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Increasing the internal combustion engine efficiency is necessary to decrease their environmental impact. Several combustion systems demonstrated the interest of low temperature combustion to move toward this objective. However, to ensure a stable combustion, the use of additives has been considered in a several studies. Amongst them, 2-Ethylhexyl nitrate (EHN) is considered as a good candidate for these systems but characterizing its chemical effect is required to optimize its use. In this study, its promoting effect (0.1 - 1% mol.) on combustion has been investigated experimentally and numerically in order to better characterize its behavior under different thermodynamic and mixture. Rapid compression machine (RCM) experiments were carried out at equivalence ratio 0.5 and pressure 10 bar, from 675 to 995 K. The targeted surrogate fuel is a mixture of toluene and n-heptane in order to capture the additive effect on both cool flame and main ignition.

In Spark Ignition engines, the heat release rate is not only piloted by the mixture reactivity but also by its sensitivity to stretch effects. Only few results can be found in the literature about flame stretch effect in SI engine configurations. For this study, three different fuels (Methane, Propane, Iso-octane) were studied, but at different air-fuel lean mixture conditions, to present almost equivalent laminar flame speeds and thermo-dynamical properties at ignition timing condition. Besides those mixtures present different Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography was then performed in an optical Spark Ignition (S.I.) engine. Using a high speed camera, flame propagation images were acquired through the piston. Thermodynamic analyses based on in-cylinder pressure traces were performed to estimate in-cylinder temperature and burnt mass fraction during the engine cycle.

Plasma sustained ignition systems are promising alternatives to conventional spark plugs for those applications where the conditions inside the combustion chamber are more severe for spark plug operation, like internal combustion engines with high compression ratio values and with intake charge dilution. This paper shows the results of an experimental activity performed on a spark ignition engine equipped alternatively with a conventional spark plug and a radio frequency sustained plasma ignition system (RFSI). Results showed that RFSI improved engine efficiency, extended the lean limit of combustion and reduced cycle-by-cycle variability, compared with the conventional spark plug at all test conditions. The adoption of the RFSI also had a positive impact on carbon monoxide and unburned hydrocarbon emissions, whereas nitrogen oxide emissions increased due to higher temperatures attained in the combustion chamber.

The current decrease in fossil energy resources requires a diversification of the liquid and gaseous fuels potentially consumable in internal combustion engines. The use of these fuels modifies the combustion process and the heat released as well. In a Spark Ignition (SI) engine, the heat released is mainly piloted not only by the mixture reactivity but also by its sensitivity to stretch effects. Only a few results can be found in the literature about stretch effects for SI engine configurations. The purpose of the present paper is to evaluate stretch effects on the flame front propagation in an optical SI engine and to investigate the relative importance of these effects depending on the fuel considered. Different air-fuel mixtures presenting different flame stretch sensitivities were selected. Four different engine regimes (1400, 1600, 1800 and 2000 rpm) were studied for all the mixtures in order to evaluate the impact of different turbulence intensities.