An experimental and computational study of fuel injection, mixing and combustion in diesel engines

Ricart, L. M. An Experimental and Computational Study of Fuel Injection, Mixing and Combustion in Diesel Engines. University of Wisconsin-Madison, 1998.

An experimental and computational study of fuel injection, mixing and combustion was carried out to improve and validate models in the multidimensional code KIVA-II. Experiments were carried out in a single-cylinder, heavy-duty, diesel engine using an endoscope-based imaging system. The experimental data obtained included natural flame luminosity images and spray penetration measured via elastic Mie scattering. The cases studied included a typical 75% load case with single injections and pilot injections or light loads. Maximum liquid penetration lengths beyond which the fuel is completely vaporized were observed for all the cases studied. The maximum lengths varied from 23 mm to 28 mm for the different start-of-injection timings. The location of the luminous ignition was found to be close to the injector nozzle, but it moved further downstream from the injector nozzle as the injection duration was shortened. Rapid inflammation or flame propagation was observed after ignition.

The experiments were modeled using an improved version of the KIVA-II code. The spray breakup process is modeled to be due to Kelvin-Helmholtz (KH) instabilities, or a combination of KH and the Rayleigh-Taylor (RT) instabilities. A concept of breakup length is introduced to account for the dense core region of the diesel spray. A wide range of parameters, which included in-cylinder pressure, apparent heat release rate, liquid-fuel penetration and luminous flame images were used to assess the spray models. It was found that a combination of the KH and the RT mechanisms with the dense core concept results in more realistic predictions of the spray characteristics and hence, the diesel combustion process. In particular, the limited liquid fuel penetration observed experimentally was captured by including these two competing mechanisms in the spray model without compromising the penetration of the vapor fuel ahead of the liquid spray.

With the introduction of the breakup length model it was found possible to model the effects of nozzle inlet geometry on engine combustion and emissions. In particular, with the longer resulting breakup length for the rounded-inlet nozzles it was possible to explain the large increase in soot observed experimentally when compared to results obtained with the sharp-edged nozzle.