Variable Camshaft Timing (VCT)

General
The use of the new variable camshaft timing optimizes the combustion procedure by improving gas exchange in the cylinders. As a result of this the exhaust emissions regulations can be met without the use of a pulse air system or an Exhaust Gas Recirculation (EGR) system.

Mechanical Operation

Mechanical Design Of The VCT Unit:

The mechanical principles behind Variable Camshaft Timing (VCT) are relatively simple. The valve timing for opening and closing of the exhaust valves is influenced by a hydraulic cylinder (3) which is riveted to the pulley (1). This cylinder connects the pulley (1) with the camshaft (8) through a hydraulic piston (4) and inner gear bush (6). The hydraulic piston is guided along its axis by the helical gears (6,7) on the exhaust camshaft and the hydraulic cylinder. The helical gear transfers the up and down motion of the hydraulic piston into a rotary motion. As the pulley is fixed in place by the timing belt, the position of the exhaust camshaft is rotated in relation to the pulley, and the exhaust valve timing is therefore adjusted. The hydraulic piston is moved by supplying pressurized engine oil from the engine oil circuit to both pressure chambers of the hydraulic cylinder. The engine management system controls a solenoid valve which in turn supplies the pressurized oil to the pressure chambers.

The return spring (3) ensures that the hydraulic piston goew returns towards its original position when the engine is switched off.

Emergency mode:
^ If oil pressure is to low, the VCT will be regulated to the "END-STOP" position.
^ In cause of an electrical error, the solenoid will regulate to the "END-STOP" position.

Hydraulic Operation

VCT Unit In Original Position And At Maximum Extension - 1 Of 2:

VCT Unit In Original Position And At Maximum Extension - 2 Of 2:

The VCT unit is supplied with engine oil (3) from the engine oil circuit and controlled by the engine management system. The engine management system regulates the solenoid valve (2) through a Pulse Width Modulation (PWM) from 0 - 100%.

With rising current (0-1A) the displacement of the control piston (4) in relation to the return spring (6) increases, and vice versa. As a result of this motion the control edges and return bores (5) of the control piston open and close the oil flow to and from the two pressure chambers (11, 12) of the hydraulic cylinder (9).

When the engine is switched off the return spring returns the control piston back to its original position. At the same time the return bores in the front pressure chamber (12) are opened, which allows the return spring (11) to slide back the piston in the hydraulic cylinder towards its original position.





The hydraulic unit can be infinitely adjusted in the hydraulic cylinder. However, the control piston can only move into three positions.