This study examines fuel auto-ignitability and shows a method for determining fuel performance for HCCI combustion by doing engine experiments.
Previous methods proposed for characterizing HCCI fuel performance were assessed in this study and found not able to predict required compression ratio for HCCI auto-ignition (CRAI) at a set combustion phasing. The previous indices that were studied were the Octane Index (OI), developed by Kalghatgi, and the HCCI Index, developed by Shibata and Urushihara. Fuels with the same OI or HCCI Index were seen to correspond to a wide range of compression ratios in these experiments, so a new way to describe HCCI fuel performance was sought.
The Lund-Chevron HCCI Number was developed, using fuel testing in a CFR engine just as for the indices for spark ignition (research octane number and motor octane number, RON and MON) and compression ignition (cetane number, CN). By running the engine in HCCI mode, the required compression ratio for achieving auto-ignition with a combustion phasing of CA50 3° after TDC was determined for various gasoline surrogate fuels prepared from blends of n-heptane, iso-octane, toluene, and ethanol. To study the effect of different operating conditions, five inlet air temperatures ranging from 50°C to 150°C were used to achieve different pressure-temperature combinations in the engine, and the compression ratio was changed accordingly to keep a constant combustion phasing, The experiments were carried out in lean operation with a constant equivalence ratio of 0.33 and with an engine speed of 600 rpm.
The basis of the Lund-Chevron HCCI Number is the required compression ratio of primary reference fuels (PRF), from PRF 60 to PRF 100. For each evaluated fuel, at each condition, the Lund-Chevron HCCI Number is set equal to the octane number of the PRF having the same CRAI value at the same conditions.