Douglas Allen Dobson

Nitrous oxide emissions from a medium-duty diesel truck exhaust system

Starting in 2010, medium-duty diesel trucks in the USA were introduced with aftertreatment systems that contained precious metal oxidation catalysts, soot filters, and selective catalytic reduction (SCR) systems for control of nitrogen oxides (NO, NO2). While modern diesel aftertreatment systems have high performance for meeting hydrocarbons, carbon monoxide (CO), NOx, and particulate matter, there is some concern over emission of nitrous oxide (N2O) that can be formed within the exhaust system. N2O has an atmospheric lifetime of approximately 114 years and is 298 times more effective than CO2 at trapping heat in the atmosphere. In this study, the sources of N2O were compared in the laboratory flow reactor and at the system level on diesel trucks. The interactions of HC with NOx on the DOC and NOx with NH3 within the SCR catalyst were the predominant mechanisms for N2O formation. The composite N2O mass emission was calculated to be approximately 43 mg/mi, resulting in an equivalent CO2 penalty of about 2%, similar to the 1% to 3% penalty estimated for the global light-duty vehicle fleet.

Gasoline Particle Filter Development

Gasoline particle filter (GPF) development includes optimization of multiple, competing targets: low backpressure, high clean filtration, acceptable strength, high oxygen storage capacity, small size, and low cost. A three-way catalyst + GPF system needs to meet targets for hydrocarbons, carbon monoxide, and nitrogen oxides in addition to particle mass and/or particle number. GPFs behave differently than diesel particle filters (DPFs) in terms of regeneration and ash loading behavior due to vastly different operating conditions. In a relatively clean exhaust condition on GDI relative to diesel, an empty GPF can have filtration efficiencies on the order of 60%. This was improved to 80–90% with a small amount of soot and/or ash on the filter walls, or higher catalyst washcoat loading. In the course of this work, models were developed to predict backpressure, filtration, and chemical performance.

Impact of Fuel Metal Impurities on the Durability of a Light-Duty Diesel Aftertreatment System

Alkali and alkaline earth metal impurities found in diesel fuels are potential poisons for diesel exhaust catalysts. A set of diesel engine production exhaust systems was aged to 150,000 miles. These exhaust systems included a diesel oxidation catalyst, selective catalytic reduction (SCR) catalyst, and diesel particulate filter (DPF). Four separate exhaust systems were aged, each with a different fuel: ultralow sulfur diesel containing no measureable metals, B20 (a common biodiesel blend) containing sodium, B20 containing potassium, and B20 containing calcium, which were selected to simulate the maximum allowable levels in B100 according to ASTM D6751. Analysis included Federal Test Procedure emissions testing, bench-flow reactor testing of catalyst cores, electron probe microanalysis (EPMA), and measurement of thermo-mechanical properties of the DPFs. EPMA imaging found that the sodium and potassium penetrated into the washcoat, while calcium remained on the surface. Bench-flow reactor experiments were used to measure the standard nitrogen oxide (NOx) conversion, ammonia storage, and ammonia oxidation for each of the aged SCR catalysts. Vehicle emissions tests were conducted with each of the aged catalyst systems using a chassis dynamometer. The vehicle successfully passed the 0.2 gram/mile NOx emission standard with each of the four aged exhaust systems.

Modeling and Measuring Exhaust Backpressure Resulting from Flow Restriction Through an Aftertreatment System

This paper describes the pressure loss characteristics of a variety of substrates (with and without washcoat) that have different cell densities, lengths, and diameters. Both experimental and analytical approaches were used to determine pressure loss characteristics. Engine dynamometer testing was conducted as an experimental approach to measure pressure losses at several different speed and load points. A simple, but comprehensive, analytical model was also developed to estimate pressure loss and equivalent power loss in an exhaust system. The model provides for losses due to the substrate resistance and the inlet/outlet headers. The experimental approach demonstrated that the model was an effective tool to provide assistance during the screening of exhaust system design alternatives.