Endoscopic high-speed particle image velocimetry for flow and turbulence measurements in multi-cylinder engines

Abstract

Demands for high efficiency spark ignition engines have led to intake port and combustion chamber design achieving enhanced flow and turbulence. The present study reports a significantly improved version of the original endoscopic high-speed particle image velocimetry (eHS-PIV) developed and implemented in a high-tumble production engine to show flow structure details (Kim et al., 2020) and then their variations with engine operating conditions in several follow-up studies. The eHS-PIV has been improved for a finer pixel resolution while the PIV laser coverage is expanded to show the entire pent-roof region and upper half of the cylinder. This enables the time-resolved analysis of flow field and turbulence intensity distribution for the intake flow issued through the intake valves and its interaction with the piston. The new eHS-PIV also measures the in-cylinder flow/turbulence during the compression stroke. For the eHS-PIV measurement, a 532 nm Nd:YAG laser beam is supplied through a rigid endoscope wherein a series of rod lenses producing a 60-degree diverging planar laser sheet. Two laser endoscopes are installed on the spark plug hole vertically and the exhaust-side of the cylinder head horizontally. Normal to this plane, a camera endoscope is installed to capture the PIV signals. For PIV seeding, hollow-glass spherical particles are supplied at the upstream of the throttle body. The new HS-PIV diagnostic was applied to a selected multi-cylinder, naturally aspirated 1.6-litre engine operating at 1600 and 2000 revolutions per minute. The intake air flow rate and throttle position were controlled to simulate a fixed 60 Nm load condition. For each engine speed, a total of 100 motored cycles were recorded and the spatial filtering method with 7 mm cut-off length was used to extract turbulence intensity from each individual cycle image. The eHS-PIV was successfully demonstrated for the selected engine and operating conditions. It was measured that the high-tumble engine produces lateral intake flow and intense tumble flow that is skewed towards the exhaust side for both engine speeds with the higher engine speed resulting in higher intake flow magnitude and more lateral flow direction. During the piston compression, the upward flow motion is measured, which is also stronger at higher engine speed. The resulting tumble vortex formation is enhanced at higher engine speed and its interaction with the remaining upward flow vectors becomes more significant, leading to a more complex flow structure near top dead centre. This leads to not only higher flow magnitude but also increased turbulence intensity at higher engine speed. The turbulence is particularly higher at the interface between the tumble vortex and the remaining upward flow vectors.