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EK-Motor : 2017/MAR/08 – 2018/DEC/01 – 2022/DEC/04 – 2023/DEC/07
Question
Q1. With reference to poor ignition quality fuel:
(a) Explain how it can affect combustion in BOTH slow speed and medium speed diesel engines; (8) (b) Explain how the effects on BOTH diesel engine types in part (a) can be reduced. (8)
Asked in Year/Month/Ques No.
Synopsis
>> The ignition of the fuel is dependent on the ease of which the hydrocarbon molecule can be broken away or 'attacked'. The long thin chains of the paraffin are more easily broken down than the compact ring structures of the aromatics (benzene ring). Thus, the amount of aromatics within the fuel structure has a direct relationship on the ignition performance. However, measuring fuel aromaticity is difficult, so the concept of CCAI (Calculated Carbon Aromaticity Index) has been used for indicating ignition quality.
Marker's Feedback
2018/DEC A number of candidates seemed to have little understanding of fuel ignition quality and many wrote about fuel properties in general; presumably believing that fuel ignition quality must be in there somewhere and that it was up to the examiner to sort out what was relevant .
2017/DEC Q8 has a stem “With reference to poor ignition quality fuels” and a large number of candidates answered in terms of overall fuel quality not ignition quality.