Abstract
This study experimentally investigates the effects of pilot injection on first stage and overall ignition behavior for different fuel blends. A conventional petroleum-derived fuel (F-24) with a Derived Cetane Number (DCN) of 48.5 and gasoline (GAS), which has a low DCN of 21.4 but high volatility, were blended at different volumetric ratios. These blends were injected into a high temperature, pressure vessel using either a single or pilot-main injection strategy. Four optical diagnostics (formaldehyde PLIF, OH* chemiluminescence, schlieren, and Mie scattering) were employed to visualize the ignition process and to evaluate liquid length, penetration distance, heat release rate, the first stage, and overall ignition delays. It is shown that the higher GAS blend ratios result in longer ignition delay and less overall heat release attributed to the poor reactivity of GAS. In such cases, the contributions of the extended first stage ignition delay to the extended overall ignition delay decrease with GAS blend ratio. Furthermore, some of the high GAS blend fuels exhibit no overall ignition at all due to the excessive fuel-air mixing. The study then shows that this decrease in ignition performance with GAS can be mitigated by introducing pilot injection, which shortens the ignition delays and provides more overall heat release. Specifically, at low GAS blend ratios, the ignition exhibits mixing controlled combustion where the ignition of pilot fuel-air mixture induces the main fuel ignition. For pilot-main injections, the decrease in first stage ignition delay accounts for the average of 87 % of the reduced overall ignition delay, suggesting that the reduced ignition delay with pilot injection is primarily due to the reduced time for fuel atomization, vaporization, and decomposition of the main fuel. These results will contribute to the development of ignition models capable of capturing the impact of fuel composition and pilot injection on ignition performance.
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