That's how you build an engine....

Cam chains and pushrods fail as well, the most obvious difference there is that some sort of actuator would generally fail with the valve held shut, meaning the engine continues running with one cylinder inoperative until the fault is fixed. When a cam chain fails it leaves some of the valves held open, resulting in the destruction of the engine more often than not.

Reply to
antispam
Loading thread data ...

It's a good system - the valve lift can be adjusted from to the maximum for the cam, and it can be done on a per-cylinder basis. However, it takes ~

1/3 second to make a full scale adjustment (worm drive), so it's not suited to per-cycle adjustments.
Reply to
Andrew Kirby

Doh! "...from zero to...."^^^^^^^

Reply to
Andrew Kirby

No- at the moment the valvetronic only allows control of all the inlet valves at once- NOT per cylinder. - There is only one actuator. The system isnt fast enough to move the valve timing / lift between each cylinders induction strokes as yet.

Tim..

Reply to
Tim (Remove NOSPAM. Registry corupted, reformated HD and l

How does it work?

It's easy enough to adjust the basic valve timing by having a tensioner wheel either side of the cam and to move them in and out to take up the slack on one side or the other, but that would move the opening time by the same advance and in the same direction as the closing time, not extend the duration of the cam.

Reply to
antispam

Infinite acceleration requires infinite power.

Lotus have (at least they did 8 years ago - I got an old version of the brochure below) a single cylinder solenoid operated 4 valve head. It's about 20cm x 20cm x 20cm, so a straight 4 would be 80cm long. The bore centers on a normal engine are around 10cm so if electronic only your engine will be twice as long as any normal engine and 5cm taller.

formatting link
AVT electro hydraulic system is under development forproduction.

Can't avoid the power drain on the engine due to valve operation. Example for a 10mm lift, 100g valve and piston operating for 240 degrees with constant acceleration / de-accel to max lift, accel to return and de-accel on to seat, at 6000rpm. Power = work done per sec. Work = force * distance, force = mass * accel. For initial accel from seat to 1/2 lift. Accel = 2 * (lift/2) / t^2. t is 1/4 of the time for 240 degrees rotation = 1/4 * (240 / 360) * (60 / 6000) =

0.0016666 sec. Accel = 2 * 0.005 / 2.777e-6 = 3600 m/s^2. Force = 0.1 * 3600 = 360N. Work = 360 * 0.005 = 1.8J. For full valve motion = 4 * 1.8 = 7.2J, for all 16 valves to cycle once = 115.2J. At 6000 rpm there are 1/2 * 6000 / 60 cycles per second = 50 cycles / sec. Power = work * cycles / sec = 115.2 * 50 = 5760 J/s = 5.76Kw (7.68hp).

Not too bad - now do it for a 12,000 rpm bike engine or for your fast opening, long duration at full lift, quick shut valve operation. Say

1/2 the time to reach full lift, 120 deg at full lift, twice as fast to shut. t = 0.000833 sec, Accel = 14400m/s^2, force = 1440N, Work = 7.2N, total work per cycle = 460J, Power = 23Kw (30hp). Power goes up to the square of the speed of valve operation. It also goes up by the square of lift as the lift appears in accel and work but is directly proportional to mass. This means moving 4 small valves with more total mass over a smaller distance can work out the same as 2 big valves over a big opening.

Long duration cams with smooth lift and return reduce the power required due to slower valve operation - got to be worth a fraction of a bhp in themselves. Or the same lift and return as a standard cam with a dwell at full lift will not cost more power.

With a mechanical cam you get losses due to work done against spring preload and friction in cam / follower. (General Motors once claimed that a million barrels of oil were saved when they switched from conventional fulcrum-type rockers to roller rockers in the 3800cc V6 engine alone) In an electro hydraulic system you will have pump losses, and flow losses neither of which will be small. Either way a good working value would be 50% loss.

The Desmo system if made using roller followers should give near zero losses. If you took a head off, wrapped a length of string round the cam drive wheel and pulled like a cord starter it would continue running for some time, a normal cam would just drop into the next trough between two cams once the string came off.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Reply to
Peter Hill

Ahh someone who speaks my language :-)

Reply to
Ben Organ

"Look for ultra-stout GRP connecting rods in this combo." GLASS REINFORCED PLASTIC???

Not quite the same but still what the heck are these? "billet aluminum GRP rods" billet aluminum GLASS REINFORCED PLASTIC rods? Not an easy combination.

