Fuel design real-time to control HCCI combustion

Abstract

In order to achieve lower emissions and extensive load in the homogeneous charge compression ignition (HCCI) engine system, a novel fuel design concept that high-octane number fuel and high-cetane number fuel are mixed real-time to control HCCI combustion is proposed in this study. HCCI combustion fueled with iso-octane/n-heptane mixtures controlled real-time on a single-cylinder HCCI combustion engine is studied. The test results show that the equivalence ratio of n-heptane in mixtures decides ignition and controls the combustion phase of HCCI combustion. The addition of iso-octane extends knocking limit in equivalence ratio somewhat, but knocking occurrence mainly depends on the total concentration of mixture. Although operating range in equivalence ratio becomes narrow with the increasing proportion of iso-octane, the maximum load of HCCI combustion fueled with iso-octane/n-heptane mixtures controlled real-time is increased about 80% more than that of pure n-heptane. When iso-octane/ n-heptane mixtures are controlled in optimized method, it is proved that the load of HCCI combustion can be fully extended and emissions can be decreased remarkably, while at the same time the higher indicated thermal efficiencies are obtained over the extensive operation range.HCCI combustion is receiving increasing attention for its potential to improve both the efficiency and emissions of the engine [1] . In an HCCI combustion engine, the homogenous fuel-air mixture is compressed to attain auto-ignition close to top dead center (TDC). As the ignition process depends on a spontaneous autoignition at many points and HCCI combustion has no flame propagation, the whole bulk burns almost simultaneously and the combustion rate becomes very fast.Highly diluted mixtures are necessary to avoid knocking. Compared with conventional engines, it is fundamentally more challenging to control the start of combustion and the rate of heat release over a wide range of engine operating conditions [1][2][3] , because the ignition is sensitive to very small changes in temperature in HCCI combustion mode, while conventional engines have a direct control over the start of ignition through a spark for spark ignition mode and fuel injection timing for diesel engines.The ignition timing and combustion rate significantly depend on the interaction between the chemical kinetics of the fuel-air-residual gas mixture and their temperature-pressure history during the compression process. Two primary factors to influence HCCI combustion can be deduced. First, the environmental conditions such as pressure, temperature and components of air-residual gas mixture are important to HCCI combustion. Hence, exhaust gas recirculation (EGR), variable compression ratio (VCR) and variable valve timing (VVT) [4][5][6] measures that can influence the environmental condition of HCCI combustion, have led to some improvement on controlling the ignition timing and combustion rate of HCCI combustion.On the other hand, the fuel properties are more important to HCCI combustion. Fuel...

Active fuel design—A way to manage the right fuel for HCCI engines

“…Therefore, researchers strived to take advantage of the fuel chemical properties in HCCI engine control and optimization. Lü et al [23][24], Hou et al [25] and Huang et al [26] proposed a fuel design concept for HCCI combustion, in which fuel components and physiochemical properties could be optimized according to engine operation conditions. First, two primary reference fuels (n-heptane and isooctane) and their blends were studied on an HCCI engine.…”

“…In addition to simply blending mixing basic fuels, the fuel design concept was further realized in a more practical way. By adopting two independent fuel supply systems, the realtime control of dual-fuel injection was achieved to control ignition timing and combustion phasing of HCCI [25,26]. Wilhelmsson et al [27] investigated an n-heptane/natural gas dual-fuel HCCI engine equipped with a variable geometry turbo charger.…”

Experimental study on dual-fuel compound homogeneous charge compression ignition combustion

“…Ogawa et al 22 directly injected water and low-ignitability fuels early in the compression stroke to suppress HCCI combustion of premixed light naphtha, and investigated the chemical effects of these ignition inhibitors on HCCI reactions. Hou et al, 23 Huang 24 and Wilhelmsson et al 25 adopted two independent port fuel supply systems to attain real-time adjustment of the injection amount of two different fuels, named the active fuel design method, to control ignition timing and combustion phasing of HCCI. These active fuel design methods allow flexible fuel-property adjustment in HCCI engines and a real-time response could be made according to the operation conditions.…”

Experimental study of n-heptane ignition delay with carbon dioxide addition in a rapid compression machine under low-temperature conditions

“…A conventional diesel engine has the advantage of a higher combustion efficiency than that of a gasoline engine, but it cannot simultaneously reduce particulate matter (PM) and NO x because of the partially rich or lean combustion in the combustion chamber. Homogeneous charge compression ignition (HCCI) has become the focus of research because of its excellent efficiency and low emission levels [1][2][3]. To control HCCI combustion, it is essential to have sufficient understanding of its ignition time and combustion speed.…”