Toyota T-VIS System
Variable Air Induction Systems
Toyota engines have taken advantage of four valves per cylinder technology throughout the later half of the 1980s. This cylinder head and valve arrangement allows better engine performance at high rpm by improving the engine's volumetric efficiency. Enlarging the port area of the intake valves does little for engine breathing unless the intake manifold is enlarged as well By enlarging the intake manifold runners and plenum area, a greater volume of air is available to the intake valves at high engine rpm.
At lower engine rpm, however, an enlarged intake runner has a negative effect on volumetric efficiency due to reduced air velocity at the intake valve. This characteristic causes a four valve engine to have a very healthy torque curve at high engine speeds but a comparatively weak one at lower engine speeds. The variable induction system is designed to give a four valve engine the best torque characteristics at both low and high engine speeds. This is accomplished by changing either the effective length or diameter of the intake runner through the use of an intake air control valve. This valve is activated by an ECU controlled Vacuum Switching Valve (VSV) and vacuum actuator.
The intake manifold feed for each cylinder is divided into two separate runners. The main runner is provided for low speed operation while the other is provided as the variable induction runner. Each intake runner is purposefully designed to flow approximately half of the air volume required by the engine at full power. The variable induction runner is equipped with an intake air control valve. The main intake runner supplies air to the intake valves at low speeds. Intake air flows at high velocity due to the long and narrow runner design. The intake air control valve opens the variable induction runner when adequate engine rpm is reached, thereby providing sufficient air volume for high speed operation. This design makes it possible to maintain strong engine torque at both low and high engine rpm. The intake air control valves, installed between the cylinder head and intake manifold, are all closed simultaneously by the vacuum actuator when vacuum is applied. When vacuum is relieved from the vacuum actuator, the air control valves return to their fully open position.
When the engine is running below the 4000 to 5500 rpm threshold, manifold vacuum from the vacuum storage tank is supplied to the actuator through the ECU controlled VSV. The vacuum storage tank is required as a vacuum reservoir to hold the intake as control valves open whenever the engine rpm is below the operating threshold but manifold vacuum is too low to hold the intake air control valves closed. When the pre-programmed rpm is reached, the ECU signals the VSV to switch vacuum away from the actuator and open an atmospheric bleed. This action causes the actuator to release the air control valves to their open position, allowing maximum air volume to enter the cylinders.
