Numerical Studies of Methanol PPC Engines and Diesel Sprays

Sammanfattning: The global environment suffers from utilizing fossil fuels to powering internal combustion engines (ICE), due to the massive amounts of released CO2. Besides the global impact, the local environments experience high concentrations of harmful pollutants such as NOx, CO, soot and particulate matter (PM). The automotive industry is continuously striving to find new solutions to decrease fuel consumption and also to develop cleaner and more advanced combustion systems, i.e., low-temperature combustion (LTC) engines.The goal of this thesis is to employ computational fluid dynamics (CFD) simulations to investigate methanol under the partially premixed combustion (PPC) regime, which is one of the advanced LTC concepts alongside HCCI and RCCI. The benefit of PPC engines is the reduced average combustion temperature, which results in optimized emission rates of UHC/CO and NOx, maintaining high thermal efficiency. Interesting properties of methanol, such as a low stoichiometric air to fuel ratio and high latent heat of vaporization as well as non-sooting combustion, may enable further improvement of the PPC concept. Studies have been carried out by employing RANS and LES models to simulate mixing and ignition processes. It was found that methanol PPC can be achieved at relatively later injection timings (similar to those in diesel engines), in comparison to gasoline. Late injection timings can ease injection targeting into the piston bowl and utilize strong wall-spray interaction to help control the in-cylinder flow and therefore reduce the wall heat losses. The well-stirred-reactor (WSR) approach fails to predict pressure traces at highly stratified mixture compositions, such as $0.3 < \phi < 2.5$. Instead, the partially-stirred-reactor (PaSR) model, after model constant calibration, was employed to improve the prediction of combustion behavior. The ignition kernel of methanol starts in the fuel leanest mixtures, and continues as an ignition front propagation towards the fuel rich mixtures, consuming the remaining fuel that has originated from the fuel rich side, in the diffusion flame mode. In the second part of the thesis, the focus is set on the diesel spray - wall interaction. LES is employed to study the air entrainment mechanisms, such as flame lift-off length, impingement mixing and entrainment wave, to identify their importance on the soot oxidation process. The free jet and wall impingement jets at 30 mm and 50 mm distance between the nozzle and the impingement wall are considered. The main finding is the non-monotonic mixing enhancement during combustion. The soot formation mixtures ( 1600K2$) can be accumulated in the near-wall region until the impingement vortices are developed, which then accelerates the mixing rate. Both wall jets resulted in more entrained air after the end of injection, which is considered to be the main reason for the faster oxidation of soot, with comparison to the free jet, which is in agreement with experimental measurements of the optical soot thickness KL.