Simulation of n-heptane and fuels for advanced combustion engines (FACE) surrogates in a single-cylinder compression ignition engine

Abstract

A CFD model of a HATZ diesel engine was developed for the purpose of simulating low temperature combustion (LTC) of surrogate diesel fuels for the Fuels for Advanced Combustion Engines (FACE). Initial validation of the model was performed using n-heptane data from a single cylinder HATZ diesel engine. N-heptane was initially chosen for several reasons: (1) several well validated mechanisms are available, (2) there is a wealth of data for n-heptane ranging from constant volume to engine experiments, (3) n-heptane is also used as the simplest representative of diesel fuel, because of the similarity in cetane numbers resulting in ignition characteristics (e.g. ignition delay) that are representative of diesel fuel without the added complexity of modeling several hundred components. Simulations were run with both a detailed n-heptane mechanism and several reduced mechanisms to determine the suitability of using a reduced mechanism and ensure that the main ignition characteristics were captured. Due to the computational time, CFD combustion models are limited to using a reduced version of the detailed mechanism. It was found that a 173 species n-heptane mechanism predicts start of combustion (SOC) within 0.5 crank angle degrees of the detailed 561 species mechanism. The 173 species mechanism required 27 hours of computational time to reach the end of the simulation whereas the 561 species detailed mechanism required 41 hours of computational time under the same conditions. There were two additional reduced mechanisms that were used which contained 85 species and 35 species and were used with reasonable accuracy with a computational time of 8 hours and 2 hours, respectively. Due to the varying physical and chemical properties of the FACE surrogates, a sensitivity analysis of the effects of the physical properties was conducted by changing the n-heptane physical properties to n-hexadecane physical properties while keeping the chemistry the same. As expected, when the fuel properties of n-hexadecane (which is less volatile than n-heptane) were assigned to the n-heptane chemistry, SOC was delayed and the net heat release rate was reduced. This is mainly due to the droplets requiring more time for evaporation and less of a premixed combustion phase along with combustion occurring later in the cycle. The FACE fuels were developed to fulfill the need for research grade fuels that are able to represent common refinery stream fuels [1]. Due to the FACE fuels consisting of hundreds of fuel components and the lack of availability of a chemical mechanism for some of the fuel components make it extremely difficult to model these fuels in a fullscale engine model. Thus, surrogates are required for the FACE fuels as well as a reduced mechanism capable of capturing the chemical reactions of each individual fuel component. A detailed diesel mechanism was reduced from 4016 species to a single chemical mechanism with 1046 species to match the characteristics of the surrogates for the FACE fuels 1, 3, 5, 8, and 9. The surrogates were used in simulation because they experimentally behave the same as the FACE fuels, and they only contain 4-7 species, compared to the hundreds of species found in the FACE fuels. Using the single chemical mechanism to represent the five surrogates FACE fuels it was found that 200 degrees C of air preheat was required to achieve autoignition in the HATZ model compared to the 100 degrees C of air preheat required experimentally. Initial runs have found that there were similar trends between the FACE fuel surrogate experiments and simulations for the respective fuels. Future work will require improvements on the single chemical mechanism to represent the five surrogate FACE fuels.