Combustion in homogeneous charge compression ignition engines : experimental analysis using ethyl esters and development of a method to include detailed chemistry in numerical simulations

Contino, Francesco [UCL]

(eng) In the global energy context, many solutions are suggested to change our energy dependence and solve our CO2 emission issues. For transportation, biofuels are mentioned as a possible alternative to fossil fuels. Yet many people argue that they could hardly replace oil. The low efficiency of the conversion from biomass to biofuels implies other main issues like land use and food competition. More simple conversion routes, using less energy-consuming steps, could be used. However, the major drawback is then the lack of compatibility with conventional engines. Instead of adapting the fuel to the engines, the opposite has been put forward recently with the homogeneous charge compression ignition (HCCI). This engine does not intrinsically rely on specific fuel properties. Given that a fairly homogeneous air-fuel mixture is prepared, it can be operated on a large range of fuels. In the first part of this thesis, we analyzed the esters produced from fermentable wastes by a biochemical process. We investigated experimentally the impact of these esters on the HCCI. In practice, the development of new engine concepts and the characterization of fuels rely more and more on numerical simulations. However, the accurate and comprehensive modelling of the highly nonlinear combustion processes of realistic fuels is extremely demanding. It requires detailed mechanisms that involve hundreds of chemical species and thousands of elementary reactions. The computational cost of these simulations is generally prohibitive. This implies, first, that fairly detailed CFD simulations can only include global mechanisms, which only describe the main steps of the combustion; and second, that very detailed reaction mechanisms are mainly limited to simplified systems. In the second part of this thesis, we developed a new method that significantly mitigates the computational burden of detailed chemistry in complex CFD simulations. The developed numerical method, named tabulation of dynamic adaptive chemistry (TDAC), speeds the CFD simulations up to 900 times with a very small simulation error compared to the direct numerical integration of the combustion mechanisms.