Numerical investigation of methanol ignition sequence in an optical PPC engine with multiple injection strategies

Methanol is a genuine candidate on the alternative fuel market for internal combustion engines, especially within the heavy-duty transportation sector. Partially premixed combustion (PPC) engine concept, known for its high efficiency and low emission rates, can be promoted further with methanol fuel due to its unique thermo-physical properties. The low stoichiometric air to fuel ratio allows to utilize late injection timings, which reduces the wall-wetting effects, and thus can lead to less unburned hydrocarbons. Moreover, combustion of methanol as an alcohol fuel, is free from soot emissions, which allows to extend the operation range of the engine. However, due to the high latent heat of vaporization, the ignition event requires a high inlet temperature to achieve ignition event. In this paper LES simulations together with experimental measurements on an heavy-duty optical engine are used to study methanol PPC engine. After a successful calibration of the pressure trace in terms of required intake temperature and combustion model, the optical natural luminosity data is used to validate prediction of ignition kernel and vapor penetration length. Moreover, it is shown that the inlet temperature requirement is reduced by 47 K degrees when applying multiple injection strategy. Changing the injection strategy also affects the average temperature of combustion and thus the emissions rates. Additionally, an ignition sequence analysis is performed to identify the mode of combustion and the heat release (HR) distribution depending on the local equivalence ratio, recognizing characteristics of PPC regime. Based on this analysis, a conceptual heat distribution model for PPC engine and other low temperature combustion (LTC) engine concepts is proposed.