Cooled Versus Uncooled Exhaust Gas Recirculation in
Inclusive Dates: 05/21/2001 - 09/21/2001
Background - The U.S. Environmental Protection Agency has promulgated engine and vehicle emissions standards that are very stringent, starting in the year 2002, and becoming even more stringent for 2007. These emissions standards will most likely require dramatic changes in diesel combustion. This research project is an experimental investigation of the effects of exhaust gas recirculation (EGR) temperature, main injection timing, and partial premixed charge compression ignition (PCCI) combustion on emissions and combustion in a heavy-duty diesel engine.
Approach - A Caterpillar CAT C-12, on-highway, heavy-duty diesel engine was used for the experiments. The experiments were conducted over the AVL eight-mode test set points. The engine was fitted with a high-pressure EGR loop with and without EGR cooling. The EGR rates were set to result in a weighted brake specific oxides of nitrogen (BSNOx) emissions over the eight-mode test of 2 grams per horsepower-hour (g/hp-hr), with cooled EGR. When cooled EGR was used, the EGR temperature set point was 150 °F. The partial premixed charge was obtained using air-assisted port fuel injection (PFI). The port injectors were automotive fuel injectors installed in atomizer caps previously designed at SwRI and adapted to this particular application. The fuel used for port injection was the same ultralow sulfur, number 2 diesel fuel (4 parts per million sulfur) used for the main in-cylinder direct injection. The amount of port-injected fuel was 0, 10, and 20 percent of the total fueling rate for the engine at each operating condition. A baseline main injection timing map was established for the eight-mode set points. The baseline timing map was then retarded and advanced by 3° CA to assess the main injection timing effect.
Accomplishments - Compared to the stock engine (without EGR) at the baseline timing map, the addition of EGR resulted in a 32- and 38-percent reduction of the weighted BSNOx for uncooled and cooled EGR, respectively. The reductions (compared to the same baseline) obtained by retarding the injection timing by 3° CA were 43 and 50 percent, respectively. The brake specific unburned hydrocarbons (BSHC), carbon monoxide (BSCO), and particulate matter (BSPM) emissions and the brake specific fuel consumption (BSFC) showed modest increases compared to the stock engine (without EGR) at the baseline timing map. As expected, cooled EGR and retarded injection timing resulted in the lowest BSNOx emissions over the eight-mode test (2.09 g/hp-hr).
When port injection was added, PCCI combustion resulted in further NOx reductions over the eight-mode. The lowest weighted BSNOx achieved was 1.90 g/hp-hr and corresponded to the retarded main injection timing map, using cooled EGR and 20 percent PFI. This value represents a 54-percent reduction compared to the stock engine (without EGR) at the baseline timing map. BSHC and BSCO emissions increased by factors of eight and ten, respectively. Assuming that the stock engine was at 0.1 g/hp-hr BSPM, the PM emissions increased by a factor of four. BSFC also increased by 7.7 percent. In general, BSHC, BSCO, and BSPM emissions and BSFC increased dramatically with the amount of port-injected fuel.
With uncooled EGR using the retarded timing and 10-percent PFI, the lowest BSNOx achieved was 2.29 g/hp-hr (which represents a 45-percent reduction compared to the stock engine without EGR, and the baseline timing map). The increases in BSHC, BSCO, BSPM, and BSFC were consistent with the trends observed with cooled EGR and PFI.