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The differential between  VTEC system for Honda Jazz and other carís series.

Variable Timing

 

   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.  

 

 The incredibally simple Porsche VarioCam system advances and retards the intake cam by simply moving the chain tensioner up and down.

 

 

    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.

 

 

vvti.jpg (103795 bytes)

 

 

    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.

 

    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.

 

 

The Future

 

   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.

 

BMW Electronic VVT.jpg (285708 bytes)

 

 

    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.

 

  

BMW Mechanical VVT.jpg (495797 bytes)

   

    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.

 

 

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