Search Results

The autoignition resistance of a practical gasoline is best characterized by the Octane Index, OI, defined as RON-KS, where RON and MON are respectively, Research and Motor Octane Numbers, S is the sensitivity (RON-MON) and K is a constant depending on the pressure and temperature history of the fuel/air mixture in an engine. Experiments in knocking SI engines, HCCI engines and in premixed compression ignition (PCI) engines have shown that if two fuels of different composition have the same OI and experience the same pressure/temperature history, they will have the same autoignition phasing. A practical gasoline is a complex mixture of hydrocarbons and a simple surrogate is needed to describe its autoignition chemistry. A mixture of toluene and PRF (iso-octane + n-heptane), TPRF, can have the same RON and S as a target gasoline and so will have the same OI at any given K value and will be a very good surrogate for the gasoline.

As SI engines strive for higher efficiency they are more likely to encounter knock and fuel anti-knock quality, which is currently measured by RON and MON, becomes more important. However, the RON and MON scales are based on primary reference fuels (PRF) - mixtures of iso-octane and n-heptane - whose autoignition chemistry is significantly different from that of practical fuels. Hence RON or MON alone can truly characterize a gasoline for its knock behavior only at their respective test conditions. The same gasoline will match different PRF fuels at different operating conditions. The true anti-knock quality of a fuel is given by the octane index, OI = RON −KS where S = RON − MON, is the sensitivity. K depends on the pressure and temperature evolution in the unburned gas during the engine cycle and hence is different at different operating conditions and is negative in modern engines.