Yahoo Autos How Does Variable Valve Timing Work?
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Back in the day, a car’s intake and exhaust valves opened a specific amount at a specific point in the four stroke cycle, and for a specific amount of time. It was that simple. Nowadays, however, many engines can not only change when their valves open, but also how much they open and for how long – that is, new cars can change valve timing, valve lift, and valve duration. Let’s have a look at how variable valve timing (VVT) works.
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For a lot of you this is review, but if we want a new generation of car enthusiasts to care about cars, it can’t hurt to explain how things actually work.
Variable Valve Timing
A typical engine’s intake and exhaust valves are opened via the lobes on a camshaft. With dual overhead cam engines, there are separate camshafts for exhaust valves and intake valves. These camshafts are made of hardened iron or steel and are connected to the crankshaft via timing belts, chains, or gears. Since modern gasoline engines incorporate the four-stroke cycle, this means that the camshafts rotate once for every two crankshaft rotations.
To reinforce this point, consider the intake stroke of an engine. The intake valve is open, meaning 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 bottom dead center. Once the engine has reached bottom dead center, the crankshaft has rotated 180 degrees. Then the piston moves up to compress the fuel mixture. Once the piston has reached top dead center, the crankshaft has made a full rotation.
Next, the spark plug ignites the fuel mixture, sending the piston back down to bottom dead center. At this point, the crankshaft has turned one-and-a-half full rotations. The exhaust valve opens and the piston moves back up to top dead center. The crankshaft has now rotated two full revolutions.
With that piston 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. Thus, after two revolutions of the crankshaft, the camshaft has rotated once.
Different Strokes for Different... Engines
In the 1960s, automakers began developing variable valve timing systems that allowed intake and exhaust valves to open earlier or later in a 4-stroke cycle. The aim was to improve volumetric efficiency, decrease NOx emissions, and decrease pumping losses.
Today, there are two major types of variable valve timing: cam phasing and cam changing. With cam changing, the engine control unit (ECU) selects a different cam profile based on engine load and speed, whereas with cam phasing, an actuator rotates the camshaft, changing the phase angle. There are dozens of ways to go about varying valve timing, lift, and duration, so we’re just going to look at Toyota’s VVT-i, and Honda’s VTEC.
Before we look at VVT-i, let’s briefly talk about sensors. VVT systems use all sorts of sensors, but the most important are the camshaft and crankshaft position sensors (which are often hall effect sensors). The ECU uses these sensors to 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 trigger valve lift events. Thus, with the information from the crankshaft and camshaft position sensors, the ECU can learn how fast the engine is rotating and the relative positions of the piston and the intake and exhaust valves.
