Experimental Study On The Control Of HCCI Combustion And Emissions By Means Of Fuel Design Concept

Recently, Homogenous Charge Compression Ignition (HCCI) combustion is receiving more attention for its potential to improve efficiency and reduce emissions when it is used in engines. In contrast 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 operating range. Relevant basic investigations need to be further done in order to put HCCI technology into the practical engines in the future.It is very import to understand the low-temperature oxidation mechanism of the primary reference fuels, because HCCI combustion is controlled by the chemical kinetics. In this paper, the different performance of HCCI combustion was investigated between pure n-heptane and pure iso-octane by means of numerical simulation. Afterwards, the HCCI combustion with different iso-octane/n-heptane mixtures and the producing process of relevant productions were investigated when intake temperature and fuel/air equivalence ratio are same. The simulation suggested that the mixing with high-cetane fuel and high-octane fuel can influence the combustion phase and cylinder pressure in HCCI combustion.The four-cylinder, four-stroke high-speed direct injection (DI) diesel engine was employed as prototype engine. One cylinder of the prototype engine was reformed for operating with premixed HCCI combustion. The combustion and emissions characteristics of HCCI fueled with the difffernt PRFs with specific iso-octane-to-n-heptane ratios were studied and the relevant operating range in fuel/air equivalence ratio was sketched. It was found that the fuel properties may control the heat release in the low temperature range to influenced of HCCI combustion. However, there were fundamental defects for high-octane fuels and high-cetane fuels. The high-cetane fuels had been proven to ignite easily at the early combustion phase before TDC and were easily knocking at higher load in HCCI combustion. On the contrary, the high-octane fuels were difficult to ignite and had good anti-knocking performance in HCCI combustion. However, high-octane fuels had deteriorated emissions in lower loads.For the limitations of high-octane fuel in HCCI combustion, the samll DTBP was added as the ignition improver. The paper studied HCCI combustion fueled with PRF80 when small DTBP was added. It was found that samll DTBP can improve the ignition of HCCI combustion fueled with PRF80 and extended its low load operation range. At the same time, the emissions of HCCI fueled with PRF80 were remarkable reduced when some DTBP is added.