Air and Fuel Characteristics in the Intake Port of a SI Engine 1999-01-1491
The interaction of fuel sprays and airflow in the intake system of a port fuel-injected spark-ignition engine has been examined experimentally in a pulsating-flow rig which comprised the cylinder head and intake manifold of a production engine connected to a large-capacity plenum chamber, with the camshaft of the intake valves driven by an electrical motor at engine speeds between 1000 and 5000 rpm and with air sucked through the system by a suction fan. Static pressure measurements in the intake port showed periodic pulsations with frequencies of 360 and 200 Hz with open and closed valves, respectively, and these corresponded to quarter- and half-waves in the manifold and were independent of engine speed. Cycle-resolved measurements of air velocity quantified the damped oscillations in the port during the period when the intake valves were closed, with the same characteristic frequency as that of the pressure and with amplitudes decreasing from about 10 to 5 m/s; the velocity increased rapidly to 70 m/s as the valves opened and the frequency of the velocity and pressure oscillations increased to about 360 Hz. A one-dimensional calculation method was also employed to provide accurate estimates of pressure and velocity variations in the intake runner.
Fuel (iso-octane) was injected into the port at three time intervals relative to the position of the intake valves so that the spray arrived when the valve was closed, opening or fully open. The duration of fuel injection was constant at 5 ms and the velocity of fuel droplets and air at various locations in the intake port of one cylinder with were measured together with the wall static pressure in the port and of the mass flow at exit from the plenum. The results showed that the trajectory of the spray was directed towards the lower wall of the port with injection against the closed valves and that droplets passed through the measurement volume over more than twice the injection duration. Larger droplets, with higher inertia, arrived with the spray tip and smaller droplets arrived later, slowed by the aerodynamic drag. With open valve injection, the fuel crossed the control volume as a compact spray and the velocity increased with airflow but the droplets were unable to follow the steep velocity gradients leading to relative velocities between air and fuel of up to 30 m/s. A large portion of the fuel was lifted by the co-flowing air towards the upper half of the port and around the partition between the two intake ducts, and this was confirmed by visualisation.