Experimental And Numerical Simulation Study On The Effect Of Fuel Additives On The Ignition For HCCI Engine

Homogeneous charge compression ignition (HCCI) has been attracting growing attention due to its high efficiency and low NOX and particulates emissions. However, it is very difficult to control the ignition timing and extend the operation range of HCCI engine.First, the HCCI combustion characteristics and the influence of additives on the HCCI engine ignition were researched on a 4-cylinder diesel engine. One of the cylinders was modified for HCCI combustion with additives. The experimental results show that under the steady operating conditions, the ignition timing of the base fuel (PRF90-vol.90% isooctane +10% n-heptane) without additives delays and the peak value of the heat release rate is low compared with the base fuel with additive (DTBP) which advances ignition timing and increases the peak of the heat release rate obviously.Furthermore, to study the HCCI combustion process, a numerical simulation study on effective of additive (DTBP) on the ignition for HCCI engines was performed by using single-zone model coupled with a detailed kinetic model. The results show that the base fuel with additives will improve the HCCI combustion remarkably. The ignition timing advances with the increase of DTBP mass fraction. However, excessive additive addition is not good for engines to operate normally.The HCCI combustion presents two-stage combustion process, it is consist of low temperature reaction (LTR) and high temperature reaction (HTR). Two times of O₂ addition reactions of species produced by H-atom abstraction from hydrocarbon fuel are very important for the LTR stage. The main product in the LTR stage is H₂O₂, and the HTR is controlled by H₂O₂ decomposition, which is also the main source of OH. DTBP is known to undergo rapid thermal decomposition at low temperature. The CH₃, which is decomposed by DTBP, reacts with oxygen to produce OH, and then the OH promotes the fuel decomposition. Therefore, the CH3 play a dominant role in the LTR.In addition, In order to achieve the best performance at low engine load, the mass fraction of DTBP additive in PRF90 should be increased with engine speed. Meanwhile, the HCCI combustion characteristics were also quite sensitive to the changes of engine operating parameters. Increasing intake temperature, intake pressure and compression ratio cause advanced ignition while raising engine speed will delay the ignition. Accordingly, for the limit of engine speed, it will be better to operate at low speed.