Liquid and Vapor Fuel Distributions within a High Speed Direct Injection (HSDI) Diesel Engine Operating in HCCI and Conventional Combustion Modes

An optically accessible single cylinder small-bore HSDI diesel engine equipped with a Bosch common-rail injection system was used to study the effects of multiple injection strategies on the in-cylinder combustion processes. The operating conditions were considered typical in the metal engine under moderate load conditions. In-cylinder pressure traces are used to analyze heat release characteristics. The combustion modes transit from the Homogeneous Charge Compression Ignition (HCCI)-like combustion mode to conventional diesel combustion by changing injection parameters. The whole cycle combustion process was visualized through a high-speed digital video camera and the combustion images clearly show the combustion mode transition. Laser-Induced Exciplex Fluorescence (LIEF) technique was used to obtain simultaneous liquid and vapor fuel distributions within the combustion chamber, with tetradecane-TMPD-naphthalene as the base fuel-dopant combination. The effects of injection timing, injection fuel quantity, and injection pressure on the first injection were investigated. Vapor is seen throughout the jet cross-section regardless of the injection parameters. The majority of the vapor is seen in the central region. Early injection timing results in a larger spray jet cone angle and more dispersed spray structure; however, longer liquid existing time, namely the liquid signal duration, is seen for early injection timing at low injection pressure. High injection pressure leads to shorter injection duration and a narrower injection spray cone angle. The liquid existing times in the combustion chamber are only slightly different for various injection timing under high injection pressure and liquid signal disappears a short time after the end of injection. Heat release rate curves show that the combustion of the first injection with earlier injection timing or high injection pressure is not complete due to the low local equivalent ratio. It is found that the start of the low temperature reaction of the first injection event is mainly determined by in-cylinder gas temperature with little dependence on the global equivalence ratio seen from the results. The chemical reaction process of the first injection determines the ambient pressure and temperature at the start of injection for the second injection and plays an important role for the combustion mode of the second injection.