Variable Valve Timing or VVT engine is an engine technology that allows the timing of a vehicle's intake and/or exhaust valves to be adjusted continuously or variably. VVT aims to optimise the engine's performance, efficiency, and emissions across different operating conditions.

VVT engine technology has become increasingly prevalent in modern engines and is found in many gasoline and diesel-powered vehicles. In addition, it is a crucial element in achieving the balance between power, efficiency, and emissions control, contributing to better overall engine performance and improved driving experience.

Valve timing refers to the precise timing of the opening and closing of an internal combustion engine's intake and exhaust valves. Proper valve timing is crucial for the engine to operate efficiently and generate power effectively.

There are different types of VVT systems used in engines, such as:

1. Intake Valve Timing (IVT)

IVT Adjusts the timing of the intake valves, influencing the amount of air-fuel mixture entering the cylinders.

2. Exhaust Valve Timing (EVT)

This type controls the timing of the exhaust valves, impacting the expulsion of exhaust gases from the cylinders.

3. Dual Variable Valve Timing (DVVT)

This is the combination of both IVT and EVT and provides more control over engine performance and emissions.

The valve timing process involves four key events in the engine's four-stroke cycle: intake, compression, power, and exhaust strokes. Here are the steps that help to comprehend the valve timing process:

Intake Stroke

The intake valve opens, allowing a mixture of air and fuel to enter the combustion chamber during the intake stroke. The piston moves downward in the cylinder, creating a vacuum that draws the air-fuel mixture from the intake manifold into the cylinder.

The intake valve remains open for a specific duration to allow the proper amount of air-fuel mixture into the cylinder based on the engine's needs.

Compression Stroke

With the intake valve closed, the piston moves upward in the cylinder during the compression stroke. This compresses the air-fuel mixture inside the combustion chamber, preparing it for ignition. The compression ratio determines how much the air-fuel mixture is compressed before ignition.

Power Stroke

The spark plug ignites the compressed air-fuel mixture just before the piston reaches the top of the compression stroke. This ignition causes a rapid expansion of gases, forcing the piston downward with significant force. As the piston moves downward, it transfers energy to the crankshaft, creating rotational motion that drives the vehicle's wheels.

Exhaust Stroke

The exhaust valve opens as the piston reaches the bottom of the power stroke. The expanding gases push the piston back up the cylinder, expelling the burned exhaust gases out of the combustion chamber and into the exhaust manifold. 

The exhaust valve remains open for a specific duration to allow efficient evacuation of the exhaust gases from the cylinder.

The purpose of the VVT engine is to maximise the engine's performance, efficiency, and emissions by adjusting the timing of the intake and/or exhaust valves continuously or variably. Here are the following additional functions of the VVT engine:

1. Improved Power and Torque

VVT can advance or retard the timing of the intake valves, ensuring that the right amount of air-fuel mixture is supplied to the cylinders at the right time. This results in improved torque and power output across a wider range of engine speeds, providing better acceleration and overall performance.

2. Enhanced Fuel Efficiency

At certain engine speeds and loads, retarding the intake valve timing can reduce the amount of air-fuel mixture entering the cylinder. This "late intake valve closing" strategy allows for higher compression ratios and reduced pumping losses, leading to improved fuel efficiency.

3. Reduced Emissions

Precise control of valve timing helps enhance the combustion process, reducing unburned fuel and harmful emissions. Moreover, the functions of VVT sensors also include enabling exhaust gas recirculation (EGR) to occur at specific times, further reducing nitrogen oxide (NOx) emissions.

Some of the key special conditions in valve timing include:

Late Intake Valve Closing (LIVC)

In certain engines, especially those with high compression ratios, the intake valve closing can be delayed, allowing the piston to compress the air-fuel mixture for a longer period before ignition. This "late intake valve closing" strategy reduces the effective compression ratio, which can help prevent knocking and reduce fuel octane requirements, thereby promoting better fuel efficiency.

Early Intake Valve Opening (EIVO)

In some cases, particularly in engines with direct injection or lean-burn technologies, the intake valve opening can be advanced. This strategy allows the engine to draw in a portion of the air-fuel mixture during the exhaust stroke, which helps cool down the combustion chamber and reduce NOx emissions.

Variable Valve Lift (VVL)

Apart from variable valve timing, some engines feature variable valve lift, which allows the lift of the intake and/or exhaust valves to be adjusted. The engine can optimise airflow by varying the valve lift and achieve different performance characteristics at various engine speeds and loads.

Exhaust Valve Opening Deactivation (EVOD)

In some engines, particularly in hybrid vehicles or those equipped with cylinder deactivation, the exhaust valve opening can be deactivated during certain operating conditions. This deactivation prevents exhaust gases from entering the cylinder, effectively turning the cylinder into an "air pump," reducing pumping losses and improving efficiency.

Variable Valve Timing in Forced Induction Engines

In turbocharged or supercharged engines, the timing of the intake and exhaust valves can be optimised to work in tandem with the forced induction system. This ensures optimal air-fuel mixture management and exhaust gas scavenging, enhancing the engine's performance.

Fixed valve timing refers to a traditional valve timing configuration in an internal combustion engine where the timing of the intake and exhaust valves remains fixed or constant throughout the engine's operating range. In such engines, the timing of the valves is determined by the camshaft's profile, which has fixed lobes or cam profiles for each valve.

The main characteristics of fixed valve timing are as follows:

Simplicity

Fixed valve timing systems are straightforward and relatively simple in design. They consist of a camshaft with fixed cam profiles that actuate the valves at predetermined timings based on the engine's design.

Limited Optimisation

While fixed valve timing can be well-matched to certain engine operating conditions, it may not be optimal across the entire range of engine speeds and loads. As a result, fixed timing can lead to some compromises in performance, fuel efficiency, or emissions control at certain engine operating points.

Variable Valve Timing offers several significant advantages for internal combustion engines. Here are the main advantages of variable valve timing:

Improved Performance

VVT engine allows engines to optimise valve timing based on the engine's operating conditions. By adjusting the valve timing, engines can achieve better power and torque output, leading to improved acceleration and overall performance.

Enhanced Fuel Efficiency

This can optimise valve timing for different engine speeds and loads, allowing the engine to operate more efficiently. By adjusting the timing of the intake and exhaust valves, VVT reduces pumping losses, increases thermal efficiency, and improves fuel economy, especially during light-load and cruising conditions.

Reduced Emissions

Precise control of valve timing helps improve combustion efficiency, reducing unburned fuel and harmful emissions. VVT engines can also facilitate exhaust gas recirculation (EGR) at specific times, further reducing nitrogen oxide (NOx) emissions.

Smoother Engine Operation

VVT can optimise valve timing to reduce engine vibration and improve engine stability at idle and low speeds, leading to a smoother and quieter engine operation.