Abstract
Background
In recent years, fuel has rapidly diversified in correspondence to global changes such as CO2 reduction and depletion of energy resources. Additionally, corresponding fuel injection systems are also required. Our diesel injection system must accurately operate under high pressure and high speed for further high performance. In this environment, we were confronted with the problem that the injection quantity changed due to the deposit that adhered to the injector.
Objective
This paper describes the establishment of the deposit reproduction technology by the clarification of the adhesion mechanism in the deposit formation process and the effect of the countermeasure material to deposit formation.
Analysis and hypothesis
We collected several injectors which exhibited change to the injection quantity, and analysed both the deposit and the injector material to thoroughly understand every detail. We carried out the componential analysis at the molecular structure level and observed both the nano-morphology and the surface state. The deposit was sodium oleate. Its morphology differs from reagent grade which is just synthesized in solution, with lamellar structure. The surface oxidation state of the ferrous injector material to which the sodium oleate had adhered proceeded. Therefore, we assume that the sodium oleate which was soluble in the fuel becomes a peculiar morphology by the environmental change and precipitates; thereby adhering to the oxidized ferrous material which has a high affinity to sodium oleate.
Verification
Sodium oleate in the fuel gradually self-assembles at the temperature as the assumed internal injector situation, becoming a gel when the temperature is around 100 degrees or higher. The solubility of sodium oleate to fuel decreases rapidly at 100 degrees or more also, showing sodium oleate easily precipitates at high temperature. An affinity between sodium oleate and the oxidized surface of the ferrous material is evaluated by measuring surface free energy. The energy difference between the two is small. This result teaches us that sodium oleate tends to adhere to the oxidized injector. To verify this hypothesis, a reproduction test is performed. Specimen plates are prepared, with surfaces pre-oxidized and consisting of the same material of the injector. The plates are soaked into fuel which is saturated with dissolved sodium oleate. At 120 degrees or more, the sodium oleate precipitates and adheres to the surface of the plate. In addition, the morphology of sodium oleate adhered to this plate has a lamellar structure, similar to the actual deposit.
Countermeasure
We selected the engine test condition based on the clarified mechanism to reproduce the injection quantity change. We succeeded in reproducing the phenomenon. As one of the solutions to deposit formation, DLC applied onto the surface of the sliding part of the injector was effective. DLC has low affinity to deposit, and we have confirmed that DLC prevents the change of injection quantity due to deposit.
KEYWORDS – deposit, fuel, surface free energy, solubility, morphology