System Simulation of Partially Premixed Combustion in Heavy-Duty Engines : Gas Exchange, Fuels and In-cylinder Analysis

Sammanfattning: The concept of partially premixed combustion (PPC), applied to conventional diesel engines, has shown to yield high gross efficiencies and low emissions of oxides of nitrogen and soot. PPC emerged from the knowledge gained from homogeneous charge compression ignition (HCCI) research. To extend the load range and thus reduce cylinder pressure rise rates, the fuel is directly injected during the compression stroke, instead of in the intake port as is common with HCCI. In contrast to conventional diesel combustion, there is a separation between the fuel injection event and the start of combustion in PPC. Furthermore, the PPC concept relies on a high degree of dilution with exhaust gas recirculation (EGR) and air. This dilution and premixedness lead to a lower global temperature, which reduces NOx emissions and wall heat transfer which therefore results in a high thermodynamic efficiency. However, a high level of dilution reduces the exhaust temperature and thus leads to a lower gas exchange efficiency because the turbine needs to compensate by generating a higher exhaust back pressure. This thesis therefore focuses on expanding the system boundary of PPC, to facilitate a commercial application. This has been conducted in several studies which targeted the brake efficiency, instead of the gross. The work was conducted with a combination of engine modeling and simulations. Moreover, the in-cylinder combustion was taken from experimental measurements or predicted using a stochastic reactor model. The first part of the work investigated the influence of dilution on the gas exchange performance. The gas exchange efficiency was seen to decrease exponentially at high levels of dilution. In addition, a low inlet temperature led to an increase in both brake and gross efficiencies. Furthermore, an evaluation of turbocharger configurations revealed that, although a two-stage turbocharger only negligibly increased the brake efficiency, it enabled a substantially higher engine load than the two single-stage turbochargers. Finally, the gas exchange efficiency was increased with 4 %pt. by using a combined low and high-pressure EGR system. The second part focused on optimizing the engine boundary conditions, choice of fuels, and injection strategy. The results showed that by using methanol, an increase in brake efficiency of 2.2 %pt. was possible compared to gasoline. The reason was a higher gross efficiency which resulted from an improved compromise between combustion duration, heat transfer, and NOx emissions, as well as lower compression work and favorable ratio of specific heats. Increasing the engine's compression ratio, facilitated a lower inlet temperature with methanol and this led to a 1.4 %pt. further increase in brake efficiency.