Diesel Engine Emissions and Control Equipment Lukas Willits University of Idaho ABSTRACT This technical paper explores the major exhaust pollutants of the 4-stroke compression ignition (CI) diesel engine and how the diesel engine compares to the spark ignition (SI) gasoline 4stroke engine. Government regulations are becoming increasingly strict on pollutant emissions of diesel engines. The negative consequences of these emissions to the environment and human health will also be examined. Additionally, the differences between these engine designs and why they require different emissions control equipment will be addressed. Finally, the function of control equipment employed to reduce these pollutant emissions in diesel engines will be discussed. EXHAUST POLLUTANTS Here are the major exhaust pollutants we generally focus on reducing and more specifically why we want to reduce them. Nitrogen oxides (NOx) contribute to the formation of smog by reacting with volatile organic compounds (VOCs), which includes unburned hydrocarbons, to create tropospheric or, “ground level” ozone. Nitric oxide also depletes the ozone layer in the stratosphere. Although ozone is harmful to our health in the atmosphere closest to the Earth’s surface due to smog formation, stratospheric ozone is what protects us from excess UV radiation. [1] Carbon monoxide is another ingredient in the formation of tropospheric ozone and smog formation. Carbon dioxide is a greenhouse gas contributing to man-made climate change. Also, it is estimated that one-third of all man-made carbon dioxide emissions have been absorbed by our oceans, which increases their acidity [2]. This poses a threat to aquatic life that is sensitive to pH balance. Unburned hydrocarbons include volatile organic compounds, as well as methane and ethane. VOCs are highly reactive and a major ingredient in tropospheric ozone and smog formation, while methane and ethane are less reactive but still contribute to smog. Methane is also a greenhouse gas. Particulate matter and soot cause a variety of human health problems, such as asthma and lung cancer. [3] Soot, or black carbon, also causes positive radiative forcing like greenhouse gases. Positive radiative forcing means the earth receives more incoming energy from sunlight than it radiates to space. The radiative forcing of greenhouse gasses and particulates are shown in Figure 1. Figure 1. Radiative forcing in watts/meter2 of greenhouse gasses and particulates. [4] COMPARISON OF ENGINE DESIGNS DIESEL ENGINE EMISSIONS Figure 2 shows a comparison of emissions between compression ignited and spark ignited engines. Compression ignited engines have onefifth the unburned hydrocarbon emissions, higher nitrogen oxide emissions, lower carbon monoxide emissions, and comparable carbon dioxide emissions. [5] The lean combustion operation of compression ignition engines adds complexity to emissions reduction because the three-way oxidation reduction catalysts used in spark ignition gasoline engines aren’t effective at reducing nitrogen oxides due to the excess oxygen in compression ignition engine exhaust. These differences are mainly due to the lean combustion and higher compression of compression ignition engines, resulting in higher temperature and excess oxygen combustion conditions. Instead a diesel oxidation catalyst (DOC) is used for oxidizing carbon monoxide and unburned hydrocarbons, while other methods are used to reduce nitrogen oxides. The main methods of reduction are selective catalytic reduction (SCR) and diesel exhaust fluid (DEF) injection, and lean NOx traps (LNT). The higher concentration of soot found in compression ignition engines’ exhaust, which is due to direct injection of diesel fuel into the cylinder, is reduced by using a diesel particulate filter (DPF). DIESEL ENGINE EMISSIONS CONTROL EQUIPMENT DIESEL PARTICULATE FILTER Figure 2. Comparison of the composition of exhaust emissions between CI and SI engines. [6] Diesel particulate filters collect and remove around 85% of soot from diesel exhaust, however they will eventually clog or create excessive backpressure if the soot is not burned out of the filter with sustained high exhaust temperatures. The process of burning the soot is called regeneration and is achieved passively and actively. Passive regeneration occurs during extended highway driving where exhaust temperatures reach the light-off temperature required for regeneration, about 550 oC or higher. Active regeneration can occur by the engine control unit signaling an extra diesel fuel injector upstream of the DPF to dose and ignite fuel in the DPF, by using an electric heater, or by using microwave heating. Roughly every 200,000 miles, the DPF must be removed and professionally cleaned. [7] SCR. Smaller passenger diesel powered