Abstract
Different projections suggest that in the coming decades, a) the worldwide demand for transport fuels will increase significantly, b) transport energy will still come substantially (around 90% share) from petroleum-based fuels and c) the demand increase will be significantly skewed towards commercial vehicles. On the current fuel/engine technology trajectory, there will be a massive shift in demand towards diesel and jet fuels. Hence there is likely to be a surplus of lighter, less-processed fuels such as straight run gasoline from the initial distillation of crude.
Current diesel engines are efficient but expensive and complicated because they try to reduce NOx and soot simultaneously while using conventional diesel fuels which ignite very easily. Gasoline-like fuels with high ignition delay make low-NOx/low soot combustion very much easier. Studies on fuel volatility/auto-ignition quality effects on GCI combustion show that the most important fuel property is the autoignition quality which determines the ignition delay. Moreover, the RON of the optimum fuel for GCI engines is likely to be in the range of 70-85 and hence much lower than that of current gasolines. Also, the fuel does not need to have high volatility and the final boiling point can be higher compared to current gasolines to enable low soot/ low NOx combustion as long as the ignition delay is sufficiently large.
The advantages of the GCI concept are -
i) The engine will be at least as efficient and clean as current diesel engines but will be less complicated and hence cheaper (lower injection pressure, after-treatment focus on CO and HC rather than soot and NOx).
ii) The optimum fuel will be less processed and hence easier to make compared to current gasoline or diesel fuels.
iii) It provides a path to mitigate the global demand imbalance between heavier and lighter fuels that is otherwise projected. The alternative is investment in refineries to make the required diesel fuel while the diesel engine continues to be expensive in order to meet increasingly stringent NOx/soot requirements. The concept has been well-demonstrated in research engines but development work is needed to make it feasible on practical vehicles e.g. on cold start. In the long-term scenario it will also require the availability in the market of a new, albeit a simpler fuel.
KEYWORDS – Compression Ignition, Low NOx, Low Soot, Transport Energy, Fuel Efficiency