A new constant volume combustion chamber (CVCC) apparatus is presented that calculates the cetane number (CN) of fuels from their ignition delay by means of a primary reference fuel calibration. It offers the benefits of low fuel consumption, suitability for non-lubricating substances, accurate and fast measurements and a calibration by primary reference fuels (PRF). The injection system is derived from a modern common-rail passenger car engine.
The apparatus is capable of fuel injection pressures up to 1200 bar and requires only 40 ml of the test fuel. The constant volume combustion chamber can be heated up to 1000 K and pressurized up to 50 bar. Sample selection is fully automated for independent operation and low levels of operator involvement. Capillary tubes employed in the sampling system can be heated to allow the measurement of highly viscous fuels.
For primary reference fuel calibration, ignition delay times of six mixtures with defined CN in the range of 35 to 70 are measured and correlated to their CN using a mathematical best fit curve. First tests showed good correlation with conventionally determined engine CN for several diesel fuels.
Methanol and ethanol with a CN below 20 were measured using a special low-CN calibration and higher chamber temperature and pressure. Two oxygenates with a high CN above 100 were measured after mixing with a low-CN primary reference fuel. This blending CN method allows extrapolating the CN for samples above the calibrated measurement range.