![]() ![]() A spring then returns the cam back to its normal position when oil pressure is relieved. This typically retards cam timing 20 to 30 degrees. When oil pressure is applied to the gear mechanism inside the phaser, it forces the helical gear to move. These are used on many of the older applications and are essentially “on” or “off” actuators. The phaser is part of the cam drive system and is mounted on the cam drive gear or sprocket. The cam phaser changes camshaft and valve timing by rotating the relative position of the camshaft slightly fore or aft compared to its normal base timing setting. Most of the VVT systems you are likely to encounter are those with hydraulically actuated cam phasers, although a few (such as Lexus) use an electric actuator to advance or retard cam timing. This rotates the cam to change valve timing. Volkswagen camphaser: The rotor inside the cam phaser changes position when oil pressure is routed into the chambers. On SOHC and pushrod engines that have VVT, the same cam operates both the intake and exhaust valves, so advancing or retarding the cam simultaneously changes both the intake and exhaust timing. This changes the centerline between the cams and the lobe separation angle, which, in turn, changes valve overlap and duration. On DOHC engines with separate cam phasers for the intake and exhaust cams, the powertrain control module can independently advance or retard the intake and exhaust cams. Valve timing can also be retarded when the engine is under load to reduce NOx emissions (and eliminate the need for an EGR valve). Valve duration can be increased at higher engine speeds to improve breathing and power. Valve timing can be advanced at low RPMs for better throttle response and torque. For this article, we’re limiting our discussion to applications that use some type of cam phasing to change valve timing. ![]() ![]() Some applications also have the ability to continuously vary valve lift on the go, such as BMW’s Valvetronic, Nissan’s CVTC and VVEL and Toyota’s Valvematic. Some applications combine both cam phasing and cam changing such as Audi’s Valvelift, BMW’s VANOS, Honda’s i-VTEC, Mercedes’ Camtronic, Porsche’s Variocam Plus and Toyota’s VVTL-i. In a cam changing VVT system, the position of the rocker arms changes when oil pressure is applied so the rockers will follow a different set of lobes on the cam for increased lift, duration and power. This differs from other VVT systems such as Honda’s VTEC and Nissan’s VVL that are “cam changing” systems. Cam phasing systems essentially rotate the camshaft fore or aft to advance or retard timing. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |