Continuously Variable Valve Timing Control System

Without any advancements, the automotive industry will have been doomed by now. But thankfully the R&D of this sector across the globe has got it covered. Engineers work their kith and kin and produce some really unique and state of the art masterpieces. Well, they work to make the engine refined, efficient, powerful and environmentally friendly. One of the innovative advancements that brought a drastic change to the industry is the VVT (Variable Valve Timing) and VVL (Variable Valve Lift).

Variable Valve Timing | Image Source (1)

Many of the vehicles, some even on the Indian road come with the engine running VVT, VVL or both the techs. But what exactly are VVT and VVL? How does it help? and How does it work? So, without further wasting any time, let's get going.

But without going into the detail of VVT we first need to look at what is valve timing.

Valve Timing

To understand valve timing, let us take you back to "what are camshafts?" here. In simple terms, camshafts are responsible for the opening and closing of the valves.

• And the valves are like the doors for the ports from which the air or air-fuel mixture enters from the intake and exits the chamber from the exhaust valve (when it opens).
• With that clear, let's move to the valve timing. As said earlier the cam or the lobes on the camshaft control the opening and closing of the valves.
• They ride the valves at definite intervals and with utmost extreme precision.
• For you to know, the valve timing is measured in degrees that correspond to the piston's position inside the combustion chamber.
• Defining the valve timing of an engine is one of the most typical processes.

Valve Timing Process

Valve Timing Process Diagram

Not going into the detail, here is a brief about valve timing. But before doing so, here are some technical terms used in the automotive world. TDC (Top-Dead Centre) when the piston is at its topmost position. BDC (Bottom-Dead Centre) is when the piston is at its lowest position.

1. Firstly, the inlet valve opens to let the air or air-fuel mixture enter the combustion chamber. This valve opens a few degrees before the piston reached TDC. This is when the engine is about to complete its exhaust stroke.
2. The inlet valve closes after the piston reaches tad bit further than the BDC. The piston's movement from the BDC to the TDC creates a vacuum inside the combustion chamber forcing the air or air-fuel mixture to enter. Also called the suction stroke.
3. The inlet valve closes just before the compression stroke. 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. Let's save ignition timing for some other day.
4. Coming back to the valve timing, the exhaust valve opens way before the piston reaches the BDC after expansion stroke or power stroke.
5. Now through BDC to all the way up to a few degrees more than TDC the exhaust valve is open after which it closes. and the cycle repeats itself stroke after stroke.

Special Conditions in Valve Timing

Now the question comes why isn't the intake and exhaust valve opening and closing in sync with the pistons TDC and BDC? And why at TDC both the valves are open for some duration? The reason likes in the three crucial factors called Blow Down, Overlap, and Ram Effect.

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. 'Blow-down' is the process where the exhaust valve opens before the piston reaches the BDC. This releases the excess pressure from the combustion chamber. This also confirms that there is no other pressure exerting on the piston during its motion to BDC. If it the exhaust valve were to remain close till the BDC, some engine power had to be wasted in order to assist the piston to move from BDC to the TDC.

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. This is no way a manufacturing fault and the opening of the intake valve slightly before TDC and closing of the exhaust valve later after TDC is deliberately done. This is to assist in pulling the fresh charge from the intake manifold into the combustion cylinder just like the Siphon Effect. 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. Like the others, this is also intentionally done to let in more air into the combustion chamber. How is this possible? You may ask. This is a physical phenomenon wherein a large amount of air entering the cylinder rapidly cannot stop itself. In simple terms, the air is rammed inside the combustion chamber. This is why high revving engines tend to keep the intake valve open for a longer duration to let the air in. But this is not so prominent at low speeds and the piston will push some of the air out of the cylinder.

Interesting Fact: Well, the ram effect is the most important variable to keep in mind as this will definitely have a huge impact on the engine's performance, definitely more than the other two.

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So, with the basics of valve timing clear, let's move to the variable valve timing. In order to understand the effects of VVT. So, let's have a look at how the valve timing behaves without this tech.

Fixed Valve Timing (NO-VVT)

DOHC setup With Constant Valve Timing

Majority of the IC engine on the road have a fixed valve timing. The engine running on a constant valve timing has to be intelligently tuned. What does that mean? The motor has to be designed for a specific function. Of course, one has to compromise on the other aspects.

• A people's mover or a van usually travels at low speeds and does have no relation with high speeds. 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. If not, let's explain.

• 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?

Variable Valve Timing Having 2 Cam Profiles | Image Source (1)

As said before, VVT changes the valve timing when the engine is running. But how does the engine do the same? Long story short, VVT optimized the timing depending upon the RPMs, to get the most out of the engine for the desired RPM.

First Gen VVT Tech

• The VVTs first generation uses a two-step variation that boosts the performance at two different RPMs.
• The first variation of valve timing is fixed for up to 3500 rpm and the other phase is the one with full load for more than 3500 rpm. (low and medium cam profile)
• VVT, in short, offers the best of both worlds, where an engine with VVT tech has good low-end torque and a lot of power at high revs. (High-Speed cam profile)

Variable Valve Timing Working | At High Speeds | Image Source (1)

Advanced VVT Tech

It's been a while and the technology has advanced with leaps and bounds. With that, the VVT tech too reaches new heights.

• One of the advanced VVT technology includes a CVVT or Continuously Variable Valve timing. 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.
• The ECU controls all valve timing to produce the maximum possible power and efficiency at a certain RPM. Well, there are many mechanisms of CVVT but the basic one includes a variable timing camshaft which is operated by a solenoid valve.
• There are even more advanced technologies when it comes to variable valve timing, one such is the 'Dual VVTi'. 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!

Companies VVT Tech
VVT-Acronym	Vehicle Manufacturer	Full-Form
CVVT	Renault, Volvo	Continuously Variable Valve Timing
VCT	Ford	Variable Cam Timing
VVT	Suzuki, Volkswagen	Variable Valve Timing
i-VTEC	Honda	Intelligent – Variable Valve Timing and Lift Electronic Control
VVTi	Toyota	Variable Valve Timing (intelligent)
VTVT	Hyundai	Variable Timing and Valve Train
N-VCT	Nissan	Nissan Variable Cam Timing

Variable Valve Lift (VVL)

Going by its name, an engine head with VVL allows the valves to have a different sized opening, again, depending upon the revolutions of the engine.

• 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.
• But having more travel on at low speeds will have its counter effects on the performance as it will affect the swirl motion which is necessary for a better air/fuel mixture.

Let us know in the comment section below if you have a question regarding VVT. Tell us about which car technology you want to know about.

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Source: https://gomechanic.in/blog/vvt-variable-valve-timing-explained/

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