Investigation of local inhomogeneous ignition in fuel/air mixtures in a high-pressure shock tube

Stringent emission requirements around the world, particularly in the automotive market lead to increasing efficiency of internal combustion (IC) engines and major developments in the exhaust after-treatment. The set targets require an increase in efficiency with the simultaneous use of renewable energy like biomass -derived or synthetic fuels. Technologically, the requirements have led to two trends in IC engines design: Downsizing, i.e., reducing the stroke volume while maintaining the same power, and down-speeding, i.e., reducing the engine speed (RPM) while increasing the torque. The increase in power density is achieved by increasing the boost pressure and the associated temperature rise. In conjunction with these changes in operating parameters, abnormal ignition phenomena have been observed in spark ignition (SI) engines that can lead to detonation with extreme pressure oscillations and engine damage. These ignition phenomena differ from classical knock on the one hand by much higher pressure rises and on the other hand by their random occurrence independent of the ignition angle. Because this abnormal combustion occurs randomly under operation of low engine speed and high load, it is called low-speed pre-ignition (LSPI).

It is assumed that local ignition sources like particles, lubricant oil droplets or hot surfaces can initiate ignition in the compression stroke. In the present work, local ignition in homogeneous gas mixtures and the influence of local ignition sources like particles, hot surfaces and lubricant oil on the ignition delay time were investigated in a high-pressure shock tube for the first time. Shock tubes ideally simplify the conditions in an engine (approaching the influence of transport processes), which allow the investigation of reactive fuel mixtures at constant pressure and temperature after nearly instantaneous compression.

In a first step, the ignition delay time, spot and homogeneity of the ignition of the primary reference fuel PRF95 (95 vol. % isooctane and 5 vol. % n-heptane) and of ethanol in air at equivalence ratios (f) of 1 was investigated under engine-like conditions (p5 = 20 bar, T5 = 750–950 K). Therefore, the high-pressure shock tube was equipped with an optic accessible endwall. The ignition was observed by two high-speed cameras one in the visible (VIS) and one in the ultraviolet (UV) spectrum simultaneously.

Investigations of the gas-phase ignition have shown, that depending on the temperature range, the ignition is not fully homogeneous for the undiluted mixtures. Local ignition near the wall in the vicinity of the endwall as well as upstream (caused by hot particles) were observed. The hot membrane particles were found not to influence the ignition delay time. In contrast, local gas-phase ignition resulted in a reduction of the ignition delay times. Targeted dilution of the reactive gas mixture with components that either increase the heat capacity or the thermal diffusivity of the mixture enabled the prevention of local ignition in the pure gas phase.

The influence of defined inhomogeneities on the auto-ignition of the mentioned fuel/air mixtures was analyzed. The influence of a hot surface on the ignition delay time was determined by inserting a glow plug in the sidewall of the shock tube and the associated non-uniformity of the temperature field. Non-uniform temperature distribution induced by the hot surface led to local ignition for gas temperatures behind the reflected shock wave (T5) below 850 K and surface temperatures Ts above 1000 K. No influence on the ignition delay time was observed.

Previous studies have shown that the formulation of lubricant oil influences the frequency of LSPI events in engine experiments. It is unclear whether the lubricant oil is decomposed, and oxidation products favor the local ignition or if processes in the liquid phase of the droplet are more important. For clarification, the problem was investigated in this work in three steps.

In a first step, lubricant oil samples were analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in combination with quadrupole mass spectroscopy (QMS). Ignition delay time measurements of PRF95/air mixture at f = 1 plus up to 1000 ppm of molecules identified as potential promotors containing sulfur showed no effect.

To mimic the process of oil droplets that are hurled into the combustion chamber under representative and repeatable conditions, single droplets were injected behind the reflected shock wave through a sidewall port into the high-pressure shock tube. The droplets contained n-dodecane, lubricant base oil, and/or different single additives and/or liquid PRF95. The injected droplets were observed by laser induced fluorescence in combination with a high-repetitive imaging. Therefore, each sample was doped with 0.1 mmol/l of pyrromethene 597. The local ignition, propagation and volumetric ignition was observed and ignition delay times over temperature were determined for each sample. Droplets of n-dodecane and base oil with the addition of an anti-wear additive based on zinc diphosphate (ZDDP) or a detergent additive based on calcium showed significantly faster local ignition delay times than the presence of pure base oil droplets and accelerated the volumetric ignition delay time significantly compared to cases without injection of droplets.


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