Combustion of Alternative Fuels: Opto-Mechanical Design and Optical Investigation

There is a considerable amount of research that presents observations and measurement results obtained from optically accessible reactors such as piston engines and constant volume chambers. However, the aspect of design and relevant information regarding the optical access are not sufficiently explored in the existing publications. A review of literature on optical access for high-pressure and high-temperature design cases was conducted and is presented along with a comparison of optical materials.
It was found that the optical materials have sufficient mechanical strength; however, there were certain limiting factors of design, namely the working temperature, the required electromagnetic (EM) wave range, and cyclic loading. It is especially difficult to carry out an accurate design for cyclic loading, as the literature lacks the relevant data for optical materials. As a result of the significant uncertainty arising from the inconstancy of design data, it is suggested that the value of the safety factor should be between two and five, depending on the probability of failure and risks. The design process is shown in detail for two case studies of the opto-mechanical design of pressure vessels.
Utilising the optical access, a high-speed imaging system was used to record the ignition and flame kernel formation in the internal combustion engine. A range of fuels were investigated, including gasoline, isooctane, and a few alternative renewable fuels: E85, M85, and hydrogen. The experiments were conducted at stoichiometric conditions for the liquid fuels and φ=0.67 for hydrogen at various engine speeds and compression ratios.
A novel analysing method was proposed to process the acquired raw optical data, where ellipses, rather than conventional spheres, were fitted onto image projections of the visible light emitted by the flames. A cross-comparison of the results with the available data from the literature was also conducted. The large amount of optical data allowed the statistical evaluation of the flame area, flame speed and a flame-shape descriptor.
The image analysis showed that the ellipse fitting method provided a 50–100 % better fit and thus allowed a more accurate description of the flame propagation properties. The results indicated that gasoline and isooctane had similar flame propagation behaviour, but a significant difference was observed between these fuels and E85, M85 and hydrogen. Similarities were found between the propagation characteristics of M85 and hydrogen, showing the fastest propagating flames among all the fuels. The statistical analysis found that the precision of the flame speed measurement and the roundness of the flames increase with the engine speed, compression ratio, and time elapsed after ignition.