So, the advantage of (VVT) – variable valve timing, 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. Consequently, this is where, (VVT) – variable valve timing takes a front seat.
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.
So, Where Are Automakers, Going 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.
(VVT) – Variable Valve Timing, 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 faults.
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.
Thank You !