Reducing Diesel Emissions
Pollutants Formation MechanismsTo reduce emissions from internal combustion engines, optimizing combustion and improving aftertreatment of exhaust gases are crucial. Four primary pollutants that need to be minimized include hydrocarbons (HCs), nitrous oxide (NOx), sulfur dioxide (SO2), and carbon monoxide (CO). It’s worth noting that carbon dioxide (CO2) is not considered a pollutant but rather a greenhouse gas. There are two main types of internal combustion engines: the spark-ignition (SI) gasoline engine and the compression-ignition (CI) diesel engine. Compared to SI engines, CI engines have the advantage of producing one-fifth of the HC emissions due to their lean burning nature. Despite non-homogeneous mixtures that contain rich and lean spots during combustion, both engine types exhibit impressive combustion efficiency, with CI engines reaching 98% and SI engines ranging from 95% to 98%.
When the compression ratio (rc) is higher, which is typically between 16 to 20 for compression-ignition (CI) engines and 6 to 10 for spark-ignition (SI) engines, more fuel tends to escape past exhaust valves and into crevice volumes. The gap size is larger when the engine is cold, allowing up to 3% of the fuel to be trapped. Unfortunately, these fuel-rich zones lead to the formation of soot, and the high temperature and pressure during combustion results in the production of significant amounts of nitrous oxide (NOx). The maximum temperature is reached at an expansion ratio (ER) of 1. However, due to the short time available for each engine cycle, incomplete mixing occurs, and the highest amount of NOx formation happens at ER = 0.95. CI engines typically operate with a lean ER, providing an abundance of oxygen for NOx formation.
The diesel engine’s power is governed by the amount of fuel injected, in contrast to an SI engine where the air supply regulates the power output. At idle, an SI engine restricts the air supply with a nearly closed throttle, resulting in insufficient oxygen to combust the fuel, leading to poor emissions. In contrast, the CI engine does not throttle the air supply and has adequate oxygen to burn the fuel, resulting in lower emissions at idle. However, under high engine loads with a wide open throttle, the CI engine runs on a rich mixture, leading to decreased fuel efficiency and significant emissions.
Fortunately, the black smoke emissions from CI engines have significantly improved since 2000, resulting in less pungent exhaust odors due to reduced sulfur content. During combustion, over 90% of carbon particles are consumed. However, due to the higher rc and temperatures, CI engines require more lubricating oil, which vaporizes and accounts for 25% of the carbon in soot. At low engine loads, the expansion cooling creates a soluble organic fraction (SOF) of up to 50%. However, at high engine loads, SOF reduces to only 3%. Although CI engines offer benefits such as reduced HC emissions at light loads due to being non-air-limited, they produce more SOF due to the high amounts of oil used and cool temperatures that cause expansion cooling.
Exhaust Gas RecirculationExhaust gas recirculation (EGR) is a technology used for reducing NOx emissions by recirculating a portion of the exhaust gas back into the incoming air. This recirculated exhaust gas serves as a diluent that helps prevent the dissociation of nitrogen and oxygen in the air by reducing peak combustion temperatures. In compression ignition (CI) engines, high compressive heating can lead to very high temperatures, which can cause nitrogen to dissociate from diatomic to monatomic (N2 -> 2N). This dissociation process is highly temperature-dependent, with the most significant amount of nitrogen produced in the range of 2,500 – 3,000°C. In addition to this, other reactions, such as O2 -> 2O and H2O -> OH + H2, also contribute to the formation of NOx.
NOx is a major contributor to the development of photochemical smog, which has emerged as a significant problem in major cities worldwide. To address this issue, most modern compression-ignition (CI) engines utilize exhaust gas recirculation (EGR) which has the capacity to eliminate a vast majority of NOx emissions. Additionally, EGR also has the capability to reduce the amount of CO resulting from the dissociation of CO2 into CO and O. However, it is important to note that EGR leaves behind a considerable amount of particulate matter (PM) in the form of unburned black soot, which must be filtered.
By recirculating exhaust gases, the specific heat capacity (T = Q / cp) of incoming air and downstream air-fuel mixture (AFR) increases, resulting in a reduction of the adiabatic flame temperature and an increase in volumetric efficiency. While this lowers the combustion temperature, it recovers waste heat in the soot and leads to less fuel consumption, albeit with a slight decrease in combustion efficiency. To cater to the different engine conditions that require a higher peak combustion temperature during high engine loads and a lower temperature during low engine loads, the EGR flow can be adjusted accordingly, reducing its usage during high load conditions. Both internal and external EGR types are commonly used in conjunction with a turbocharger (turbine-compressor). To combat compressive heating, an intercooler is installed after the compressor, and a portion of the EGR passes through a separate EGR cooler. This mixture is then directed into the combustion chamber to undergo combustion at a lower peak temperature.
