Auto-Ignition Quality of Practical Fuels and Implications for Fuel Requirements of Future SI and HCCI Engines

The auto-ignition or anti-knock quality of a practical fuel is defined by the Octane Index, OI = (1-K)RON + KMON where RON and MON are the Research and Motor Octane numbers and K is a constant depending only on the pressure and temperature variation in the engine. K decreases as the compression temperature in the unburnt gas at a given pressure in the engine decreases and can be negative if this temperature is lower than in the RON test. As spark ignition (SI) engine designers seek higher efficiency knock becomes more likely. Moreover such initiatives - direct injection, higher compression ratios, downsizing and turbocharging - will reduce the unburnt gas temperature for a given pressure and push the value of K downwards. In Europe there is evidence of a monotonic decrease in the average K value from 1987 to 1992. In 37 different Japanese and European cars (34 models) equipped with knock sensors that have been tested K has been found to be negative in most cases. Thus for a given RON, a fuel of lower MON has higher OI and will give better acceleration and higher power in a car equipped with a knock sensor. Such fuels are also the most appropriate for Homogeneous Charge Compression Ignition (HCCI) engines. Current fuel specifications as well as other initiatives like the World Wide Fuels Charter impose restrictions on fuel composition in order to control emissions. However modern engine and catalyst technology is more tolerant of fuel composition than before. Hence such restrictions might not be as important in controlling emissions as they were when they were first proposed especially if other fuel specifications, such as low sulphur, are left in place. Such pressures will force gasolines to become less sensitive and also make it more difficult to attain high RON. This trend is directly opposed to the fuel requirements of future engines. These issues need to be considered jointly by the various stakeholders to align future fuels with the requirements of future engines. The paper is a review of these points.