The effect of Biodiesel fuel blend rate on the Liquid-phase fuel penetration in Diesel engine conditions

A study was conducted to investigate the evolution of liquid phase penetration of evaporating sprays under engine-like conditions, with diesel and biodiesel fuel blends. This study has been performed in a facility based on a single cylinder two-stroke direct injection Diesel engine operating at low rotational speed which provides a quiescent thermodynamic environment around TDC, when fuel is injected, realistic for current D.I. Diesel engines. Due to the absence of inlet or exhaust valves, very easy optical access to the combustion chamber can be provided through the cylinder head. Pure nitrogen is supplied to the engine as intake gas, in order to avoid combustion. The injection event is carried out by an electronically controlled common rail system. The injector is equipped with real 8-hole nozzles, with a hole diameter of 0.115 mm. Injection pressures in this study ranged between 30 and 160 MPa and different in-cylinder peak pressure and temperature values were considered.

Evaporative liquid spray evolution has been analyzed on the basis of images taken with a high speed CMOS camera at a rate of up to 30000 frames per second and purpose developed software. Thus, single injection events evolution can be tracked avoiding uncertainties associated to shot-to-shot fluctuations, and a more precise analysis of the evaporation characteristic length and time scales. From these experimental results, scaling laws based on the hypothesis of mixing limited evaporation have been evaluated for the biodiesel blends, extending the results available in the literature. Results have allowed quantifying the influence of the biodiesel content upon the liquid length.