Developments in internal combustion engines and implications for combustion science and future transport fuels

Changes in engine technology, driven by the need to increase the efficiency of the SI engine and reduce NOx and soot from diesel engines, and in transport energy demand will have a profound effect on the properties, specifications and production of future fuels. The expected increase in global demand for transport energy is significantly skewed towards heavier fuels like jet fuel and diesel compared to gasoline. Abnormal combustion such as knock and preignition will become more likely as spark-ignition (SI) engines develop to become more efficient and fuel antiknock quality will become more important. In current and future SI engines, for a given RON (research octane number), a fuel of lower MON (motor octane number) has better antiknock quality. Current fuel specifications in several parts of the world assume that MON contributes to antiknock quality and will need to be revised as the mismatch with engine requirements widens. Diesel engines need to maintain efficiency while reducing emissions of soot and NOx. Soot and NOx can be controlled more easily if such engines are run on fuels of extremely low cetane. In the long term compression ignition engines could run on fuels which require less processing in the refinery compared to today's fuels. Such an engine/fuel system could be significantly cheaper and also help mitigate the expected demand imbalance in favour of heavier fuels. The review concludes with a list of issues for combustion science that are relevant to this fuel and engine development.