A direct swirl ratio measurement in a diesel engine combustion chamber under an engine motoring condition

The intake flow within an operating engine exhibits unsteadiness. Most engine manufacturers presently measure the swirl ratio at specific valve lift positions under steady flow conditions. However, this approach fails to accurately represent the actual swirl flow inside the engine cylinder. Despite these limitations, this method has been used due to the substantial challenge of measuring swirl ratio under real engine operation conditions. To address this issue, our study developed a measurement system capable of measuring the swirl ratio in an operating diesel engine. This system involved the use of a BLDC motor to forcefully drive the diesel engine, connected via a reducer that increased torque by reducing the motor’s rotation speed to a 5:1 ratio. The maximum engine speed achieved was approximately 720 rpm. A blade assembly that could detect the torque induced by the swirl flow within the combustion chamber in a non-contact state was designed. The swirl ratio was calculated by measuring the cylinder pressure and torque according to the crank angle change at a constant engine speed. The newly developed measurement system reliably measured the swirl ratio under actual engine operating conditions. Within the crank angle range of −10° to 10° (where 0° corresponds to the top dead center (TDC) and typically encompasses the fuel injection phase in direct injection diesel engines), there was little difference in the swirl ratio Rs value with engine speed changes.