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Non-catalytic oxidation kinetics of diesel engine soot and more than a dozen commercial carbon black samples was investigated using non-isothermal and isothermal thermogravimetric analysis (TGA) experiments. The effect of various operating parameters, such as oxygen flow rate, initial sample mass, oxygen partial pressure, crucible type, and ramp rate, on the oxidation rate was investigated. Three types of TGA experiments (non-isothermal single-ramp rate, non-isothermal multiple-ramp rates, and isothermal) were conducted and analyzed to extract the kinetic parameters for oxidation. Activation energies for oxidation of carbon black samples ranged from 125 to 257 kJ/mol, whereas that for soot oxidation was ∼155 kJ/mol. Furthermore, oxidation rate trends were explained on the basis of structural characteristics, such as scanning electron microscopy (SEM)-based average particle size and Brunauer–Emmett–Teller (BET) surface area. In general, a low particle size and high surface area were associated with a higher oxidation rate and vice versa. A thorough understanding of the non-catalytic oxidation kinetics developed in this work along with the correlation of the oxidation rate with the structural parameters may assist in efficient oxidation of diesel engine soot during the regeneration of diesel particulate filters.

The particulate matter (PM) emissions of gasoline vehicles were much lower than those of diesel vehicles until the introduction of diesel particulate filters (DPFs) in the early 2000s. At the same time, gasoline direct injection (GDI) engines started to become popular in the market due to their improved efficiency over port fuel injection (PFI) ones. However, the PM mass and number emissions of GDI vehicles were higher than their PFI counterparts and diesel ones equipped with DPFs. Stringent PM mass levels and the introduction of particle number limits for GDI vehicles in the European Union (EU) resulted in significant PM reductions. The EU requirement to fulfill the proposed limits on the road resulted to the introduction of gasoline particulate filters (GPFs) in EU GDI models. This review summarizes the evolution of PM mass emissions from gasoline vehicles placed in the market from early 1990s until 2019 in different parts of the world. The analysis then extends to total and nonvolatile particle number emissions. Care is given to reveal the impact of ambient temperature on emission levels. The discussion tries to provide scientific input to the following policy-relevant questions. Whether particle number limits should be extended to gasoline PFI vehicles, whether the lower limit of 23 nm for particle number measurements should be decreased to 10 nm, and whether low ambient temperature tests for PM should be included.

<div class="section abstract"><div class="htmlview paragraph">This review covers some of the major advances pertaining to reducing tailpipe emissions of greenhouse gases and criteria pollutants. Discussed are both new and upcoming regulations, and technologies being developed for improving engines and after-treatment systems.</div><div class="htmlview paragraph">There is clearly a focus on reducing greenhouse gas emissions in major countries, implemented through ambitious CO₂ and electrification targets. Several mature IC engine (ICE) technologies are reviewed which promise to deliver double digit reductions in CO₂ emissions. We cover some of these in detail, including gasoline compression ignition, pre-chamber combustion, water injection, and cylinder deactivation. Electrification of the powertrain and synergistic gains with advanced engine technologies are examined. The case is made for the need for cradle-to-grave analyses when evaluating various powertrain choices, and highlight the role hybrids can play in achieving significant and immediate CO₂ reductions. For the first time, also briefly discussed are the role of advanced fuels and their potential for improving emissions.</div><div class="htmlview paragraph">On criteria pollutants, the focus remains on reducing NOx and particulates. California is leading the charge on an omnibus rulemaking for heavy-duty trucks, targeting a 90% reduction in NOx emissions, and elements of the proposal are discussed. The challenge is to achieve this reduction without affecting CO₂ emissions. Various studies are underway and technological pathways are being proposed and we cover the leading concepts. These include close-coupled SCR with dual dosing, innovative solutions for low temperature urea dosing, cylinder deactivation, advanced combustion techniques, electrification and natural gas engines.</div><div class="htmlview paragraph">For light-duty vehicles, real-world driving emission (RDE) norms seem to be addressing the discrepancy between lab and on-road NOx emissions, and the relevant data are discussed. Discussions are ongoing for post Euro-6 light duty regulations, and both the possible regulatory changes as well as after-treatment system developments are covered. A major focus will remain on reducing cold-start emissions and studies for both diesel and gasoline engines are reviewed.</div><div class="htmlview paragraph">Particle number standards in Europe and China have enforced gasoline particulate filters (GPFs) in those markets, and improvements in that technology are summarized. Filtration efficiency requirements are expected to increase with further tightening of regulations, and GPFs are also expected to be required for port fuel injected vehicles in the future.</div></div>

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