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Control Technologies & After-treatment Systems

Pollutants Formation Mechanisms

Four main pollutants that should be minimized by optimizing combustion and improving after-treatment of the exhaust are hydrocarbons (HCs), nitrous oxide (NOx), sulfur dioxide (SO2), and carbon monoxide (CO). Carbon dioxide (CO2) is not listed as a pollutant but is a greenhouse gas harmful to the planet.

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 rather surprising considering the non-homogeneous mixture, with rich and lean spots present during combustion.

The higher the compression ratio (rc = 16 to 20 for CI and rc = 6 to 10 for SI), the more fuel leakage past exhaust valves and into crevice volumes. Up to 3% of the fuel is trapped, bearing in mind the gap size is larger when the engine is cold. Unfortunately, these fuel-rich zones cause soot to form. Also, due to the high temperature and pressure during the combustion process, large amounts of NOx are formed. The max temperature occurs at an expansion ratio (ER) of 1, but due to the brief time available for each engine cycle, incomplete mixing occurs, and max NOx formation occurs at ER = 0.95. Also, CI engines operate with a lean ER, so there is plenty of oxygen available to form NOx.

The diesel engine power is supplied by controlling the amount of fuel injected, rather than the air supply as is done in an SI engine. At engine idle, in an SI engine, the throttle is near closed and the air supply is restricted, resulting in not enough oxygen supplied to burn with the fuel and the emissions are poor. However, the CI engine is unthrottled and does have enough oxygen to burn the fuel, so there are fewer emissions at idle. However, when operating under high engine loads (wide open throttle – WOT), the CI engine operates with a rich mixture. This results in poor fuel economy and significant amounts of emissions. Fortunately, this black smoke has become much cleaner since 2000 and the exhaust odor today is not as pungent due to the reduction in the sulfur content. Over 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, is increased SOF due to high amounts of oil used and cool temperatures that cause more expansion cooling. Also, there is a trade-off between having a high efficiency (a function of high rc and subsequent considerable expansion cooling) which increases engine performance but also results in cooler exhaust temperatures which may be undesirable for thermally activated aftertreatments.

Exhaust Gas Recirculation (EGR)

A NOx reduction technique known as exhaust gas recirculation (EGR) recirculates a portion of the exhaust gas to mix with incoming air. This exhaust acts as a diluent to prevent the dissociation of nitrogen and oxygen in the air by decreasing the peak combustion temperatures (high temperatures encountered in CI engines due to high compressive heating). The dissociation of diatomic nitrogen into monatomic nitrogen (N2 -> 2N) is highly dependent on temperature, with a much more significant amount of N generated in the 2,500 – 3,000 K range. Other reactions that contribute to the formation of NOx are O2 -> 2O and H2O -> OH + H2.

NOx is one of the primary causes of photochemical smog, which has become a major problem in many large cities in the world. Fortunately, EGR can eliminate all but a fraction of a percent of NOx. Another harmful exhaust product that EGR can reduce is CO, created when CO2 dissociates according to CO2 -> CO + O. Most modern CI engines use EGR during engine operation. Unfortunately, EGR still leaves significant amounts of particulate matter (PM) black soot left unburned in the power stroke that must be filtered.

The exhaust increases the specific heat capacity (T ~ Q/ cp) of the incoming air and downstream air-fuel mixture (AFR), which lowers the adiabatic flame temperature and increases the volumetric efficiency. Even though the combustion temperature decreases, there is waste heat recovery in the soot and therefore less fuel is burned, with the net result of a minor reduction in combustion efficiency. During high engine load the peak combustion temperature needs to be high; the opposite is true during low engine loads. By tailoring the EGR flow to the engine conditions, less EGR can be used for high load conditions.

Both internal and external EGR types exist. A turbocharger (turbine-compressor) is almost always used in conjunction with the EGR recirculation. Since there is compressive heating due to the compressor, there is the addition of an intercooler after the compressor and a separate amount of EGR passes through this EGR cooler. This mixture then flows into the combustion chamber for impending combustion at a lower peak combustion temperature.

Diesel Particulate Filter (DPF)

Stricter regulations on emissions, specifically the high levels of particulate matter (PM) that result from EGR, require a diesel particulate filter in the exhaust system. One issue with using a filter is keeping the filter from getting clogged. The filter is cleaned, or regenerated, by oxidizing the soot that gets trapped in the filter. Thermal regeneration can be achieved by using either an active or passive system. Active systems spray air and fuel into the exhaust to increase the temperature. The obvious downside to combusting fuel in the exhaust manifold with the sole purpose of decreasing emissions is the fuel penalty and further emissions from burning this added fuel. Fortunately, other forms of heating the exhaust exist like electrically-assisted diesel particulate filter (EADPF). When the backpressure reaches 150 mbar the fuel is injected and combusted so the exhaust reaches ~550 C; this allows the active DPF system to function as intended. Conversely, 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. Additionally, thermal stresses on the system are avoided. Since it is impractical to require the driver to clean the filter out with compressed air or other means, regeneration is necessary.

Selective Catalytic Reduction System (SCR)

Another after-treatment used to convert NOx to N2 and H2O with the aid of a catalyst is a selective catalytic reduction (SCR). A reductant is a chemical that donates electrons (addition of a hydrogen molecule). This chemical is sprayed into the catalyst chamber and mixes with the exhaust. 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, aka diesel exhaust fluid (urea in water), before 2% – 6% urea is added to fuel. Its use as an effective reductant is attractive in diesel engines since it reduces NOx by 70% – 95% [2]. The 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 has 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 containing sulfur dioxide is a constituent 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 which makes them practical for retrofit, but they can get plugged which reduces their life. These systems reduce NOx up to 98%, PM 40% – 60%, total HC by 80%, and CO by more than 90%, along with being a highly effective diesel oxidation (DOX) catalyst.

Tier 4 Diesel Emissions Standards

The Environmental Protection Agency (EPA) sets the regulations for engine emissions among many other processes and chemicals used or burned which degrade the environment. People (and buildings) are vulnerable to the damage that acid rain causes due to the small percent of sulfur in fuel. More importantly, people cannot avoid the health hazards that result from breathing air filled with harmful particles that get trapped in their lungs. For Tier 1 – 3 there was no regulation on the sulfur content in diesel fuel , and it was at 3,000 ppm (0.5% wt, max). By June 2007 it was 500 ppm. By June 2010 for nonroad fuel it was 15 ppm and by June 2012 for locomotive and marine fuels it was 15 ppm. 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 noticeable achievement of Tier 4 standards was the reduction of PM and NOx emissions by ~90%.

References

  1. Bright Hub Engineering. 2008
  2. Mo, Yanbin. “HCCI Heat Release Rate And Combustion Efficiency: A Coupled Kiva Multi-Zone Modeling Study. Dissertation”. The University of Michigan. 2008
  3. Teo, Alvin. “Exhaust Flow In An Automobile”

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