Physico-chemical and toxicological characterization of regulated and unregulated emissions of diesel and advanced dual-fuel engines

Abstract

At present, the transportation sector predominantly utilizes conventional engines fueled by petroleum-based fossil fuels. However, with concerns over decreasing fossil fuel reserves, stringent emission norms, energy security, and the adverse environmental impact of combustion-generated pollutants and carbon-containing compound emissions, there is a growing momentum towards adopting low-carbon alternative fuels and advanced combustion mode engine technologies. Before adopting new engine technologies and alternative fuels, assessing their potential adverse effects on human health and the environment is essential. Introducing alternative fuels or advanced dual fuel combustion methods alters the combustion processes within the engine, leading to changes in the exhaust species formation and emission characteristics compared to conventional fuels and combustion modes. Analyzing the physical and chemical properties of regulated and unregulated emissions provides a better estimate and more precise understanding of their impact on human health and the environment. The present study aims to ensure that emerging technologies support environmental goals and safeguard public health. This study is also in accordance with the US-EPA charter that states that any new technology must not emit more toxic pollutants than existing ones. Experimental and numerical investigations are conducted to meet the proposed objectives. The experimental investigation was conducted on a single-cylinder automotive diesel engine representing light-duty applications. The engine was modified to run on advanced dual-fuel combustion mode utilizing gasoline and methanol as low reactive fuel and diesel as high reactive fuel. To modify the conventional engine to operate in advanced dual fuel mode, the intake port is modified to install a port fuel injection system to deliver low-reactive fuel during the intake stroke. Numerical investigation was done on an experimentally validated Cummin N-14 series engine for heavy-duty applications, using diesel and hydrogen-diesel dual-fuel modes. A detailed reaction mechanism is used to predict the exhaust emission species for hydrogen-diesel dual-fuel combustion in numerical simulation. Regulated and unregulated gas phase emissions were measured using an AVL-manufactured FTIR emission analyzer. Nanoparticle emissions were measured using a DMS 500 (Manufacturer: CAMBUSTION) particle analyzer. For the physicochemical and toxicological characterization of PM samples, the aggregate PM sample was collected in a partial flow dilution tunnel using a pre-conditioned filter paper. The physical characterization of PM emission includes nanoparticle size distribution, number concentration, mass distribution, and aggregate PM surface morphology analysis using SEM imaging. Chemical characterization encompasses gas phase unregulated (saturated, unsaturated, carbonyl and aromatic) emissions, soluble organic fractions associated with PM, and traces of heavy metals in the PM samples. The study found that methanol-diesel dual fuel (MD-RCCI) combustion mode reduces total particle number (TPN), nucleation mode particle number (NMPs) and accumulation mode particle number (AMPs) emissions significantly compared to conventional diesel combustion (CDC) and gasoline-diesel dual fuel (GD-RCCI) mode. The concentration of nucleation mode particles (NMPs) decreases as engine load increases, while accumulation mode particles (AMPs) increase for both CDC and MD-RCCI combustion modes. With increased fuel premixing ratio (RP), the NMPs in the MD-RCCI and gasoline diesel dual fuel GD-RCCI engines are increased. The chemical characterization results found that as the combustion mode transitioned to the RCCI, the saturated, unsaturated, carbonyl, aromatic (toluene) and soluble organic fraction emissions increases. Formaldehyde emissions are observed to be highest in MD-RCCI engines. The metal trace emission is significantly lower in RCCI than in CDC. The forecast for lung loading reveals that nanoparticles emitted by the CDC show a higher lung retention than MD-RCCI. RCCI combustion demonstrates lower lung retention in all tested conditions compared to CDC. The lung loading of particles of MD-RCCI decreases significantly with an increase in the premixing ratio for a constant load compared to GD-RCCI. The MD-RCCI engine shows higher cancer risk potential at a lower engine load than CDC and GD-RCCI engines due to higher formaldehyde emissions. Interestingly, cytotoxicity potential varies with engine load; medium engine loads decrease toxicity potential in MD-RCCI engines but increase it in GD-RCCI and CDC engines. PM cytotoxicity increases with increased RP in both RCCI engine types. Methanol reduces cytotoxicity and particle inhalation toxicity at medium engine load. The transitions of combustion mode from CDC to RCCI significantly decrease global warming, acidification, eutrophication and ozone-forming potential. Adding hydrogen in diesel engines decreases the carcinogenicity and mutagenicity potential under all the simulated conditions. Alternative fuels such as methanol and hydrogen have the potential of enhancing environmental sustainability and public health. However, careful consideration and additional exhaust after-treatment are essential to fully realize the benefit of the MD-RCCI engine across all operational conditions.