Simulation of the Formation Process of Diesel Exhaust Particle Emissions

Tämän diplomityön tarkoituksena oli kehittää ANSYS FLUENT -virtauslaskentaohjelmistoa varten malli, jolla voidaan simuloida dieselpakokaasun hiukkaspäästöjen muodostumisprosessia. Mitattujen hiukkaskokojakaumien ennustaminen mallilla vaatii nukleaationopeuden ja kondensoituvien kaasujen pitoisuuksien sovittamisen, mutta eri parametrien vaikutuksia muodostuneeseen jakaumaan voidaan tutkia jo sovitetun tilanteen ratkaisun perusteella. Mallin toimintaa tutkittiin vertaamalla simuloituja linja-auton ja henkilöauton hiukkaspäästöjä mitattujen kanssa. Linja-autosimulaatioissa havaittiin, että syntynyt hiukkasjakauma on hyvin herkkä pakokaasun rikkihappo- ja hiilivetypitoisuuksille, ja että suurin osa nukleaatiosta ja kondensaatiosta on tapahtunut 0.1-0.2 s aikana pakoputkesta poistumisen jälkeen. Vielä kauempanakin kaasufaasiin on jäänyt vielä suuri osa kondensoituvia kaasuja, mutta näiden nopea laimeneminen hidastaa kyseisten prosessien jatkumista eteenpäin. Henkilöautosimulaatiossa havaittiin, että eri mallien toiminta ja tulokset poikkeavat hieman toisistaan, mutta kaikkien tulokset ovat kuitenkin lähellä mitattuja tuloksia. Laivapäästöjen simulointiin nukleaationopeuden laskentaa olisi jatkokehitettävä laivapolttoaineiden suurien rikkipitoisuuksien vuoksi. Laboratorioskaalaisessa laimentimessa, joka voisi tuottaa samanlaisen hiukkasjakauman kuin vastaavassa todellisessa ulkoilmalaimenemisessa syntyisi, tapahtuva hiukkasten muodostumisprosessi ei matalan turbulenttisuuden vuoksi voi olla täysin samanlainen kuin ulkoilmatilanteessa. Tällöin kaasujen kulkemat jäähtymis-laimenemisreitit laimentimessa eivät vastaa ulkoilmatilanteen reittejä. Simulaatioiden perusteella laimentimen optimaaliset käyttöparametrit löytämällä voidaan kuitenkin tuottaa samanlainen hiukkasjakauma yksinkertaisellakin laimentimella. Turbulenttisella laimentimella hiukkasia muodostuu enemmän, mutta voimistunut seinämädepositio tulee haasteeksi.The purpose of this Master's Thesis was to develop a model for ANSYS FLUENT CFD-software, which can be used to simulate the formation process of diesel exhaust particle emissions. To predict measured particle size distributions by the model, fitting of the nucleation rate and the concentrations of condensing gases is needed, but the effects of different parameters to the formed distribution can be examined by using the solution of an already fitted case. The functioning of the model was examined by comparing the simulated particle emissions of a bus and a passenger car with the measured ones. It was observed in the bus simulations, that the formed distribution is very sensitive to the concentrations of sulfuric acid and hydrocarbons in exhaust, and that the major part of nucleation and condensation is occurred within a time of 0.1-0.2 s after exhaust is released from the exhaust pipe. Even further, a high amount of condensing gases has still remained in the gas phase, but rapid dilution of these gases decelerates the continuing of these processes. It was observed in the passenger car simulation, that the functioning and the results of different models differ slightly, but the results are, however, near the measured ones. To simulate marine diesel emissions, the calculation of the nucleation rate should be developed further because of the high fuel sulfuric content of marine fuels. In a laboratory-scale diluter, which could produce a particle distribution corresponding to that formed in a real-world outdoor dilution case, the formation process of particles cannot be completely the same as in an outdoor case. In the laboratory case, cooling-diluting paths of gases in a diluter are different than in an outdoor case. According to the simulations, a similar particle distribution can, however, be produced even with a simple diluter if the optimal parameters for operating the diluter are found. More particles are formed in a turbulent diluter, but increased wall deposition will become a challenge

Challenges and solutions in determining dilution ratios and emission factors from chase measurements of passenger vehicles