Diesel Particulate FilterDue to stricter emissions regulations, particularly concerning the high levels of particulate matter resulting from EGR, a Diesel Particulate Filter (DPF) is now mandatory in the exhaust system. The filter collects soot, which can be cleaned or regenerated by oxidation. Thermal regeneration can be achieved through either an active or passive system.
Active systems introduce air and fuel into the exhaust manifold to raise the temperature and reduce emissions. However, this method incurs a fuel penalty and generates additional emissions from the burned fuel. The active DPF system can function as intended once the back pressure reaches 150 mbar, at which point the fuel is injected and combusted, raising the exhaust temperature to 550°C. Luckily, electrically-assisted Diesel Particulate Filters (EADPFs) provide alternative means to heat the exhaust gas.
On the other hand, passive systems employ materials that act as oxygen catalysts, lowering the soot oxidation temperature and enabling filter regeneration without a fuel penalty. Additionally, thermal stresses on the system are minimized. Since it is impractical to expect drivers to clean the filter themselves using compressed air or other methods, regeneration is necessary.
Selective Catalytic Reduction SystemSelective catalytic reduction (SCR) is an aftertreatment technique used to convert NOx into harmless nitrogen (N2) and water (H2O) through a catalytic reaction. This process involves the use of a reductant, which is a chemical compound that donates electrons to the NOx molecules, resulting in their reduction. The reductant is injected into the catalyst chamber and mixed with the exhaust gases to facilitate the reaction. The most common reducing agents used in SCR systems are typically anhydrous or aqueous ammonia and urea, while less commonly used ones include cyanuric acid and ammonium sulfate.
In diesel engines, urea is a particularly effective reductant, capable of reducing NOx emissions by 70% to 95%. However, before it can be used as a reductant, urea must first undergo thermal decomposition into a purified form called diesel exhaust fluid or automotive-grade urea. This fluid, containing 2% to 6% urea in water, is then added to the fuel supply. The use of urea as a reductant in SCR systems is an attractive solution to NOx emissions reduction, as it can achieve significant reductions while maintaining engine performance.2
Active catalytic components in SCR systems are typically composed of oxides of base metals (V, W) or precious metals. However, base metals are not highly thermally durable, a critical property for automotive engines, and have a high potential to oxidize sulfur (2SO2 + O2 = 2SO3 and SO3 + H2O = H2SO4). Sulfur oxidation is damaging to the SCR system since it is highly acidic.
Fortunately, iron- and copper-exchanged zeolite catalysts are effective alternatives that overcome both of these shortcomings. These catalysts are commonly used in SCR systems and are typically configured as honeycomb or plate geometries, with the less common option of a corrugated geometry. The honeycomb configuration is smaller but has higher pressure drops and can clog more easily than the plate configuration.
SCR systems can be retrofitted independently of the engine controller, making them practical for retrofitting older vehicles. However, they can get clogged over time, reducing their lifespan. Despite this, SCR systems are highly effective, reducing NOx by 98%, PM by 40%-60%, total HC by 80%, and CO by 90%. They also function as an effective diesel oxidation catalyst (DOX).
Tier 4 Diesel Emissions StandardsThe Environmental Protection Agency (EPA) has established rigorous standards for engine emissions, as well as other industrial and chemical processes that pose a threat to the environment. Acid rain, caused by the presence of sulfur in fuel, can have harmful effects on human health and infrastructure. To mitigate this risk, ultra-low sulfur diesel is now required for 2010 car models to reduce sulfur content in exhaust gases.
In the past, there were no regulations on the sulfur content in diesel fuel for Tier 1-3 engines, allowing up to 3,000 ppm (0.5% wt, max) of sulfur. However, this changed with new regulations that mandated a reduction in sulfur content. By June 2007, the limit was lowered to 500 ppm, and by June 2010, nonroad fuel was required to have a limit of 15 ppm. Locomotive and marine fuel followed suit and were required to meet the 15 ppm limit by June 2012.
To further reduce emissions, Tier 4 emission standards were introduced in May 2004, with a phase-in period from 2008 to 2015. These standards apply to all nonroad diesel engines used in construction, agriculture, and industrial equipment of all sizes. The most notable achievement of Tier 4 standards was the 90% reduction in PM and NOx emissions, representing a significant step forward in reducing harmful emissions.