Load-Dependent Emission Factors and Chemical Characteristics of IVOCs from a Medium-Duty Diesel Engine

Abstract

A detailed understanding of the climate and air quality impacts of mobile-source emissions requires the characterization of intermediate-volatility organic compounds (IVOCs), relatively-low-vapor-pressure gas-phase species that may generate secondary organic aerosol with high yields. Due to challenges associated with IVOC detection and quantification, IVOC emissions remain poorly understood at present. Here, we describe measurements of the magnitude and composition of IVOC emissions from a medium-duty diesel engine. Measurements are made on an engine dynamometer and utilize a new mass-spectrometric instrument to characterize the load dependence of the emissions in near-real-time. Results from steady-state engine operation indicate that IVOC emissions are highly dependent on engine power, with highest emissions at engine idle and low-load operation (≤25% maximum rated power) with a chemical composition dominated by saturated hydrocarbon species. Results suggest that unburned fuel components are the dominant IVOCs emitted at low loads. As engine load increases, IVOC emissions decline rapidly and become increasingly characterized by unsaturated hydrocarbons and oxygenated organics, newly formed from incomplete combustion processes at elevated engine temperatures and pressures. Engine transients, including a cold-start ignition and engine acceleration, show IVOC emission profiles that are different in amount or composition compared to steady-state combustion, underscoring the utility of characterizing IVOC emissions with high time resolution across realistic engine operating conditions. We find possible evidence for IVOC losses on unheated dilution and sampling surfaces, which need to be carefully accounted for in IVOC emission studies.

Technical note: Use of an atmospheric simulation chamber to investigate the effect of different engine conditions on unregulated VOC-IVOC diesel exhaust emissions

“…The SpVOC-IVOC emissions were observed to decrease with increasing engine load, with SpVOC-IVOC emission rates of 1019±65, 365±24, and 70±4 mg kg −1 at 30, 40, and 53 % load, respectively (see Table S7). This trend of decreasing VOC emission rates with increasing engine load has been observed in a number of previous studies for light-duty and medium-duty diesel vehicles (Cross et al, 2015;Shirneshan, 2013;Chin et al, 2012;Yamada et al, 2011) and can be explained by considering the engine operation. At low engine temperatures (i.e.…”

“…By 2050, this population is expected to grow to 6.34 billion people, with an estimated 66 % of the world's population living in urban environments (Prüss-Üstün and Corvalán, 2006;United Nations, 2014). Road transport emissions are a dominant source of urban air pollution (DEFRA, 1993;Colvile et al, 2001;HEI, 2010) with common road-traffic pollutants including gaseous hydrocarbons (including volatile organic compounds, VOCs), nitrogen oxides (sum of NO + NO 2 ), carbon oxides (CO and CO 2 ), and particulate matter (PM), with secondary reaction processes resulting in the formation of ozone and secondary aerosol (WHO, 2006;HEI, 2010). Exposure to road-traffic air pollutants, both primary and secondary, are of a major health concern (United Nations, 2014; WHO, 2006;HEI, 2010).…”

“…Dynamometer engines or chassis dynamometers can afford compositional insight into exhaust emissions with varying engine conditions, providing a high degree of control and reproducibility (Tadano et al, 2014;Louis et al, 2016). Several studies have used dynamometers to investigate compositional changes in unregulated exhaust gas emissions with the use of different transient driving cycles, for petrol engines (Pang et al, 2014;Baldauf et al, 2005), diesel (Yamada et al, 2011;Cross et al, 2015;Schauer et al, 1999;Zhao et al, 2015;Ballesteros et al, 2014;Nelson et al, 2008;Siegl et al, 1999;Westerholm et al, 1991), or both (Alves et al, 2015;Chirico et al, 2014;Alkurdi et al, 2013;Caplain et al, 2006;Schmitz et al, 2000;Louis et al, 2016). Driving cycles (often performed with chassis dynamometers) are designed to simulate real-world driving conditions, allowing the exhaust emissions from individual vehicles to be investigated.…”

“…However, these driving cycles offer limited information on the effect of combustion or specific engine conditions (e.g. engine load, speed) on unregulated exhaust emissions, due to the averaging of emissions over entire driving cycles and the lack of steady-state engine conditions (Cross et al, 2015), compositional information which is easily obtained with the use of a single-engine dynamometer rig (Chin et al, 2012;Cross et al, 2015;Zhu et al, 2013;Schulz et al, 1999;Machado Corrêa and Arbilla, 2008). Recent studies have focused on the measurement of intermediate-VOCs (IVOCs) in diesel exhaust emissions, primarily due to advances in instrumentation to allow the detection of these species.…”

“…At low engine loads, the exhaust gas composition was dominated by saturated hydrocarbons from unburnt fuel. At high engine loads, however, the exhaust gas composition changed, including newly formed unsaturated hydrocarbons and oxidised compounds from incomplete combustion (Cross et al, 2015). Furthermore, Chin et al (2012) found the composition of VOC and IVOC diesel exhaust emissions (generated under steady-state engine conditions) depended on engine load, fuel type (ULSD and biodiesel), and emission control devices.…”