VARIABLE VALVE TIMING SYSTEM (VVT)

THE FUNCTION OF THE VVT SYSTEM

Since the speed of automotive engines is not constant, the timing diagram should ideally vary with it. In other words: the valve closing and opening angles should be adapted to values that maximize the processes of cylinder emptying and filling, according to the engine speed.

The VVT system influences the timing of the valves with the help of a phase shifter located at the head of the camshaft. This part is activated by the engine control unit using oil flow controlled by the?solenoid valve.

The main advantages of this system are:

• reduced fuel consumption
• increased torque and power
• reduced emissions

MAINLY IN PETROL ENGINES

The?VVT system?was introduced in Asian and European vehicles in the late 80s and early 90s. In the mid-2000s, this system became more popular and started to be used by all the main car manufacturers. Nowadays, this system is normally installed in petrol engines (though not in all), but can also be found in some diesel engines. While the actual?name used for the system?varies from manufacturer to manufacturer, and, although there may be small differences, the principle of operation is practically the same:

• Honda: VTEC
• Toyota: VVT-i
• BMW: VANOS
• Ford: Ti-VCT
• Kia-Hyundai: CVVT
• Porsche: VARIO CAM
• VAG: TGV

The theoretical 4-stroke?engine operating cycle?consists of the intake stroke, the compression stroke, the power stroke and the exhaust stroke. For the sake of simplicity, the same theoretical cycle is used for all automotive engines. But to make sure every cylinder is filled with and emptied of gases in the most efficient way possible at each specific moment (even at different speeds and with different loads),?the angle?at which the valves open and close has to diverge from the theoretical cycle. This is where the VVT or variable valve timing system and?VVT solenoids?come in. Read on to discover the function and geometry of these important parts.

?DELAY AND INERTIA

As explained in the introduction, the valve?opening and closing times?have to differ slightly from the theoretical cycle to optimize the process of gases entering and escaping from the cylinder. And that, in turn, is affected by both delay and inertia:

• Delay:?Valves don’t open instantly. It can take between 20-30° of crankshaft rotation for them to be completely open. Without any corrective actions, this would cause delays in both the intake cycle and the exhaust cycle:
• if, in the intake cycle, the piston starts to descend and the intake valve is not yet open due to the above-mentioned delay, a?vacuum?is created in the cylinder. This makes it more difficult for the piston to start its downward stroke and therefore reduces engine performance.
• in turn, if the piston starts its upward movement in the exhaust cycle, and the valve is not open due to the delay, the?pressure?in the cylinder opposes the rise of the piston, again reducing the performance of the engine.
• Inertia:?Moreover, when a valve opens, the gases slightly ‘hesitate’ to start moving. This also generates a short delay at the beginning of the process (the cylinder filling or emptying).

CREDITS TO:

• Gates TechZone - visited on 04/11/2021.