A quantitative analysis of nozzle surface bound fuel for diesel injectors

Abstract

In a fuel injector at the end of the injection, the needle descent and the rapid pressure drop in the nozzle leads to discharge of large, slow-moving liquid structures. This unwanted discharge is often referred as fuel 'dribble' and results in near-nozzle surface wetting, creating fuel-rich regions that are believed to contribute to unburnt hydrocarbon emissions. Subsequent fluid overspill occurs during the pressure drop in the expansion stroke when residual fluid inside the nozzle is displaced by the expansion of trapped gases as the pressure through the orifices is equalised, leading to further surface wetting. There have been several recent advancements in the characterisation of these near nozzle fluid processes, yet there is a lack of quantitative data relating the operating conditions and hardware parameters to the quantity of overspill and surface-bound fuel. In this study, methods for quantifying nozzle tip wetting after the end of injection were developed, to gain a better understanding of the underlying processes and to study the influence of engine operating conditions. A high-speed camera with a longdistance microscope was used to visualise fluid behaviour at the microscopic scale during, and after, the end of injection. In order to measure the nozzle tip temperature, a production injector was used which was instrumented with a type K thermocouple near one of the orifices. Image post-processing techniques were developed to track both the initial fuel coverage area on the nozzle surface, as well as the temporal evolution and spreading rate of surface-bound fluid. The conclusion presents an analysis of the area of fuel coverage and the rate of spreading and how these depend on injection pressure, in-cylinder pressure and in-cylinder temperature. It was observed that for this VCO injector, the rate of spreading correlates with the initial area of fuel coverage measured after the end of injection, suggesting that the main mechanism for nozzle wetting is through the impingement of dribble onto the nozzle. However, occasional observations of the expansion of orifice-trapped gas were made that lead to a significant increase in nozzle wetting. KeywordsDiesel; end of injection; dribble; surface wetting; image processing Introduction Increasingly strict emissions standards place considerable pressure on the automotive industry to increase the efficiency of, and reduce the emissions of the engine. This is primarily done by optimising the fuel-air mixing processes that occur inside the combustion chamber. Since the fuel air mixing is affected by the fuel injection equipment (FIE), much of the research efforts have been in improving the injector design and spray characteristics. This has resulted in the numbers of holes in the injector tip increasing with subsequent designs, as well as increases in the operating pressure, with typical systems operating with common rail pressures of over 200 MPa. These increasingly harsh conditions are believed to accelerate the formation and growth of deposits in and arou...

Quantification of diesel injector dribble using 3D reconstruction from x-ray and DBI imaging

“…6), suggesting that the fuel volumes estimated from our morphological reconstructions are consistent. In agreement with previously published studies [6,7], this investigation revealed a high cycle-to-cycle deviation in the quantity of fuel delivered after the end of injection. No clear dependence of the fuel dribble on injection pressure was observed (Fig.…”

“…Recently published studies [6][7][8][9][10] demonstrate different aspects of the EOI fuel dribble based on a qualitative and quantitative analysis of experimental images of the injection process. The following important factors affecting the mechanism of the fuel dribble were studied: peak injection velocity [7], needle closing speed [7], in-cylinder pressure [6,7], injection pressure [6], fuel mass expulsion [9], bubble ingestion at the EOI [10], liquid length recession at the EOI [8] and different flow characteristics at the EOI [11]. The present study is dedicated to a quantitative analysis of the fuel dribble with the focus on dribble volumes estimated by processing of images from high-speed X-ray phase-contrast and diffused back-illumination (DBI) imaging techniques.…”

Visual Analyses of End of Injection Liquid Structures and the Behaviour of Nozzle Surface-Bound Fuel in a Direct Injection Diesel Engine

“…Whilst there are many unexplored fluid dynamic processes occurring in and around the injector nozzle at the macroscopic scale [33], our investigation is focused on the nozzle wetting processes that occur on the nozzle surface at the microscopic scale, as the cycle progresses. Our previous work confirmed that fuel would deposit and spread on the surface of the injector nozzle through both the impingement of fuel droplets and the overspill of nozzle-trapped liquid fuel [26,27]. The investigations were performed on a 2-stroke reciprocating rapid compression machine (RRCM), therefore the 4-stroke optical diesel engine utilised here provides further validation of the conclusions previously drawn and introduces the interactions of realistic in-cylinder gas flow.…”

“…Although the approximated peak ICT is 439 K (still below the octane vapour saturation temperature at 45 CAD), the nozzle surface temperature would be much lower than this due to thermal inertia, the relatively short period of maximum ICT, and the engine head temperature of around 358 K [37,38]. During a previous investigation we measured the nozzle tip temperature with an embedded K-type thermocouple inside a 6-hole Siemens VCO injector [27]. The experiments were carried out in a motored reciprocating rapid compression machine with an engine speed of 500 rpm.…”

“…This leads to two problems. First of all, the liquid often lands on near-nozzle surface where it is then exposed to the high pressures and temperatures inside the cylinder [26,27]. The locally rich regions of surface-bound fuel may undergo incomplete combustion or degradation leading to the production of deposit precursors.…”