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Source and characterization of particulate matter from a pilot-ignited natural gas fuelled engine Jones, Heather L.

Abstract

With growing concern over particulate matter and gaseous emissions and stringent new emissions standards upcoming in 2007 for heavy duty transport vehicles, it has become essential to study the parameters that affect heavy duty vehicle emissions. At the University of British Columbia emissions from a heavy-duty engine running with Westport Innovations' high pressure direct injection of natural gas (HPDI) technology are studied. In this engine, a small quantity of diesel pilot fuel is injected prior to a main natural gas injection event to provide ignition. This study focuses on the effects of pilot fuel quantity, overall equivalence ratio and exhaust gas recirculation on particle mass, particle size, particulate soot fraction and on gaseous emissions. With HPDI, there is the question of how much of the particulate matter is pilot-derived; this question is addressed by using carbon-14 (¹⁴C) tracing of biodiesel pilot fuel derived soot. It was found that EGR, equivalence ratio and pilot quantity play an important role in particulate matter emissions, where the emissions increase by increasing any of these quantities. Particulate matter mass concentration is an order of magnitude higher at 25% EGR and high equivalence ratio than at low equivalence ratio without EGR. Particulate matter from this engine generally contains a larger fraction of soluble organic species than that from a typical diesel engine. The percentage of black carbon in particulate samples was found to range from approximately 5% at low load conditions without EGR to approximately 60-80% at high load with 25% EGR. Particle structure can generally be described as agglomerates consisting of spherical primary particles of average size 10-25 nm. Particle size distributions definitely show evidence of a nucleation mode of particle formation (many small particles) and also a large number of larger particles likely formed due to particle-particle interaction (coagulation). EGR results in a larger average particle size. Fossil fuels contain no ¹⁴C so using a biodiesel pilot fuel, naturally containing current carbon (¹⁴C/C = 1.2 x 10⁻¹²), enabled us to apportion the pilot fuel derived soot mass. The pilot fuel was found to contribute between 4 and 40% to soot emissions under the measured operating conditions. The highest contribution is found at low load without EGR. A very significant finding is that the pilot fuel is promoting black carbon formation from other sources within the combustion chamber (most likely natural gas). An increase in pilot fuel flow of 75% causes an increase in soot emissions not derived from the pilot fuel of somewhere between 25% and 45% depending upon the engine operating condition. While the pilot-derived soot does not increase a large amount with increasing pilot fuel quantity, the soot derived from fossil sources does. This indicates that pilot fuel amount should be minimized and kept under tight control in HPDI combustion.

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