Knock is a major technological constriction of natural gas spark ignition engines. Nowadays, it is widely accepted that knock is due to auto-ignition in the end gas region. Knock can occur for different reasons, which could be related to the engine itself (design and settings) or to the gas composition (or the gas quality). In a previous study the effect of engine settings on knock in a C.F.R. SI engine fuelled by pure methane was established by using a knock indicator, based on the evaluation of the energy of end gases.
The paper deals with knock limit prediction from natural gas quality in a C.F.R. engine. A 2-zone thermodynamic model was developed in order to predict knocking conditions by evaluating a knock indicator.
The model relies on some standard assumptions. Ignition delay was expressed as a function of engine settings, and a physical correlation for the heat release rate model was used. Effect of gas quality is integrated to the correlation of the laminar flame speed by considering an original assumption adopted by the authors “two gases having the same knock tendency (i.e. same gas quality) are expected to have the same combustion behaviour (Laminar flame speed)”. Chemical scheme is the well known GRI-3.0, involving 325 equations and 53 species.
First simulations with pure methane show good agreement with literature. In a second step, a natural gas database is used to evaluate the knock criterion, showing linearity with gas quality. Finally, a preventive technique is described, resulting in a correction of knocking conditions (caused by a low grade gas) through an optimal control of engine settings.