Variable Valve Timing By Scott “Gonzo” Weaver 2011
History of the Variable Valve Timing system
From the early days of steam power there has been some
form of variable valve timing in existence. In the late 60's,
Fiat was the first automotive manufacturer to patent a version
of variable valve timing for production automobiles. The system
used hydraulic pressure to vary the cam followers. The hydraulic
pressure changed the cam followers by way of the engine speed
and intake pressure.
Since then there has been many different design changes and
improvements. GM actually has a patent on a variable valve timing
systems dating back to 1975 but with the many development
changes and advanced computer capabilities it wasn't till 2005
when GM had a system that could go into production on their LZE
and LZ4 engines.
Alfa Romeo had the first full production model with a VVT system on their 1980 2.0 liter Spider. 1989 introduced the US market with the Honda VTEC system, although Honda had already worked the kinks out of their system with the introduction of a VVT system on their 1983 CBR400F motorcycle. (which was only sold in Japan). By the late 80's early 90's almost all automotive manufacturers had a successful VVT system in production. Not all of them called their system a “variable valve timing” system. Here's a list of the current engine nomenclatures that I've found. Alfa Romeo - Twinspark technology Audi – VVT BMW - Valvetronic, VANOS and Double VANOS Ford - Variable Cam Timing GM - Double Continuous Variable Cam Phasing (DCVCP), Alloytec and Variable Valve Timing (VVT) Honda - VTEC, iVTEC and VTEC-E Hyundai - MPI CVVT Lexus - VVT-iE Mazda - S-VT Mitsubishi - MIVEC Nissan - N-VCT, VVL , CVTC and VVEL Porsche - VarioCam and VarioCam Plus Subaru - AVCS and AVLS Toyota - VVT, VVT-i and VVTL-i Volkswagen - VVT Volvo - CVVT
How's it Work
On a conventional engine both the exhaust and intake valves open or closed position depends on their fixed position related to the chain or belt that is driven by the crankshaft. The pattern and timing cannot be altered so there is no way to increase or decrease the amount of valve lap. (when both valves are open at the same time) As engines go, some have great low end (near idle) performance while others have high end (full throttle) performance. (If you've ever heard an old street rod or dragster popping and rumbling at an idle, you've heard the lope from the cam, this “lumpiness” in the cam is what gives it that particular sound at idle. Because these engines are designed to have maximum performance at the top end the cam is “cut” for better performance at the high end so sacrifices are made at the idle end.) With variable valve timing, the valve duration can be matched to the engine speed, torque requirements, and valve overlap. Now an engine can produce both low and high end performance with out any erratic idle condition or high end loss. This also enables an increase in mpg's throughout the engines power band by controlling valve timing, making the engine more fuel efficient. One great advantage of the VVT system is the way it can take the force needed to expel the burnt mixture out of the exhaust valve. To push the exhaust gas out of a cylinder requires some of the force that has been generated during the combustion stroke. Opening the exhaust valve when there is still some pressure left in the cylinder allows a small portion of the exhaust gas to escape before the piston starts its upwards travel. This reduces the exertion from the crankshaft and piston and provides a smoother more even engine at every rpm level. Leaving the intake valve partially open at the right point also allows fresh air to enter the cylinder while the exhaust valve is doing its job of removing the already burnt gasses. This slight intake valve opening creates a low pressure and aides the piston to push out the remaining gases and be ready for the next turn of the crankshaft. This is all due to the configuration and shape of the exhaust ports and manifolds. All of which work together and make the whole process seamless. Honda VTEC system Honda's term for a variable valve system is the i-VTEC system.
The i-VTEC system uses a oil pressure solenoid activated electrically
by the PCM to allow oil to pass into the rocker arm between the two
normal rockers, this in-turn "locks" the normally used two intake
valves together with a set of pins that are pushed outwards into the
two intake rockers and transfers their motion to a higher eccentric
lobe (operated by the middle rocker arm) This higher lobe gives the
engine the needed boost in power at rpms higher than 4500.
When the rpm level drops below 4500 rpms the oil pressure solenoid
shuts off blocking off the oil pressure and returns the engine to the
normally operated two intake valves.
2008 Accord take this to a whole new level of valve control,
allowing the engine to go from 6 cylinders down to 4 and even down to 3 cylinders. It uses a solenoid to “unlock/lock” the cam followers on one bank and allows the follower to float freely while the valve spring keeps the valve in the closed position. Vehicles equipped with Honda's VCM (Variable Cylinder Management) systems also include Active Noise Cancellation (ANC) and Honda's Active Control Engine Mount (ACM) system. The ANC and ACM systems work in cooperation to cancel both noise and vibration that could occur in relation to the cylinder deactivation process. The ANC system uses the audio speakers to cancel out noise by using an opposite phase sound. This whole process is controlled by the computer system and becomes imperceptible to the driver. (They won't even know it's happening under the hood at all.) These systems use a mechanical/electrical solenoid that operates with oil pressure to accomplish the range of variable valve timing. The VTECE system is slightly different in it's configuration than the VTEC. The emission qualities have been increased but at the same time performing the same functions as the VTEC system does.
