Variable Air Induction Systems

Purpose of Variable 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.

Toyota Variable Induction System (T-VIS)

T-VIS System Components

The T-VIS system is the fast variable induction system used on Toyota engines built for use in the U.S.A. The system is used on the 3S-GE, 3S GTE, and 4A-GE engines. The induction system consists of the following components:

• Four individual intake air control valves supported on a common shaft
• Vacuum actuator which rotates the shaft
• ECU controlled Vacuum Switching Valve (VSV)
• Vacuum storage tank

T-VIS System Operation

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.

T-VIS Vacuum Switching Valves (VSV) and Operating Strategies

There are two different VSVs used for T-VIS control depending on application. The 3S-GE engine uses a VSV with a normally open valve. This VSV passes vacuum to the actuator when de-energized by the ECU. During low speed operation, the ECU keeps the VSV de-energized to keep the actuator applied, closing the air control valves. At high speed operation, the VSV is energized, blocking the vacuum supply to the actuator and bleeding any trapped vacuum. This allows the air control valves to open.

The 3S-GTE, 4A-GZE, and 4A-GE engines use a VSV with a normally-closed valve. This VSV passes vacuum to the actuator when energized. When the engine is operating at low speed, the VSV is energized, allowing vacuum to apply the actuator and hold the air control valves closed. When the ECU parameters are met to open the air control valves, the ECU de-energizes the VSV to block the vacuum signal to the actuator and bleed trapped vacuum from the actuator diaphragm.

3S-GTE Fuel Judgment Strategy Using signals from the knock sensor, the 3S-GTE engine incorporates a fuel judgment strategy to control maximum turbo charger boost pressure and T-VIS operation. When fuel is judged to be of premium grade, the T-VIS system functions as outlined above. The ECU will de-energize the VSV when engine speed reaches approximately 4200 rpm. When fuel is judged to be of regular grade, however, the ECU operates the air control valves based on throttle position. When the IDL contact is closed (closed throttle), the VSV will be energized, allowing vacuum to pass to the actuator, closing the air control valves. When the IDL contact opens, the ECU de-energizes the VSV, blocking the vacuum supply to the actuator, causing the air control valves to open.

Acoustic Control Induction System

The Acoustic Control Induction System (ACIS) is used on the 7M-GE, 3VZ-FE and 2JZ-GE engines. As with the T-VIS system, the purpose of this system is to improve engine torque throughout the engine rpm range. The system consists of the same basic components as T-VIS and operates similarly. The ACIS system uses a single intake air control valve located in the intake as chamber which effectively changes intake runner length as it opens and closes.

ACIS Air Control Valve

The main difference between T-VIS and ACIS is the single butterfly air control valve used in the ACIS system. This valve is located in the center of the intake manifold plenum and is designed to divide the plenum into two sections, a front chamber and a rear chamber. By dividing the manifold plenum into two chambers, the effective manifold money length can be controlled by opening and closing the valve.

ACIS System Operation

The ECU controls the position of the intake air control valve based on input signals from throttle angle (Vta) and engine rpm (Net. The VSV which controls the vacuum supply to the actuator, is normally closed and passes vacuum to the actuator when it is energized by the ECU.

By energizing the VSV vacuum is passed to the actuator, closing the air control valve. This effectively lengthens the intake manifold run- the velocity of intake air flow at the intake valve. By de-energizing the VSV, vacuum to the actuator is blocked and trapped vacuum is bled off of the actuator diaphragm. Without vacuum, the air control valve opens, effectively shortening intake runner length. This logic is the same as that used on the 3S-GTE T-VIS system.

The following charts represent ECU operating strategy for the ACIS system.

The T-VIS and ACIS systems are wired to the ECU as shown in the electrical wiring diagram. During troubleshooting, a voltmeter can be used to monitor the ECU command to the VSV. Voltage measured at the T-VIS or IACV terminal of the ECU will be high (battery voltage) until the ECU energizes the VSV. The ECU logic to drive the air control valve VSV, and the voltage measured, will differ depending on the applications and strategies outlined above.