The analysis of fuel spray penetration and distribution in a medium-speed diesel engine using optical techniques

The investigations described were initiated in order to provide information and understanding on the mechanism of combustion in a medium speed diesel engine. In this way more economic engine development could be achieved by prediction or the fuel injection and combustion system performance. Work carried out at Loughborough University of Technology comprised a literature survey, indicating the areas of knowledge that required elucidation and a study of diesel nozzle spray characteristic's. The penetration and distribution of a fuel spray in a cold bomb at engine densities and atmospheric temperatures was measured, using high speed photography. The work differed from previous investigations by using an actual injection system. The feasability of using a double pass schlieren technique in a working engine was tested on a small three-inch bore high speed diesel engine, with a mirror on top of the piston and a window in the cylinder head. This optical system allows the different phases of the cylinder charge (i.e., the air and the liquid, vapour and burning fuel) to be identified, and greatly facilitates study of the total penetration of the fuel spray, air swirl, combustion and scavenging.processes. The knowledge and experience obtained in the prosecution of this work showed that it was feasible to apply this system to a large medium speed engine. Although difficulties with piston tilt and oil passing up the bore and obscuring the piston mirror were encountered, methods to overcome these were found, as also was a method of mounting the sensitive schlieren mirror, so that the engine vibrations were minimized. Application to the large engine meant major design modifications to allow a quartz ~dndow to be fitted into the cylinder head. Experiments were carried out using various optical arrangements to change the range and sensitivity of the schlieren system in order to distinguish all the phases of tuel distribution. Electronic instrumentation was developed which allowed measurement of transient cylinder and injection system conditions during direct and schlieren high speed photography tests. From the cold bomb work an equation is proposed...This allows the prediction of unevaporated spray penetration from an actual injection system taking into account nozzle upstream conditions and changes in length to diameter ratio. The large engine work shows that no appreciable air movement exists. Fuel spray penetration is radically affected by evaporation in hot gas but·the vapour continues to penetrate as a spray. The penetration of tpe vapour is identical to that of a spray without evaporation thus the proposed equation also predicts the total penetration of a hot gas. Using this equation a concept of combustion is proposed in which the fuel spray is deliberately allowed to impinge upon the piston 0 as opposed to the accepted theory of non-impingement. A 12 1/2-inch bore engine using this form of combustion has demonstrated that an improvement in efficiency can be obtained at the same time as substantial reductions in mechanical loading of the injection system.