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There was a time when engines had to be big to be powerful. There was a time when engines could either be tuned for low-rpm torque or high-rpm power, but not both. There was a time was a time when a specific output of 100 hp per liter was the stuff of racecar fantasies. Today these limitations are all but gone. Getting 100 hp for each liter of displacement is now possible on cars that have to get good gas mileage, emit clean air, act civilized enough for your grandmother to drive them and sell for under $20,000. So what happened? Variable valve timing.
Camshaft profile is probably the single most important engine design parameter determining the personality of an engine. One cam profile can make an engine a trucky torque-monster, another can make it a peaky race engine, but no single profile can give you both. For decades engine designers have wrestled with the compromises that are inherent in cam design. A cam that idles well and offers clean emmisions typically won't make much power, while a serious high-horsepower cam can make the engine belch smoke at idle and be balky at low rpm. Some method of changing cam profile on the fly has always been the ideal solution; if an engine needs a different personality for different parts of the powerband, why not give it a split personality? For most of the history of the internal combustion engine this has been an impossible solution - the VTEC arrived.
Teb years ago variable valve timing was exotic technology; today it's so commonplace that some automakers have even forgotten to brag when they use it. Like all wheel drive, supercharging, and virtually every other automotive technology, variable valve timing can be achieved through a surprising variety of methods. and for several different purposes.
There are two basic types of variable valve timing systems. variable timing and lift systems can typically switch between completely different cam profiles. most systems, however, vary only timing by advancing and retarding a standard set of cams.
Variable Timing and Lift
Honda broke the ice when the NSX debuted in 1991 as the first production car with a variable valve timing system. Honda's VTEC (which sort of stands for Variable valve Timing and lift Electronic Control) system, which has basically remained unchanged since then, is still one of the most effective systems for making ultra high specific output.
The concept is incredibly simple. So simple, in fact, that you have to wonder why nobody thought of it earlier. Basically each pair of valves has three cam lobes, two that operate the valves at low-rpm, and a third that takes. over at high rom. During low-rpm operation, the two rocker arms riding the low-rpm lobes push directly on the top of the valves. In most cases, the cam profiles of the two intake valves will be slightly different, promoting swirl in the combustion chamber for better driveability. At high rpm (usually 4500 rpm to 6500 rpm range, depending on the engine) the ECU sends a signal to an oil control valve that allows oil pressure to flow into the low-rpm rocker arms. A third, high rpm rocker arm sits between the two low-rpm arms and follows a much more aggressive lobe. When oil pressure arrives, two hardened steel pins pop out of the sides of the low-rpm rocker arms and slide into sockets in the high-rpm arm, and the valves start folowing the larger cam profile. Just when you thought the engine was going to run out of power, output starts climing againas the engine "comes up on the cam" for the second time.
The fact that the pins in the rocker arms have to line up perfectly has caused problems in the past when people tried to make even hotter cams. When a performance cam is reground from an existing stock camshaft, the base circle - the round backside of the cam where the valve is closed - is ground down to a smaller diameter, while the peak of the cam lobe is left at the same height. (Ideally, the base circle would be left alone and the peak raised, but try adding metal with a grinder some time). If the three cam lobes are not ground to exactly the same base circle, the high-rpm cam will not be able to engage. The only time the pins and the holes will line up is when the valves are closed, but if the base circles don't match, the holes will never be aligned. Inexperienced cam grinders making this mistake are probably responsible for the popular rumour that it's impossible to make aftermarket cams that work for a VTEC engine. It is possible, you just have to make them carefully.
Until recently, VTEC was a unique system in the automotive world. The came VVL, Nissan's new variable valve lift system that, to the untrained eye, looks identical to Honda's VTEC system. In fact, even to the trained eye, it looks the same. This system is currently available exclusively on Japan only versions of the SR engine series (of which the SR20DE in the Sentra SE and G20 is the only US version), so information is hard to come by, but it appearsthat the only significant difference between the VTEC and VVL is the fact that the VVL switches the intake and exhaust at slightly different engine speeds in an effort to smooth the transition between cam profiles. There is no question that the system is incredibly effective, no matter who is making it. The 1.8 liter non-VTEC Integra makes 140 hp, while the VTEC-equipped version makes either 170 or 195 hp (depending on whether you look at the GS-R or Type R). On the Nissan side, a US model SR20DE also makes 140 hp, while the VVL equipped SR20VE makes 190 hp. Stretch the Nissan engine's development to the Type R level and you have the hyperactive 1.6 liter SR16VE that was in the Nissan Pulsar N-1, power output: 200 hp.
Systems that change only timing are far simpler then either the VTEC or VVL systems. It is generally well known that adjusting cam timing with adjustable cam sprockets can yield significant gains at certain points in the powerband, but always at the expense of power somewhere else. If you are designing engines that have to meet LEV (Low Emissions Vehicle), ULEV (Ultra-Low Emissions Vehicle), or even SULEV (Super Ultra-Low Emissions Vehicle) standards, you have other concerns that can be affected by cam timing as well. Emissions, drivability, cold startability - the effects of cam timing are far reaching. A sophisticated cam sprocketthat can advance and retard intake and/or exhaust cams on the fly eliminates most of the compromises inherent in cam timing.
