This topic details the most common types of Variable Valve Timing (VVT) systems fitted to vehicles, including the Toyota VVT - i & VVTL - i, to provide a better understanding of a particular vehicle fault.

There are various manufacturers’ systems covering Variable Valve Timing (VVT) systems:

These include:

There are two different principles involved in VVT systems. The basic VVT system operates by advancing the Camshaft timing (usually the inlet camshaft ) by using the engine’s oil pressure to move (switch) the camshaft between two different points at pre-determined engine speeds.

The advanced VVT systems operate by not only advancing either (or both) the inlet and exhaust camshafts as per the basic system, but can also change the amount of valve lift by altering the cam profile on the camshaft.

Basic principles of Valve timing

Until recently, a manufacturer used one or more camshafts to open and close an engine's valves. The camshaft / camshafts were turned by a timing chain (or belt) that connected to the crankshaft. As the engine rpm speed increased or decreased, the crankshaft and camshaft would turn faster or slower to keep valve timing relatively close to what was needed for efficient engine operation.

Unfortunately, the dynamics of airflow through a combustion chamber changes radically between 2,000 rpm and 6,000 rpm. Despite the manufacturer's best efforts, there was no way to maximise the valve timing for high and low rpm with a simple crankshaft-driven valve train system.

Instead, manufacture’s had to develop a "compromise" system that would allow an engine to start and run when pulling away from rest but also allow for strong acceleration and motorway cruising. However, because of the "compromise" nature of standard valve train systems, few engines were ever running at there "peak efficiency," which resulted in wasted fuel (higher emissions) and reduced vehicle performance.

Variable valve timing has changed all that. By coming up with a way to alter valve timing between high and low rpm, Honda, Toyota and BMW and many more manufacturers can now tune valve operation for optimum performance and efficiency throughout the entire engine rev range.

History of VVT systems

Honda was the first to offer what it called VTEC in its performance engine models like the Prelude, Accord  and Civic models. VTEC stands for Variable Valve Timing and Lift Electronic Control. VTEC uses two sets of camshaft profiles one for low to mid-range rpm and one for high rpm operation.

An electronic switch shifts between the two profiles at a specific rpm to increase peak engine power and improve torque. While this system does not offer continuously variable valve timing, it can make the most of high rpm operation while still providing solid driveability at lower rpm levels.

Toyota uses a similar VVT system but instead of the ON / OFF system that VTEC employs, Toyota decided it wanted a continuously variable system that would maximize valve timing throughout the rpm range. VVT - i stands for Variable Valve Timing with intelligence. Toyota uses a hydraulic rather than mechanical system to alter the intake camshaft phasing.

The main difference from the Honda VTEC is that VVT - i maintains the same cam profile and alters only when the valves open and close in relation to engine speed. Also, this system works only on the intake valve while the Honda VTEC has separate settings for the intake and the exhaust valves.

Ferrari has a different system for changing the camshaft timing. The camshafts on some Ferrari engines are designed with a three-dimensional profile that varies along the length of the cam lobe.
At one end of the cam lobe is the least aggressive cam profile, and at the other end is the most aggressive.

The shape of the cam lob smoothly blends these two profiles together. A mechanism can slide the whole camshaft laterally so that the valve engages different parts of the cam. The shaft still spins just like a regular camshaft, but by gradually sliding the camshaft laterally as the engine speed and load increases, the valve timing can be optimised.

Several other manufacturers, including Ford, Lamborghini and Porsche have also designed different types of VVT systems. BMW has also used a cam phasing system, called VANOS (Variable Onckenwellen Steuerung).
Like the other manufacturers, this system only affected the intake cams. However since 1999, BMW has designed a Double VANOS system on the 3 Series (E46). Double VANOS changes both the intake and exhaust camshaft timing to provide efficient operation at all rpm.

VVT system precautions before carrying out engine repairs

Most VVT systems operate by using additional gearing mechanisms connected between the timing chain and camshafts to rotate one or both of the camshafts. Before removing the timing chain it is recommended to first thoroughly read the manufacturer’s recommendations on use of special tools to lock the camshafts and gearing mechanism in place.

It is also strongly advised to mark up plenty of reference points for the timing chain, gearing mechanisms and camshaft positions to make disassembly and reassembly of the camshaft parts as simple as possible.

Failure to note these precautions has led to numerous calls (especially on Rover VVC systems) where it has been impossible to reassembly the VVT system correctly without special VVT camshaft alignment tools. These special VVT camshaft alignment tools do not appear to be readily available, resulting in the vehicle having to be returned to the Main dealer for expensive reassembly charges.

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Toyota VVT-i system details

One of the more common VVT systems fitted is the Toyota VVT - i (Variable Valve Timing - intelligent system). This system is referred to as “Intelligent” as the amount of intake camshaft timing (intake valve overlap) change is in proportion to engine speed, intake air volume, throttle position and coolant temperature.

Basic VVT systems only vary the camshaft valve timing by operating an hydraulic valve at a pre-determined engine rpm which acts like an ON / OFF switch for moving the camshaft between two fixed positions.

The Toyota VVT - i system constantly varies the camshaft timing by using the engine management system ECM to calculate the optimum intake valve timing. The camshaft valve timing is then altered by the engine management system ECM changing a control signal to an oil control valve which is located on the top part of the camshaft assembly. The oil control valve is used to control the oil pressure in the VVT - i controller assembly to retard or advance the intake camshaft. Feed back on the position of the intake camshaft is supplied to the engine management system ECM by the camshaft position sensor.

The VVT - i controller consists of a housing which drives the intake camshaft via a vane which is  located on the end of the exhaust camshaft. The engine management system ECM controls the position of the oil control valve to distribute the engine oil pressure to either the retard or advance sections in the vane of the VVT- i controller.

