Modeling knocking combustion in hydrogen assisted compression ignition diesel engines

In the present study, effects of hydrogen induction on combustion characteristics of a compression ignition diesel engine were investigated and a comprehensive model for identifying knocking combustion was developed. This was done by defining number of critical local regions within the CFD (computational fluid dynamics) computational domain for a hydrogen assisted compression ignition engine. Regional parameters such as local pressure rise rate, local heat release rate and local concentration change of specific chemical species were used for knock identification. Comprehensive chemical kinetics mechanisms of diesel and hydrogen fuels were used enabling detailed chemistry predictions. After validation of the model for extensive diesel operating conditions; 1%, 3%, 5% and 7% hydrogen induction in volume in intake air was considered for a single case to investigate knocking combustion. Using the developed knock prediction model, results showed knocking combustion for hydrogen-air premixed charges richer than 5% by volume. This was well captured by the regional pressure rise rate and heat release rate diagrams. Moreover, regional data showed that knock occurred in central parts of the piston bowl and above the piston crown, whereas location near to cylinder wall did not show the same trend. In former locations, very high rate of production and consumption for HO₂ as a free radical was resulted. This was coincided with higher hydrogen consumption and temperature rise.