So, the advantage of variable valve timing (VVT), is that all the factors traditionally associated; with a given camshaft grind; are no longer fixed.
As a result, 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)
So, How Does Valve Overlap Affect Performance:
More Valve Overlap:
- 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
Less Valve Overlap:
- 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. But, different automakers, use different variable valve timing (VVT) strategies; for different purposes.
Some of the (VVT) – variable valve timing types are:
- Cam Changing (VVT)
- Camshaft Phasing (VVT)
- Cam Changing + Cam Phasing (VVT)
- (VVC) System ( Unique To Rover )
So, 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. Finally, as a means to reduce fuel consumption and emission.
Consequently, the exhaust valves do not close; until the intake valves have been open for a while. Therefore, 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.
Above All, Automakers, Need To Get Together
Because, 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
So, 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. Because, 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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