Note. On Toyota engines the VVT- i controller is located on the end of the intake camshaft.

By varying the position of the oil control valve, oil pressure can be diverted to either the retard or advance sections of the vane in the VVT - i controller housing which results in the rotation of the intake camshaft, so that the intake valve timing can be retarded or advanced as necessary for engine load conditions.

To improve engine starting, when the engine is switched “OFF”, the oil control valve is de-energised which results in the vane in the VVT - i controller positioning the intake camshaft in the maximum retard position.

Phases of VVT-i operation

Toyota have designed 7 phases (ranges) of camshaft valve timing control for the VVT - i system for all operating ranges of the engine. These include engine off, idle, light load, medium load etc.

The following chart illustrates the different phases (ranges) of camshaft valve timing control.

Phases (ranges)	Valve Timing Position	Condition	Effect
At starting / stopping engine		No valve overlap to prevent intake blow back	Improved start ability
At idle conditions		No valve overlap to prevent intake blow back	Stable idle engine speed & better fuel economy
At low engine temperatures		No valve overlap to prevent intake blow back & allow for reduction of fuel increase in low engine temperatures, and improved fast idle control	Stable fast idle speed & better fuel economy
At light engine load		Decreasing (retarding) valve overlap to prevent intake blow back	Improved engine reliability
At medium engine load		Increasing (advancing) valve overlap to improve EGR efficiency	Better fuel economy  & improved emission control
At low to medium speed with heavy engine load		Increasing (advancing) valve overlap to improve engine efficiency	Improved torque in low to medium speed ranges
At high speed with heavy engine load		Decreasing (retarding) valve overlap to improve engine efficiency	Improved engine output

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Toyota VVTL-i system details

Introduction

The Toyota VVTL - i (Variable Valve Timing & Lift - intelligent) system has been designed to provide improved fuel economy and engine performance, and also to reduce exhaust emissions.

In addition to the VVT - i system (explained previously), the VVTL - i system has designed a cam changeover mechanism that varies the amount of the lift for both the intake and exhaust valves.

The VVTL - i system uses rocker arms, and the intake and exhaust camshafts each have two different types of cam profiles on them. One profile is used for low to medium speed, and the other is used for high speed.

Camshafts

The intake and exhaust camshafts are designed with a low to medium speed cam lobe profile and a high speed cam lobe profile for operating the VVTL - i system. The VVT - i controller is located on the end of the intake camshaft and the camshaft position sensor with timing rotor is located at the other end of the intake camshaft.

Rocker Arm assembly

A camshaft profile changeover mechanism is designed into the rocker arm assembly. Its main components are a pad, locking pin and roller. The locking pin is operated by oil pressure which is controlled by an additional VVTL - i oil control valve (located close to the camshaft position sensor).

The VVTL - i oil control valve is controlled by the Engine management system ECM.

Low to medium speed operation of VVTL - i system

Engine speed below 6000 RPM /  Water temperature below 60 degrees C

During low to medium speed engine operation, the VVTL - i oil control valve is de-energised, and the locking pin in the rocker arm assembly is in its rest position.

In this condition, there is excessive clearance between the pad and high speed cam profile.

With the pad free to move, the action of the high speed cam profile pushes the pad up and down but does not push downward onto the valves. As a result the valve lift is governed by the low to medium cam profile and not affected by the high speed cam profile.

High speed operation of VVTL - i system

Engine speed Above  6000 RPM /  Water temperature above 60 degrees C

During high speed engine operation, the engine management system ECM supplies a control signal to the VVTL - i oil control valve. This control signal energises the VVTL - i oil control valve and opens a port to allow oil pressure to be directed to the locking pin in the rocker arm. The oil pressure is used to raise and lock the pin in the rocker arm assembly.

When the locking pin is raised, the pad in the rocker arm assembly is also raised and locked in the UP position. This allows the high speed cam profile to push downward onto the valves before the low to medium speed cam profile.

The action of the low to medium speed cam profile has no affect on the rocker arm and valves, and the cam timing and valve lift  is governed by the design of the high speed cam profile.

Note. The engine management system ECM will only energise the VVTL - i oil control valve when the water temperature is above 60 degrees C. Below 60 degrees C, the engine management system ECM will keep the additional VVTL - i oil control valve de-energised.

Once the water temperature is above 60 degrees C, as the engine speed increases to over 6000 RPM the engine management system ECM will supply a signal to energise the VVTL - i oil control valve. This locks the pad on the rocker arm, causing the high speed cam profile to push downwards onto the valves.

The engine management system ECM receives feedback that the camshaft has changed over to high speed cam profile from a signal received by an oil pressure switch located next to the VVTL - i oil control valve.
 
When the engine speed falls to below 6000 RPM, the engine management system ECM turns “OFF” the control signal to the VVTL - i oil control valve. The VVTL - i oil control valve de-energises, and the oil pressure is reduced in the rocker arm assembly by the pad being pushed down against the locking pin.

As the locking pin is forced back against its seat, the pad is again free to move up and down in its chamber. The action of the high speed cam profile has no affect on the rocker arm and valves, and the cam timing and valve lift are governed by the design of the low to medium speed cam profile.

Conclusion

Regarding the previous technical bulletin on the Toyota Corolla, it can now be seen that any effect on oil pressure values in the engine, caused by either incorrect oil, low oil levels or oil that has passed its service life, can affect the operation of both the VVT - i and VVTL - i systems.

This is the reason why the Toyota technician advised the caller to carry out an oil flush and oil filter change before any electrical checks on the camshaft position sensor were carried out.

If the oil pressures in the engine are not correct, the VVT - i & VVTL - i systems will not function correctly, and this is more likely to  be the cause of the Camshaft sensor fault code.