The effects of intake pressure on in-cylinder gas velocities in an optically accessible single-cylinder research engine 2020-01-0792
Particle image velocimetry measurements of the flow in the intake pipe and cylinder in an optically accessible single-cylinder research engine were taken to better understand the effects of intake pressure variations on the flow field. At a constant engine speed of 1500 rpm, the optical research engine at Technische Universität Darmstadt was operated at six different intake pressure loads from 0.4 to 0.95 bar under motored operation. The average velocity fields show that the tumble center position is located closer to the piston and velocity magnitudes decrease with increasing pressure load. A closer investigation of the intake flow near the valves reveals sharp temporal gradients and differences in maximum and minimum velocity with varying intake pressure load which are attributed to intake pressure fluctuations. Despite measures to eliminate acoustic oscillations in the intake system, the pressure fluctuations are believed to be caused by the intake valve opening as air from the exhaust pipe in the exhaust stroke flows back into the intake pipe, exciting acoustic modes in the fluid volume. This phenomenon is much stronger in the throttled load cases because of the need for the pressures to equalize. In addition, 1-D simulations of the engine at varying operating loads provide insight into the interactions between geometry, pressure variations, and velocity variations.
This work aims to better quantify these intake pressure fluctuations to understand their effect on flow structures which dictate fuel-air mixing, early flame development, and local heat transfer affecting the late flame development. In better understanding the causes and effects of the fluctuations, measures can be taken in the design of the engine test stand to limit or control instabilities in engine operation.
Cooper Welch, Marius Schmidt, Karri Keskinen, George Giannakopoulos, Konstantinos Boulouchos, Andreas Dreizler, Benjamin Boehm