means, regeneration is necessary.
Selective Catalytic Reduction System
Selective catalytic reduction (SCR) is another aftertreatment used to convert NOx to N2 and H2O with the help of a catalyst. A reductant (reducing agent) is a chemical that donates electrons (addition of a hydrogen molecule). It is sprayed into the catalyst chamber and mixed with the exhaust gases. Typical reducing agents are anhydrous or aqueous ammonia or urea; less common are cyanuric acid and ammonium sulfate. Urea must be thermally decomposed into automotive-grade urea, known as diesel exhaust fluid. This 2% – 6% urea in water is added to fuel. Its use as an effective reductant is attractive in diesel engines, as it reduces NOx by 70% – 95%.2
Active catalytic components are usually oxides of base metals (V, W) or precious metals. Base metals lack high thermal durability, an important property in an automotive engine, and have a high potential to oxidize sulfur (2SO2 + O2 = 2SO3 & SO3 + H2O = H2SO4). Oxidizing sulfur due to its acidic nature is damaging to the SCR system. This high catalyzing potential of sulfur explains why ultra-low sulfur diesel is required for 2010 car models. The exhaust gases containing sulfur dioxide are a component of acid rain, which is harmful to marine life and building structures. Fortunately, iron- and copper-exchanged zeolite catalysts overcome both shortcomings. The most common geometries are honeycomb and plate; corrugated is less common. The honeycomb configuration is smaller than the plate, but has higher pressure drops and can plug more easily. SCR systems can be independent of the engine controller, making them practical for retrofits, but they can get clogged, which reduces their lifespan. These systems reduce NOx by 98%, PM by 40% – 60%, total HC by 80%, and CO by 90%, and are an effective diesel oxidation catalyst (DOX).
Tier 4 Diesel Emissions Standards
The Environmental Protection Agency (EPA) sets rules for engine emissions, among many other processes and chemicals used or burned that degrade the environment. People and buildings are vulnerable to the damage caused by acid rain due to the small percentage of sulfur in fuel. For Tier 1 – 3 there was no regulation of the sulfur content in diesel fuel, and it was 3,000 ppm (0.5% wt, max). It was 500 ppm by June 2007, and 15 ppm for nonroad fuel by June 2010, and 15 ppm for locomotive and marine fuel by June 2012. Tier 4 emissions standards were introduced in May 2004, with a phase-in period from 2008 to 2015. It applies to all nonroad diesel engines of all sizes used in construction, agricultural, and industrial equipment. The most remarkable achievement of Tier 4 standards was a 90% reduction in PM and NOx emissions.