A Study of Low Temperature Plasma-Assisted Gasoline HCCI Combustion

摘要

In this study low temperature plasma technology was applied to expand auto-ignition operation region and control auto-ignition phasing of the homogeneous charge compression ignition (HCCI) combustion. The low temperature plasma igniter of a barrier discharge model (barrier discharge igniter (BDI)) with high-frequency voltage (15 kHz) was provided at the top center of the combustion chamber, and the auto-ignition characteristics of the HCCI combustion by the low temperature plasma assistance was investigated by using a single-cylinder gasoline engine. HCCI combustion with compression ratio of 15:1 was achieved by increasing the intake air temperature. The lean air-fuel (A/F) ratio limit and visualized auto-ignition combustion process on baseline HCCI without discharge assistance, spark-assisted HCCI, and BDI-assisted HCCI were compared. BDI assist in the intake stroke improved the HCCI ignitability, thereby expanding the region of stable HCCI operation on the lean mixture side and on the low intake air temperature side. The effect of the BDI assist on improvement of HCCI ignitability was substantially greater than a spark assist. The HCCI combustion process was clearly different between spark assist and BDI assist according to the visualized combustion images. The BDI discharge onset timing at -315 deg. ATDC in the intake stroke was optimum in terms of promoting HCCI auto-ignition regardless of the engine operating conditions. In case of BDI discharge timing of -315 deg. ATDC, BDI electrode was filled with large volume of gas during the discharge period, so that the overall mass of hypothetical active species (O_3) produced by BDI increased and also the active species were diffused in the combustion chamber before HCCI combustion initiation. From the aspect of a chemical kinetic simulation of the engine combustion cycle, ozone formed in the cylinder during intake stroke decomposed in the middle of compression stroke to produce O radicals in the mixture, and then low temperature oxidation reactions began from a lower temperature, which presumably advanced the hot ignition timing to an earlier phase.