Vehicle chase measurements used for studying real-world emissions apply various methods for calculating emission factors. Currently available methods are typically based on the dilution of emitted carbon dioxide (CO2) and the assumption that other emitted pollutants dilute similarly. A problem with the current methods arises when the studied vehicle is not producing CO2, e.g. during engine-motoring events, such as on downhill sections. This problem is also encountered when studying non-exhaust particulate emissions, e.g. from electric vehicles. In this study, we compare multiple methods previously applied for determining the dilution ratios. Additionally, we present a method applying multivariate adaptive regression splines and a new method called near-wake dilution. We show that emission factors for particulate emissions calculated with both methods are in line with the current methods for vehicles producing CO2. In downhill sections, the new methods were more robust to low CO2 concentrations than some of the current methods. The methods introduced in this study can hence be applied in chase measurements with changing driving conditions and be possibly extended to estimate non-exhaust emissions in the future.Peer reviewe

Particle number, mass, and black carbon emissions from fuel-operated auxiliary heaters in real vehicle use

Fuel-operated auxiliary heaters (AHs) are frequent solutions to heat the vehicle engines and cabins in cold areas. Particulate exhaust emissions of AHs are unregulated; therefore, their contribution to local air quality and thus human health and even the global emissions budget is unknown. Experiments for studying the AH-originated emissions were performed under Finnish winter conditions mimicking real-world use for six selected vehicles with original AHs installed, including both gasoline- and diesel-powered heaters. We present quantitative results of particle number emissions down to 1.3 nm, particle size distributions, particulate mass, and black carbon, and compare to gaseous emissions. The start-up and shutdown phases showed the highest particle peaks, while the particle concentrations were stable between these. The mean particle number, mass and BC emission factors were found to be as high as 590 × 1012 kgfuel−1, 33 mg kgfuel−1 and mg 18 kgfuel−1 for gasoline-operated heaters and 560 × 1012 kgfuel−1, 20 mg kgfuel−1 and 12 mg kgfuel−1 for diesel-operated heaters. Comparing total number of particles larger than 23 nm emitted during vehicle preheating with AH to vehicle tailpipe emissions during drive shows that a typical heating cycle emits an equal number of particles to drive dozens or even thousands of kilometers.publishedVersionPeer reviewe

Nonvolatile ultrafine particles observed to form trimodal size distributions in non-road diesel engine exhaust

Some recent findings regarding the negative health effects of particulate matter increase the relevance of the detailed characteristics of particulate emissions from different sources and especially the nonvolatile fraction of particles. In this study, the nonvolatile fraction of ultrafine particulate emissions from a non-road diesel engine was studied. The measurements were carried out in an engine laboratory and the exhaust sample was taken from the engine-out location with various steady state driving modes. Four different fuels, including fossil fuel, soybean methyl ester (SME), rapeseed methyl ester (RME), and renewable paraffinic diesel (RPD), were used. In the sampling system, the sample was diluted and led through a thermodenuder removing the volatile fraction of particles. The measured particle size distributions of nonvolatile particles were found to be trimodal. Based on the size distribution data as well as the morphology and elemental composition of particles in transmission electron microscopy (TEM) samples, we were able to draw conclusions from the most probable origin of the different particle modes, and the modes were named accordingly. From larger to smaller in particle size, the modes were a soot mode, lubricating oil originated core (LC) mode, and a fuel originated core (FC) mode. All of these three modes were detected with every driving mode, but differences were seen, for example, between different fuels. In addition, a trade-off was observed in the concentrations of the LC mode and the soot mode as a function of the engine torque.© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.fi=vertaisarvioitu|en=peerReviewed

Engine preheating under real-world subfreezing conditions provides less than expected benefits to vehicle fuel economy and emission reduction for light-duty vehicles

