There is a growing body of evidence that Diesel fuel passing through fuel metering and pressure control nozzles in high pressure Diesel fuel injection equipment is subjected to tribological stress, which is able to alter the composition of the Diesel fuel being returned back to the fuel tank. The evidence suggests that a fraction of the aromatic molecules comprising the Diesel fuel are subject to collisional excitation, facilitating molecular combination reactions that are able to result in the formation of polycyclic aromatic hydrocarbons (PAHs). These PAHs are precursors to the formation of primary soot-like nano-particles.

The following Images and Figures refer to recent research work done in an optically accessible model magnetic solenoid operated Diesel injector return valve. The fused silica return valve assembly was micro-machined in Aachen, Germany. It consists of 2 fused silica blocks, with hemispherical and cylindrical holes bored into the blocks. The bored sections are connected via 200 micron and 220 micron nozzle holes respectively. The 2 blocks are arranged together axially, and held within an aluminium cage, with nitrile rubber gaskets located between each face. The blocks were sealed together using a steel sleeve with recesses and nitrile o-rings. The assembly is capable of being exposed to up to 500 bar upstream fuel pressure.

During early stage imaging of the Diesel flow occurring in the model pressure control system, we noticed light originating from the entrance to the second nozzle hole. Further investigation of this led to our paper “Hydrodynamic luminescence in a model diesel injector return valve” in International Journal of Engine Research (, Hydrodynamic Luminescence in a Model Diesel Injector Return Valve ( The luminescence appears to be caused by tribological stress induced in the Diesel fuel at the entrance to the second nozzle, and is dominated by the three spin-allowed π* → π transitions (1E1u1A1g, 1B1u1A1g, and 1B2u1A1g) in the aromatics comprising the Diesel, which are subject to bathochromic shifting through orbital conjugation.

Image 1: High-pressure recirculation flow rig connected to optically accessible model Diesel injector pressure control valve. Light emitted from the Diesel fuel flowing through the nozzles is focused onto a spectrophotometer, which resolves the emitted light into its constituent wavelengths.
Image 2: Optically accessible model Diesel pressure control valve with adjustable needle in the foreground, the focusing lens, and the Newport Oriel spectrophotometer and detector in the background.
Image 3: High resolution photograph of the interior nozzle layout in the optically accessible model Diesel injector pressure control valve.
Image 4: Hydrodynamic luminescence emitted from Diesel flowing into the second control nozzle. It is believed that this luminescence is caused by the decay of excited aromatic molecules comprising the Diesel.
Figure 1. The mean uv-visible emission spectrum obtained from the tribological luminescence signal obtained from the Diesel fuel shown in Image 4 above. The mean spectrum spans the ultra-violet and visible violet-blue part of the visible light spectrum, containing six broad peaks in the 350nm to 460nm range. The spectrum is believed to be dominated by the three spin-allowed π* → π transitions (1E1u1A1g, 1B1u1A1g, and 1B2u1A1g) in the aromatics comprising the Diesel fuel, subject to bathochromic shifting through orbital conjugation.
Video 1: Short video showing hydrodynamic luminescence originating from Diesel fuel flowing into the second nozzle of the model pressure control valve. In the middle of the video, the camera aperture is increased, revealing the multi-phase cavitating flow occurring in the second nozzle. The luminescence appears to originate from an annulus immediately downstream of the nozzle entrance, located near the interface of the liquid and vapour phase.