Toyota came out with the Valvematic system in 2008. This system uses an intermediate shaft to achieve a continuous variable valve lift. The intermediate shaft consists of followers on either side of a roller bearing. These followers rotate with respect to the roller member and “finger followers” (small followers) by means of an internal gear and electric motor attached to the shaft. As the shaft moves the roller member and followers will move in opposite directions (either closer or further apart) . As the angle increases the the valve lift does too. This system has can vary the valve timing to any angle needed. In 2007 the VVT-ie system was introduced on the Lexus LS460. This system is electrical and hydraulic. The exhaust valve is still controlled by way of an oil pressure solenoid while the intake is controlled by a electric motor on the front of the cam. This allows valve timing to be adjusted with no regards to engine temperature or oil pressure. Toyota's early VVT system was relatively simple in comparison. The PCM signaled an oil control valve (OCV - oil control valve) to open and let oil pressure into a special gallery in the first cam bearing area, this routed oil into the VVT pulley where a small piston would move causing the pulley to turn in relation to the pressure and a helical spline in the pulley. This would force the cam into an advanced or retarded position depending on the engine requirements (determined by the PCM).
Common Problems associated with VVT
The two most common codes I've ran across are P0011 and P0021 (Cam shaft position “Bank 1” sensor and Cam shaft position sensor “Bank 2”) These codes (like any code) doesn't entirely mean the sensor is faulty, however the diagnostic charts will tell you to look at the VVT system for a fault AND check the sensor as well. Some of the common areas to look into are: Valve timing, Oil control valve, Oil control valve filter screen, Cam shaft timing/gears, and of course the electrical side of the operation as well as the PCM.
The very first thing I want to look at before getting into turning nuts and bolts is to check the oil. Oil is an essential part of most VVT systems. Dirty oil and lack of regular oil changes can leave a build up of sludge or debris in the passages leading to the pressure control valve that operates the variable valve timing. If the oil is dirty and enough sludge accumulates at the valve ports the sludge can be passed on through the cam and the valve assembly. Now the oil passages in the cam can be comprimised and could result in a cam failure due to scored journals. Keep in mind that the VVT system is not operated at normal driving condition rpm's. For example the Honda VTEC system doesn't operate below 4500 rpm's so if you have someone who lives in a city area and never gets the car out on the highway and never changes their oil you can have a potential problem waiting to happen if and when the car is rev'd up above 4500 rpms the next time it heads onto an on ramp of thier local interstate highway.
There is a code P0521 (Oil pressure sensor/switch range/performance) that could be an indication of the quality of the oil in the engine. Maybe not the best diagnostic answer but I've found on several vehicles when I've seen this code that the oil was black and neglected. The code can also indicate (in some cases) that the wrong type of oil has been used. I wouldn't use this as the final answer to the problem with a variable valve timing but more of an indication of things to come. (It's a good time to talk to your customer and find out what type of oil, how often it's changed and whether or not you need to proceed any further with your diagnostics.)
Lack of regular maintenance seems to be the big factor in most of these systems. Unlike vehilces from years gone by where certian maintenance issues could be left undone for quite some time these newer engines and newer systems require the utmost in care. Engine longevity and perfomance can directly be associated with the care given to the vehicle over it's natural life span. Stressing this point to your customers will increase your profits and their vehilce by performing the required basic maintenance per the manufacturers schedule. This will reduce the amount of unwanted mechanical failures and keep your customer coming back again and again.
Variable valve timing, (in my opinion) will be as common as a spark plug in the near future. I'm sure we'll see even more and more of them. Reducing emissions, reducing the need for an EGR valve, improving fuel economy, and getting more performance out of smaller engine sizes tells me that the VVT systems are here to stay. The tool requirements for installing a new timing belt on some of the models can be quite expensive, but I'm sure those prices will come down in time also. There's no doubt the overall concept of using oil pressure or rpm levels will be enhanced and changed with newer ideas and engineering. The next generation of VVT systems are on the drawing boards now, and it won't be long before they'll be in the marketplace. As a technician the constant upgrading of your knowledge base is a never ending challenge. We can all wish for those simplier times we all rememeber but the automotive world is constantly changing and bringing new ideas into use. With the latest requirements in fuel economy coming up engines like the variable valve timing engine will become the norm. It's all for the better, so we better be prepared for it.