HCCI Engine Combustion Control using EGR : Gas Composition Evolution and Consequences on Combustion Processes 2007-24-0087

Due to the new requirements on exhaust pollutant emissions, studies on new engine processes are currently being conducted. New combustion systems such as homogeneous charge compression ignition (HCCI) appear promising as they have the potential to achieve near zero particulate and NO_x emissions. Nevertheless their main drawback is their unstable ignition start, inducing changes in the operating condition, because ignition is controlled only by the chemical kinetics, this has a limiting effect on the operating range of HCCI engines. Several strategies to improve the combustion initiation have been proposed, such as the adaptation of technologies (modification of the injection strategy or the bowl geometry), use of high EGR (Exhaust Gas Recirculation) rates or changes in the fuel formulation. This paper presents works done in the last two items.

The chemical influence of CO₂, NO_x, H₂O, CO and CH₂O on combustion has been fully described in former studies, but the exact composition of the recirculated exhaust gas before intake is still unknown. To be able to determine this composition precisely, a study on the burned gas in the recirculation circuit was carried out. This required the adaptation of a specifically designed gas trapping system to well identify a large range of species. The trapping and analyses were performed at four locations on the EGR circuit (from exhaust to plenum) to obtain a global and precise vision of the gas composition throughout the whole process. Knowing exactly these compositions, enables the determination of the relationship between EGR composition and the fuel formulation, or EGR readmitted species and the combustion control.

Several operating conditions were investigated with various fuels. The results showed that the gas composition of EGR changes before admission, so an evolution profile could be determined for each species. Along the EGR pipe, a decrease in unburned HC (including aldehydes, ketones and light PAH (Polycyclic Aromatic Hydrocarbons)) was observed, and also an increase in CH₄, heavy PAH, NO_x and CO concentrations. This study also determined the exact composition of admitted gases (fresh gas mixed with recycled gas) through the knowledge of gas evolution due to deposition, high temperature gradient from heat exchangers and chemical reactions and consequently determined the possible impact on combustion.