Modulation of integral length scales of turbulence in an optical SI engine by direct injection of gasoline, iso-octane, ethanol and butanol fuels

Abstract

In-cylinder air flow structures are known to play a major role in mixture preparation and engine operating limits for spark-ignition engines. In this paper PIV measurements were undertaken in an optical spark-ignition at 1500RPM, part-load with 0.5bar intake plenum pressure. One of the PIV planes was vertical, cutting through the centrally located spark plug (tumble plane). The other plane was horizontal 1mm below the spark plug ground electrode (swirl plane). The effect of engine head temperature was also examined by using engine-head coolant temperatures of 20°C and 80°C. The flow field was examined late in the compression stroke at typical ignition timing. The study was conducted under air-only motoring engine conditions but also under fuelled conditions in the early intake stroke using direct injection of gasoline, iso-octane, ethanol and butanol fuels. The flow field under air-only motored conditions showed velocities between 3 and 5m/s predominantly from intake to exhaust. Little differences were observed between hot and cold engine-head temperature; typically ∼10% larger mean velocity and turbulent kinetic energy was seen on the intake side. Integral length scales were on the tumble plane between 2 and 4.5mm in the vertical and 4–7mm in the horizontal direction. The swirl view showed scales between 4 and 10mm, larger at cold than at hot engine-head conditions. The vertical length scales appeared to be limited by the clearance height, scaling typically by about 10–15%. The horizontal components scaled to the cylinder bore diameter by about 5–12%. The fuel injection process in the early intake stroke led to little differences in the general mean flow structure at ignition timing, except for a small increase in the maximum velocities, ∼10%. The turbulent kinetic on the tumble plane was highest towards the exhaust side of the engine under non-fuelled engine conditions but fuel injection resulted in highest values on the intake side. The integral length scales with fuel injection were of the same order of magnitude to those of air only measurements on the tumble plane, but showed distinctly larger areas with length scales up to 9mm on the swirl plane. Differences between fuels for their fuel specific injection duration were small, with average length scales between 4 and 6mm. Ethanol exhibited typically largest scales and butanol smallest.