Fuel-reforming effects on a gasoline direct injection engine under a low-temperature combustion mode: Experimental and kinetics analyses

Dedicated in-cylinder reforming where gasoline fuel is injected directly into a rich mixture can control the combustion of gasoline engines. This work investigates the effects of injection parameters and in-cylinder fuel reforming on a gasoline direction injection (GDI) engine operated under low-temperature combustion mode. The single-cylinder research engine is modified from a dedicated 4-cylinder spark-ignited engine used in subcompact passenger vehicles. Experimental and kinetics results on direct fuel injection into an oxygen-depleted environment of the engine's recompression interval are analyzed under 0.53 MPa indicated mean effective pressure and 1500 RPM. Zero-dimensional kinetics simulations show that the recompression reactions are net endothermic under near-stoichiometric environments. Substantial short-chain hydrocarbons (CH₄, C₂H₄, C₃H₆) are reformed, while CO and H₂ are small under the O₂-depleted environment. These kinetics reformates strongly depend on the start of injection timing (SOI). Consequently, the kinetics species shorten predicted ignition delays and play a significant role in advanced combustion phasing. Validated 3D-CFD results further indicate the effects of these reformates on advanced combustion. Experimentally, SOI, injection pressure P_inj = 5 and 6 MPa, single/double pulses injection, and mixture equivalence ratios ϕ are varied in the recompression and intake strokes. Underϕ = 0.97, excessive ringing intensities (RI) over 5 MW/m² are observed under advanced SOIs. The fierce combustion, excessive combustion noise metrics, NO_x emissions can be reduced significantly by retarding SOI, double-pulse injection, and ϕ variations. Under ϕ < 0.67, measured NOx emissions are below 10 ppm. For all pulse-injection strategies, the peak recompression pressures are dropped by up to 13% lower than 2.0 MPa of the motoring pressure, indicating that the net endothermic reactions occur during the recompression. Therefore, the reformed products chemically control the main combustion, and 41 % indicated thermal efficiency is achievable.