Variable Valve Timing:
The engine’s power largely depends on the camshaft. If it has cams that are long and oval, the valves remain open longer. More air can enter and exit the engine, which increases power. If the cams are shorter and pointier, the valve will not open as far and will close sooner, allowing less air to flow, resulting in less power. The advantage is that fuel consumption can be reduced.
Low engine speeds with low loads require:
- Intake valves opening late and closing early.
- Exhaust valves opening late and closing early.
High engine speeds with heavy loads require:
- Intake valves opening early and closing late.
- Exhaust valves opening early and closing late.
Automakers always seek a compromise. Variable valve timing adjusts the camshaft to the required position at the engine speed it’s operating at. The image shows two valve timing diagrams.
The left valve timing diagram shows the “normal” situation without adjustment, the right after adjusting both the intake and exhaust camshafts. Here it can be seen that with the adjustment, the intake valve opens 46 earlier and closes 46 later. The exhaust valve also opens 46 earlier and closes 46 later.
Camshaft Adjustment:
In this form of variable valve timing, the camshaft rotates relative to the gear driven by the timing belt or timing chain (see image below). This system ensures valves open earlier or later, but cannot regulate valves to stay open longer, as the camshaft profile remains unchanged. The system shown below operates hydraulically. The engine, and thus the camshaft gear, rotate clockwise. In the camshaft gear in the image below, two positions are marked, where the camshaft will be in relation to the gear at idle (pink) and at full load (purple). On the cams themselves, the positions for early opening at full load (purple) and the neutral position (pink) are also marked.
In low-speed driving situations, with low loads, the valves will open later. The pink markings are applicable. When accelerating hard with full throttle, the purple section applies. Oil is then pumped to the adjustment point in the camshaft gear, causing the inner part to rotate to the right. The camshaft is then rotated relative to the gear, causing the cams to push the valves open and closed earlier.
The image shows the oil chambers in the adjustable camshaft gears. Filling the space with oil causes the camshaft to adjust. The timing gears are turned relative to the camshaft attachment.
The image shows the two adjustable camshaft gears with the oil channels. The oil channels are colored yellow.
The oil pump in the oil pan supplies the solenoid valves with oil pressure. When the solenoid valves are actuated by the engine control unit (the ECU), they switch the oil pressure to the camshaft gears. The solenoid valves ensure the camshaft gears adjust.
Possible malfunctions in the above system include:
- Solenoid valves failing to supply the camshaft gears with oil pressure. This is often due to contamination. The filter screen in the solenoid valve can be clogged with black sludge or other dirt particles. Cleaning often provides a solution.
- Camshaft gears failing to adjust correctly. This is also often caused by contamination.
Contamination usually occurs from driving too long with old engine oil.
MultiAir:
MultiAir is a fully electronically controlled system to regulate the opening and closing of the intake valves independently via electronic-hydraulic actuation. With MultiAir, both the valve lift height and timing of the intake valves are controlled. MultiAir is applied to both gasoline and diesel engines. An adjustable camshaft is no longer necessary.
In MultiAir, there is a hydraulic connection between the camshaft and the intake valve. The camshaft drives via a roller finger follower the piston of the MultiAir high-pressure cylinder. The piston in this high-pressure cylinder compresses the oil to the electronically controlled valve; also known as the solenoid or the solenoid valve. The oil volume passed to the oil chamber above the intake valves is varied by the electronically controlled valve. Less oil volume means the intake valve opens less. This electronically controlled valve is precisely actuated by the engine control unit (the ECU). As a result, both the timing and the lift height can be finely adjusted to the engine’s air demand.
When engine power is needed, such as when driving at higher speeds and higher engine loads, the intake valve will fully open.
During start-up and idling, the intake valve will open later. There will be a large vacuum in the combustion chamber, causing the airspeed to increase when the intake valve opens. This results in better mixture formation and better combustion.
At medium engine speeds and partial engine loads, the intake valve will close earlier. This helps prevent interference in the intake manifold and reduces gas exchange losses, resulting in higher engine torque.
In medium engine speeds and partial engine loads, there is also “multilift”. In multilift, the exhaust valves open multiple times per power stroke, enhancing combustion quality.
MultiAir also allows the intake valves to open briefly at the end of the exhaust stroke; this adds a small portion of exhaust gases to the intake air, creating an internal EGR.
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