EMPI - Reactive Fluids

The experimental investigation of the ignition behavior of fuels and the description of the results with reaction kinetics mechanisms form the basis for the optimization of processes in internal combustion engines and for the use of alternative fuels. Understanding ignition is also essential for the expansion of engine combustion to very fuel-rich conditions as part of a polygeneration concept for the simultaneous generation of mechanical energy and valuable chemicals. Corresponding measurements are primarily carried out in a high-pressure shock tube under pressure and temperature conditions similar to those in realistic engines. The ignition delay time is determined by measuring the CH* and OH* chemiluminescence. Gas-phase products formed are collected using very fast sampling and determined using GC/MS.

Ignition experiments are also carried out in shock tube 3 at pressures of up to 5 bar. The progress of ignition is detected here using IR spectroscopy by determining both CO concentrations and the temperature in a time-resolved manner, providing the basis for the validation of reaction kinetics mechanisms.

Ignition in the presence of oil droplets is highly relevant for modern internal combustion engines with high power density in the context of low-speed pre-ignition (LSPI). This process was investigated in the high-pressure shock tube by injecting oil droplets of various compositions into a reactive fuel/air mixture and determining their influence on ignition using chemiluminescence measurements and high-speed camera measurements.           

Non-ideality in shock tubes
Shock tubes are frequently used to determine ignition delay times in homogeneous fuel/air mixtures due to their quasi-instantaneous heating and well-defined conditions behind the reflected shock wave. There are, however, limitations to this desired ideal behavior that need to be understood to prevent misinterpretation of the measurement results.
Shock tubes are often regarded as ideal zero-dimensional reactors with a constant pressure and a constant temperature during the measurement time. This assumption is justified for elementary kinetics investigations with low initial concentrations, low pressures, and short measurement times. When determining ignition delay times, however, these ideal conditions are often no longer given, as significantly longer measuring times (> 10 ms) are considered and the high reactant concentration leads to non-negligible heat release, which can lead to the formation of detonation waves and thus also influence the ignition in more distant points of the shock tube. One point that is comparably easy to take into account is the gas dynamic pressure increase (dp/dt) in the measuring plane, which is determined using piezo pressure transducers. By implementing this pressure increase in the simulations, the agreement between simulations and experiments can be greatly improved, especially for fuels such as hydrogen, which have a very high activation energy. The pressure increase can be significantly reduced by structural measures. Much more difficult is the consideration of non-ideal ignition by membrane particles or in regions farther away from the end flange, which occur especially with long ignition delay times and undiluted mixtures. These are investigated with fast high-speed cameras or with pressure/chemiluminescence detectors mounted at various positions along the shock-tube axis. By adding inert gases with high thermal conductivity and by using smaller volumes of reactive gas (constraint volume), the non-ideal ignition effects can be reduced.