ABSTRACT
Modern high-pressure Diesel fuel injection equipment (FIE) employ fuel recirculation through sequential nozzles located inside fuel injectors to control the pressure, flow rate and amount of fuel injected into the engine. Common rail accumulator pressure is often controlled through relief valves that return excess fuel back to the fuel tank.
Diesel fuel that is recycled from the FIE back to the tank passes through nozzles that produce large shear stresses and cavitating flows, which may result in an alteration to the chemical composition of the recycled fuel through molecular collisions producing pyrolysis-like chemical reactions.
High-pressure recirculation flow experiments have been conducted on various Diesel fuel samples, using in-situ time-resolved 405nm optical extinction measurements and 2-column gas chromatography (2d-GC) analysis of the fuel samples taken before and after the flow tests. Recirculation flow tests were conducted for 40 hours duration, using new commercial injectors of the same type for each fuel sample, in a customized 630 bar high-pressure recirculation flow rig. The 405nm optical extinction and 2d-GC measurements were employed to identify and determine any variation in the composition of the fuel samples that occurred during the flow tests.
The optical extinction coefficients were observed to increase consistently over the duration of the recirculating flow, suggesting a flow-induced alteration to the composition of the Diesel fuel samples. The 2d-GC measurements suggested that the mono-, di- and tri-aromatics comprising the Diesel samples were reacting with other compounds to form particulates and polymeric lacquers.
An optically accessible sequential stepped nozzle system was developed and deployed in an alternative high pressure recirculation flow rig to model the flow inside an injection pressure control system. The flow in the high-pressure nozzle was observed to remain single-phase, while the flow in the low-pressure nozzle was observed to develop into a cavitating flow at the entrance to the second nozzle and at the step. A blue-white light emission appeared to originate from the low pressure nozzle entrance. A spectral analysis of the emitted light suggested that the emission was produced by excited aromatic molecules comprising the Diesel fuel sample, supporting earlier conclusions.
E-Diesel fuels employed in future high-pressure FIE will be subjected to similar conditions, and must be chemically stable and resilient to these types of flows.296
