Condensational Growth of Particulate Matter from Partially Premixed Low Temperature Combustion of Biodiesel in a Compression Ignition Engine

Abstract

Condensational growth is not typically assumed to be significant compared with adsorption for conversion of unburned hydrocarbons in the exhaust of diesel engines to the particulate phase. However, when partially premixed low temperature combustion (LTC) modes designed to simultaneously reduce soot and NO X emissions are implemented, unburned hydrocarbon (UHC) concentrations in the exhaust are an order of magnitude higher than for conventional combustion modes, increasing the likelihood of gas to particle conversion by condensation. In this work, two LTC operating conditions are compared with conventional diesel combustion using a multi-cylinder direct-injection diesel engine using low-sulfur fuel, a soy-based biodiesel and a 50% by volume biodiesel blend. Gaseous emissions of unburned hydrocarbons were measured and particulate samples were taken using a partial-flow dilution tunnel. Gravimetric analysis of the collected filters, Soxhlet extraction of particulate and speciation using GC-FID was performed for all operating conditions. Elemental carbon (EC) emissions were measured using a thermal optical analyzer and particle size distribution was analyzed using a differential mobility spectrometer. For increasing biodiesel concentration in the fuel, mass emissions of both EC and UHC decreased for all combustion modes compared with petroleum diesel. However, for biodiesel use in LTC modes of operation, particulate mass significantly increased following exhaust dilution. Low vapor pressure methyl esters found in the exhaust of biodiesel LTC increases heterogeneous condensation onto soot particles in the exhaust compared with unburned species from petroleum diesel fuel operation. A model estimating this condensation mechanism accurately predicts the exper-

Volatility characterization of nanoparticles from single and dual-fuel low temperature combustion in compression ignition engines

“…In-cylinder formation mechanisms and speciation of HCs from LTC have been studied (Knafl et al 2006;Colban et al 2007;Han et al 2008). Although solid particle mass concentrations are very low for LTC, high concentrations of semivolatile HC in LTC exhaust are known to form nanoparticles through nucleation and growth mechanisms upon primary dilution (Northrop et al 2011a). Further, we have shown in previous work that volatile material can slip through a close-coupled aftertreatment system composed of a diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) and form particles under certain conditions (Lucachick et al 2015).…”

“…Unburned fuel is thought to heavily contribute to LTC-derived primary particles, but previous work has shown unburned fuel alkanes are not solely responsible for the increase in volatile particle emissions (Northrop et al 2011a). Other contributing species include low-volatility components like methyl ester in biodiesel and polycyclic aromatic hydrocarbons (PAHs) present in diesel fuel and from combustion.…”

Concentration and Size Distributions of Nanoparticle Emissions during Low Temperature Combustion using Fuels for Advanced Combustion Engines (FACE)

“…Particle Size Distribution for PCCI and HCCI Combustion Mode [60] Similar results concerning SOF of engine out PM emissions as well as the shift in particle diameter towards nanoparticles during advanced combustion were obtained in an investigation on a GM 1.7L common-rail diesel engine comparing conventional diesel operation with partially premixed early and late injection LTC strategies [61].…”

Effect of Cetane Number on Specific Fuel Consumption and Particulate Matter and Unburned Hydrocarbon Emissions from Diesel Engines

“…High viscosities provide larger droplet diameters and high penetration of the fuel jet. The use of high viscosity fuels hinders vaporization, favoring the formation of large diameter droplets and causing incomplete combustion due to the high penetration of the fuel jet, hindering cold starts and increasing the emission of unburned hydrocarbons (HCs) and particulate matter (PM) [9][10][11].…”

“…The temperature of 10% of the volatilized gas fractions recovered reflects the ease of vaporization, while the temperature of 90% of these fractions indicates the presence of high molecular weight compounds that will be difficult to vaporize completely, favoring the emission of particulate matter (PM) and unburned hydrocarbon (HCs) [6], and deposits in the engine [12]. Fuels with low CN may also increase PM emissions, since combustion begins in the final stage of the expansion cycle when the temperature inside the chamber diminishes, reducing the speed of oxidation, which in turn increases the concentration of unburned HC's that condense on the surface, causing the mass of particulate matter to increase [9,[13][14][15].…”