Fuel autoignition variability : shock tube measurements, a reduced kinetic model and diesel engine studies

Abstract

The shock tube facility in the Combustion Energy and Systems Lab at RPI was used to make ignition delay measurements for conventional and alternative jet and diesel fuels, namely JP-5 (US Navy petroleum jet fuel), HRJ-5 (hydro-processed Camelina jet fuel), F-76 (military diesel fuel) and HRD-76 (hydro-processed diesel fuel from algal oil). The experiments were carried out for a range of temperatures (650 - 1300 K), pressures (10 and 20 atm) and equivalence ratios (0.5 and 1). These results augment an already vast library of ignition delay measurements obtained earlier in our lab, and showed similarities in the gas phase ignition characteristics of high molecular weight straight chain hydrocarbon compound fuels. Particularly, the high temperature ignition times showed very little variation between fuels, whereas the low and intermediate temperature regimes displayed different reactivity characteristics. The derived cetane number (DCN) was used as a parameter to develop a correlation to describe the fuel reactivity variability, by incorporating it into an empirical three-Arrhenius model for ignition delay. A commonality among ignition times for all fuels indicated that the ignition times decreased as the DCN increased, as expected.

Further, a reduced chemical kinetic model was developed, applicable to the entire library of fuel ignition data obtained above. The model is based on a seven-step reduced chemistry model for n-heptane available in the literature (Zheng, J.; Miller, D.L.; Cernansky, N.P. "A Global Reaction Model for the HCCI Combustion Process", SAE Paper 2004-01-2950, 2004). The DCN is used as a common parameter to account for fuel reactivity variability, and is incorporated into the reaction mechanism. This helps to account for and predict ignition delay times for different fuels, and is particularly useful in the low and intermediate temperature regimes. The shock tube data were chosen as the target data set and the chemical kinetic mechanism model parameters were adjusted to obtain best fit curves. The model successfully predicted all the experimental results to within acceptable limits, with the standard deviation between the predictions and the experimental results being ±21%. The model accounts for changes in DCN, temperatures, pressures and fuel / air equivalence ratios across all temperature regimes.

In addition to the above, engine experiments were carried out for the fuels, testing for CO and NO emissions, ignition delay times, and fuel consumption. A Yanmar single-cylinder direct-ignition diesel engine was chosen for experimental runs. Ignition delay times were measured at a fixed engine speed. The results show, as expected, a decrease in ignition delay as the DCN increased. The DCN did not appear to have any significant impact on or correlation with engine loading. CO emissions show a decrease with increasing DCN, as did the BSFC (brake specific fuel consumption), and NO emissions show a decrease with increasing H/C ratio.

This thesis presents 1) ignition delay time measurements carried out using the shock tube method for jet and diesel fuels of interest to the US Navy; 2) a reduced kinetic model to describe the ignition of jet and diesel fuels; and 3) single-cylinder diesel engine ignition, emissions, and fuel consumption measurements carried out for jet and diesel fuels. This combination of work provides both fundamental and applied characterization of jet and diesel fuel combustion behavior and a reduced kinetic model that can describe fuel ignition variability in computational fluid dynamics simulations of combustion engines.