Abstract
The conversion of existing heavy-duty diesel engines to lean natural-gas (NG) spark ignition can be achieved by replacing the diesel injector with a spark plug and fumigating the NG into the intake manifold. While the original fast-burn diesel chamber will offset the lower NG flame speed, it will result in a two-stage combustion process (a stage inside and another outside the bowl). However, experimental data at more advanced spark timing, equivalence ratio of 0.8, and mean piston speed of 6.5 m/s suggested an additional combustion stage (i.e., three combustion stages). A three-dimensional (3D) computational fluid dynamics (CFD) simulation and a zero-dimensional triple Wiebe-function model were used to better understand the phenomena. While 78% fuel burned inside the bowl, burning rate reduced significantly when the flame approached the squish entrance and the bowl bottom. Moreover, the triple Wiebe-function indicated that the burn inside the squish was also divided into two separate combustion stages, due to the particularities of in-cylinder flow before and after top dead center. The first stage was fast and took place inside the compression stroke. The second took place in the expansion stroke and produced a short-lived increase in the burning rate, probably due to the increasing squish height during the expansion stroke and the increased combustion-induced turbulence, hence the third heat-release peak. Overall, these findings support the need for further investigations of combustion characteristics in such converted engines, to benefit their efficiency and emissions.
Introduction of innovative technologies in friction units of heavy-duty tribosystems and monitoring of their condition