vehicles use LNTs as a cheaper and simpler method of NOx reduction, removing the necessity of the operator to refill the DEF storage tank. Another benefit of LNT’s is in medium and heavy-duty applications they can be calibrated to generate ammonia for use in a downstream SCR system, reducing the amount of DEF being consumed. One downside of LNT’s is they gradually become poisoned by sulfur dioxide in the exhaust and require high temperature regeneration periods like the DPF to regain effectiveness of reducing NOx. [8] Figure 3. Diesel Particulate Filter [3] SELECTIVE CATALYTIC REDUCTION Selective catalytic reduction (SCR) converts NOx into nitrogen and water by injecting diesel exhaust fluid (DEF) into the exhaust. DEF is a mixture of water and urea, and the urea thermally decomposes into ammonia in the exhaust and is absorbed by the SCR catalyst, allowing the chemical reactions in figure 4 to occur. Urea is used as opposed to ammonia as it is safer to handle and store. SCR has a conversion efficiency of about 90% for NOX. [7] Figure 5. Diagram of standard emissions control equipment employed in diesel engines. [7] SUMMARY Diesel engines differ from 4-stroke gasoline engines due to their high compression combustion ignition. The high compression and lean air/ fuel mixture inherent to the diesel 4stroke engine creates the need for alternative exhaust pollution reduction equipment. Figure 4. Chemical reactions occurring within the SCR system. [7] LEAN NOX TRAPS Lean NOx Traps are another method used for reduction of nitrogen oxides in diesel engines and can either be a replacement for Selective Catalytic Reduction or used in conjunction with The pollution control equipment used are the diesel particulate filter to remove and burn soot, selective catalytic reduction with DEF injection to reduce nitrogen oxides into nitrogen, and a diesel oxidation catalyst to oxidize carbon monoxide and unburned hydrocarbons into carbon dioxide and water. REFERENCES 1. “Tropospheric Ozone.” Wikipedia, Wikimedia Foundation,en.wikipedia.org /wiki/Tropospheric_ozone. 2. Mathez, Edmond A., and Jason E. Smerdon. Climate Change: The Science of Global Warming and Our Energy Future. Columbia University Press, 2009. 3. Reşitoğlu, İ.A., Altinişik, K. & Keskin, A. The pollutant emissions from diesel-engine vehicles and exhaust aftertreatment systems. Clean Techn Environ Policy 17, 15– 27 (2015). https://doi.org/10.1007/s10098014-0793-9 4. “Causes of Climate Change – Online Course.” FutureLearn, University of Bergen,www.futurelearn.com/courses/caus es-of-climate-change. 5. Heywood, John. Internal Combustion Engine Fundamentals. McGraw Hill, 1988. 6. Motor Vehicle Exhaust Emissions. Volkswagen AG, www.volkspage.net/technik/ssp/ssp/SSP_2 30.pdf. 7. Alozie, N. S., & Ganippa, L. C. (May 29th,2019). Diesel Exhaust Emissions and Mitigations. doi: 10.5772/intechopen.85248 8. Johnson, Timothy V. “Diesel Emissions in Review.” SAE International Journal of Engines, vol. 4, no. 1, 2011, pp. 143–157. JSTOR, www.jstor.org/stable/26278142. ME 433 Project Evaluation Rubric Student: Lukas Willits Title: Diesel Engine Emissions Deliverable: Research Paper Exemplary DELIVERY CONTENT Date: 8/9/20 Style Fully Meets Criteria Background Provides clear context with compelling purpose and strong inquiry questions. Analysis Appropriately describes relevant engineering concepts and tools. References Cites robust set of credible resources, appropriately cited and formatted. X Organization Logically ordered with effective transitions between sections. X Visuals Graphics, tables, and equations used to support project narrative. X Language, sentence structure, and layout conveys professional meaning. X Partially Meets Criteria Minimally Meets Criteria X X Comments: The background category is satisfied with a solid set of inquiry questions which determined the structure of the paper. The analysis category is where I see the greatest weakness in the content of this paper. I read about a lot of technical topics along the lines of diesel engine emissions, such as engineering concepts and tools/ equations. The problem was that I couldn’t decide how to incorporate them into my research paper in a coherent manner. My set of references are robust and include multiple credible sources like the Heywood text we used for the class, as well as a few papers published by SAE. I believe I organized my paper in such a way that the reader progresses through the inquiry questions I set out to answer in a logical manner. I added more visuals and diagrams to aid in understanding of the concepts and systems being discussed throughout the paper. Style is not one of my strengths, but I feel I did an adequate job using proper language for a technical research paper and avoided obvious errors by rereading my writing