A Simple Method to Predict Knock Using Toluene, N-Heptane and Iso-Octane Blends (TPRF) as Gasoline Surrogates

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.

In this paper, a method to define the composition of a TPRF to match both RON and MON of a target gasoline is presented. The appropriate TPRF as a surrogate for a particular gasoline, which has been extensively tested in a knocking SI engine, is identified using this method. A chemical kinetic model is used to calculate ignition delays at different pressures and temperatures for this surrogate TPRF. From these data, a simple Arrhenius type equation with a pressure correction to predict ignition delays is identified. This equation is used to find the ignition delay as a function of crank angle and calculate the Livengood-Wu integral, I, for a number of individual knocking cycles covering a wide range of operating conditions using the gasoline in a single cylinder engine. Knock is predicted to occur at the crank angle when the integral, I, reaches unity. The crank angle at which knock is predicted to occur using the simple equation for ignition delay for the surrogate TPRF agrees very well with the experimentally observed value for the gasoline for all the cases considered. Finally, using the chemical kinetic model for TPRF, simple equations which can be used to estimate ignition delay are presented for a range of RON and sensitivity. Such equations can be used to predict when knock occurs during the cycle for these gasolines if the pressure and temperature development with crank angle is known.