Spectroscopic Measurements of Low-Temperature Heat Release for Homogeneous Combustion Compression Ignition (HCCI) n-Heptane/Alcohol Mixture Combustion

The effects of the alcohol fuel fraction on the diesel homogeneous combustion compression ignition (HCCI) combustion are explained by analyzing simultaneously chemiluminescence data and cylinder pressure measurements. In parallel, experimental results are compared to the simulation results from a zero-dimensional (0D) model. n-Heptane was selected to represent diesel fuel, while ethanol and I-butanol were the considered alcohol fuels. The results showed that the main combustion was delayed when the alcohol fuels were blended, but species radiation results showed that the impacts on chemical processes were not distinguishable during the main combustion. This delay was linked to the impact of the blended fraction of alcohols on the low-temperature heat release (LTHR). This influence can be separated in two ways. First, the heat release rate results showed that the LTHR quantity decreased, inducing a decrease of the air-fuel mixture temperature after LTHR. Thus, the mixture required more time for the temperature to rise sufficiently to initiate the main combustion processes. Second, the chemiluminescence intensity of CH2O* decreased with an increase in the fuel fraction of alcohols. Therefore, the reaction processes during the main combustion were slower because of less supplied active intermediate species. Moreover, the simulation results from the model showed a similar trend. The results can be used to increase the understanding of the chemical aspects of diesel HCCI combustion.