Metal matrix composite (MMC) maybe. Carbon composite maybe. MAD flouropolymer impregnated ceramic coated MMC maybe. even graphite fibre reinforced aluminum maybe. But GLASS and PLASTIC !!?

sniff sniff... google not a trace of GRP rods or billet alloy GRP rods. sniff sniff... something smells.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Reply to
Peter Hill

I used to have a couple of Lister / Petter 2 cyl diesel engines, they had a gear cam which you could use as a PTO shaft i.e. the gears on it could handle at 18 hp...

The problem with gear driven cams on OHV engines is that the crank and cam shafts are some distance apart, the heads are removable and get skimmed. Making a gear system to connect the two is expensive, and would be heavy.

For an industrial engine (e.g. a generator set) they tend to have modest specific power outputs and don't have a requirement to have a high power to weight ratio like a car or even an aero engine does, so they can have heavy gear driven cams and pushrods.

-- James

Reply to
James

Light accelerates infinitely quickly. But that's not really got much to do with "as quickly as the kit can do it".

You're misunderstanding what you're looking at on the brochure.

The piece of kit described is a testbed, the valves are opened and closed on the testbed to evaluate different cam profiles in an engine, measure the power curves and so forth. Then, the results from the tests tell you what cam profile works best in a given engine configuration. Then you build that cam and put it into your engine in the normal way.

The testbed's not intended to be put into a car and the size is of no relevance whatsoever, they've not even thought about it and nor need they, any more than a aircraft wind tunnel needs to be lightweight.

On any engine using valves.

I needn't check your figures because they apply well enough to any engine using valves, obviously, although you'll find in practice that the springs used have the biggest effect on the valve train drag and you haven't included them at all. It's easiest by far to simply measure this directly.

If you turn an engine by hand, and then disconnect the cam chain and turn it over again, you will immediately realise that the valve train is a huge mechanical drag on the engine. Also, if you take all the valve train components off and weigh them, you'll find that is also a tidy weight. You can try and push open a valve by hand, rather than using a spring compressor, not exactly easy. Start by removing all that friction and weight.

Any alternative system for opening the valves will need to add some weight back and introduce some sort of load, whether it is more or less than the existing one is open to question.

Whatever the answer to that question is, it wasn't the reason for doing it, the reason for doing it was to be able to dynamically vary the valve timing and duration (and also the opening / closing speeds) which gives you the benefit of being able to have what amounts to different sorts of cam in your engine depending on what you are using it for at the time.

If you think about the possibilities you can see where there is some scope for serious power gains without having to compromise out torque. For instance, you've got a computer in the car already and it's controlling fuelling and similar. Were you to have the inlet valve duration widen under computer control up to the point where cylinder pressures are just beginning to tail back off, you can dynamically adjust the exact timing by feedback from sensors in the engine and get the maximum benefits from duration possible with that engine at that time at that specific rpm.

Another advantage boils down to the fact that the upper rpm limit on an engine is often set by the valve train.

Reply to
antispam

Seems diesels do tend to have pushrods and gear driven cams.

Yeah, I can see that. I was thinking of a cog driven by the flywheel but it has to be quite large or the cam spins too fast, obviously. So then you think smaller crank cog and have a few more between them, and yeah, it starts to get heavier and stuff.

I just tried to find a picture of a rotary from the war, it had a ridiculous number of cam gears, about 36 in total. The sort where the flywheel stays put and the engine rotates around it.

Reply to
antispam

Honda used gears on the VFR750 and VFR800 V4's. They had to, it broke cam chain tensioners - the hi-Vo inverted chain was OK. The engine has a 180 degree crank, so on a pair of cylinders all the cams are in about 180 degrees of cam rotation and there is big off load period then whack! the first cam comes back on full load. Normal multicylinder cams have two open, one opening and one closing. The massive variation in load and tension did the VF750 cam chain tensioner no good at all. They used multicylinder design procedures to design the tensioner and should have used single cylinder methods. They have a long history of getting this wrong, CB250/325 K4, CB250/260G5, CJ250/360, all 180 degree twins CX500/600/650 - 80 degree V twins. Honda (and most Japanese) haven't heard of Dr Lanchester (dummy spring loads) or are only too willing to steal and patent his inventions (like balancer shafts). On the 250's and most of the range they went overboard and fitted Hi-Vo inverted chains, massivly over engineered tensioner systems but they didn't need any of it as they had fixed it by going to a 360 degree twin.

-- Peter Hill Spamtrap reply domain as per NNTP-Posting-Host in header Can of worms - what every fisherman wants. Can of worms - what every PC owner gets!

Reply to
Peter Hill

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.