What is vvt variable valve timing

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

Thus, after two revolutions of the crankshaft, the camshaft has rotated once. Check out this gif to see it all in motion. In the s, automakers began developing variable valve timing systems that allowed intake and exhaust valves to open earlier or later in the 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 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. Before we look at VVT-i, let's 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 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. Cam phasing advances or retards valve lift events by rotating the camshaft, typically over a range of about 60 degrees relative to the crankshaft angle.

Let's say our intake valve normally opens 5 crankshaft degrees before top dead center and closes crankshaft degrees after top dead center 5 degrees after bottom dead center. There are numerous different methodologies used to vary valve timing. Each manufacturer has its own name for its own VVT system. In this system, the ECU receives signals from the camshaft position sensor, crankshaft sensor, oil temperature sensor, mass air flow sensor MAF , and the engine coolant temperature sensor and uses the information to adjust its output signal to an oil control valve.

This valve acts as a hydraulic actuator, rotating a rotor which is connected to the camshaft inside a housing, which is connected to the crankshaft via a timing chain. This is when both the valves are shut inlet as well as the exhaust valve. If the engine is FI Fuel Injected , the fuel is atomized sprayed into tiny droplets inside the combustion chamber before the piston reaches TDC and then fuel is ignited in SI engine Spark Ignition engine. This also depends on the ignition timing.

Coming back to the valve timing, the exhaust valve opens way before the piston reaches the BDC after expansion stroke or power stroke. Blow Down Both the valves remain shut to perform the combustion process efficiently during the compression stroke all the way up to the power or expansion stroke.

Overlap In the diagram, it is very prominent that when the piston reaches TDC in the exhaust stroke, both the valves, intake and exhaust, are open. Not doing so can cause some burnt exhaust gasses to remain inside the combustion chamber and dilute the air-fuel mixture Ram Effect This is the situation where the intake valve closes at a few degrees after the BDC. So the engineers have to tune the valve timing with less overlap. This is to reduce the escaping of the fresh charge during the suction stroke.

On the other hand, a racing motorcycle or car has to run on high speeds and the engine has to keep on the boil. Hence, the intake and the exhaust valve has to have an increased overlap period. As said earlier, it helps in the suction of fresh charge from the intake manifold at high revolution.

The third situation is the road cars. This is where the car run on both, in the city and out on the highway. So, performance is best in the middle of the rev range. Hence the city cars have their valve timing calculated in these conditions in mind. Variable Valve Timing By now you might have an idea what the variable valve timing would be.

In simple words, a traditional engine with no VVT operated on only a single cam profile. In other terms, the valve timing remains constant throughout its rev range. But an engine with a variable valve timing, the cam might have 2 or sometimes 3 cam profiles in order to control the valve timing.

The cam train might also have different timing gear to shift the valve timing. It all depends on the cam train mechanism. So, not going into the mechanism, but what effect the variable valve timing has on the engine, in terms of performance, efficiency, and emissions. How does Variable Valve Timing Work?

The first variation of valve timing is fixed for up to rpm and the other phase is the one with full load for more than rpm. As the name suggests this tech constantly changes the valve timing in sync with the cars ECU. In short like the CVT these too have infinite variations of valve timings. Well, there are many mechanisms of CVVT but the basic one includes a variable timing camshaft which is operated by a solenoid valve.

Like the others, this system also has varying valve timings. Additionally, this can vary the inlet and exhaust valve timing independently. Different companies use different mechanisms hence different names.

Here are some! Long story short, at high speeds the VVL, provides more room for the fresh charge to enter from the intake valve and to expel more gasses from the exhaust valves.