The focus on downsizing powerplants in many racing series, for example Formula One and sportscar racing, has seen direct-injection (DI) gasoline engines become more common. The development of such engines is no easy task and requires extensive simulation and testing to ensure that systems are working to their optimum.
When it comes to testing and analysing factors such as injector spray patterns and the combustion process itself, much reliance is placed on CFD simulation. However, there is still no substitute for real-world data, which is where combustion imaging systems come into play. As has been covered in previous RET-Monitor articles, in-cylinder pressure sensors can provide useful information of the combustion process, although pressure data does not deliver any insight into the spatial character of combustion, which can only be obtained by optical analysis, which allows the actual injection and combustion process to be captured visually, enabling the fuel distribution and flame front travel within the combustion chamber to be assessed.
Imaging the combustion process is by no means a new development, and for many years high-speed photography, combined with bespoke test engines featuring viewing windows, have been used for such tasks. While effective, the cost of such development is very high, a factor that is now being addressed with the introduction of non-intrusive imaging systems that can be retrofitted to existing engines.
One such system uses an endoscope to provide images from within the combustion chamber. This can be used to gather both visible and ultraviolet light from the combustion process. Mixture distribution within the combustion chamber can be observed using the visible light spectrum, while the ultraviolet spectrum is used to view the combustion event itself.
Different types of processing equipment are required to record images of these two processes. To capture images within the visual spectrum a CCD (charge-coupled device) very similar to those found in a regular digital camera is used. For in-depth analysis of combustion though, a CCD cannot capture all of the UV spectrum, so instead the images are directed at a series of photomultipliers that can record the entire UV spectrum. The endoscope includes about 10,000 optical fibres in an inner tube, while an outer, concentric tube provides additional mechanical protection; the space between them is filled with compressed air for cooling.
The compact dimensions (4 mm in outer diameter) mean the system can easily be adapted to series engines without any major modifications in engine design. The installation of the endoscope needs only a small access hole in to the combustion chamber, similar to the ones used for pressure transducers. The endoscope is mounted into a probe, which is protected by a fused silica window. The endoscope allows signals to be captured within an observation cone of about 80º. When only images within the visual light spectrum are being captured, the combustion chamber needs to illuminated; this is provided by a secondary fibre optic illumination probe.
The ability to image the combustion chamber on any engine – provided of course that access can be created for the probe – is highly beneficial. Not only can the spray pattern of injectors be assessed but conditions such as knock can also be identified and isolated, which is of particular importance given the ‘lean’ nature of DI engines.
Written by Lawrence Butcher