The Fuel and Engine Bible - variable valve timing. i-VTEC, VVT-i, Valvetronic, Valvelift and more.
The Fuel & Engine Bible
Variable Valve Timing
Time to confuse and educate you further now, because the concept of a static or single-mode combustion engine is a little out of date in today's world. Because of the nature of fuel injection, carburettors, the 4-stroke cycle and valves, the internal combustion engine only really works really well at one particular range of speeds. Any higher or lower and you start to cock up fuel efficiency, reliability and power. To overcome this issue, and to try to make engines more useable throughout their rev ranges, manufacturers invented various different types of variable valve timing. The idea is simple - alter the timing and/or size of the intake and exhaust ports at different engine RPMs to ensure that the engine is as efficient as possible throughout it's range of operating speeds. Thanks to a day of piss-poor weather and a lot of spare time on my part, I now bring you an explanation of some of the range of variable valve timing methodologies and how they work. I'm not going to go into all the different variations, combinations and permutations because there are so many now. However, a top-level overview ought to tell you what you need to know. So without further delay:
VTEC stands for Variable Valve Timing and Electronic Lift Control. Not sure why it isn't VVTEC other than it would look like a typo, but there is another variation called i-VTEC, meaning Intelligent-VTEC. The basic functionality of VTEC is surprisingly simple, and hence well-used and very reliable. In its simplest form, VTEC allows the valves to remain open for two different durations. A short opening time for low-speed operation to give good torque and acceleration, and a larger opening time for higher speeds to give more power. To do this, the camshaft has two sets of cam lobes for each valve and a sliding locking pin on the cam follower that determines which lobe is operating the valve. The locking pin is moved by a hydraulic control valve based on the engine speed and power delivery requirements. The two lobe shapes are referred to as fuel economy cams and high power cams, meaning that Honda engines with this technology are really two engines in one - a performance engine and an economical engine. The two animations below show a pair of cam lobes and followers to demonstrate the fundamental operation of VTEC. The left animation is fuel economy mode - the blue locking pin is not engaged so the two followers run on their respective cam lobes independently. The yellow one is the main cam follower which pushes on the valve. On the right, in power mode, the blue locking pin is engaged so now the red cam follower is locked to the yellow one which, now being driven by the red one, no longer contacts the lower profile cam lobe. Because the red follower is running on a higher profile cam lobe, it now forces the valve to stay open longer.
Go on - guess what VVT-i stands for. If you guessed Variable Valve Timing - Intelligent, then step forwards and claim a gold star. VVT-i is the second approach to variable valve timing. The problem with Honda's idea is that you really only have two modes - economy and power. The VVT-i system goes a step further and allows a continuously variable engine operating profile, so rather than simply having economy and power modes, there's an infinite number of positions in between that can be selected on-the-fly in fractions of a second. This means that the engine can be kept in it's sweet spot for a far broader range of operating conditions and demands. To do this, VVT-i doesn't have two sets of cam lobes, rather it can dynamically adjust the timing of the entire camshaft instead. This means that whilst the actual duration that the valves are open never changes, their timing in relation to all the other engine operations can be adjusted. In a simple engine, the timing belt or chain from the crankshaft loops up and around a camshaft pulley that turns the camshaft. With VVT-i, the timing belt loops around a pulley that contains hydraulic fluid or oil. The camshaft itself has vanes on the end of it that sit inside the fluid, so in this system, the camshaft is not directly linked to the timing belt pulley. By altering the oil pressure through a series of valves, the position of the camshaft vanes can be altered inside the pulley housing. The cutaway animation below shows an example of this. The pulley housing is rendered in yellow - that's the pulley which is driven by the toothed timing belt looped around the the outside of it. Inside you can see the red vanes of the camshaft (also in red) which are free to rotate a certain amount back and forth. When rotated all the way in one direction, the engine is in economy mode. When rotated all the way in the other direction it's in power mode.
BMW VANOS & Valvetronic Variable Valve Lift
BMW have two technologies they can put in their engines. VANOS is their VVT system that varies the timing. It uses high oil pressure to articulate pistons with helical gears at the end of each camshaft. Depending how far each piston is moved either back or forth, due to the helical gears, the camshaft is rotated one way or the other. This has the effect of changing the position of the TDC orientation of the camshaft, which can advance or retard the timing.
BMW also use a system of variable valve lifing too, to alter how far the valves open. This is their Valvetronic system and in simple terms, it alters the pivot point of the cam follower. Different pivot points result in different deflections of the tip of the follower for any given position of the cam lobes. That larger movement increases the size of the valve opening which allows more gas to pass through. The mechanism by which it does this is a lot more complicated than I'm going to illustrate here, but the animation below should give you some idea how it works. The left one shows the engine running in fuel economy mode. The cam acts on the red cam follower which presses against a roller bearing in the yellow valve lever. The tip of that lever is what pushes the valve open. In power mode, on the right, the blue pivot bearing has rotated around and altered the pivot point of the cam follower. This means that the resulting action on the yellow valve lever is more severe, forcing the valve to be open further and for longer. Simple eh? And very precise, being German.
It seems only fair to mention two German solutions given there are two Japanese solutions at the top of the page. Audi's system is very similar to Honda's in that is uses a relatively simply mechanism to choose between two differently-shaped cam lobes. In principle, it works just like i-VTEC, the only real difference is the mechanism used to select the cam lobes. Instead of having two cam followers and a locking pin, Audi use a pair of sliding cam lobes on a splined part of the camshaft. The sliding lobe carrier is moved along the camshaft to determine which of the two lobes will be used to operate the valves. The animation below is non-looping, and shows this. On the first rotation, the low-profile cam lobe activates the cam follower. At the beginning of the second rotation, the blue activator pin drops into the spiral groove in the sliding lobe holder. During that rotation, the lobes are moved along the camshaft to their new position so that on the third rotation, after the blue pin retracts, the high-profile cam lobe now activates the cam follower instead. To slide the cam lobes back to the original position, the second activator pin is used instead, engaging in the second spiral channel. Frankly, I think this is much cleverer than the BMW system, but I'm sure some sort of Teutonic engineering war would erupt if they heard me say that out loud.
Are there more types?
Sure there are. Every manufacturer and their respective dogs have some form of variable valve timing now. Hopefully the four designs on this page will give you some idea of the different engineering solutions to the same problem. I'm not going to cover all the other types - they're all variations of the four already covered here. So next time the motoring bore in the pub asks you if you know how VVT works, you can geek out on him. Won't get you many women, but at least you'll sound like you know what you're talking about.