Throttle Body at Engine Idle - Tolerance Effect on Flow Rate

A small airflow rate at engine idle is required to maintain a low engine speed and to save fuel consumption. Since the throttle plate is almost closed at idle, the plate and bore tolerance becomes important in determining the plate open area and thus the airflow rate. The objective of this work is to use computational fluid dynamics (CFD) analysis as a tool to aid throttle body design and to find out how the tolerance affects the airflow rate. Also, the conventional equation for calculating the throttle plate open area is modified to include the leakage area which is no longer negligible at idle.

Throttle bodies with plate closed angles of 4.0 and 4.5 degrees under tight and loose fit conditions were studied. The flow regions above and below the plate are connected by a narrow region between the plate and the bore. This sudden change in flow area creates a big pressure loss across the plate. Therefore, even though the pressure ratio is smaller than the critical value (0.528) for a sonic flow under the 1-D isentropic assumptions, the predicted flows are subsonic at idle. Pressure distributes almost uniformly above and below the plate. Loose fit produces a flow rate 4 to 5 times larger than that under tight fit.

CFD result was verified by comparing test data at a plate angle of 10 degrees where the tolerance effect on the plate open area is much smaller. Weak supersonic flows exist across the plate. The discrepancy between the predicted and the measured flow rates is 7.8% which is reasonably good.