PanterroR
Lap Time Luminary
"The goddamned thing doesn't even run right until it gets over 100 miles an hour," was how one local marshal described an impounded Hudson Hornet, caught running 'shine from the hollers of Franklin County, Virginia, through the fog-covered Blue Ridge Mountains in the early hours of the night. Designed for maximum speed when outrunning the law, the cantankerous Hudson was a victim of the same problem that modern engineers face: The qualities that make high-performance engines free-breathing and gnarly render them unusable for the street, while a street engine, with its smooth idle and easy driveability, is too meek to be used in a sports car. The solution to making an engine that is tame enough to putter across town but switch to an aggressive tune when given the beans lies in variable valve timing. Chances are you have been confronted by that term at least a hundred times and somewhat understand the principle behind it, but can't put your finger on exactly what kind of voodoo happens when the VANOS actuates or the hot cam switches over. Even more daunting are the reasons why Johnny Tran was puckering up and rocketing toward the horizon when the VTEC just kicked in, yo. Well, cheer up! The science behind variable valve timing isn't as mentally taxing as you'd expect it to be.
Before we go any further, let's take a minute to bone up on what's happening as the pistons pump up and down in the cylinder. In nearly every car on the road, the piston completes two full up-and-down strokes for one complete power cycle. Very briefly, those four strokes are:
The intake stroke: When the piston is at the top of the cylinder, the intake valve opens. The piston starts moving downwards, sucking in a charge of air mixed with a fine mist of gasoline.
The compression stroke: The intake valve shuts and the piston starts
moving back up. The piston compresses the fuel/air mixture into an area nearly 1/10th its original space. It's this compression that makes the next stroke more powerful.
The power stroke: The intake and exhaust valves are both closed, and the piston is at the very top of the cylinder. An electric arc jumps across the gap of the spark plug, igniting the gasoline charge in the cylinder, which then explodes. The force of that explosion drives the piston down.
The exhaust stroke: As the piston bottoms out in the cylinder and starts to head back up, the exhaust valve opens all the way, and the upward motion of the piston pushes burnt exhaust gases out of the cylinder. With the piston now back at the top of its travel, it starts back down another intake stroke and the cycle begins anew.
Now that you're familiar with the basics of the motions involved, let's expound a bit on what the valves are doing during those four strokes and why their motions are so critical to an engine's performance.
It's after the power stroke — after the fuel/air mixture has been compressed and detonated and the pressure of combustion has sent the piston on its merry way down the cylinder bore — that our journey begins. The cylinder is full of spent gases and byproducts. Just before the piston reaches the bottom of its travel, the exhaust valve is cracked open and the remaining pressure in the cylinder begins to rush out into the exhaust manifold. Soon after, the piston begins its journey back up the cylinder and forces any straggling exhaust gas out the exhaust port.
As the piston nears the top of its exhaust stroke, the intake valve begins opening, so both the intake and exhaust valves are open at the same time. The period that both valves are open is called overlap and, while it seems counterintuitive, it actually helps load the cylinder with a fresh fuel/air charge. As exhaust gas speeds out of the exhaust port, it creates a negative-pressure area in the combustion chamber, and when the intake valve opens, the fuel/air mixture in the intake manifold rushes in to fill the void. The incoming surge also helps purge any remaining exhaust gas out of the cylinder in a process known as scavenging. As you'd suspect, the moment that the intake valve comes off its seat is critical: If it opens too early, the cylinder hasn't been fully evacuated and the rising piston will force exhaust gas back into the intake manifold. But if it opens too late, there won't be any negative pressure left in the combustion chamber and the fresh charge won't be sucked in. As engine speed increases, the dynamics of airflow require the valves to open at different times in order for the scavenging effect to happen properly — but with a fixed cam, engineers have to compromise and settle on one design that is merely okay for most situations.
