For a diesel engine, injection timing is a major parameter that sensitively a ects the engine performance, emission and durability. The physical and chemical properties of vegetable oils are known to be different from those of diesel. This study examines the changes in the behaviour of waste cooking oil (WCO) with changes in injection timing of a direct injection (DI) diesel engine, compared with those of diesel. The aspects taken into consideration were the effects of injection timing on combustion, performance and emissions. The results reveal that WCO and diesel responded identically to injection timing changes. To reduce NOx emission, one of the methods is to retard the injection timing from MBT timing. The engine used in this research follows this technique and had its original injection timing set at 15° before top dead centre (BTDC). With injection timing advanced by 4°, the engine produced better e ciency by 1.6 per cent for WCO and by 1.1 per cent for diesel, reduced CO emission, by averages of 9.9 per cent for WCO and 44.9 per cent for diesel, but su ered increased NO emission of 76.6 per cent for WCO and 91.4 per cent for diesel. In all instances, WCO had shorter ignition delays than diesel, but the ignition delay for WCO was more sensitive to load and injection timing than that for diesel. The test engine could run on WCO with the original injection timing, and altering the timing could result in a trade-o between performance and emission.
The use of waste cooking oil (WCO) as an alternative to diesel in engines has advantages from both economic and environmental standpoints. Typical of vegetable oils, WCO has a higher viscosity, leading to a general perception that its use is likely to have an adverse effect on the fuel injection system and consequent combustion process. In the present investigation, tests were carried out to determine engine performance and combustion analysis as well as emissions for both WCO and diesel. It was observed that because of the shorter ignition delay the premixed combustion phase of WCO was less intense than that of diesel. However, because of the corresponding smaller combustion volume, the peak pressures were on average 1.5 bar higher and occurred 1.1°-3.8° earlier than for diesel. This early peaking characteristic requires careful attention to ensure that, while running with WCO, the peak pressure takes place marginally after top dead centre for efficient operation. In terms of emissions of CO, NO and SO2, the level was higher for WCO compared with diesel.
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