Gasoline direct injection (GDI) and negative valve overlap (NVO) are common strategies to control combustion characteristics and exhaust gas emissions in homogenous charge compression ignition (HCCI) engines. The combustion and emissions of the GDI-HCCI engines are highly dependent on injection parameters such as timings, pressure, pulse injection, and available oxygen concentrations of the NVO or recompression interval. The injected gasoline into hot trapped residuals in NVO can be pyrolyzed differently into light hydrocarbons due to the oxygen levels. The NVO-reformed species can kinetically or thermally control the main combustion phases, depending on whether the NVO reaction is net endothermic or exothermic.In this experimental investigation, data from a single-cylinder research engine operated under the GDI-HCCI mode are analyzed. The engine is modified from a production-based four-cylinder engine used in a subcompact vehicle. A custom piston is used to reach a compression ratio of 13.0. It is operated at near-unity O2 concentration (phi = 0.95) and is boosted to reach a mid-load condition in which the net indicated mean effective pressure IMEPn = 0.55 MPa. Overall, the engine indicated thermal efficiency reaches 41%. A side-mounted injector is used to inject primary reference fuel with octane number 90 directly into the combustion chamber during the NVO period. The measured results show that the early start of injection (SOI) timings during the NVO advance combustion phase CA50 (crank angle degree at which 50% of the fuel is burnt). Peak recompression pressures are lower than the motoring, emphasizing that the NVO reactions are net endothermic. Therefore, the CA50 are chemically controlled by the NVO-reformed species. Zero-dimensional chemical kinetics calculations with a gasoline surrogate mechanism are conducted to understand the recompression reactions, and the results show that classical reformate species increase almost linearly as a function of SOI timings. Predicted ignition delays are shortened by adding these reformates to the mixture.This work also demonstrates the effects of intake boosting (Pint = 0.12 - 0.14 MPa), injection pressure &nbsp;&nbsp;(Pinj = 5 - 6 MPa), and single versus double pulses injections on the combustion characteristics such as combustion phase CA50, burn duration CA10-90, peak cylinder pressure, maximum pressure rise rate (MPRR), and ITE. Effects of the injection parameters on the emissions are also included. Single-digit NOx emission can be achieved at late intake stroke injection. Under advanced SOI timing, the MPRR is over the engine's constraint limit at 600 kPa/deg. The MPRR and NOx emission can be reduced significantly with double-pulse injection.

Experimental Investigation on the Effects of Direct Fuel Injection Into Low-O2 Recompression Interval of an HCCI Engine

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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