Experimental Investigation of Direct Fuel Injection Into Low-Oxygen Recompression Interval in a Homogenous Charge Compression Ignition Engine

Abstract

This work analyzed measured data from a single-cylinder engine operated under the gasoline direction injection homogenous charge compression ignition (GDI-HCCI) mode. The experiments were conducted at a 0.95 equivalence ratio (ϕ) under indicated mean effective pressure of 0.5 MPa and 1500 rpm. A side-mounted injector delivered primary reference fuel (octane number 90) into the combustion chamber during negative valve overlap (NVO) interval. Advanced combustion phase CA50 were observed as a function of the start of injection (SOI) timings. Under ϕ = 0.95, peak NVO in-cylinder pressures were lower than motoring for single and split injections, emphasizing that NVO reactions were endothermic. Zero-dimensional kinetics calculations showed classical reformate species (C3H6, C2H4, CH4) from the NVO rich mixture increased almost linearly due to SOI timings, while H2 and CO were typically low. These kinetically reformed species also shortened predicted ignition delays. This work also analyzed the effects of intake boosting pressure and single versus double pulses injections on CA50, burn duration CA10-90, peak cylinder pressure, combustion noise metrics, thermal efficiency, and emissions. Advanced SOI (single-injection) generated excessive combustion noise metrics over constraint limits, but the double-pulse injection could significantly reduce the metrics (ringing intensity (RI) ≤ 5 MW/m2, maximum pressure rise rate ≤ 0.6 MPa/CA) and NOx emission. The engine's net indicated thermal efficiency (ITE) reached 41% under GDI-HCCI mode against 36% of SI mode for the same operating conditions. Under GDI-HCCI mode and without spark ignition, late fuel injection in the intake stroke could reduce NOx to a single digit.