The advantage of variable valve timing (VVT); is that all the factors traditionally associated with a given camshaft grind are no longer fixed.
Consequently, making it possible to change valve timing in response to; changing engine speeds and operating conditions.
So, valve timing determines when the intake and exhaust valves open. Also, how long they remain open. And, when they close.
As a result, variable valve timing (VVT) provides a way of getting around the limitations of fixed timing.
Consequently, this affects:
- Intake and exhaust flow
- Intake manifold vacuum
- Running compression
- Volumetric efficiency
- Throttle response
- How much horsepower and torque the engine develops at any given (RPM)
How Valve Overlap Affects Performance:
- Reduces oxide of nitrogen (NOx) emissions under load
- Increases Exhaust Gas Recirculation (EGR) flow; lowering combustion temperatures
- On some engines (VVT) can eliminate the need for an (EGR) valve
- Smooth idle and more slow speed torque
- Poor high speed performance
- Better engine breathing; at high speeds
- Poor performance; at low speeds
- Rough idle
- Higher exhaust emissions
So, there are a variety of different (VVT) systems in use today. In contrast, different automakers use different variable valve timing (VVT) strategies; for different purposes.
Some of the types are:
- Cam Changing (VVT)
- Camshaft Phasing (VVT)
- Cam Changing + Cam Phasing (VVT)
- (VVC) System ( Unique To Rover )
In theory, maximum overlap is needed between intake and exhaust valves opening at high speed. However, when the car is running at medium cruising speed; maximum overlapping may also be useful. Consequently, as a means to reduce fuel consumption and emission.
The exhaust valves do not close until the intake valves have been open for a while. The exhaust gases are recirculated back into the cylinder; at the same time as the new fuel/air mix is injected.
So, as part of the fuel/air mix is replaced by exhaust gases; less fuel is needed. Because, the exhaust gas is comprised of mostly non combustible gas; such as (CO2), the engine runs properly at the leaner fuel/air mixture.
Automakers Need To Get Together
So, it seems like they are all trying to set the bar higher; with different types of technology. I think the end goal is the same; but we need to have some type of common ground.
Variable Valve Timing (VVT) Common Problems
On (VVT) systems that use oil pressure to actuate the cam phaser, oil quality, viscosity and contamination problems; can affect the operation of the phaser.
Also, the phaser may not work properly if:
- The phaser does not receive adequate oil pressure
- Oil is the wrong viscosity
- The oil is dirty
So, this can hurt engine performance, fuel economy and emissions. Also, turning on the check engine light setting a (VVT)-related fault code.
Consequently, any codes could be the result of a; bad cam phaser; oil flow control valve or wiring fault.
So, don’t jump to any conclusions regarding the variable valve timing (VVT) system. If an engine is idling rough, or not developing power; you should also consider other possible causes such as:
- A large vacuum leak
- Heavy carbon buildup on the intake valves
- Dirty fuel injectors
- Low fuel pressure
- Ignition misfires
- Exhaust restrictions
- Loss of compression
- Turbo problems
So, as you might have already guessed (VVT) diagnostics is very application specific. And, it also depends on the configuration of the phaser and system electronics.
Above all, oil quality, viscosity and contamination problems can all affect the operation of a; hydraulically actuated (VVT) cam phaser. Finally, this, in turn, will affect engine performance, fuel economy and emissions.
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