Experimental evaluation of a diesel engine using amide-functionalized carbon nanotubes as additives in commercial diesel and palm-oil biodiesel

Abstract

Nanomaterials used as additives to diesel, biodiesel, and its blends have shown promising results in improving thermal efficiency and reducing pollutant emissions in compression ignition engines. However, the stability of dispersions has been noted in the literature as a primary challenge to the effective use of nanomaterials as fuel additives. Between nanomaterials used as additives to fuels are carbon nanotubes (CNTs). These carbonaceous materials have shown positive and negative results concerning engine performance and emissions, and the differences could be associated with the dispersion stability of the nanomaterials in the fuel. Therefore, there is still much uncertainty regarding the relationship between the stability of nanomaterial dispersion and the impact of nanofuels on engine performance. Specifically, the current literature lacks sufficient information regarding the impact of incorporating amide-functionalized carbon nanotubes (CNT) on the stability of diesel-biodiesel blends and their influence on particulate matter emissions in ICE. In this study CNT and amide-functionalized CNT were dispersed in Colombian commercial diesel (10 % vol. of palm oil biodiesel) and palm oil biodiesel, and its stability was evaluated. The most stable blends were used in a stationary Ignition Compression Engine (ICE) to evaluate the impact of the nanomaterials on thermal efficiency and pollutant emissions. Results show that it was not possible to disperse CNT in Colombian commercial diesel (10 % vol. of palm oil biodiesel and 90 % vol. of petroleum diesel) due to its high rate of agglomeration and sedimentation. In contrast, the opposite behavior was shown using biodiesel as a base fuel. An amide functionalization of the CNT process was carried out to generate a repulsion of CNT and improve its stability in commercial diesel fuel. Amide-functionalized carbon nanotubes (CNTF) were obtained using pristine CNT and Oleylamine as nitrogen functional group precursors. X-ray Photoelectron Spectroscopy confirmed the amide functional groups on the CNT surface, and a loss weight of 14 % was identified in the temperature range of 150–550 °C, indicating a high grade of functionalization. Functionalized and non-functionalized CNT were dispersed in Colombian commercial diesel and palm oil biodiesel using a concentration of 100 mg/L to evaluate the dispersion's colloidal stability. This stability was examined for three days by visual inspection and variation of the hydrodynamic diameter using dynamic light scattering. The most stable dispersions were prepared at two concentrations (50 and 100 mg/L) and used as fuels in a stationary compression ignition engine. Colloidal stability was enhanced when CNTF were dispersed in diesel compared to the unstable, pristine CNT dispersion. However, in biodiesel, the dispersion behavior of the CNT and CNTF was similar. Compared to diesel, the diesel-CNTF dispersion did not considerably improve the engine´s thermal performance at 6 kW, and only a minor gain was observed at 12 kW at a concentration of 100 mg/L. However, at 6 kW, they reduced particulate matter emissions by up to 39.8 %. In the case of biodiesel, utilizing both functionalized and non-functionalized carbon nanotubes as additives, the thermal efficiency was increased, and CO emissions decreased.