Malfunction due to solid particle contamination can be separated into two types. Type I malfunctions involve large particles (>150 micron), most often the remnants of manufacturing, and generally lead to early life repairs. Type II contamination includes much smaller particles (<50 micron), most of which are either generated wear debris, or particles ground up from larger Type I debris. These Type II particles are responsible for erratic valve performance, poor cooling and lubrication, and the accelerated degradation of Automatic Transmission Fluid (ATF), all of which promote transmission mid-life failures. This paper focuses on Type II contamination only.
Hydraulic system cleanliness has a significant impact on shift timing, feel and general transmission durability. In order to manage the system cleanliness, a uniform repeatable measurement system must be available. Further, the effects of operating conditions must be known and considered. Once operating effects are accounted for, and reliable measurement techniques can be applied, improvements in system cleanliness may be developed and evaluated in a methodical manner.
Using ultra-high efficiency (Ex = 99.96%) bypass micro filters dramatically reduces the contamination level of solid particles in the ATF. Intensive road tests with and without the additional application of such filters are compared. On average, the ATF cleanliness level was measured to be 4-5 points lower on the ISO 4406 standard scale when using NTZ filters. This is equivalent to a reduction by a factor of 16-32 in the number of solid particles responsible for Type II malfunctions.