There’s no doubt that engine specialists will begin seeing more variable valve timing (VVT) designed engines in their shops, as the generation of vehicles equipped with this technology begins to make its way into the service industry, writes Engine Builder’s Gary Goms. In fact, the current versions of VVT were popularly introduced into the US domestic production about 10 years ago.
The theory behind variable valve timing is simple. Imagine a column of air speeding through a two-inch pipe at 250 feet per second. Suddenly, the airflow is blocked off by a valve at the end of the pipe.
The kinetic energy of the air keeps it moving until a compression wave begins to develop at the valve. The optimum time to open the valve and achieve the greatest airflow is when this compression wave reaches its peak. In contrast, the best time to open the exhaust valve is when a vacuum wave develops at the valve.
Variable valve timing takes advantage of these pressure and vacuum waves to achieve a greater airflow through a given size of engine. Advancing valve timing increases low-speed engine torque while retarding valve timing increases high-speed torque. The powertrain control module (PCM) determines the valve timing position through data supplied by the camshaft position sensors or by valve timing sensors. Be aware of this terminology because some vehicles can use both types of sensors on a single engine.
The part that actually controls the camshaft position and the valve-timing event is called a ‘phaser.’ VVT phaser design includes piston and vane-type configurations. In either case, the phaser uses engine oil pressure to push the piston or rotating vanes against a strong spring. With the vane-type phaser, a clock spring returns valve timing to a ‘default’ position during engine start-up or if the VVT system fails. Another part, called a valve timing solenoid, meters engine oil pressure into the phaser.
The VVT solenoid is supplied key-on voltage and the PCM momentarily grounds the circuit to meter oil pressure into the phaser until the valve timing reaches the desired value. The valve timing solenoid also includes a very fine-mesh screen to prevent sludge and debris from entering the mechanism.
Since correct lubrication is critical to the operation of the VVT phasers and solenoids, it’s doubly important that the correct viscosity of oil is used in a VVT engine.
Because VVT designs use a metered oil orifice to adjust valve timing, oil with a higher than specified viscosity can cause false VVT trouble codes to be stored in the PCM. In addition, the oil must have the correct additive package to keep the engine’s oil passages, phasers, and VVT solenoid screens clean.
Low speed high load conditions: Under low speed high load condition, the engine should increase its intake advance angle in order to have a better dynamic performance. When the engine is running at a low speed, the air in the intake manifold has relative low inertia, backflow of cylinder gas into the intake manifold are like to occur at the end of intake phases due to the high pressure in the cylinder. VVT system closes the intake valve in advance to suppress this kind of backflow, as shown in Figure 1.
High speed high load condition: When the engine running under high speed high load condition, the air in the intake manifold has relative high speed and inertia, the VVT system postpones the close of the intake valve to magnify the amount of air that flows into the cylinder, as shown in Figure 2. This change in valve timing provides improves the engine’s dynamic performance.
Partial load condition: when the engine is running at partial load, VVT system raises the engine’s EGR rate to improve the emission performance. And the VVT system also minimizes the pumping loss during the intake phase to optimize the fuel economy. To achieve these two purposes, the VVT system open the intake valve in advance to create a bigger valve overlap, as shown in Fig-3
Low temperature start and idling condition: VVT system diminish the valve overlap during idling and low temperature start, as shown in Figure 4. When the engine is idling, the vorticity in the cylinder decreases. VVT system postpones the open of intake valve so there will be a bigger pressure difference between the intake manifold and the cylinder when the valve is opened. This results in better combustion in the cylinder. This is also the solution for low temperature start.