Performance Evaluation of Dedicated EGR on a 12 L Natural Gas Engine 2019-01-1143
Southwest Research Institute (SwRI) converted a Cummins ISX 12 G in-line six-cylinder engine to a Dedicated EGR (D-EGR) configuration. D-EGR is an efficient way to produce reformate and increase the EGR rate. Two of the six cylinders were utilized as the dedicated cylinders. This supplied a nominal EGR rate of 33% compared to the baseline engine utilizing 15-20% EGR. PFI injectors were added to dedicated cylinders to supply the extra fuel required for reformation. A new VNT turbo was sized for the D-EGR configuration with exhaust flow from only four of the cylinders through the turbine. Multiple ignition systems were tested for their effect on improving EGR tolerance including high energy dual coil offset (DCO) ignition, pre-chamber spark plugs and advanced fast-igniters.
The initial results from the D-EGR conversion show a 4% relative BTE improvement compared to the baseline due to the increased EGR rate at 1270 rpm, 16 bar BMEP. At this test point the DCO system was able to operate the dedicated cylinders up to an equivalence ratio of 1.4. At 1836 rpm, 15.5 bar BMEP the efficiency of the D-EGR engine without dedicated cylinder enrichment was the same as the baseline. As the dedicated cylinders were enriched from stochiometric to an equivalence ratio of 1.05 the efficiency improved from 35 to 35.5%. Without dedicated cylinder enrichment the ignition timing was at the limit of being over-advanced. The enrichment allowed the ignition timing to be retarded and the combustion phasing to be advanced toward MBT. At low loads the main cylinders become more stable as the dedicated cylinder equivalence ratio was increased from improved 0-2% burn duration. The dedicated cylinders were the first to become unstable due to the rich limit.