Ferrari VVT

sid_galt

Why isn't Ferrari Variable valve timing - they have a 3d cam with variable profile along its length and shift the camshaft to provide variable valve timing, why isn't it popular?
Is it because of the cost?

brewnog

Yeah, it's not an easy thing to do cheaply. I would imagine there are some pretty major wear and reliability issues which would you would need to overcam (sorry) to make it viable for mass production. Most cars aren't Ferraris, and if any power uprates (or rather, tuning the torque and power curves) are needed then they're generally much easier to achieve by alternative means.

It's still a nice touch though!

chroot

Various other variable valve-timing implementations are relatively popular. Honda has VTEC, present on quite a number of its production models, while Toyota has VVTL-i. These systems aren't true "3D" cams, but rather cams with two sets of lobes, providing two different valve timings for low- and high-rpm operation. They are relatively fail-safe: if the cam's solenoid fails to operate, you're just stuck with the low-end cam until you get it fixed.

- Warren

Danger

I don't know whether or not any are in commercial production, but there are also hydraulic and/or solenoid operated valves that eliminate the cam entirely and can be infintely varied by on-board computer control. That sidesteps any wear issues.

brewnog

One variant (and I'm not sure whether Lotus toyed with this for a while, it's potentially on a current Toyota engine, anyone?) is to use a secondary mechanism to hold the valves open after the nose of the cam lobe has started to fall.

sid_galt

True but they bring a host of problems

1. High power consumption = high power alternator = high cost
2. valve seating velocities
3. plain expensive

sid_galt

BTW, how much approxiamately does a camshaft with cams cost?

Danger

The first item isn't a factor if they managed to perfect the 'helinoid' system. I haven't heard of it in over 30 years, but it seemed promising. It used finned, conical plungers in a conical, internally-finned, helically-wound solenoid. The stroke was only a quarter-inch or less, but a helinoid around 1 1/2 or 2 inches in diameter could pull a couple of hundred pounds that distance in a few thousandths of a second on 12VDC. There was some sort of interaction between the internal and external fins that vastly amplified the normal magnetic field. I'd have to dig through a ****load of old Car Craft and Hot Rod magazines to find the article, but I might have time one of these days.
There should be some method available to dampen the final closing speed, such as tiny hydraulic shock absorbers (which is a built-in function of the hydraulically-activated valve system).
Expensive, yes... but I'd expect that the cost could be kept within the same sort of range as selective cylinder deactivation, which is quite common nowadays.
As for the price of a cam and lifters, I couldn't begin to tell you. For one thing, prices are different here than in the US or Europe. Also, the last time that I bought one was 26 years ago. That was a Crane unit, and it cost $120.

brewnog

A camshaft with cams, as opposed to one without?!

I could buy a brand new performance camshaft for my Ford engine for about £120, but recently bought one at work which was about £4500.

Danger

I thought that I'd just let that go by. Apparently, you're not as generous as I am.

brewnog

Well, it would definitely make the things much cheaper to machine and grind!

Danger

Not to mention cutting down on lifter wear and valve spring fatigue.

brewnog

Valve seat wear would definitely be improved.

Have a funny feeling that fuel consumption and emissions would be down quite a lot too.

Danger

Along with horsepower.

sid_galt

Thank you for the responses.

I don't think that valve seating velocities are as much of a problem in hydraulic systems (I am not sure). They are a major problem in electromagnetic system.
Then again, hydraulic systems are more complex.

The problem is that these things will be required to move the valves every cycle.
I don't think cylinder deactivations are done on a cycle to cycle basis. Therefore they are easier to implement.

sid_galt

Oops!

£4500!!!!!!!! Was it a camshaft or a whole engine????

Seriously, what was so special about it? Was it handmade or something?

Stingray

This information is rather old (~10 years), but Lotus was designing an active valvetrain at one point. Apparently, the electrohydraulic actuators cost about US$1000, and couldn't even manage much more than 4-5000 rpm. That's $1000 per valve!

I'm sure things have improved since, but it's probably still too difficult to get cheap reliable systems to work at reasonable redlines. Maybe this could be partially worked around by first applying the technology to diesels or other engines that operate at lower speeds. Then it could be advertised as an improvement in fuel economy rather than performance.