Effect of Start of Injection on the Combustion Characteristics in a Heavy-Duty DICI Engine Running on Methanol

Methanol as an alternative fuel in internal combustion engines has an advantage in decreasing emissions of greenhouse gases and soot. Hence, developing of a high performance internal combustion engine operating with methanol has attracted the attention in industry and academic research community. This paper presents a numerical study of methanol combustion at different start-of-injection (SOI) in a direct injection compression ignition (DICI) engine supported by experimental studies. The aim is to investigate the combustion behavior of methanol with single and double injection at close to top-dead-center (TDC) conditions. The experimental engine is a modified version of a heavy duty D13 Scania engine. URANS simulations are performed for various injection timings with delayed SOI towards TDC, aiming at analyzing the characteristics of partially premixed combustion (PPC). The simulations are based on a relatively detailed chemical kinetic mechanism and a well-stirred reactor (WSR) approach, accelerated using a so-called chemistry coordinate mapping (CCM). The injection of the fuel is treated with Lagrangian Particle Tracking (LPT) method. A baseline case with SOI of -20 after TDC (ATDC) was studied experimentally; this case was chosen to validate the model and a good agreement between the experiments and the simulation is found after adjustment of the initial pressure and temperature condition. In all injection conditions the combustion phasing is kept the same, i.e. with the 50-percentage heat release at the same crank angle (CA50) by adjusting the intake temperature. It is shown that as SOI is delayed the combustion characteristics changes significantly leading to a high maximal pressure-rise-rate (MPRR). The SOIs between -20 and -7 ATDC results in a combustion process governed by auto-ignition with propagating ignition fronts. The MPRR increases with SOI due to the rapid heat release caused by ignition at lean but increasingly richer conditions towards stoichiometry. The diffusion controlled, diesel like combustion (CDC), starts to occur around SOI -3 ATDC. The first portion of injected fuel ignites with a delay at leaner conditions, and then forms a diffusion flame. The amount of fuel consumed in the ignition process is larger than the amount of fuel consumed in the diffusion flame. Thus, contribution to the total heat release from the ignition process is larger and more rapid from that when using diesel or gasoline in the same CDC injection. Such behavior is attributed to a longer ignition delay time, large latent heat value and higher stoichiometric mixture fraction for methanol than hydrocarbon fossil fuels. It is concluded that a single main injection strategy of methanol may not be preferable due to the high MPRR thus other injection strategies, e.g., multiple injections should be used.