Characteristics Of The Diluent Gas Impact Of The Fuel Ignition And Combustion Process

A rapid compression machine (RCM) was developed at Tsinghua University. A creviced piston was used and the piston stroke is500mm with compression duration of about30ms. Compression ratio can be change from6to22.5. Several experiments were conducted by using this device to make sure the experiment data is believable and the ignition process is homogeneous. The results show that the repeatability of this device is rather acceptable and the ignition process is very homogeneous. Data were compared with the results from RCM of Michigan University with the maximum fractional difference being less than9.68%. The results demonstrate that TU-RCM is very well developed and it is suitable for fuel combustion kinetics research.Experimental and numerical studies of the thermal and chemical effects of the buffer gases on low temperature ignition for both iso-octane and n-heptane have been performed.Experiments were conducted by using the RCM in the temperature range of600-850K. Three buffer gases were studied including N2, Ar, and a mixture of Ar and CO2at a mole ratio of65.1%/34.9%. Iso-octane was studied at20bar,φ-1, and a dilution level of buffer gas to O2of3.76:1(mole ratio). n-Heptane was studied at9bar,φ=1, and a dilution level of5.63:1(mole ratio). For experiments where two-stage ignition was observed, the buffer gas composition had no impact on the first-stage ignition delay time, but caused differences in the total heat release, pressure and temperature rise after the first-stage ignition. As a consequence, significant differences were observed for the total ignition delay time, with up to40%faster for iso-octane using Ar compared to the N2mixture and up to42.5%faster for n-heptane using Ar compared to the N2. The chemical effects of the buffer gas composition were studied experimentally by comparing the results of the N2and Ar/CO2(65.1%/34.9%) mixtures, where the Ar/CO2mixture has the same heat capacity as N2. Results showed negligible chemical effects on the first-stage and total ignition delay times.Numerical simulations were carried out over a wider range of temperatures for pure N2, Ar, and CO2as buffer gases. The results showed that thermal effects dominated at negative temperature coefficient and two-stage ignition conditions, whose characteristics were similar with the experiment results. However, the chemical effects (especially CO2) became more important than the thermal effects at temperature higher than835K, which cause shorter total ignition delay time. Buffer gases do have huge effects on fuel ignition process and the results can be used for the further development of a new generation of internal combustion engine (HCCI-LTC) and other element researches.