Effect of engine speed on in-cylinder flow and in-flame flow in an energy-assisted compression ignition engine

Abstract

The present study applies high-speed particle image velocimetry (HS-PIV), OH* chemiluminescence imaging and flame image velocimetry (HS-FIV) to a blend of 60% conventional jet fuel and 40% synthetic fuel in a small-bore optical compression-ignition engine. The engine is equipped with an ignition assistant plug (IAP) to enhance ignition and combustion of the low reactivity fuel (∼CN 35.5). This study aims to assess the combustion impact of engine speed, which affects the heat transfer conditions and thus thermal distributions and performance of IAP. For engine speed varied between 1000 and 1600 rpm, HS-PIV is performed in one tilted plane and two horizontal planes for in-cylinder flow structure and swirl analysis. High-speed OH* chemiluminescence and flame images are recorded to evaluate the changes in start of combustion and flame structure associated with engine speed variation and IAP activation. Furthermore, flame movies are post-processed to derive in-flame flow vectors using HS-FIV. The results show intake-induced chaotic flow settles to form a clear swirl structure during compression stroke, with stronger swirl flow vectors at higher speed. The swirl centre shifts off from the geometric centre but not affected by engine speed. The OH* chemiluminescence images indicate earlier start of combustion at increased engine speed; however, the signal intensity decreases. The same trend is observed from FIV results with decreased bulk flow magnitude and turbulence intensity for higher engine speed. This is due to a flame-swirl interaction near piston-bowl wall that suppresses the flame flow more at increased engine speed and stronger swirl flow conditions. Regarding IAP activation, the start of combustion is more advanced, with reduced cyclic variations of pressure and bulk flow compared to IAP turned-off condition due to heat addition in the cylinder. However, the benefit of reduced cyclic variations becomes less significant at higher engine speed due to increased heat loss.

Keywords

flame image velocimetry synthetic fuel ignition assistant plug engine speed swirl

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