Modeling of Chemical Processes in a Diesel Engine With Alcohol Fuels

Abstract

Methanol utilization in a compression ignition engine has held tentative promise for a number of years, and, in fact, the concept has seen large scale field trials intended to demonstrate this option as a precursor to commercial implementation. However, results from those tests have identified some of the practical problems encountered with this fuel, namely, (1) its difficulty of vaporization and (2) its high autoignition temperature. Luminosity promoting additives, which facilitate radiative transport as a component of flame spread (because pure alcohol burns with little luminosity, continuum radiation as a reaction transport mechanism is essentially absent), intake air heating, active and passive heat sources, etc., represent some of the attempts to overcome limitations of these two factors. Except for intake air preheat, these augmentation methods have been noted to result in poor off-load thermal cycle efficiency. Focusing on the case of intake air preheat (which can be achieved by elevated compression ratio), and to model the chemical reaction kinetics, the partially stirred reactor model in CHEMKIN was used. This approach provided examination of the chemistry and reaction rates associated with an actual trial in which methanol was the fuel under study. To initiate this simulation, literature available reaction mechanisms were obtained, and then the experimental cylinder pressure history was matched by control of heat release rate via the partially stirred reactor model. This is represented within the reactor model by changing the turbulent mixing intensity factor. The overall reaction sequence, which models cylinder pressure, and attendant extent of reaction were the major focus. The minor focus included production of emission gases, e.g., the aldehydes and unburned fuel. Not only are the model results consistent with actual findings, they also support a method for addressing causes of off-load inefficiency and engine failures due to engine oil dilution with fuel.