Combustion Behaviour of Blends of Synthetic Fuels in an Optical Single Cylinder Engine 2021-24-0038

The reduction of carbon footprint of compression ignition engines for road transport makes it necessary to search for clean fuels alternative to diesel and to evaluate them under engine conditions. For this reason, in this paper, the combustion behaviour of different blends of synthetic fuels has been analyzed in an optical single cylinder engine of Medium Duty size (0,8 liters per cylinder) by means of optical techniques. The aim is to evaluate the effect of synthetic fuels, both partly or completely fossil diesel, in terms of combustion behaviours and soot formation. Therefore, different blends of oxymethylene dimethyl ether (OME_X) with diesel and neat hydrotreated vegetable oil (HVO) were studied. A conventional common rail injection system and a single injection strategy was used. In addition, special care was taken to ensure that conditions inside the engine cylinder at the injection start were as close as possible to the conditions used in previous studies. This would allow to validate that previous finding are still valid under engine conditions.

To understand the combustion process, and enhance the differences between fuels, high-speed imaging of OH^* chemiluminescence and of natural luminosity (NL) of the flame were recorded, simultaneously. In addition, the in-cylinder pressure was also registered and processed. These tools provide a detailed description of the combustion process with high temporal resolution.

On the one hand, results confirm that the increase of OME_X in the mixture, produces a decrease in the natural radiation intensity, which indicates a decrease in soot formation. Besides, it is accompanied by a faster oxidation reaction which compensates the lower heating value (LHV) of this synthetic fuel. This results in a similar energy release which makes these mixtures suitable for replacing neat diesel.

On the other hand, the combustion process of HVO was found to be very similar to diesel, so it could be a great substitute for fossil fuel. However, in this study, a decrease of soot formation is not observed.