The number of engines equipped with some sort of adjust-on-the-fly cam sprocket is huge, and growing rapidly. nissan has used a system called VTC (Valve Timing Control) on the Infiniti Q45 and the turbocharged Nissan Silvia, among others. Toyota has begun using VVT-i (Variable Valve Timing with intelligence) extensively on their high end US models (most of which are under the Lexus badge), but as they refine the system and reduce the costs associated with it, VVT-i is bound to find it's way down to the four cylinder models very soon. Ford has already brought their variable valve timing down to the 2.0 liter Zetec engine, but unfortunately only on the exhaust cam where it improves emissions but does almost nothing for horsepower.
The actual mechanismsused to advance and retard the cams vary enormously, but two systems in particular deserve a closer look for how incredibally simple they are.
Porsche's VarioCam, used first on the 968 and know used without the fanfare on all (both) of their engines is as simple as it gets. With VarioCam, the exhaust cam is driven by the crank, and the intake cam is driven, via a short chain, by the exhaust cam. In order to advance and retard the intake cam, the chain tensioner on that short chain simply shifts up and down, moving the extra length in the chain from the tight side to the slack side. When the tight side has no extra chain (ie. the chain is straight), the intake cam is fully advanced, as more chain is shifted to the tight side, the cam is retarded.
Toyota's newest version of VVT-i is also quite simple, though it may look otherwise on initial inspection. Again, the exhaust cam is driven from the crank, while the intake cam is driven off the exhaust cam. This time, the drive is via gears, and a mysterious cylinder behind the drive gear on intake cam controls cam timing. Inside this mysterious cylinder is a simple three fluted rotor that actually drives the cam. By pumping oil into the chambers on either side of the three flutes, the hydraulic pressure can force the cam to advance or retard. This replaces the previous VVT-i system which was basically an incomprehensible little box of gears, springs and splines. (for a description of the older VVT-i go to http://members.aol.com/juanaday/gs400/vvti.htm)
The VVT-i system can change the intake cam timing over a 60 degree range, changing valve overlap from absolutely zero (for smooth idle, easy starting and better cold start performance), to severely overlapped for a natural EGR effect at medium load (eliminating the need for an Exhaust Gas Recirculation valve), to whatever is ideal for maximum power at any point on the powerband.
If all this exotic variable valve timing technology is commonplace now, what does the future hold? Currently we are limited to either adjusting overlap by moving a standard camshaft, or switching between two fixed cam profiles. There is no reason (other then cost) why both systems could not be used in parallel on one engine, but the benifits may be limited. The true future of variable valve timing is infinite adjustability of both lift and timing.
The idea of opening and closing the valves with large electrical solenoids has been bounced around for several years. Many different manufacturers from Cummins to BMW have proposed such systems, and even made running prototypes. There are a few problems with electronically operated valves, but let's look at the advantages first.
Current gasoline engines control part throttle airflow via a throttle plate, essentially lowering the air pressure in the intake manifold by choking it off with a partially closed throttle plate. This causes significant pumping losses as the engine fights to suck air in from the manifold, and ultimately reduces the efficiency of the engine. If you limit airflow by reducing the time that the intake valve is open, though, pumping losses could be significantly reduced.
At wide open throttle (or full down pedal in the case of a throttleless engine) the valve timing could be constantly adjusted for maximum power - with no worries about one valve timing profile having to work from idle and redline like they do know.
Now the downside. Eliminating all of the valvetrain reduces the cost and complexity engine and reduces the internal drag, but not as much as you might think. Opening and closing the valves takes a certain amount of power, whether that power comes through a timing belt or a wire, it has to come from somewhere. In this case, the power would have to come from a huge alternator. This is not really a problem, but it is an extra cost that isn't initially obvious. More serious is the question of how to close the valves quickly but still have it land on the valve seat gently. With a cam, you just shape the lobe so it drops shut quickly and then slows down just before the seat. With a solenoid operating the valves, it takes sophisticated electronic controls
Finally, there is the matter of rpm limitations. Ideally, with strong enough solenoids, the engine would be able to spin even faster then a conventional engine, but the current solenoids have a hard time working that fast. On the flip side, though, because the valve opening speed is not engine related, mid rpm performance could be greatly improved by having the valves slam to full open much faster than a conventional valvetrain, improving volumetric efficiency and making more power.
Finally, BMW has actually designed a mechanical system that still uses conventionalcam lobes, but is still able to vary lift and timing by moving the fulcrum point of the rocker arm. The system is incrediblydifficult to visualize (I still don't get it.) but it could offer most of the advantages of electrically actuated valves without delving so far into unexplored technologies.
With the current variable timing technologies, the 100 hp per liter hurdle has been cleared. Honda, in particular, has knocked the hurdle over and stomped it into the ground. With the technologies on their way, 150 hp per liter is on the horizon. I, for one, can't wait.