Simulation Of Low-temperature Autoignition Mechanism In A Homogeneous Charge Compression Ignition Engine Fuelled With N-butanol-gasoline Blends

Homogeneous charge compression ignition (HCCI) combustion is effective in thefuel economy improvement and NOXemission reduction of conventional gasolineengines. Bio-butanol, known as n-butanol, is a renewable alternative fuel of engines.The combination of n-butanol and HCCI combustion mode can effectively reduce theconsumption of fossil energy. The effect of valve timings/lifts and engine speeds onthe low-temperature chemical reaction kinetics of n-butanol-gasoline blends used on aHCCI engine was simulated. The research tasks and results are as follows:Simplified chemical kinetics of n-butanol used on the HCCI engine werepresented by the sensitivity analysis and reaction rate analysis with the CHEMKINsoftware. On this basis, a simplified chemical reaction kinetics mechanism ofn-butanol-gasoline surrogates was built to simulate the autoignition of the HCCIengine. The simulation results about the autoignition timings of the HCCI enginewere validated by experiments.Coupling the CHEMKIN software and the BOOST software, the influence ofengine speeds, exhaust valve closing (EVC) timings, exhaust lift, formaldehydeconcentration in the residual gases and the magnitude of intake pressure on theautoignition characteristics of the HCCI engine fuelled with n-butanol-gasolinesurrogates were simulated. The results show that,The dehydrogenation reaction of gasoline surrogates and n-butanol throughhydroxyl (OH) radicals is the mainly consumed pathways of the fuels. At lowtemperatures, the pathways of dehydrogenation reaction of n-butanol and the amountof OH radicals generated are more than those of gasoline surrogates, which is theprimary reason why the autoignition timings of the HCCI engine advances when thecontent of n-butanol in blends is increased.Exhaust valve lifts and exhaust valve closing timings mainly affect theautoignition timings of the HCCI engine by the change in the residual gas fraction andactivity of fuels at low temperatures. When engine speeds, intake valve timings andthe exhaust valve closing timings are unchanged, the amount of residual gases in thecylinder decreases with increasing exhaust lifts, lowering its heating effect on freshcharge, so that the autoignition timings of the HCCI engine retard. When intake and exhaust valve lifts and intake valve timings keep constant at given engine speeds, withdelayed exhaust closing timings, the dilution effect of residual gases on the mixture inthe cylinder weakens, which promotes the initial reactions of fuels. As a result, theautoignition timings of the HCCI engine advances.Formaldehyde in residual gases promotes the oxidation of fuels, so that theautoignition timings of the HCCI engine advances with formaldehyde concentrationin the residual gases.In the cases of fixed intake and exhaust valves timings and lifts, there iscompeting effect between the heating effect and dilution effect of residual gases onfresh charge. At low engine speeds, heating effect plays a dominant role in theautoignition and thus the HCCI engine can operate. Once engine speed is beyond acertain value, the dilution effect of residual gases dominates, and the engine can’tachieve HCCI combustion. This is the main reason why the maximum speed of theHCCI engine is limited. However, the critical engine speed under HCCI combustionmode for n-butanol is higher than that for gasoline. The increase of intake pressurecan raise the engine speed operating in HCCI combustion mode within a certainrange.