Effects of direct water injection timings on knock suppression, combustion, and emission performance of high compression ratio hydrogen-enriched natural gas engine

Abstract

In this study, an approach of direct water injection was applied to suppress knock in the high compression ratio (CR) hydrogen-enriched natural gas (NG) engine. A CFD model for the high CR hydrogen-enriched NG engine was established and its effectiveness was verified by experimental data. On this basis, effects of water injection (WI) timings on knock mitigation, combustion, and emission features of the hydrogen-enriched NG engine were studied in detail. Results show that due to the slower flame propagation speed, monitoring points No.3 and 4 (the area near the intake valve) is more likely to knock accompanied by the higher flow velocity and turbulent kinetic energy. Besides, with the advance of WI timing, KI (knock intensity) keeps nearly unchanged firstly and then has a little increase after the second phase of reduction, whose minimum occurs at −150 °CA. Meanwhile, IMEP, CA10-90, and indicated thermal efficiency (ITE) have a similar change tendency with the maximum at −150 °CA, and ITE reaches its peak of about 40.9% with an increment of 3%. From −30 °CA to −180 °CA, NOx decreases a lot, while HC and CO emissions increase to some extent. Therefore, proper WI timing can not only effectively reduce KI and NOx emission, but also improve thermal efficiency and realize high CR in spark-ignition NG engines further.