Variable Induction System: Description and Operation
CONTROL OF VARIABLE INTAKE SYSTEM
The Ignition Control Module (ICM) operates the variable intake system, which consists of:
^ Two manifold pipes (1 and 2) of different lengths and diameters, arranged in parallel. The configuration is optimized for maximum volumetric efficiency at different engine loads and speeds;
^ A vacuum actuator (3) which opens and closes the damper in the shorter intake pipes (2);
^ A solenoid valve (8/14) which opens and closes the vacuum line from the engine. The valve is, in turn, operated by the ICM in response to the prevailing engine speed and throttle opening signals;
^ A vacuum reservoir (4);
^ A non-return valve (5) installed in the vacuum line to the manifold.
Under normal driving conditions, the solenoid valve is not activated by the ICM. The connection between the vacuum reservoir and servo is closed, and the servo spring exerts an upward force on the pushrod, opening the dampers in the short pipes through the linkage.
The ICM activates the solenoid valve at speeds between 1800 and 4200 rpm, if the throttle is simultaneously open by more than approx. 80%. The connection between the vacuum reservoir and servo is opened, and the vacuum overcomes the servo spring force, pulling the diaphragm push rod downward and closing the dampers.
The solenoid valve is not activated at speeds below 1800 rpm or over 4200 rpm and the dampers remain open regardless of the throttle position.
In the event of an electrical or vacuum system fault, the dampers remain fully open and maximum power is maintained at the expense of low-speed torque.
INTAKE SYSTEM
Each cylinder has two intakes of different lengths and diameter. One short one (1) and one which is about twice as long (2) and has a slightly smaller diameter than the short one.
Both intakes start from a plenum chamber (3). The shorter intakes have a damper (4) upstream of the cylinder head.
These dampers are connected and controlled by a vacuum actuator (5) via a lever. The vacuum in the regulator is controlled by a solenoid valve which is controlled by the ignition control module (ICM).
The steel dampers have a rubber seal. When open, the dampers fit into a recess in the intake so as not to interfere with the inlet air flow.
The dampers can only be either fully open or fully closed. This is controlled by RPM and throttle opening signals sent to the ICM.
The intake manifold is mounted directly on the cylinder head. The manifold has a 2.5 liter plenum chamber and is balanced for each cylinder. This ensures uniform gas flow and charging of the combustion chambers.
Injectors are located on the intake manifold close to the inlet valves to ensure optimum fuel/air mixing.
There are two types of Throttle Body (TB) located on the intake manifold - one for the 2.5 liter engine which is 65 mm in diameter and one for the 2.0 liter engine which is 55 mm in diameter.
The TB for the 2.5 litre engine has a plastic profile on the throttle disc. This reduces airflow when the throttle opening is small ensuring the exact correspondence of throttle opening and accelerator pedal movement. This means that air volume, and therefore engine output, is more closely related to Accelerator Pedal (AP) movement.
The 2.0 liter engine has a smaller TB which means that air flow better coincides with AP movement, making a plastic profile unnecessary.
INTAKE MANIFOLD PULSES
The intake manifold is designed to exploit pulses in the intakes at different speeds to better charge the cylinders, allowing the engine to deliver an even, high torque over a wide speed range.
A At RPMs below 1,800 rpm both intakes are open regardless of throttle opening. This allows a rapid increase in airflow when the throttle is opened, which gives a rapid response at low RPMs - when starting and accelerating at low speed.
B At RPMs between 1,800 and 4,200 rpm and if the throttle is more than 80° open, the dampers on the shorter intakes are closed, and only the longer intakes are used. These are tuned for this speed range so that the air in the plenum chambers and intakes is made to resonate with the movement of the pistons and valves. This means that air in the intakes pulses in time with the opening of the valves and is pumped into the cylinder when the intake valve opens. This charges the cylinders better than a conventionally aspirated unit.
A new parameter has been introduced for model year 1994 which takes "load" into account. This means that the dampers on the shorter intakes can be closed before 80°.
C At RPMs above 4,200 rpm both in-takes are open regardless of throttle position. Air can flow through both intakes to the cylinders doubling airflow. At high RPMs the combination of the long and short, larger diameter intakes provides the right resonance for maximum torque.
The long intakes are designed for high volumetric efficiency at RPMs between 1,800 - 4,200 rpm depending on the type of engine. Using the long intakes alone at high RPMs would restrict air supply because the air pulses would no longer correspond to the opening of the valves.
The damper spindles (a long spindle for three cylinders and a short spindle for two cylinders) are supported in bearings in the intakes. The linkage mechanism is designed so that dampers seal completely as leakage between damper and seat will impair engine torque at those speeds where the dampers are closed.
NOTE: In the event of vacuum system faults, the dampers remain open, resulting in reduced torque at intermediate speeds.