Measuring tappet rotation in a valvetrain rig when lubricated in a fully formulated oil containing MoDTC-type friction modifier

Abstract

In a direct acting valve train configuration, tappet rotation plays a key role in improving lubrication, reducing wear and friction. However, to the best of the authors' knowledge, no studies were found to investigate the rotation of tappet under the effect of different coatings, thicknesses of tappets and formulations with Molybdenum Dialkyl Dithiocarbamate (MoDTC) which has been recently reported to be detrimental to Diamond-Like Carbon (DLC) wear. In this work, a new technique of measuring tappet rotation has been developed. A giant magnetoresistance (GMR) sensor coupled with a split pole ferrite disk magnet was used. The sensor was installed very close to the tappet/bucket while the magnet was mounted into the underside of the tappet. Experiments were performed using standard production steel tappets coated with Mn-phosphate (MnPO4) and diamond-like carbon (DLC) coatings. In general, results showed that the tappet rotation is strongly dependant on oil formulation, clearance, speed/temperature, and surface roughness of the coating. MoDTC promoted the rotation of the tappet under both coatings. In addition, DLC inserts showed an increase in tappet rotation as compared to MnPO4 inserts. Nevertheless, regardless of the type of coating, the thickest tappets showed the highest rotation.

The effect of clearance between tappet insert and camlobe on the tribological and tribochemical performance of cam/follower surfaces

“…7, the smaller clearance of tappet inserts generally gave a higher friction torque, suggesting a high asperity interaction at the cam/follower interface along with less reactivity to lubricant additives. More recently, the authors of this work have reported that tappet rotation is affected by changing the clearance of the tappet inserts [16]. However, regardless of tappet rotation, the tappet insert with smaller clearance offered high friction torque.…”

“…Further, the experimental details have already been presented in [16]. Basically, the rig was used in a non-fired mode (i.e.…”

“…For MnPO 4 tappet inserts, two oil temperatures (100 °C and 130 °C) were applied. For DLC tappet inserts, however, the test was only carried out at 100 °C due to limited availability of sensors used in tappet rotation measurement; further details are available in reference [16]. Molybdenum Dialkyl Dithiocarbamate (MoDTC) additive has been extensively used in engine oils as a friction modifier since the 1960s.…”

“…boundary/mixed regime). The tappet insert with smaller clearance was discussed and understood to provide a longer contact area between the camlobe and the follower [16]. This could potentially lead to a higher friction torque for the thicker tappet insert.…”

“…the tappet insert with smaller clearance generally showed higher friction torque). This is best described in [16] where the percentage of the cycle increases as the tappet clearance decreases. Also, the average friction torque when using DLC coating was decreased compared to inserts coated with MnPO 4 .…”