Infineum: Imaging boosts injection

Ever tightening global emissions standards have demanded  significant changes to diesel engines in the past decade. Specifically,  manufacturers have had to gain very precise control over the fuel injection  process. Diesel engines require a fine mist of fuel to be injected into the  combustion chamber to facilitate good mixing of fuel and air, which in turn  means more efficient burning of the fuel. To aid this process, injection  systems today commonly operate at pressures of around 2000 bar and temperatures  above 100 °C. They  are also machined to extremely high precision: injection spray holes are  commonly between 80–150 µm  in diameter.

During the operation of such finely engineered systems, however, carbonaceous deposits can alter the operation of the injector or flow of fuel and thus affect the combustion process, making it difficult to control emissions and fuel consumption. For this reason, diesel sold throughout Europe contains additives designed to clean and maintain the fuel system.

Analysis of the deposits is vital if we are to understand their impact on injector operation and develop better chemical additives to reduce them. This is commonly done by cutting the injector open or disassembling it, and then analysing the deposits with typical laboratory techniques. Cutting destroys the apparatus and disassembly modifies its performance when the components are put back together. Therefore, since injectors are often part of on-going test programmes, a non-destructive means of analysis is desirable.

The ESRF’s ID19 station offers unique possibilities for characterising the deposit phase, providing phase-contrast imaging and high spatial resolution while maintaining high-enough X-ray energies to penetrate up to 1.5 cm of steel. These factors were crucial to a two-year long study performed by Infineum with Paul Tafforeau of the ESRF and Ali Chirazi of the Institute for Condensed Matter Chemistry of Bordeaux. Using such methods we were able to determine the volumetric deposit map for the injector and compare this to the parameters measured during engine operation. The experiment also revealed that the deposits inside the injector spray holes contained metal, showing that the chemistry of the deposit phase inside the injector is different to that seen on the tip in the combustion chamber.

In addition to spray-hole experiments, scanning the internal parts of the injector was of interest to see if deposition had occurred on the fast-moving parts responsible for fuel flow. Deposition on these parts can affect the synchronisation of components because the engine’s electronic control unit can no longer maintain the same precision control. However, since the deposition can occur in several places it was necessary to scan the majority of the body of the injector (which measures 12 cm in length), at a resolution of 6 µm – generating a 900 GB dataset. An even higher resolution scan would be desirable, but would render the data volume too large to realistically process.

In collaboration with Lyle Pickett of Sandia National Laboratories at Livermore and Alan Kastengren of Argonne National Laboratory, both in the US,  we have also employed the techniques available at ID19 to scan the spray hole zone at 0.65 µm resolution in order to generate detailed internal surface geometry models. The science of machining injector spray holes is an area of intense interest due to its importance in controlling the flow, spray and thus combustion behaviour, and modelling is an integral part of such studies.

Computational fluid dynamics allows us to understand the effects of flow and cavitation on the structure of the spray, and one of the challenges is to obtain detailed structural geometry of the spray holes inside the injector at a resolution high enough to generate accurate surface models. It is hoped that the ESRF will help improve this work by allowing much more accurate inclusion of the effects of internal surfaces on flow behaviour. Meanwhile, Infineum continues to work with the ESRF to utilise and evaluate the techniques in other areas relevant to our business aims.

Peter Hutchins, Infineum

This article originally appeared in ESRFnews, March 2013. 

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Top image: Tip of a 12 cm long diesel injector (full unit also pictured) showing carbonaceous deposits. Image credit: Infineum