The low- and high-temperature oxidation mechanisms of n-heptane have been extensively studied in recent and past literature because of its importance as a primary reference fuel. Recent advanced analytical methods allowed for the identification of several intermediate oxygenated species at very low-temperature conditions in jet-stirred reactors. On these bases, new classes of successive reactions involving hydroperoxide species, already discussed for propane and n-butane oxidation, were included in the low-temperature oxidation mechanism of n-heptane. These new reactions allowed for the improvement of the overall mechanism, not only obtaining a satisfactorily agreement with reaction products, such as organic acids, diones, and ketones, but also in terms of system reactivity. Moreover, deeper attention was also paid to the formation of ketohydroperoxides, rarely experimentally measured. Because of n-heptane importance as a primary reference fuel, the overall POLIMI kinetic mechanism is validated in a wide range of conditions, in both the high- and low-temperature regimes. Moreover, the reliability of the updated oxidation mechanism is further proven in a couple of more complex applications, such as the autoignition of nheptane droplets in microgravity conditions and the oxidation of lean n-heptane/air mixtures in homogeneous charge compression ignition (HCCI) engines

This work introduces a new predictive multi-zone model for the description of combustion in Homogeneous Charge Compression Ignition (HCCI) engines. The model exploits the existing OpenSMOKE++ computational suite to handle detailed kinetic mechanisms, providing reliable predictions of the in-cylinder auto-ignition processes. All the elements with a significant impact on the combustion performances and emissions, like turbulence, heat and mass exchanges, crevices, residual burned gases, thermal and feed stratification are taken into account. Compared to other computational approaches, this model improves the description of mixture stratification phenomena by coupling a wall heat transfer model derived from CFD application with a proper turbulence model. Furthermore, the calibration of this multi-zone model requires only three parameters, which can be derived from a non-reactive CFD simulation: these adaptive variables depend only on the engine geometry and remain fixed across a wide range of operating conditions, allowing the prediction of auto-ignition, pressure traces and pollutants. This computational framework enables the use of detail kinetic mechanisms, as well as Rate of Production Analysis (RoPA) and Sensitivity Analysis (SA) to investigate the complex chemistry involved in the auto-ignition and the pollutants formation processes. In the final sections of the paper, these capabilities are demonstrated through the comparison with experimental data

Pursuing a sustainable energy scenario for transportation requires the blending of renewable oxygenated fuels such as alcohols into commercial hydrocarbon fuels. From a chemical kinetic perspective, this requires the accurate description of both hydrocarbon reference fuels (n-heptane, iso-octane, toluene, etc.) and oxygenated fuels chemistry. A recent systematic investigation of linear C2-C5 alcohols ignition in a rapid compression machine at p = 10-30 bar and T = 650-900 K has extended the scarcity of fundamental data at such conditions, allowing for a revision of the low temperature chemistry for alcohol fuels in the POLIMI mechanism. Heavier alcohols such as n-butanol and n-pentanol present ignition characteristic of interest for application in HCCI engines, due to the presence of the hydroxyl moiety reducing their low temperature reactivity compared to the parent linear alkanes (i.e. higher octane number). The promising performances of ethanol in a HCCI engine have been recently discussed by Bissoli et al. (Energy & Fuels, 2017, Submitted), observing wider stable operability conditions in terms of fuel/air load (λ) and exhaust gas recirculation (EGR) extent compared to PRF80 and PRF100. The aim of this study is to present briefly the reliability of the updated POLIMI mechanism for heavier alcohols and to investigate the fundamental role of chemical kinetics on the performance maps of HCCI engines fueled with n-butanol and n-pentanol, in terms of operability limits and engine efficiency.

A new multi-zone model for the simulation of HCCI engine is here presented. The model includes laminar and turbulent diffusion and conduction exchange between the zones and the last improvements on the numerical aspects. Furthermore, a new strategy for the zone discretization is presented, which allows a better description of the near-wall zones. The aim of the work is to provide a fast and reliable model for carrying out chemical analysis with detailed kinetic schemes. A preliminary sensitivity analysis allows to verify that 10 zones are a convenient number for a good compromise between the computational effort and the description accuracy. The multi-zone predictions are then compared with the CFD ones to find the effective turbulence parameters, with the aim to describe the near-wall phenomena, both in a reactive and non-reactive cases. Finally, the model is validated against experimental data of HCCI combustion of fossil and bio-fuels (n-heptane, methyl-hexanoate, and methyl-decanote)

The aim of this paper is to investigate the fundamental role of chemical kinetics on the performance maps of homogeneous charge compression ignition (HCCI) engines in terms of operability limits, engine efficiency, and emissions. The work focuses on a Ricardo E6 engine, highlighting the impact of different fuels (PRF80, PRF100, and ethanol) on ringing, misfire, and partial burn limits, as well as on several performance variables and pollutant emissions. The operability maps are calculated assuming proper criteria to identify the limits of the map in terms of ringing, misfire, and partial burn. Sensitivity analysis and rate of production analysis highlight the role of H2O2 in sustaining the combustion of ethanol at high exhaust gas recirculation (EGR) and air dilution with respect to PRF100 and PRF80 mixtures. The multizone model confirms that thermal stratification and crevices are the main factors responsible for the emissions of CO and unburned species. NOx are produced mainly via a thermal mechanism. Interaction of N2O with H and O radicals also plays a role, while a prompt mechanism does not significantly affect NOx emissions. Ethanol shows greater flexibility, lower pollutant emissions, and wider operability conditions with respect to engines fed with primary reference fuels. The paper highlights the potential of this multizone model in reproducing the engine performance. Nonreacting Computational Fluid Dynamics (CFD) simulations are first used to estimate heat and mass transfer coefficients. Then, the proposed model does not require further empirical or tuning parameters. Only the thresholds defining the operability maps are derived from the experiments and are the same for all the fuels and operating conditions investigated. The extensive comparison with a large set of experimental data shows the capability of the model to describe the effect of fuel composition and EGR the operability map, highlighting how such a tool can play an important role in understanding the chemistry controlling fuel reactivity and pollutant emissions in the different conditions. These information can support not only fuel and engine operation selection, but also their optimal design. As an example, the effects of boost and engine speed on the HCCI combustion are critically investigated, in terms of the extension of the operability region, engine thermal efficiency, and exhaust emissions.