Introduction:
Variable valve lift is a technology that allows the lift of the valve to be adjusted when the camshaft opens the valve. This controls how far the valve opens. That is beneficial for both fuel consumption and engine power. Variable valve lift is only applied to the intake camshaft. There are several technologies used by different manufacturers. This page describes a few of these technologies.
The image shows how the intake valve is opened further at a higher engine speed.
Axial displacement of the camshaft:
In this system, the camshaft is shifted in the longitudinal direction. The green section on the camshaft indicates the cam height at low revs and low power. At that section, the intake valve will not open very far. The red section on the camshaft indicates the cam height at high revs and/or high power. The valve will then open further. This system is used by Honda and is known as “V-tec”.
The camshaft is shifted because a solenoid valve supplies oil into an oil chamber.
Valvetronic:
BMW’s Valvetronic system makes it possible to adapt the height to which the valve is opened to the operating conditions. With an idling engine, the valves will only open a few millimetres; due to the small air gap, the air flowing into the cylinders will reach a high speed. The high air speed also enables the creation of better swirl. This swirl will distribute the fuel mist more evenly, so that combustion is optimal.
At higher engine speeds or higher loads, the valves will open further.
The Valvetronic system completely takes over the task of the throttle valve, which would normally open further as engine speed and load increase. The throttle valve is therefore redundant and not present on engines that use Valvetronic.
When the engine is idling, the camshaft (1) will push against the intermediate lever (2). The intermediate lever will pivot at the top around the eccentric shaft. As a result, the lower side will move to the left. Because the lower side pushes the roller finger follower (3) downwards, the intake valve will open. When the camshaft rotates further and the cam is at the top, the spring (4) will push the intermediate lever back to its original position. The intake valve closes.
At increased engine speed or increased load, the electric motor will rotate the worm gear and thus also the gear ring of the eccentric shaft (5). Because the eccentric shaft has rotated, the thick section will now tilt the intermediate lever at an angle. The intermediate lever will therefore be positioned at an angle. As a result, the lower side of the intermediate lever will push the roller finger follower of the intake valve a greater distance downward. The intake valve will now open further.
By continuously changing the position of the lever according to engine speed and load, the valves will open alternately further and less far.
MultiAir:
MuliAir is a fully electronically controlled system for controlling the opening and closing of the intake valves individually via an electro-hydraulic actuation. With MultiAir, both the valve lift and the valve timing of the intake valves are controlled. MultiAir is used on both petrol and diesel engines. An adjustable camshaft is no longer needed.
With MultiAir there is a hydraulic connection between the camshaft and the intake valve. The camshaft drives the piston of the MultiAir high-pressure cylinder via a roller finger follower. The piston in this high-pressure cylinder forces the oil towards the electronically controlled valve; also known as the solenoid valve. The oil volume that is passed to the oil chamber above the intake valves is varied by the electronically controlled valve. Less oil volume means that the intake valve opens less far. This electronically controlled valve is precisely actuated by the engine control unit (the ECU). This allows both the timing and the lift height to be accurately matched to the air demand of the engine.
When engine power is required, such as when driving at higher speeds and with higher engine loads, the intake valve will open fully.
During starting and idling of the engine, the intake valve will open later. A high vacuum will be present in the combustion chamber, which increases the air speed when the intake valve opens. This results in better mixture formation and better combustion.
At medium engine speeds and partial engine load, the intake valve will close earlier. This prevents interference in the intake manifold and reduces gas-exchange losses, which ultimately results in higher engine torque.
At medium engine speeds and partial engine load there is also “multilift”. With multilift, the exhaust valves are opened several times per power stroke, which benefits the quality of the combustion.
MultiAir also makes it possible to briefly open the intake valves at the end of the exhaust stroke; this adds a small portion of exhaust gases to the intake air and creates an internal EGR.
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