System Level 1-D Analysis of an Air-System for a Heavy-Duty Gasoline Compression Ignition Engine 2019-01-0240

A detailed study of various air system configurations has been conducted for a prototype gasoline compression ignition (GCI) engine using a Cummins MY2013 ISX15 heavy-duty diesel engine as the base platform. The study evaluated the configurations with the assumption that RON80 gasoline would be used as the fuel and the combustion chamber would have a geometric compression ratio (CR) of 16.5.

Using 3-D computational fluid dynamics (CFD) simulations, a high efficiency & low engine-out NOx GCI combustion recipe was developed across the five engine operating points from the heavy-duty Supplemental Emissions Test (SET) cycle: A100, B25, B50, B75, and C100. The CFD generated air-thermal boundary conditions and the combustion burn-rate & injector rate-of-injection profiles were imported into a calibrated 1-D engine model for the air-handling systems analysis. For the RON80 GCI concept, an engine-out NOx range 1-1.5 g/kWh was targeted and this drove a need for higher boost pressure and EGR rates with intake temperatures in the 65°C-70°C range. The production air system comprising a single stage turbocharger and a high-pressure exhaust gas recirculation (HPEGR) system was evaluated under these GCI boundary conditions and established as a benchmark for comparison.

To aid in future integration efforts with the base engine, only proven production and/or off-the-shelf single-stage boosting configurations were investigated. For the 1-Stage boosting systems, both variable geometry turbine (VGT) and waste-gated (WG) turbine options were examined. Multiple exhaust gas recirculation (EGR) systems were also considered and these included a dedicated high-pressure EGR system, a dedicated low-pressure EGR (LPEGR) system and a combination of the two with flow split between the HP and LP (DLEGR) systems. For DLEGR, a flow split sweep between the HP and the LP loop was carried out to optimize the pumping losses, separately for each load point. For an optimal compressor-turbine pairing, changes in the EGR strategy mandated variations in trims for the compressor and turbine wheels.

Based on the 1-D system modeling results, all the off-the-shelf 1-Stage boosting options were able to meet the GCI air-system targets, when coupled with an appropriate EGR strategy. Also, the production turbocharger when used with a dual-loop EGR strategy showed improvements in pumping losses, compared to the stock benchmark. Of all the feasible configurations, the best air-system was highlighted on the basis of minimized overall system losses.