Toyota's DOHC VVTL-I
Toyota's first variable valve timing with intelligence system, dubbed VVT-i, first appeared on the road in the Lexus GS/LS/SC 300/400 in 1998. The addition of variable valve timing made it possible to control intake valve timing for optimal performance. VVT-i can be seen as a high-tech adjustable cam sprocket. Unlike conventional adjustable cam sprockets, which have to be set prior to starting the engine, the VVT-i is an on-the-fly proposition.
The VVT-i system uses engine speed, intake air volume, throttle position and water temperature to calculate optimal cam timing. The ECU can advance or retard the intake cam depending on light, medium or heavy load, optimizing output throughout the entire powerband. Like Honda's VTEC technology, Toyota's VVT-i system was already incorporated in Japan for a period prior to its introduction into the U.S. automobile arena.
The system was taken one step further in 2000, by incorporating a changeover mechanism, which varies the amount of lift seen by the intake and exhaust valves while the engine is operating at high speeds. The new VVTL-i system first debuted in the Toyota Celica GT-S (2ZZ-GE engine). By joining the innovative VVT-i system with variable lift, the 2ZZ-GE powerplant produces a very broad powerband.
Toyota's VVT-i controller works in five different ranges.
Variable Lift Activation
There are certain variables that have to come into play before the VTEC and VVTL-i system are activated. The Honda B18C VTEC engine requires the right rpm input, water temperature, throttle position signal, oil pressure signal and vehicle speed sensor reading before the VTEC system is engaged. If any one of the five variables is out of sync or not functioning, the system will not engage.
Toyota's VVTL-i system is far less complicated and only requires water temperature, crankshaft position sensor and engine rpm before the lift mechanism is activated.
The B18C VTEC changeover point is a surprisingly low 4400 rpm compared with Toyota's 6000-rpm changeover point.
Valve Lift Mechanism and Cam Design
Both Honda's VTEC and Toyota's VVTL-i utilize oil pressure to activate a locking mechanism to achieve higher valve lift. However, the two manufacturers achieve this from two different avenues. The major difference can be found in the cams.
The B18C VTEC cams utilize a total of 24 lobes (12 on the intake side and 12 on the exhaust side). The 16 primary lobes are directly on top of the 16 valves. The other eight VTEC lobes, which are sandwiched between the primary lobes (one for every two primary lobes) are positioned between the two valves (see photo A). As the VTEC motion assembly is locked in position, it only requires one VTEC lobe to actuate both valves.
The 2ZZ-GE VVTL-i cams only utilize a total of 16 lobes (8 on the intake and 8 on the exhaust). Unlike the VTEC system however, which requires 16 lobes to actuate the primary 16 valves. The VVTL-i system only requires eight lobes. This is made possible by a linked rocker arm assembly (see figure B). At any given time, the rocker arm assembly is being actuated by only one cam lobe per every two valves. The VVTL-i system reduces the need for a third rocker arm assembly like the VTEC counterpart, reducing weight from the motion assembly.