In-Situ Real-Time Characterization of Particulate Emissions from a Diesel Engine Exhaust by Laser-Induced Incandescence

Diesel engines face tightening particulate matter emissions regulations due to the environmental and health effects attributed to these emissions. There is increasing demand for measuring not only the concentration, but also the size distribution of the particulates. Laser-induced incandescence has emerged as a promising technique for measuring spatially and temporally resolved particulate volume fraction and size. Laser-induced incandescence has orders of magnitude more sensitivity than the gravimetric technique, and thus offers the promise of real-time measurements and adds the increasingly desirable size and morphology information. The usefulness of LII as a diagnostic instrument for the precise measurement of particulate concentration and primary particle size has been demonstrated. Measurements have been performed in the exhaust of a single cylinder DI research diesel engine. Simultaneous gravimetric filter measurements were made for direct comparison with the LII technique. Quantitative LII is shown to provide a sensitive, precise, and repeatable measure of the particulate concentration over a wide dynamic range. LII and gravimetric measurements are shown to correlate well over a wide range of operating conditions. A novel method for determining the primary particle size is shown to be precise enough to distinguish particle sizes for different engine operating conditions, and subsequently the number density of primary particles was determined. LII has also been shown to be sensitive in differentiating the PM performance between four different fuels. The LII technique is capable of real-time particulate matter measurements over any engine transient operation. The wide dynamic range and lower detection limit of LII make it a potentially preferred standard instrument for particulate matter measurements. INTRODUCTION From an environmental perspective, there is an urgent need to decrease the total emissions from transportation engines. The undesirable exhaust emissions include CO2, NOx, and particulate matter (PM). CO2 is a recognized greenhouse gas, and as a result of the Kyoto Protocol, industrialized countries have committed to reducing emissions of CO2. This can be primarily achieved by reductions in fuel consumption, and diesel engines offer the highest efficiency for road-going vehicles. The concession is that the emissions reduction systems for other pollutants are not as well developed for diesel engines as they are for spark-ignited engines. Demand for improved environmental performance has led to increasingly restrictive emission regulations for diesel-powered vehicles throughout Europe, North America, and Japan. Proposed regulations indicate that this trend to lower emissions levels will continue for the foreseeable future. Although PM is regulated for environmental reasons, from an operational point of view, particulate formation is not desirable. A significant portion of atmospheric particulates arises from combustion of fuels in various engines and furnaces. In urban areas, mobile sources are major contributors to ambient PM concentrations. The particulate emissions from diesel engines are in the form of complex aerosols consisting primarily of soot and volatile organics. For regulatory purposes, particulate matter emissions are defined as the mass of the matter that can be collected from a diluted exhaust stream on a filter kept at 52°C. This includes the organic compounds that condense at lower temperatures, but excludes the condensed water. This measurement provides the timeaveraged PM emissions over the period during which the particulates are collected on the filter, making measurements of the transient behavior of PM emissions impractical. Since the collected PM and other