What Is Variable Valve Timing, And How Does It Work?

There is so much going on with internal combustion engines (ICEs) these days, it can give you a headache just reading about them. That's why we do the research for you and then cram it into bite-size chunks that are easy to digest. This time around, we want to talk about variable valve timing and how variable valve timing (VVT) works. 

VVT technology first appeared in vehicles all the way back in 1924, when a patent was filed for a valve with a variable opening duration to better control the intake and exhaust valves of an internal combustion engine. Back in the day, the intake and exhaust valves would continuously operate in the same fashion regardless of the speed or load of the vehicle. When cars were simpler, this was fine and dandy. Today, car emission regulations have become stricter, and many auto manufacturers need to make eco-friendly alternatives with reduced emissions.

On top of all this, drivers want their engine to operate differently based on its RPMs, with the general goal of having the engine deliver more power at high RPMs and better fuel economy at low RPMs. That's why we have this technology in modern engines, so let's take a look at how VVT works.

As we've seen, VVT is an automotive technology with a long history behind it. Over time, many auto manufacturers have come up with their own version of it to get ahead of the competition. 

Take Toyota's VVT-I (Variable Valve Timing with Intelligence) system, where the car's electronic control unit manages when to open and close valves. Likewise, Honda has developed a VTEC (Variable Valve Timing and Lift Electronic Control) system, where valves are managed based on the engine's current rpm. Ultimately, they're both designed to adjust the behavior of the intake and exhaust valves to control how air enters and exits the combustion chamber of an engine.

In these systems and many others, the electronic control unit (ECU) of the vehicle uses sensors that monitor the location of the pistons and valves, to advance or slow the timing of the VVT system. This provides the car with better acceleration or better fuel economy. Confused yet? Let's break it down.

Many modern VVT systems can control up to three characteristics — valve timing, valve duration, and valve lift. Valve timing refers to at what time during the engine's rotation valves open and close, typically before and after combustion. Valve duration determines how long the valve stays open, while valve lift controls how much the valves physically open during this action. In terms of power and fuel economy, let's pay a little more attention to valve duration.

Here's an example of how variable valve timing works — if the VVT increases the valve duration of the exhaust valve, then more exhaust is expended from the cylinder. In turn, there is more space for air and fuel to fill the cylinder during the intake stroke. The more air and fuel introduced to the combustion chamber, the more power generated.

On the opposite end of this, if the VVT system shortens the valve duration of the exhaust valve, then some of those fumes stay in the cylinder. This leads to less space for air and fuel to be introduced into the cylinder for the next rotation. In turn, less fuel is burned, and that's beneficial for extending fuel economy.