An experimental and numerical investigation on the influence of external gas recirculation on the HCCI autoignition process in an engine: Thermal, diluting, and chemical effects

Abstract

In order to contribute to the solution of controlling the auto-ignition in a Homogeneous Charge Compression Ignition (HCCI) engine, parameters linked to External Gas Recirculation (EGR) seem to be of particular interest. Experiments performed with EGR present some difficulties in interpreting results using only the diluting and thermal aspect of EGR. Lately, the chemical aspect of EGR is taken more into consideration, because this aspect causes a complex interaction with the dilution and thermal aspects of EGR. This paper studies the influence of EGR on the auto-ignition process and particularly the chemical aspect of EGR.The diluents present in EGR are simulated by N 2 and CO 2 , with dilution factors going from 0 to 46 vol%. For the chemically active species that could be present in EGR, the species CO, The fuels used for the auto-ignition are n-heptane and PRF40. It appeared that CO, in the investigated domain, did not influence the ignition delays, while NO had two different effects.At concentrations up until 45 ppm, NO advanced the ignition delays for the PRF40 and at higher concentrations, the ignition delayed. The influence of NO on the auto-ignition of nheptane seemed to be insignificant, probably due to the higher burn rate of n-heptane. CH 2 O seemed to delay the ignition. The results suggested that especially the formation of OH radicals or their consumption by the chemical additives determine how the reactivity of the auto-ignition changed.

Ozone applied to the homogeneous charge compression ignition engine to control alcohol fuels combustion

“…Therefore, the presence of incomplete combustion products such as carbon monoxide, unburned hydrocarbon and nitrogen oxides play an important role. Results on CO and HC showed no significant effect on the combustion timing while nitric oxide demonstrated a promoting effect [8,38]. Dubreuil et al [39] investigated the nitric oxide effect on reference fuels and observed a promoting effect on both cool and main flames for concentration lower than 100 ppm.…”

A study on combustion control and operating range expansion of gasoline HCCI

“…To alleviate this stated problem, mixture composition should be formed to have different ignition timings depending on its position in the cylinder. Machrafi et al described the effect on ignition timing and heat release rate of NO, CO, CH 2 O in EGR of the previous cycle [2]. Shi et al reported a phenomenon that the peak pressure of a cycle in the cylinder decreases as EGR portion increases [3].…”

Numerical study of EGR effects on reducing the pressure rise rate of HCCI engine combustion

“…The main obstacles in the homogeneous charge compression ignition (HCCI) engine combustion are its timing control and narrow operation range [1][2][3][4][5][6]. Unlike spark ignition (SI) and compression ignition (CI) combustion modes, whose combustion phasing can be controlled by the timing of spark and fuel injection, respectively, HCCI combustion is an auto-ignition process controlled by a chemical reaction, which, in itself, is difficult to handle.…”

“…Moreover, EGR gases have the following important properties for controlling HCCI combustion: ① Its internal energy can be directly used to adjust in-cylinder gas temperature; ② EGR will change the heat capacity ratio of in-cylinder mixtures, thus temperature history will also change (especially, in compression stroke). The dilution of its inert components may also reduce the chemical reaction rate; and ③ There are some active species in EGR gases (especially when the combustion efficiency of the previous cycle is low), which, although small in amount, may have great influence on the combustion process of the next cycle [1,2].…”