A Study on Ignition delay and Lift-off Length under GCI Condition for Gasoline-Biodiesel Blends
- In the last few decades, the gasoline compression ignition engines show better thermal efficiency than gasoline spark-ignited combustion and approaches diesel compression-ignited combustion, as well as lower emissions of hazardous air pollutants such as nitrous oxide and particulate matter. In GCI engines fueled with gasoline biodiesel blended fuels also shown the promise to reduce soot emission significantly while maintaining high engine efficiency. The GB blends is injected at initially to obtain the same combustion phasing as with diesel fuel. The mixing process of fuel and air is improved due to extra time, so combustion occurs with much greater mixing compared to diesel fuel. However, there are few studies concerning gasoline-biodiesel blended fuels, and the spray combustion characteristics and pollutant formation mechanisms have yet to be well understood. Especially, the ignition delay and lift-off length are governing factors that impact on the combustion phasing in the engine cycle, the degree of fuel vapor/air pre-mixing required to produce the first mixture, pollutant formation, and most notably NOx and PM. This dissertation allows to understand the effects of gasoline biodiesel ratio, injection conditions, and operating conditions on its ignition delay and combustion characteristics by the optical investigation. That is important to utilize gasoline biodiesel blended fuels with higher combustion efficiency and environmental benefits.
In this work, the constant volume combustion chamber and rapid compression expansion machine were successfully established with validated safety and consistency to simulate the high-pressure and high-temperature environment in diesel engines. Both provide optical accessibility and the ability to control the ambient temperature and ambient oxygen concentration and allow for the study of spray combustion with optical diagnostic techniques. Also, a gasoline-biodiesel reaction mechanism was developed to predict the chemical ignition delay of the blended fuels. The reaction mechanism with 4285 species and 15246 reactions was validated and implemented using the CHEMKIN PRO software.
Firstly, the fuel samples were four GB blends including GB20, GB40, GB60 and GB80 corresponding to 20 %, 40 %, 60 %, and 80 % volumetric biodiesel respectively, neat gasoline, and neat biodiesel. Fuel samples were injected into the CVCC to combust using a single-hole research-grade injector. Natural soot luminous images from the combustible flame were captured by a CMOS camera to determine the ignition delay and the flame lift-off length. A self-written LabVIEW code was made to separate and smoothen the flame from background noise in the image processing. Based on experimental data, the moderate biodiesel addition (less than 20%) can improve the ability of cold-engine starting, also solve engine misfire under low-load condition due to its flammability while maintaining advantages of gasoline with great volatility and high ignition delay which significantly enhance the mixture formation process.
Secondly, the gasoline was blended with biodiesel at 5%, 10%, 15%, and 20% by volume, and then tested in a rapid compression expansion machine at a compression ratio of 11 and a temperature range of 700-850 K to observe the auto-ignition delay phenomenon under engine-like conditions. The experimental conditions are focused on improving the auto-ignition characteristic of gasoline direct-injection compression ignition combustion strategies under low load and cold start. These results revealed that a higher biodiesel fraction helps to obtain shorter ignition delay, which reduces the requirement of intake temperature. The blended fuel with 20% biodiesel showed the lowest ambient temperature at the injection timing requirement and was 80 K lower than gasoline. The combustion duration and pressure peak of every blended fuel were similar to each other after increasing the biodiesel fraction.
Thirdly, a comprehensive study was performed and focused on the ignition characteristics of the GB20 in a constant volume chamber under a wide range of experimental conditions simulating engine operating conditions: gas density (5 kg/m3 and 15 kg/m3), ambient temperature (800 K-1200 K), oxygen content (10 % - 21 %), various injection pressures (30 MPa - 130 MPa), and injection durations (400 µs – 3500 µs). As a result, the ignition delay and ignition distance heavily depend on the operating conditions. The increase of ambient temperature, ambient gas density, or oxygen concentration significantly decreases the ignition delay and ignition distance. The injection parameters, including the injection pressure and injection duration, also influence the air-fuel mixing process, thus changing the local ambient temperature and local air-fuel ratio, resulting in increasing/decreasing the ignition delay and ignition distance. Besides, this experimental study provides experimental data for the mapping of ignition delay; it is vitally important to control the internal combustion engines for obtaining high performance with gasoline biodiesel blended fuels. The ignition delay data is also the primary metric for development, calibration, and validation of CFD models for the GCI engine with GB20. The ignition delay measurement from the RCEM is longer than CVCC for the GB20 fuel due to impingement in RCEM, existence of NOx due to pre-combustion in CVCC, and higher flow rate of fuel is cooling down injection location in the RCEM.
Finally, the measured auto-ignition delay was compared to a simulated result that is predicted by the CHEMKIN-PRO software to validate the accuracy of the ignition delay for the gasoline-biodiesel blended fuel to better understand the fuel ignition characteristics. These results revealed that a higher biodiesel fraction helps to obtain shorter chemical ignition delay, which reduces the auto-ignition delay, thus the requirement of the lower intake temperature.
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