Six light-duty vehicles, both gasoline- and diesel-fueled, were driven a prescribed 13.8 km route in a real-world low-traffic environment under Finnish subfreezing winter conditions (−28. −10 °C). Cold starts, hot starts, and starts with different preheating strategies were used. Fuel consumption and emissions of particles and nitrogen oxides (NOx) were examined by a chasing method with a mobile laboratory. Both electric preheaters (0.3–1.2 kW) and fuel-operated auxiliary heaters (5 kW) were used in the experiments where a cold engine was preheated before starting. While most vehicles showed potential for reducing fuel consumption and emissions of particles (PM), black carbon (BC), and NOx during hot starts compared to subfreezing-cold starts, the benefits of preheating were relatively small and limited to only a few vehicles. The fuel consumption for the 13.8 km drive decreased less than 4% with one gasoline vehicle and one diesel vehicle by preheating. These two vehicles are both equipped with a fuel-operated auxiliary heater, and taking the fuel consumption of the heater during preheating into account leads to about 30% higher total fuel consumption, canceling the preheating benefit out. These two vehicles also showed the largest reductions in PM, BC, and NOx emissions achieved with preheating, e.g., the PM emission reductions being 72% (the gasoline vehicle) and 24% (the diesel vehicle). Whereas the NOx emission reduction for this gasoline vehicle was 41% when considering only the drive, it decreases to 15% when the NOx emissions from the auxiliary heater during preheating are also taken into account. High particle number (PN) emissions from all vehicles and NOx emissions from the diesel vehicles were detected. The PN emissions of particles larger than 23 nm were up to 2 orders of magnitude higher and the NOx emissions up to a factor of 21 higher than the corresponding limits in the European regulations for type-approval of new vehicles. The PN emissions did not depend on the start types; thus, no benefits to reduce them with preheating were detected. The limit-exceeding PN emissions are partially explained with the used measurement method for PN taking both nonvolatile and semivolatile particles into account, whereas the regulations take only the nonvolatile particles into account. The PM emissions were also observed to consist mostly of semivolatile material in most of the cases, organics being the main component of the semivolatile material.Peer reviewe

Measurement report : Atmospheric new particle formation in a coastal agricultural site explained with binPMF analysis of nitrate CI-APi-TOF spectra

The occurrence of new particle formation (NPF) events detected in a coastal agricultural site, at Qvidja, in Southwestern Finland, was investigated using the data measured with a nitrate ion-based chemicalionization atmospheric-pressure-interface time-of-flight (CI-APi-TOF) mass spectrometer. The binned positive matrix factorization method (binPMF) was applied to the measured spectra. It resulted in eight factors describing the time series of ambient gas and cluster composition at Qvidja during spring 2019. The most interesting factors related to the observed NPF events were the two factors with the highest mass-to-charge ratios, numbered 7 and 8, both having profiles with patterns of highly oxygenated organic molecules with one nitrogen atom. It was observed that factor 7 had elevated intensities during the NPF events. A variable with an even better connection to the observed NPF events is f(F7), which denotes the fraction of the total spectra within the studied mass-to-charge ratio range between 169 and 450Th being in a form of factor 7. Values of f(F7) higher than 0.50 +/- 0.05 were observed during the NPF events, of which durations also correlated with the duration of f(F7) exceeding this critical value. It was also observed that factor 8 acts like a precursor for factor 7 with solar radiation and that the formation of factor 8 is associated with ozone levels.Peer reviewe

Chemical and physical characterization of oil shale combustion emissions in Estonia

In this study, oil shale combustion emission measurements were conducted in a 60 kW(th) Circulating Fluidized Bed combustion test facility located in a laboratory-type environment. A comprehensive set of instruments including a nitrate-ion-based Chemical Ionization Atmospheric Pressure interface Time-of-Flight Mass Spectrometer, a Soot-Particle Aerosol Mass Spectrometer, and a Potential Aerosol Mass (PAM) chamber was utilized to investigate the chemical composition and concentrations of primary and secondary emissions in oil shale combustion. In addition, the size distribution of particles (2.5-414 nm) as well as concentration and composition of gaseous precursors were characterized. Altogether 12 different experiments were conducted. Primary emissions were studied in seven experiments and aged emissions using PAM chamber in five experiments. Combustion temperatures and solid fuel circulation rates varied between different experiments, and it was found that the burning conditions had a large impact on gaseous and particulate emissions. The majority of the combustion particles were below 10 nm in size during good burning whereas in poor burning conditions the emitted particles were larger and size distributions with 2-3 particle modes were detected. The main submicron particle chemical component was particulate organic matter (POM), followed by sulfate, chloride, nitrate, and ammonium. The secondary particulate matter formed in the PAM chamber was mostly POM and the concentration of POM was many orders of magnitude higher in aged aerosol compared to primary emissions. A significant amount of aromatic volatile organic compounds (VOCs) was measured as well. VOCs have the potential to go through gas-to-particle conversion during the oxidation process, explaining the observed high concentrations of aged POM. During good combustion, when VOC emissions were lower, over 80% of SO2 was oxidized either to gaseous H2SO4 (37%) or particulate sulfate (46%) in the PAM chamber, which mimic the atmospheric processes taken place in the ambient air after few days of emission.Peer reviewe

