Influence of wall-wetting conditions on in-flame and exhaust soot structures in a spark ignition direct injection petrol engine

Abstract

Great attention to the efficiency benefits of spark ignition direct injection engine has been averted due to its problematic particulate emissions. In the present study, the fundamental knowledge of wall-wetting-induced spark ignition direct injection soot particles is enhanced through direct particle sampling from pool fire on the piston top surface and cylinder liner as well as from the exhaust stream. The sampled soot particles are imaged using transmission electron microscope, and the image post-processing for statistical morphology and internal structure analysis is performed to better understand the soot formation and oxidation processes. The experiments were performed in a single-cylinder optical spark ignition direct injection engine where diffusion flame luminosity was recorded using a high-speed camera through the cylinder liner window, with which the overall sooting level was understood, and the pool fire location was identified. Given the in-flame soot sampling experiments in the spark ignition direct injection engine were new, error analysis was conducted in terms of the number of fuel injections and engine run-to-run variations. This sampling technique then was applied for various injection timings in the intake stroke. The data analysis and physical interpretation was focused on a piston-wetting condition at the most advanced injection timing of 320 °CA bTDC and a liner-wetting condition at the most retarded injection timing of 180 °CA bTDC in the present study. Between these two different wall-wetting conditions, it was found that the piston-wetting condition has larger soot primary particles and soot aggregates. The internal carbon-layer fringe shows longer length, less tortuosity and smaller gap, indicating more mature and carbonised soot. This was consistent with more significant and wider distributed pool fire and thus longer soot residence time within the flames. When the exhaust soot particles were analysed, however, it was found that the reduction in soot aggregate size was much higher and the carbonisation was more progressed for the piston-wetting condition than those of the liner-wetting condition. This suggested higher soot oxidation later in the expansion/exhaust stroke for the piston-wetting condition, which potentially can be better utilised for engine applications.

Soot particles in piston-top pool fires and exhaust at 5 and 15 MPa injection pressure in a gasoline direct-injection engine

“…To create thermally stable condition, water was heated to 363 K and circulated through the cylinder head, liner and engine block. The optical engine is naturally aspirated, and the intake air temperature was held constant at 303 K. A side-mounted six-hole injector (Continental DI XL2) supplied a conventional RON 95 petrol [16] at a fixed early injection timing of 320 crank angle before top dead centre ( o CA bTDC) to simulate significant piston-wetting conditions [17]. Typically, conventional SIDI engines run at low injection pressure for low-speed, low-load operations, which is increased with increasing engine speed and load conditions.…”

Visualization of soot formation in load transients during GDI engine warm-up

“…This leads to the formation of an inhomogeneous mixture during combustion, causing increased wetting of the cold piston head and cylinder walls and thereby increasing particulate number emissions. 15–17…”

“…This leads to the formation of an inhomogeneous mixture during combustion, causing increased wetting of the cold piston head and cylinder walls and thereby increasing particulate number emissions. [15][16][17] Particulate emissions in GDI engines are a heavily sought-after research topic in the recent year with increasing stringent regulations. Extensive studies have established reduction strategies with respect to engine technology, operating conditions, and engine aftertreatment.…”

Flow directed spark stretch and flame propagation in a high-tumble production engine

“…One can retard the fuel injection timing closer to the spark timing so that the injection-induced momentum can impact the flow and turbulence at the time of ignition. 4 However, late injection timings cause the increased soot emissions associated with fuel wall wetting (pool fire conditions) and locally rich mixtures (diffusion flames), 6,7 leaving only a narrow range of injection timings in the intake stroke and near bottom dead centre for practical engine operations. 8 Therefore, the tumble enhancement and structure optimisation are primarily dependent upon the design of intake port passages even in DISI engines.…”