ABSTRACT General Motor's new front wheel drive six speed 6T40 automatic transmission benefits from patented technology to control high speed pump cavitation that is innovative yet cost effective. An annular nozzle is created with careful pump inlet design by integrating a conical section in the filter neck to create a jet pump to prevent high speed cavitation for almost no additional cost. Excess oil from a fixed displacement pump is used to achieve an effective increase in pressure at the inlet of the rotating group during high speed operation. Control of high speed cavitation reduces pump noise and improves line pressure control stability. INTRODUCTION An attribute of all automotive transmission hydraulic pumps is a condition called cavitation. Good pump design can defer the onset of cavitation to higher engine speed where cavitation noise is normally masked by the engine. Severe cavitation can effectively limit the maximum acceptable operating speed to avoid noise or even damage to the pump. The annular nozzle described in this paper shows a method to diminish cavitation noise in fixed positive displacement rotating pumps during high speed operation. Three-axis front wheel drive (FWD) transmission architecture has severe space constraints for axial packaging due to the location of the transmission gearbox on the engine axis. Combined with a goal of reducing transmission cost, this led to the selection of an on-axis fixed displacement gear pump for the 6T40 transmission. The cast iron pump cover serves a dual role as the transmission front support for the output chain and includes the stator shaft to provide reaction torque to the torque converter ( Figure 1). Die cast aluminum is used for the pump body with many net form features to helpreduce part count in the pump assembly. The 6T40 pump / valve body does not require a spacer plate or gaskets since metal to metal sealing between the pump cover and pump / valve body is utilized to save cost and axial space. Valves were incorporated into the pump design ( Figure 2) both to keep the bypass circuit very short and to enable elimination of a layer of the main valve body for better transmission packaging to fit multiple vehicle applications. The 6T40 transmission pump demonstrated first time capable design which simply means that it worked to meet expectations from the time the very first prototype was built. This results in significant product development savings in terms of development time and cost. Use of an annular nozzle to control high speed cavitation was an enabler to quiet high speed pump operation with stable line pressure. BACKGROUND A fixed displacement pump provides a set amount of oil per each revolution of the pump; such as 20 cc per revolution. Theoretical flow rate is proportional to rotational pump speed, exclusive of internal pump leakage. The pump is typically driven at engine speed in a conventional non hybrid transmission. In practice, a limiting speed is reached where pump chambers can no longer be completely filled with oil. This limiting speed is called the “High Speed Fill Limit” (HSFL) for the pump. A typical HSFL in a conventional pump without nozzling may be 3500 rpm. The lack of completely filling the pump chambers with oil results in air being drawn thru the pump inlet and mixed with the oil. This two-phase mixture of air and oil inside the pump can create a condition called cavitation. When the pump is above the HSFL; the flow rate of the pump levels off to become independent of further increases in speed. Unfortunately, the cavitation phenomena can cause pressure instability that interferes with the transmission control valves, while the Annular Nozzle in Fixed Displacement Transmission Pump2011-01-0723 Published 04/12/2011 John Schultz General Motors Company Copyright © 2011 SAE International doi:10.4271/2011-01-0723Downloaded from SAE International by American Univ of Beirut, Sunday, July 29, 2018cavitation noise occurring inside the pump