As the piston starts heading downward again on the intake stroke, the intake valve opens fully, which brings with it another issue: valve lift. For most of the intake stroke, the largest restriction in the intake tract is the area between the backside of the open intake valve and its valve seat. The size of that area is determined by valve lift, or how far the camshaft's lobes push the valve open. Valve lift is another area where engineers have to compromise: At low engine speeds, the opening needs to be small enough so that the rush of incoming air maintains a high velocity. That velocity creates turbulence, which prevents the suspended cloud of atomized gasoline in it from coming out of suspension. All the while, that same opening has to be large enough so that the engine can breathe freely in the upper revs.
There's another factor at play with the intake valve: its duration. Duration is the amount of time that the valve spends open, from the first nanometer it lifts off its seat until it's firmly closed. The time between those two events is crucial in determining an engine's power band. At high engine speeds, the cylinder doesn't completely fill by the time the piston reaches the bottom of the cylinder, so the intake valve needs to be held open longer to let the air charge in, even up to the point that the valve doesn't close until the piston starts its compression stroke. At low speeds, though, holding the valve open that long means that the piston will begin pushing the fuel/air charge back out the intake port, resulting in less mixture left to combust, which in turn results in lower cylinder pressure and less low-end torque. By contrast, a camshaft that closes the intake valve as soon as the piston bottoms out allows it to retain more of the fuel/air charge at low speeds, and the result is an engine with mountains of bottom-end pull, but also an engine that doesn't like to rev.
So, not only does a camshaft have to suit the driving style of American commuters — who couldn't tell the difference between their car and refrigerator if the former were painted avocado green and parked in the kitchen — but it has to allow enough flexibility in design to give the car ample power when rocketing up on-ramps and sailing across the amber waves. It's little wonder that most engineers wear short hair — were it longer, striking the ideal cam design that balances the requirements of marketers, designers, and management would have them pulling it out by the roots.
Salvation for balding engineers comes in the form of variable valve timing (VVT). Far from a single technology, VVT is a catchall title; it covers systems ranging from those that simply rotate the camshaft slightly to those that eliminate throttle bodies entirely by dynamically changing valve lift.
One system, and the most widespread by far, is simple cam phasing. Instead of the camshaft and the crankshaft turning in perfect sync, cam phasers can rotate the camshaft several degrees in either direction. Most cam phasers are installed on just the intake camshaft, where it can rotate the cam back, or "retard" it, at low engine speeds — letting the intake valve close early and giving the engine more torque off the line. As the engine speeds up, the phaser will rotate the cam forward, or "advance" it — keeping the intake valve open longer during the piston's intake stroke so that it can more fully suck in an intake charge. In doing so, the cam phaser moves the engine's torque peak into the higher reaches of the tach, making the engine's torque curve broader than it would be in a traditional engine.
Some engines use cam phasers on both the intake and exhaust cams. By varying when the exhaust valves open and close, this can limit how much exhaust gas is expelled, allowing a small bit of leftover byproduct to remain in the combustion chamber as the fresh fuel/air charge is sucked in. Known as internal exhaust gas recirculation, this practice helps cut down on emissions when the engine is loafing along at a steady speed.
Cam phasers have even been adapted to cam-in-block engines, which is quite an accomplishment considering that an engine with a single camshaft has its intake and exhaust lobes fixed in relationship to each other. A phaser on GM's 3.5- and 3.9-liter pushrod V6s rotates a solid camshaft back and forth through a very limited range of motion. This setup is limited in its effectiveness since it adjusts both intake and exhaust timing at the same time, but it still gives the engines power benefits over their static-cam siblings.
