DIRECT NUMERICAL SIMULATIONS OF IGNITION OF HYDROCARBON FUEL/AIR MIXTURES UNDER HCCI/SCCI/RCCI CONDITIONS

Abstract

This study investigates the ignition characteristics of various hydrocarbon fuel/air mixtures under homogeneous charge compression ignition (HCCI) conditions using direct numerical simulations (DNS). The ignition processes of various types of HCCI combustion including HCCI with thermal stratification (TS), stratified-charge compression ignition (SCCI), and reactivity-controlled compression ignition (RCCI), and direct dual fuel stratification (DDFS) are numerically studied. HCCI engines are designed to operate under low-temperature conditions by utilizing ultra-lean, highly-diluted, well-mixed fuel/air mixture like SI engines while relying on compression ignition by an elevated compression ratio like diesel engines. Accordingly, HCCI combustion can provide high diesel-like thermal efficiency while avoiding excessive NOx and particulate emissions. Therefore, prototypes of the HCCI combustion engines have been developed as an alternative to conventional gasoline and diesel engines. However, several key issues still remains unresolved in the development of HCCI combustion engines: for instance, how to control the ignition timing and how to mitigate excessive heat release rate (HRR) under a wide range of load conditions. Therefore, several variants of HCCI combustion are developed to overcome these issues.

With the help of high-fidelity DNSs, fundamental understanding of the combustion modes, flame speeds, turbulence-chemistry interactions, key species and controlling reactions of the variants of HCCI combustion can be obtained. The main objectives of the study are (1) to provide more insights into the effect of thermal and compositional stratification levels under different temperature regimes, and fuel compositions on the ignition mechanism of HCCI combustion; (2) to investigate the relative effect of T/phi/PRF stratifications coupled with the turbulence effect on the ignition process and combustion mode of HCCI/SCCI/RCCI using several different fuels including primary reference fuels (PRFs), n-heptane, and biodiesel; (3) to identify the key species and critical reactions of the SCCI and RCCI combustion using chemical explosive mode analysis (CEMA); and finally (4) to elucidate the effect of the late-direct-injection timing on the DDFS combustion process by developing a pseudo-\ioctane\ model. The results of this study can aid in the development of the next-generation high-efficiency IC engines.

By systematically investigating the effects of T, phi and PRF inhomogeneities and their relative roles on the HCCI combustion process at the low-, intermediate-, and high-temperature chemistry regimes, the generalization of their effects on the HCCI combustion is made. It was found that the effect of thermal and compositional stratifications on HCCI combustion, in general, depends on the initial mean temperature, T0, of the fuel/air mixture. TS is most effective at the high-temperature chemistry (HTC) regime regardless of fuel types (i.e. both single- and two-stage ignition fuels). Similar to the single-stage ignition fuels, with T0 lying within the HTC regime in which only the HTC governs the ignition, the mean HRR of the two-stage ignition fuels is more distributed over time, and its peak HRR is more reduced with increasing T'. On the contrary, for the two-stage ignition fuels with T0 lying within the LTC and ITC regime, phi and PRF stratifications play dominant roles in enhancing deflagration mode, and thereby spreading out HRR and reducing the peak HRR. These results suggest that (1) TS is most suitable for single-stage ignition fuels for tailoring the rate and timing of the overall heat release of HCCI combustion (2) while SCCI and RCCI combustion concepts work better if they are operating within/near the negative temperature coefficient (NTC) regime because the ignition delays are more sensitive to phi and PRF than temperature in this regime. It is also found that high turbulence intensity with short-time scale is more likely to homogenize thermally and compositionally stratified mixtures such that the overall combustion becomes similar to the 0-D ignition with excessive HRR.

CEMA shows that at the first ignition delay, the low-temperature chemistry (LTC) represented by the isomerization of RO2 chain branching reactions of KOOH, and H-atom abstraction of $n$-heptane is predominant for both RCCI and SCCI combustion. Temperature is identified to be the predominant factor, and HTC represented by H + O2 -> O + OH is responsible for the thermal ignition. At deflagrations, temperature, CO, and OH are the most important variables while the conversion reaction of CO to CO2, and high-temperature chain branching reaction of H + O2 -> O + OH are identified to be important.

Finally, a novel pseudo-iso-octane model is developed, which has the capability to reproduce the timing, duration, and cooling effect of the late direct injection timing, tinj. The PC8H18 model was then adopted to explore the effect of DI timing on the DDFS combustion. It is found that regardless of tinj, the DDFS combustion has much lower peak HRR and longer combustion duration than the RCCI combustion. This is primarily attributed to the sequential injection of iso-octane. The combustion phasing of the DDFS combustion exhibits a non-monotonic behavior with increasing tinj due to the different effect of fuel evaporation on the low-, intermediate-, and high-temperature chemistry of the PRF oxidation.