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Vvt-i System, How It Works. Part 1


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What is VVT-i? Variable Valve timing with Intelligence and how does it work? PLease read on

Varying intake valve open/close timing according to the operating conditions of a vehicle improves engine performance and fuel economy and provides for cleaner emissions.

Without variable valve timing, valve timing was a compromise between the need to produce maximum torque at low to medium speeds, maintain idle stability and fuel ecomony while still producing low emissions. Continuously adjusting when the valves open and close, called Variable Valve Timing(VVT-i) yields significant improvements in all these areas. The ECM advances or retards the intake camshaft, changing when the valves open and close according to driving conditions.

The VVT-i system optimizes valve overlap throughout the engine's rpm range and under all operating conditions. Under high load driving conditions where high torque and output is required, intake valve timing is controlled optimally according to the engine speed, producing valve overlap and an intake inertia effect. VVT-i eliminates the traditional compromises between low-end toque and high rpm hp. By taking maximum advantage of this overlap, intake air volume is increased, thus torque and output are improved. At the same time, enhancing fuel economy and reducing Oxides of Nitrogen and Hydrocarbons in emission gas so effectively that it eliminates the need for such emissions devices as the EGR valve and provides a large increase in mid-range power.

Valve Timing

For an engine to operate smoothly under a wide range of operating conditions, it must maintain high dynamic compression pressure, proper ignition timing, and good air-fuel misture. Ignition timing is variable, engine speeds and loads have different requirements. Air fuel mixture and the inertia of combustion gas in the cylinders are factors that are considered during the design stage of an engine. To obtain maximum engine output, it is necessary to draw as much air-fuel mixture into the cylinder during the intake phase of the combustion cycle and to discharge as much post combustion exhuast as is possible on the exhuast stroke.

Valve timing is expressed in terms of the crankshaft angle from the piston at top dead center(TDC) or at bottom dead center(BDC). The optimal valve timing is pre-determined for each engine. To maximize the length of time the valves are open, a "valve overlap" has been engineered into the valve timing. The intake valve starts to open before the piston begins its intake stroke and closes after it reaches BDC. The exhuast valve open before the piston enters the exhuast stroke and closes after TDC.

Generally, a larger valve overlap gives better high-speed performance. Too large an overlap may cause unstable idle.

Smooth Engine Idle

Smooth engine idle demands very low camshaft overlap. At idle rpm, valve overlap is eliminated by retarding the intake camshaft. With the intake valve opening after the exhuast valve has closed, there is no blow-back or exhaust gases to the intake side. Combustion is stable, engine idble is smooth at the lower rpm, and fuel consumption is improved.

In the low medium speed range with a heavy load, the camshaft is advanced increasing the valve overlap. This has two effects. First, the exhuast gases help with the induction of the intake mixture. Secondly, by closing the intake early, the air/fuel mixture is not discharged back into the intake.

High Engine Speed at Full Throttle

High power output during rapid acceleration(wide-open throttle) is enhanced by valve timing with a high overlap (the intake opens while the exhaust valve is open). This effects a more filling of the cylider(using low pressure in the exhuast system to draw in additional air fuel mixture).

At high engine speed, at full throttle, the cam timing overlap is reduced to produce higher toque and more horsepower.

Exhuast Gas Recirculation (EGR) Effect

Best emissions performance during cruiseI (part throttle) is enhanced by valve timing with medium overlap (the intake opens small amount while the exhuast valve is open) allowing incoming air fuel mixture to be diluted with inert exhuast gas remaining in the cylinder. The return of exhuast gas into the cylinder reduces combustion temperature, resulting in a reduction in NOx. Additionally, HC are reduced because some of the unburned a/f mixture form the exhaust stroke is returned to the intake side (due to negative pressure) for re-combustion. Finally, C02 is reduced as a result in the decrease in fuel comsumption.

Enhanced Fuel Economy

A reduction in take stroke, resistance enhances fuel economy. In the medium-load operation range, when the valve overlap is increased, the vacuum (negative pressure) in the intake manifold is reduced. This reduces resistance to the piston moving downward on the intake stroke resulting in more energy available to propel the vehicle. At idle, with no overlap, the idble speed is lower which also enhances fuel economy.

VVT-i Actuators(controllers)

All lexus vehicles equipped with VVT-i primarily operate in a similar manner and produce the same beneficial results. However, there are two different type actuators (controllers) used in Lexus VVT-i systems.

The purpose of the actuator is to control the position of the intake cam. Directed by the ECM, and Oil Control Valve (OCV) varies oil pressure to the actuator to continually adjust the timing throughout the engine's operating range.

Helical Type Actuator

A helical type actuator is used on the V8(3UZ-FE: 1UZ-FE) engines and the In-line 6 (2JZ-GE) engine. A hydraulic piston with a helical gear drives the intake camshaft. The actuator changes the relation of the intake cam position to the crankshaft position. The timing belt drives the VVT-i actuator outer gear which is attached to the intake camshaft drive gear. The intake drive gear, in turn, drives the scissors gear attached to the exhuast camshaft. This maintains a consistent relationship with the exhuast cam.

The inner gear is affixed to the camshaft and moveable piston that is placed between the outer and inner gears. Having helical splines (twisted, grooves) on the pistons inner and outer parameters, the piston moves laterally (front-to-back) to shift the phase of the outer and inner gear causing the valve timing to continually change based upon the signals received from the ECM. This shifting of the pistons position provides up to 50 degree of advance intake cam timing in V-8 engines and up to 60 degree advance in an in-line 6 engines.

Site note: Only the intake cam moves in relation to the timing pulley. The timing belt pulley and scissors are fixed to the actuator outer tube. Exhaust cam timing does not change.

To be continue......

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