Computational Study to Identify Feasible Operating Space for a Mixed Mode Combustion Strategy: A Pathway for PCI High Load Operation
Authors: Kavuri Chaitanya , Kokjohn Sage L. 
Published: November 2017

Mixed mode combustion strategies have shown great potential to achieve high load operation but soot emissions were found to be problematic. A recent study investigating soot emissions in such strategies showed that delaying the load extension injection sufficiently late after the primary heat release makes the soot production dependent solely on the temperature field inside the combustion chamber and eliminates any dependence on mixing time and oxygen availability. The current study focuses on furthering this research to identify a feasible operating space to operate in and enable high load operation with this mixed mode combustion strategy.

A PCI combustion event was achieved using a premixed charge of gasoline (early cycle injection) and a load extension injection of gasoline was added near top dead center. CFD modeling considering polycyclic aromatic hydrocarbon (PAH) chemistry up to pyrene was used to perform a full factorial design of experiments (DOE) to study the effects of premixed fuel fraction (fraction of total fuel that is premixed), load extension injection timing and exhaust gas recirculation (EGR). The early injection timings for EGR rates less than 40% showed a soot-NOx tradeoff which constrained operating with SOI timings before TDC. The late injection timings showed reductions in soot and NOx at the expense of gross indicated efficiency (GIE). GIE increased with increasing premixed fuel until the premixed fuel quantity reached 80% of the total fuel mass. Premixed fuel quantities higher than 80% resulted in an efficiency penalty due to increased wall heat transfer losses resulting from early combustion phasing. However, at premixed fuel quantities close to 80%, the peak pressure rise rate became the dominating constraint. This confined the feasible operating space to a premix fuel mass range of 70% to 80%. For this premix fuel mass range, the feasible operating space had two regions; one in the early SOI regime before TDC at EGR rates higher than 38% and the other in the late SOI regime (SOI > 15° ATDC) across the entire EGR space. The study was repeated by splitting the premixed fuel into an early cycle injection and a stratified injection with SOI timing of −70° ATDC. The ratio of fuel in the two injections was varied in the DOE. The results showed that adding a stratified injection increases the ignition delay due to in-cylinder equivalence ratio stratification and relaxes the pressure rise rate effect on the operating space. This allows operation at high premix fuel quantities of 70% and higher with EGR rates less than 40% which yields significant increase in GIE. It was also identified that by targeting the fuel from the stratified injection into the squish region, there is improved oxygen availability in the bowl for the load extension injection, which results in the reduction of soot emissions. This allows the load extension injection to be brought closer to TDC while meeting the soot constraint, which further improves the GIE. Finally, the results from the study were used to demonstrate high load operation at 20 bar and 1300 rpm.