A Chemical and Morphological Study of Diesel Injector Nozzle Deposits - Insights into their Formation and Growth Mechanisms

Abstract

Modern diesel passenger car technology continues to develop rapidly in response to demanding emissions, performance, refinement, cost and fuel efficiency requirements. This has included the implementation of high pressure common rail fuel systems employing high precision injectors with complex injection strategies, higher hydraulic efficiency injector nozzles and in some cases <100µm nozzle hole diameters. With the trend towards lower diameter diesel injector nozzle holes and reduced cleaning through cavitation with higher hydraulic efficiency nozzles, it is increasingly important to focus on understanding the mechanism of diesel injector nozzle deposit formation and growth. In this study such deposits were analysed by cross-sectioning the diesel injector along the length of the nozzle hole enabling in-depth analysis of deposit morphology and composition change from the inlet to the outlet, using state-of-the-art electron microscopy techniques. Deposits produced in the injector nozzles of the industry standard fouling test (CEC F-98-08 DW10B bench engine) were compared with those formed in a vehicle driven on a chassis dynamometer, using a drive cycle more representative of real world vehicle conditions, to explore the effects of differing drive cycles and engine technologies. Fouling in all tests was accelerated with the addition of 1ppm zinc neodecanoate, as specified in the CEC DW10B test. This in-depth characterisation revealed a complex multi-layered system of deposits inside the diesel injector nozzle. Through analysing these layers the mechanisms enabling the initial deposit formation and growth can be postulated.

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

“…Orifice fouling reduces the flow rate through increased frictional forces caused by the rough deposit surface, promoting turbulence in the emerging spray [15,16]. The deposit thickness is normally small, typically in the region of 0.5 to 4 µm (up to 10 µm at the outlet) [15][16][17][18], yet they can lead to a considerable impact on the discharge rate due to the small orifice sizes (of around 100 µm) in modern injectors [19]. The influence of roughness on the inorifice cavitation [20] may also have a considerable effect on the spray characteristics [21].…”

Dynamics of post-injection fuel flow in mini-sac diesel injectors part 1: Admission of external gases and implications for deposit formation

“…The detail of the mechanisms involved in internal deposit formation are not well understood, and a considerable amount of research is required to resolve this difficult problem. Deposits located in the nozzle holes, mini-sac and on the base of the injector needle are most likely formed through an interaction between external combustion gases and components of the diesel fuel [28].…”

“…Metals such as Zinc (Zn) are employed as active ingredients in lubricant oils, which were used in the past in lubricity additives in diesel. Unfortunately, Zinc is indicated in accelerated internal deposition in the form of insoluble zinc salts, and is consequently employed in the CEC F-98-08 injector nozzle coking test [27,28]. Other metals such as Sodium (Na), Calcium (Ca), Potassium (K) and Phosphorous (Ph) are often present in the fuel through exposure to sea water during transportation, or their inclusion in corrosion inhibitors, anti-oxidants, de-icing agents, and salt dryers [22,25].…”

“…17,18 Deposits that are observed to form inside the injector nozzle and on the injector needle during operation have been the subject of a significant body of research over the last 10 years. 15,16,19–29 The details of the mechanisms involved in internal deposit formation are not well understood, and a considerable amount of research is required to resolve this difficult problem. Deposits located in the nozzle holes, mini-sac and on the basis of the injector needle are most likely formed through an interaction between external combustion gases and components of the diesel fuel.…”

Piezo Resistive Read-Out Contact Resonance Spectroscopy for Material and Layer Analysis at High-Aspect-Ratio Geometries

“…Gases or fluids flowing inside a hole, interact with the sidewalls and this can quickly lead to abrasions or deposits which alter the specifications of the concerned system. In the case of injection nozzles in motor vehicles, for example, fuel deposits lead to efficiency losses and higher exhaust gas values [1]. To analyse the wear process of spray holes with a diameter of around 100 µm, a nondestructive sensor is needed.…”