Towards Stoichiometric Combustion in HCCI Engines: Effect of Ozone Seeding and Dilution

Homogeneous Charge Compression Ignition (HCCI) is generally considered as an efficient solution to reduce fuel consumption and meet the pollutant requirements of internal combustion engines. Furthermore, the HCCI combustion strategy delivers drastically reduced levels of NOx and particulate matter, and combined with a post treatment device, low levels of unburned hydrocarbons (HC) and carbon monoxide (CO) can be achieved. However, affordable and widely used three-way catalytic converters require the engine to be run under stoichiometric conditions. Running an HCCI engine under an increased equivalence ratio leads to advanced combustion phasing and an excessive in-cylinder pressure rate that can affect engine operation. The dilution effect of Exhaust Gas Recirculation (EGR) represents a way to delay ignition of the mixture and reduce excessive in-cylinder pressure gradients. However, acting exclusively on dilution in order to control HCCI combustion is problematic and could lead to misfire or unstable combustion characteristics. Recent studies demonstrated that seeding the engine intake with oxidizing chemical species is a promising strategy to achieve combustion control in HCCI engines. Among many oxidizing chemical species, ozone (O₃) is one of the most promising promoters of combustion. Results showed that it enhances combustion and advances the combustion phasing. Moreover, ozone generators have become increasingly compact, making it possible to extend combustion control with ozone to commercial engines. The present work investigates HCCI combustion of iso-octane for lean to stoichiometric conditions and examines the combined effects of ozone and EGR dilution inside the combustion chamber. Experiments showed that dilution counterbalances excessive in-cylinder pressure due to the high fuel concentration, enabling stoichiometric HCCI combustion of iso-octane. Seeding ozone at the engine intake reduced engine inefficiencies and extended the HCCI engine operating range towards lower temperatures. EGR was also simulated using the main combustion products, H₂O, CO₂ and N₂. Experiments were performed to investigate possible interactions with ozone. Results showed that seeding the intake of the engine with ozone and simultaneously diluting the mixture allowed better control of HCCI combustion of iso-octane.