Effects of EGR and boosting on the auto-ignition characteristics of HCCI combustion fueled with natural gas

- Natural gas HCCI auto-ignition was analyzed under EGR and boosting using a contribution matrix.
- EGR and boosting increase the rates of H2-O2 system reactions, but result in H2O2 loop decreases.
- The optimum combination of boosting and EGR extends the operation zone without knocking.

In this study we computationally investigate the effects of exhaust gas recirculation (EGR) and boosting to clarify the auto-ignition mechanisms of homogeneous charge compression ignition (HCCI) in natural-gas-fueled engines. CHEMKIN-PRO is used to perform the thermodynamic and chemical kinetics analysis. To set the boundaries of the operating range, intake air pressures are varied from naturally aspirated operation to boost pressure (0.3 MPa) and EGR ratios are varied from 0% to 50% at an equivalence ratio of φ = 0.4, intake air temperature of Tin = 430 K, and engine speed of 1200 rpm. Natural gas has a single stage heat release, which results in a high temperature heat release (HTHR). The HTHR is composed of two parts, the thermal ignition preparation and thermal ignition range, in which fuel series reactions, H2O2 loop reactions, and H2-O2 system reactions occur. The reaction paths and contribution ratios that occur within the transient temperature can be described by a contribution matrix. We found that when the EGR ratio is increased, auto-ignition is retarded with a longer duration of combustion. The absolute heat release rate (HRR), forward and reverse reaction rates of reactions, and transient temperature during the thermal ignition stage are decreased, as are misfires. When the intake air pressure is increased, the rates of important reactions also increase rapidly. The auto-ignition points are advanced by a crank angle degree (CAD) of 6°. In the combined case of 50% EGR and 0.2 MPa boosting, all reactions are sped up and the absolute heat release rate (HRR) is higher than in the naturally-aspirated (NA) condition. However, R1 is extremely sensitive to intake air pressure. In the case of NA (no EGR and 0.1 MPa), auto-ignition occurred at a higher temperature (approximately 40 K). The absolute HRR decreased while increasing the boost and EGR ratio, and the forward and reverse reaction rates of most reactions increased.