All about Variable Valve Timing

For this purpose, a systematic system is employed that controls variables such as quantity, flow, etc are called Variable Valve Timing or VVT.

What is Variable Valve Timing or VVT??

This is a mechanism that varies the timing of the valves opening and closing depending upon the fluctuation is speeds.

This is done so as to optimize engine performance and increase the fuel economy.

The principle for this system is that during the idling speed the engine requires the least amount of fuel so the valves can be opened for a very short duration of time, whereas at higher speeds the demand for the fuel increases exponentially and thus valves need to be opened for the maximum amount of time.

For this reason, the engine performance is optimized as well as the fuel economy is increased.

Types of Variable Valve Timings

A typical engine’s intake and the exhaust valves are opened with the lobes on a camshaft.

With the engines with dual overhead camshafts, there are separate camshafts for both the exhaust valves and intake valves.

These camshafts are made from hardened iron or steel and thus are connected to the crankshaft via the timing belts, chains, or the gears.

Since modern gasoline engines incorporate the four-stroke engine cycle, this means that the camshafts rotate once for every two crankshaft rotations.

To reinforce this point, consider the intake stroke of an engine. As the intake valve is open, which means the camshaft lobe is pushing against the cam follower and opening the valve.

Let’s trace the motion of this cam lobe and compare it to the motion of the crankshaft.

While the intake valve is open, the piston moves down toward the bottom dead center.

Once the engine has reached the bottom dead center (BDC), the crankshaft has rotated 180 degrees. Then the piston moves up to compress the fuel mixture.

Once the piston has reached TDC, the crankshaft has made a full rotation. Next, the spark plug initiates the spark and ignites the fuel mixture, pushing the piston back down to the bottom BDC.

At this point, the crankshaft has turned one and a half full rotations. Now the exhaust valve opens, and the piston is moved back up to the top dead center.

At this moment the crankshaft has rotated two full revolutions. Now that the piston is at approximately top dead center, the camshaft lobe that we are monitoring comes back around and opens the intake valve and the piston moves back down.

In the mid-nineties, various automakers began developing the variable valve timing systems that allowed intake and exhaust valves to open earlier or later in the 4-stroke cycle.

Their aim was to improve volumetric efficiency, lessen the NOx emissions, and decrease pumping losses.

Today, there are two major types of variable valve timing systems ie cam phasing and cam changing.

With cam changing, the Electrical control Unit selects a different cam profile based on engine load and speed, whereas, with cam phasing, an actuator rotates the camshaft, changing the phase angle.

VVT systems use all sorts of high tech sensors, but the most important of all is the camshaft and crankshaft position sensors (which are often hall effect sensors).

The ECU uses these sensors to closely monitor the relationship between the piston’s location and the valves’ positions.

The crankshaft is connected to the rod and piston, and the camshaft’s lobes actuate the valve lift events.

Thus, with the data from the crankshaft and camshaft position sensors, the ECU can interpret how fast the engine is rotating and the relative positions of the piston and the intake and exhaust valves.

Cam Switching System

This method uses two cam profiles, equipped with an actuator to swap between the profiles (usually at a specific engine speed).

Cam switching can also provide variable valve lift and variable duration, however, the adjustment is discrete and not continuous.

The first production use of this system was Honda’s in the VTEC system.

VTEC changes hydraulic pressure to actuate a pin that locks the high lift, high duration rocker arm to an adjacent low lift, low duration rocker arm.

Cam Phasing

Many production VVT systems are the cam phasing type, using a device known as a variation.

This allows continuous adjustment of the cam timing (although many early systems only used discrete adjustment), however, the duration and lift cannot be adjusted.

Oscillating cam

These designs use an oscillating in a part cam lobe, that acts on a follower. The motion of this follower then opens and closes the valve.

Some oscillating cam systems use a conventional cam lobe, while others use an eccentric cam lobe and a connecting rod.

The principle is similar to that of steam engines, where the amount of steam entering the cylinder was regulated by the steam “cut-off” point.

Eccentric cam drive

Eccentric cam drive systems operate through an eccentric disc mechanism that can slow and speeds up the angular speed of the cam lobe during its rotation.

Arranging the lobe to slow during its open period is equivalent to lengthening its duration.

Three-dimensional cam lobe

This system consists of a cam lobe that varies along its length. One end of the cam lobe has a short duration/reduced lift profile, and the other end has a longer duration/higher lift profile.

In between, the lobe provides a smooth transition between these two profiles. By shifting the area of the cam lobe which is in contact with the follower, the lift and duration can be continuously altered.

This is achieved by moving the camshaft axially (sliding it across the engine) so a stationary follower is exposed to a varying lobe profile to produce different amounts of lift and duration.

Originally published at https://www.carengineered.com on August 4, 2020.