Le Li

Numerical simulation and validation of SI-CAI hybrid combustion in a CAI/HCCI gasoline engine

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In this study, a SI-CAI hybrid combustion model (HCM) has been constructed on the basis of the 3-Zones Extended Coherent Flame Model (ECFM3Z). An ignition model is included to initiate the ECFM3Z calculation and induce the flame propagation. In order to precisely depict the subsequent auto-ignition process of the unburned fuel and air mixture independently after the initiation of flame propagation, the tabulated chemistry concept is adopted to describe the auto-ignition chemistry. The methodology for extracting tabulated parameters from the chemical kinetics calculations is developed so that both cool flame reactions and main auto-ignition combustion can be well captured under a wider range of thermodynamic conditions. The SI-CAI hybrid combustion model (HCM) is then applied in the three-dimensional computational fluid dynamics (3-D CFD) engine simulation. The simulation results are compared with the experimental data obtained from a single cylinder VVA engine. The detailed analysis of the simulations demonstrates that the SI-CAI hybrid combustion process is characterised with the early flame propagation and subsequent multi-site auto-ignition around the main flame front, which is consistent with the optical results reported by other researchers. Besides, the systematic study of the in-cylinder condition reveals the influence mechanism of the early flame propagation on the subsequent auto-ignition.

CHEMICAL EFFECTS OF THE INCOMPLETE-OXIDATION PRODUCTS IN RESIDUAL GAS ON THE GASOLINE HCCI AUTO-IGNITION

Homogeneous charge compression ignition (HCCI) achieved by negative valve overlap (NVO) strategy can realize ignition phasing control through retaining residual gas. Except for the heating and dilution effects of residual gas, the presence of incomplete-oxidation products in residual gas also affects auto-ignition significantly. In order to investigate the compositions of incomplete-oxidation products, a fast instantaneous in-cylinder sampling system was developed. The in-cylinder sample gas was extracted just before the low temperature reaction and analyzed online quantitatively. It was found that the main incomplete-oxidation products in residual gas, which possibly have significant effects on auto-ignition, are short-chain hydrocarbons, aldehydes, and alcohols. Simulation analysis with a detailed chemical kinetic mechanism of gasoline surrogate was used to decouple the effects of these species. It indicated that CH2O, C2H4, and C2H2 play the most important roles in advancing auto-ignition. Also, the accumulation mechanisms of these main active species prior to auto-ignition are analyzed.

Gas Sampling Analysis of A Low-Temperature Compression Ignition Reaction Process in an Engine Fueled with Methanol and Gasoline

With an in-cylinder gas sampling system that was modified from a gasoline direct injection injector, a series of in-cylinder gas sampling experiments were performed in a Ricardo Hydra four-stroke single-cylinder gasoline engine to study the components and concentrations of chemical substances in the hot residual gas by using methanol and gasoline, respectively. And then the effects on low-temperature oxidation process of these chemical substances were analyzed with the experimental data and chemical reaction mechanisms. The experimental results show that there were some chemical substances, such as formaldehyde, acetaldehyde in the trapped hot residual gas achieved by negative valve overlap method when fueled with methanol or gasoline. These components activated the chemical reactions in the cylinder and promoted the low-temperature oxidation of the fuel. The concentration of formaldehyde in the hot residual gas of methanol was significantly higher than that under the same experimental conditions of gasoline because of the easier production process of formaldehyde in methanol according to reaction mechanisms. And the concentration of formaldehyde decreased in the prophase of the compression process and then increased rapidly, which indicated that formaldehyde in the mixture accelerated the oxidation process by participating in the reaction and that is one of the reasons why temperature increased rapidly and ignition timing advanced in the methanol-controlled auto-ignition combustion process. With the increasing residual gas fraction, formaldehyde concentration in the mixture decreases for lower reactant concentration and this affected the ignition timing as well as the heat release rate accompanied by the temperature.