Gasoline direct injection engine soot oxidation: Fundamentals and determination of kinetic parameters

Abstract

Current emissions legislation for road transport vehicles, including modern gasoline vehicle fleet limits the mass and the number of Particulate Matter (PM) emitted per kilometre. The introduction of a gasoline particulate filter (GPF) is expected to be necessary, as was the case for diesel vehicles, the traditionally recognised source of PM in transportation. Therefore, for the design of efficient GPFs and the regeneration strategies, soot oxidation characteristics in gasoline must be understood. Extensive research has been carried out mainly to investigate the oxidation of diesel soot, however, in the most cases soot were collected on microfiber filters and the activation energy was calculated with the logarithm method assuming mass and oxygen reaction orders equal to one. Identified limitations that lead to inconsistent and inaccurate trends and results are presented in this paper. As a consequence, a novel methodology to accurately obtain the oxidation kinetic parameters for soot emitted from a Gasoline Direct Injection (GDI) engine has been developed and presented in this paper. The particles collected in a silicon carbide wall-flow particulate filter are directly exposed to oxidation conditions in a thermogravimetric analysis (TGA) without the use of microfiber filter. The significance of more accurate and consistent calculations of soot oxidation kinetic parameters as a result of this methodology will aid modelling and experimental work of the aftertreatment systems and will lead in improving the GPF regeneration process in modern GDI vehicles. Avoiding high peak temperatures during regeneration and large thermal stress gradients and thus increasing the operating life of the filters is amongst the benefits can be seen.

Potential of Ceria-Zirconia-Based Materials in Carbon Soot Oxidation for Gasoline Particulate Filters

“…As required for diesel engines, it is possible to introduce a particulate filter to reduce emissions of soot: the so-called gasoline particulate filter (GPF) [4][5][6]. However, due to the different operating conditions of a diesel engine and a GDI engine [3], it is not possible to simply transfer the technologies optimized and developed in the last twenty years for diesel particulate filters (DPF), but it is necessary to investigate in detail the mechanisms of soot accumulation and oxidation under typical GDI operating conditions [7]. Exactly as it happens for DPFs, the accumulation phase must necessarily be followed by a regeneration phase, in order to oxidize the accumulated soot and prevent an increased pressure drop [8].…”

Analysis of Regulated Pollutant Emissions and Aftertreatment Efficiency in a GTDi Engine Using Different SOI Strategies

“…The aforementioned trend shows how the particulate produced is much lower than the particle emissions generated by the pool fire (for most advanced SOIs) when the time available for the AF mixture is reduced (most delayed SOIs). In the advanced SOIs, particle formation can persist until the end of the combustion cycle where the rate of heat release can decrease significantly, causing a considerable decrease in the subsequent oxidation phase [69][70][71].…”

Nonuniform Oxidation Behavior of Loaded Gasoline Particulate Filters

“…The potential for short-term buildup of PM on the GPF and/or the PM's interaction with catalytic active sites, in cases where the emission control catalyst and particulate filter are combined, have created an increased interest in the oxidative reactivity of PM, which would also be relevant when consumer driving behavior requires active regeneration. An understanding of the oxidative reactivity is critical to developing both system models of the powertrain-GPF interactions and operational controls for managing PM accumulation in the GPF [30].…”

“…TGA, within a given study, can highlight the effects that a set of conditions has on the activation energy of PM samples; however, variances between experimental procedures used in different reported studies can make direct quantitative comparisons less accurate. The impact of different TGA experimental procedures, such as sample mass, temperature window integration, and ramp rate, has been noted in the literature, and activation energy calculations for PM samples from the same study vary widely, ranging from 67 to 256 kJ/mol [12,[30][31][32]. Wang et al, in a non-isothermal kinetic study, showed that ramp rate and initial PM mass used in the study impacted the calculated activation energy for the same PM [31].…”

“…Bogara et al reported results from an isothermal kinetic study using separate PM samples to measure oxidation rate at different temperatures, ranging from 400 to 750°C, which gave an activation energy of 132 kJ/mol for E0 PM. In addition, they found a range of activations energies (~140-160 kJ/mol) for the same PM in non-isothermal studies at different ramp rates and sample masses, leading them to suggest that due to the impacts of ramp rate and initial mass of the sample on activation energy, these values should be used to understand trends but absolute values should be taken with caution [30]. Furthermore, since most TGA studies are on loose PM or PM on a microfiber filter, in contrast to an actual GPF which has a wall-flow, monolithic structure, the typical TGA approach must assume a uniform PM sample with a single activation energy regardless of whether the study uses a nonisothermal ramping method on a single sample or isothermal oxidations on multiple samples.…”