I was reading the stickers on new Volvos at my local dealer, and saw the newer 5 and 6 cyl engines have variable valve timing.

How does this work? How does this increase HP?

Volvo is using the VVT on exh on turbos and intake on the non-turbos.

On the turbos the VVT is activated to advance cam on cold start for a period during warm-up. Advancing the exh cam allows exh valve to close sooner decreasing overlap and reducing the raw fuel escape that occurs during rich mixture while at the same time opening the exh valves sooner to allow some escape of flame that will contribute to combusion of the raw fuel left in exhaust from last exhausting cylinder. After the warm up period the cam goes back to std position except at idle where it is advanced 7-10 degrees for better idle emissions. Exh cam range is 0-30 crank degrees advanced.

On the non-turbo the int cam is continuously variable w/o respect to temp. It is regulated for emissions optimization and HP and torque optimization based on engine speed and throttle position.

The range is 40 crank degrees but I don't know what the "standard" position is within this range.

The control is done via a hub on the cam that carries the belt gear. In the hub is a piston with an internal helical gear (piston is keyed to hub and can rotate, only move laterally). The external helical gear is inside the int helical gear and bolts to cam. As the oil pressure is regulated to be higher on one side of piston or the other the piston moves back and forth and changes the relative position of gear to the cam.

The oil pressure is regulated by a duty cycle operating pulse from ECM to an oil pressure solenoid providing the oil feed to the cam gear hub.

The cam having the VVT has a cam position sensor that gives a signal of cam postion to ECM that it compares to the RPM sensor that is tracking crank position and speed. This allows the ECM to pulse width control solenoid to obtain cam position desired.

Varaible Valve Timing. Most carmakers have models with it nowadays.

Honda was one of the first to offer it in the late eighties. They call theirs VTEC. Toyota has VVTi. Our Alfa Spider has it. It looks rather simple. It's electomagnetically actuated on the intake cam. It kicks in at 3500RPMs...or is it 4500, been too long :o( Anyways, it gives a good boost in acceleration.

They're at the point now where they can continuosly vary the valve timing for whatever they like.

Variable compression...it's on it's way.

Will Repsher

VTEC actually brings a completely new cam profile on line (usually much more agressive for power up to 9000 rpm in the power-oriented VTEC types) once it gets activated. It is not a valve-timing system in this regard.

The VTEC cam has a second cam grind on it, but these can be actuated by rocker arms which have a hydraulic 'lock' on it to enable the use of this alternative cam profile.

Bye, Arno.

Like Jim said, instead of a cam with an 'average' profile, you can vary the exact timing of each valve opening to allow the air/fuel charge to enter and then the exhaust exit at a more optimal level throughout the RPM band (i.e. less lift and shorter duration makes for a clean burn at idle and good torque off the line, where a higher lift, more duration, and maybe more overlap means greater high-RPM power.)

Volvo's implementation of VVT is mostly for improved emissions, not greater power. I think the NA engines use it on the intake side only, and the turbos use it on the exhaust side only.

I'm not sure about the new Volvo but I know that BMW and Porsche have been doing this for several years. (I gave shop manuals to #1 son for his BMW and #2 son for his Boxster. Chips don't fall far from the tree re interest in fine cars.)

Basic concept is that at each engine speed there is an optimum time, angle, for the cam to open and close the valves to obtain maximum torgue. (Remember that Torgue x RPM = power.) The variable system allows the designers to alter the timeing based on RPM, or some control method to get closer to the optimum. This as apposed to a fixed compromise value on all our older Volvos.

Usually the hardware is a hydraulic cylinder changing the effective length of timing belt or chain on either side of the cam gear. The cylinder in turn can be controled by the ECU or just a mechanical governer.

This isn't teribly specific but I hope it helps.