Measurement report : The influence of traffic and new particle formation on the size distribution of 1–800 nm particles in Helsinki – a street canyon and an urban background station comparison

Peer reviewe

Direct field evidence of autocatalytic iodine release from atmospheric aerosol

Reactive iodine plays a key role in determining the oxidation capacity, or cleansing capacity, of the atmosphere in addition to being implicated in the formation of new particles in the marine boundary layer. The postulation that heterogeneous cycling of reactive iodine on aerosols may significantly influence the lifetime of ozone in the troposphere not only remains poorly understood but also heretofore has never been observed or quantified in the field. Here, we report direct ambient observations of hypoiodous acid (HOI) and heterogeneous recycling of interhalogen product species (i.e., iodine monochloride [ICI] and iodine monobromide [IBr]) in a midlatitude coastal environment. Significant levels of ICI and IBr with mean daily maxima of 4.3 and 3.0 parts per trillion by volume (1-min average), respectively, have been observed throughout the campaign. We show that the heterogeneous reaction of HOI on marine aerosol and subsequent production of iodine interhalogens are much faster than previously thought. These results indicate that the fast formation of iodine interhalogens, together with their rapid photolysis, results in more efficient recycling of atomic iodine than currently considered in models. Photolysis of the observed ICI and IBr leads to a 32% increase in the daytime average of atomic iodine production rate, thereby enhancing the average daytime iodine-catalyzed ozone loss rate by 10 to 20%. Our findings provide direct field evidence that the autocatalytic mechanism of iodine release from marine aerosol is important in the atmosphere and can have significant impacts on atmospheric oxidation capacity.Peer reviewe

On Sulfuric Acid and Nanocluster Formation in Vehicle Exhaust

Particles existing in the atmosphere have effects on human health, climate, and visibility. The sources of airborne particles include both biogenic and anthropogenic sources. One of the main sources is direct emissions from traffic, but new particles are formed also in the atmosphere from nucleating precursor vapors. Sulfuric acid is perhaps the most important gaseous compound involved in new particle formation occurring in the atmosphere and in vehicle exhaust. It is formed via photochemistry in the atmosphere but is also formed in the exhaust systems of vehicles. It has been shown to correlate with nucleation rate, i.e., the formation rate of new critical clusters capable of growing to larger particle sizes. Several nucleation mechanisms exists but the exact mechanism occurring in vehicle exhaust has not been resolved yet. One method in resolving the nucleation mechanism is to examine the effects of precursor concentrations on the nucleation rate, which usually involve power-dependencies denoted as nucleation exponents. This thesis focuses on determining the nucleation exponents and an explicit nucleation rate function for binary sulfuric acid-water nucleation in exhaust-related conditions. Determining the nucleation rates in vehicle exhaust in this thesis involves laboratory experiments and computational simulations outputting the nucleation rates inversely using the experimental data. An aerosol model was first developed for this purpose and was further improved to a more generalized model later. The generalized model includes an important improvement for representing particle size distributions efficiently even though they are not in a trivial log-normal form using a combined power law and log-normal size distribution model, developed in this thesis. The obtained nucleation exponent for sulfuric acid is 1.9, which corresponds best with the kinetic nucleation theory, but other compounds existing in real vehicle exhaust, such as hydrocarbons, seem to have an important role in nucleation too. The obtained nucleation rate function also provides information on the effects of water and temperature on nucleation and a starting point in examining the effects of the other compounds in more detail. Traffic-originated sulfuric acid and particles were also studied by measuring them together with solar irradiance and traffic density in a highly-trafficked street canyon near the city center of Helsinki, Finland. Nanocluster aerosol, i.e., sub-3 nm particles, was found to be emitted directly by traffic, as expected, but also to not need sulfuric acid or radiation to form. Sulfuric acid, instead, was found to be formed via a secondary emission route, in which vehicle-emitted sulfur dioxide is converted to sulfuric acid via photochemistry. Additionally, sulfuric acid emitted directly by vehicles was found to be converted rapidly to particles after the emission. These results present an important update to the current view of sulfuric acid and particle formation in traffic-influenced areas