Injector Deposits

Hannu Jääskeläinen, Alessandro Ferrari

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Abstract: Diesel fuel injector fouling involves deposit formation on the external and/or internal surfaces of the injector and nozzle. Factors that affect injector deposits include properties and chemical composition of the fuel, local fuel temperature, and injector geometry. Standardized tests have been developed to quantify the tendency of a fuel and fuel injector combination to be affected by deposits. Injector deposits can have a number of negative effects on engine performance, including power loss and increased emissions.

Introduction

Fouling due to solid deposits inside a diesel fuel injector or its nozzle is an important problem experienced with diesel fuel injectors. This phenomenon arises during injector aging and consists of a series of chemical reactions whose products are deposited on the external and/or internal metallic surfaces of the injector and/or nozzle. Diesel injector fouling deposits can be broadly categorized as injector nozzle deposits and internal diesel injector deposits (IDID). The latter are also referred to as internal injector deposits (IID).

The factors that affect injector deposits basically fall into three categories: (1) properties and chemical composition of the fuel properties, (2) local fuel temperature of the geometry and (3) the injector nozzle and internal fuel wetted surfaces.

Properties and Chemical Composition of the Fuel. Fuel characteristics such as high viscosity, low volatility and reactivity of the unsaturated hydrocarbon chains (olefins, aromatics), can facilitate carbon deposits at the nozzle holes and the formation of protuberances on the injector nozzle tip. The presence of small traces of Na, Zn, Cu, and Ca in the fuel (metal contamination) has been demonstrated to significantly intensify nozzle fouling as well as internal injector deposits [3006][2219]. The additive package dispersed in the fuel also has an important effect on metal contamination and more generally on injector coking.

The presence of biodiesel in the fuel can also impact injector deposit formation. In some cases, depending on its detailed composition, biodiesel can have little impact on the accumulation of injector nozzle deposits [3011]. In other cases, biodiesel can contribute to the formation of injector deposits. Not only does biodiesel contain trace metals that can intensify injector deposit formation, biodiesel oxidation products can contribute to deposit accumulation. Carboxylic acid formed during biodiesel oxidation can corrode iron surfaces to yield an iron carboxylic salt layer. This salt layer can then trap other components found in the fuel, for example polymers that are also formed during biodiesel oxidation [3012].

Local Fuel Temperature. The influence of temperature on nozzle coking is also significant [3008]. The thermal condensation and cracking reaction kinetics of diesel fuel has been shown to accelerate the rate of deposition in the nozzle when temperatures exceed about 300°C. This value seems to be a critical threshold for coking in diesel engines.

A number of engine design parameters such as EGR and charge air cooling can influence the in-cylinder gas temperature and thus the temperature of components exposed to cylinder gases such as the injector nozzle. These design parameters can also impact nozzle fouling since chemical reaction rates, including those related to injector deposit formation, are very sensitive to temperature.

Injector Geometry. A third factor that affects injector deposit accumulation is the injector geometry. One current tendency in common rail injectors is to increase the number of injection holes (e.g., from six to eight) and to reduce their diameter [3009]. This, together with the continuous increase in the injection pressure level, has been one way of coping with Euro 5 emission limits. However, smaller diameter injector holes intensify coking effects, due to their higher sensitivity to fouling. Furthermore, hydro ground and convergent injector holes are often employed as a means of improving the nozzle discharge coefficient and inhibiting cavitation. Unfortunately reduced levels of cavitation can increase the accumulation of deposits in injector nozzle holes. Cavitation in the nozzle holes is commonly believed to help remove coking deposits.

Nozzle Deposits

[photo]
Figure 1. Nozzle Deposits on Injector Tip: (a) ‘Dry’ Sample and (b) ‘Wet’ Sample

After aging with zinc doped fuel ([Zn] = 6 ppm).

Nozzle deposits have been a problem for many years [3013]. These deposits typically form inside and around nozzle fuel flow holes on the tip of the injector, Figure 1, and can have several important consequences including:

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