Did we say that the intake and exhaust lobes on pushrod engines are fixed? Don't tell them that at Chrysler, where the SRT powertrain team was considering switching the Viper's 8.4-liter V10 to dual overhead cams so that VVT could be fitted. That is, until Mechadyne, a British engineering firm, introduced a two-piece camshaft that allows the intake and exhaust lobes to be rotated independently of each other. The exhaust lobes on Mechadyne's Viper camshaft are mounted to a hollow tube that fits over a solid shaft containing the intake lobes, allowing the two to move up to 45 degrees apart based on engine load. Unlike most systems that control intake timing, the Viper's controls the exhaust cam. Why? SRT engine team supervisor Kraig Courtney points out that the primary benefit of varying intake timing would have been more low-end torque. With 560 lb-ft already in the Viper, he says, that would simply "translate to more tire smoke."
Far out in cam phasing's left field is Porsche, which uses a proprietary system called VarioCam. Unlike other systems that use a cam phaser to rotate the camshaft, VarioCam uses a hydraulically controlled piston to exert pressure on the timing chain between the intake and exhaust cams. Pushing the chain one way causes the cam to retard, pushing it the other makes it advance.
The next systems are known as cam shifters. Unlike cam phasing systems, which change valve timing relative to the crankshaft, cam-shifting setups don't change the relationship between the cam and crank at all. They use camshafts with two unique sets of lobes, each with their own rocker arms that switch over based on engine speed. The less aggressive, or primary, cam is engaged at lower engine rpm, where it restricts valve lift. The air flowing through the smaller valve opening maintains enough velocity so that the fuel/air mixture remains agitated and atomized. As engine speed increases, a solenoid shunts oil pressure to a set of spring-loaded pins in the primary rocker arms, forcing them out and locking the arms to a second set of rockers being driven by the hotter, or secondary, cam. Because the secondary cam lobes are designed to operate when the engine is in its upper reaches, its lobes have a taller overall height that opens the intake valves further, allowing the engine to take large, gulping breaths that wouldn't be efficient at lower speeds. The physical shape of the secondary cam lobes are curved so that they open the valves more gently and close them slowly, preventing the valves from slamming shut and bouncing at high speeds. Because cam-shifting systems are wholly digital, on-or-off affairs, the switchover between cam profiles can sometimes be startling and harsh. Toyota's 1.8-liter engines with cam-shifting, as used in the last Celica GT-S and Lotus's Elise, are notorious for being gutless until they crest 6000 rpm, at which point the secondary cam engages with a violent rush of power.
As the price and complexity of adding cam phasers has decreased, manufacturers are taking the technology to its logical end by combining both cam-phasing and cam-shifting systems into a complete valve control system. Not only can these systems dynamically shift cam timing, they can shift it when the engine is on either its primary or secondary cam lobes, ensuring that the engine is always in its power band and is always making the most efficient use of its fuel, be it for power or economy.
While valve control systems are fairly straightforward, there are two outliers so wonky that they bear mention. When looking for ways to increase fuel economy, BMW engineers found that the restriction caused by sucking air through a partially closed butterfly in the throttle body was hurting mileage, so they set about a way to eliminate it entirely. The solution was Valvetronic - a system that varies the amount of valve lift so precisely that engines equipped with it run unthrottled like a diesel. Changes in valve lift alone to regulate engine speed. This is accomplished through an offset pair of rocker arms between the cam lobes and the valves. The rocker arms are pivoted by an electric motor, which changes the fulcrum point of the arms to allow to for more or less lift. Recently, Nissan revealed a similar system called Variable Valve Event and Lift, debuting on its new Infiniti G37 coupe. So effective is the system at increasing power and efficiency that Nissan claims a 13-percent improvement in highway mileage for the new car versus the outgoing G35, while gaining 32 hp in the process.
Combined with both intake and exhaust cam phasing, Nissan and BMW's systems represent the acme of valve-actuation technology, eliminating any of the steps or changeovers found in other manufacturers' cam-shifting setups, while providing superior economy versus a traditionally throttled engine. They are, arguably, the most fully realized valve control systems in production — proving that while the design of the internal combustion engine is going on 150 years old, technology still finds a way push the game forward.
Link: Motivemagazine.com - Motive Tech: Variable Valve Timing Explained
