Characterization of future fuels using an optically accessible rapid compression machine

Due to increasingly stringent environmental regulations in the field of energy and transportation systems, future fuels such as ammonia or hydrogen from renewable sources are promising solutions for internal combustion engines in various applications. The design of modern combustion systems for these fuels relies on advanced models and simulation tools which often require ignition, combustion
and knocking properties as input, e.g., laminar burning velocity or minimum ignition energy, which are highly different from conventional fuels. To avoid expensive investigations directly at the engine test bed, fundamental investigations on special test rigs are the key to acquiring the important parameters that describe the combustion behavior of these fuels at relevant engine-like conditions.
This paper presents the results of fundamental investigations to gain insight into different characteristics of the combustion of future fuels within mixtures relevant to large engines. To this end, two different optical techniques are implemented on a rapid compression machine (RCM): Schlieren imaging to visualize the propagating spherical flame front in the optically accessible combustion
chamber and Particle Image Velocimetry (PIV) for flow field measurements in a prechamber. Schlieren imaging is applied to investigate the laminar flame characteristics of NH3/H2/N2/air flames at various cracking ratios, equivalence ratios and initial pressures. Specially designed spark plug electrodes are
employed to minimize the influence of the ignition system hardware on the flame front shape. The PIV investigations use a prototype prechamber of production size that is large enough to allow optical access. Time-resolved planar velocity fields are obtained with which the nature of the flow in the prechamber is examined over the duration of the compression stroke using phase-invariant mean
velocity fields. The scope of this study includes illustrating the implementation of PIV technique in the prechamber and presenting results obtained from the application of the measurement technique.
These fundamental experiments represent an important building block in the overall development methodology of large engines that run on future fuels.