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Both valves are shut and capture a combustion charge. And they act like springs as they continue to compress then expand the captured charge. Like JerryU said.
^^^^
Hmm, coming from the industrial gas business (those used in welding) I prefer the simple system used in F1. No springs, Nitrogen gas! Acts like that captured gas in those V4 Cylinders!
In years past F1 engines operated at ~20,000 rpm! Yep valves opening and closing 330 times a second! Limited to 15,000 now by rule!
If one used a voice coil actuator durable and strong enough to reliably plunge and open a intake/exhaust valve then bouncing valves would be a thing of the past and RPM would be limited to mechanical strength and inertia limitations as well as the stability and reliability of the ignition of the mixture. The valves timing and open/closing rates could be completely computer controlled for highly optimal flow at all times.
Both valves are shut and capture a combustion charge. And they act like springs as they continue to compress then expand the captured charge. Like JerryU said.
Are both valves closed for both the compression and exhaust strokes? (I.E: Never ending compression strokes?)
Are both valves closed for both the compression and exhaust strokes? (I.E: Never ending compression strokes?)
Both valves, intake and exhaust, on each deactivated cylinder as shut (closed) for the duration of the V4 mode - closed all the time - all strokes......... so yes closed on what would otherwise be the exhaust, intake strokes, and any other part of the piston motion. The combustion charge is trapped in the cylinder - for up to 10 minutes when it time out, or sooner if more torque is required or if the car coasts. It will not stay in V4 mode while coasting with closed throttle. Its like a gas spring. Since they are not "pumping" any air, there is no "work" being done and so very little energy is lost.
A "Mickey Mouse "valve train that made a very success engine was the Penske Ilmore Indy engine. Penske saw a loophole in the Indy engine regulations where they tried to make pushrod engines more competitive. He got Ilmore to design, in secret in PA , what turned out to be a very successful race wining engine. It was a Hemi and used valve springs but a Desmodromic type mechanism, NOT to control the valves but to control the pushrod and rocker arms using cam followers!
Great read for techies with the details, called "Beast"
Just checked the Range Technologies website - no listing for a C8 corvette AFM disabler...unfortunately. I have one installed on my Escalade and it works great...guess they haven't put one out for the C8 yet. Bummer.
Disabling AFM is not about comfort or mileage....it's about avoiding damage to the engine, and also the transmission. Do some google searching....much information about Chevy/GM AFM damaging engines and also transmissions over time. There's even class action lawsuits related to it proving it's a real thing.
I'd like my C8 to be in V8 mode all the time. Hoping a reputable source puts out a good OBD II AFM disabler soon.
There are literally tens of millions, maybe hundreds of millions of GM engines with AFM, and problems are extremely rare. There were issues with the first generation, but those are long since past. And there will never be an OBD AFM disabler because of the heavily encrypted nature of the VIP electrical system. The only way to do it is to crack the ECU and tune it out. That's probably going to cost a couple of grand, easily.
If you want to stay in V8, drive with the DCT in manual, in Track mode, or in Z-mode with powertrain set in track mode.
Originally Posted by mortelec
Granted the C8 hasn't had the AFM lifter failures like the past. The AFM disabler doesn't prevent lifter failure. I had a C7 that had a range disabler from day one. There was catastrophic lifter failure at 40k miles. The disabler was a bandaid on a much bigger design problem. The only way to prevent it on the previous years was an AFM complete delete.
Lifters fail in all kinds of cars, with and without AFM. And the mechanics who work on these cars say lifter failures are no more common on the AFM cylinders than the others.
A "Mickey Mouse "valve train that made a very success engine was the Penske Ilmore Indy engine. Penske saw a loophole in the Indy engine regulations where they tried to make pushrod engines more competitive. He got Ilmore to design, in secret in PA , what turned out to be a very successful race wining engine. It was a Hemi and used valve springs but a Desmodromic type mechanism, NOT to control the valves but to control the pushrod and rocker arms using cam followers!
Great read for techies with the details, called "Beast"
Had to look that one up. There must be a little bit of spring action in that arm that hold the valve closed, don't ya think? It needs to exert some pressure to keep the valve seated some something must stretch a little bit somewhere.
It would be nice if they eliminate the camshaft and pushrods/lifters/valve springs in leu of robust electromagnetic bipolar valve actuators (I.E: extremely strong voice coils). A bipolar electromagnetic actuator could not only act as a valve spring pulling the valve closed, it could also instantly open/close any valve with extreme precision and with any type of slew rate desired. You could mimic a very shallow or very steep cam dynamically and exactly when the ECU decides to. It would also make things like AFM trivial.
Very expensive and very noisy. I think Koenigsegg uses this on a couple of their engines. And the engine costs more than an entire Corvette. Probably many times over.
It’s what it does to the motor’s top end. Ask me, my motor lunched as a result of AFM, my neighbour across the street had her motor lunched for the same reason. Dump the AFM as soon as you can…it is expensive ($7K) to repair and it always happens after warranty ends.
Very expensive and very noisy. I think Koenigsegg uses this on a couple of their engines. And the engine costs more than an entire Corvette. Probably many times over.
Voice coils are silent and cheap. There might be weight cost associated with the amount and size of the neodymium magnets, the size of the coils needed, and possibly a cooling system for the coils; however, I doubt it's much more than the weight of the camshaft, the drive gear/chain, the pushrods, the roller rockers, etc. The voice coil actuators would/could likely be built into the heads and act directly on the valve stems. The output stage electronics that provide the high levels of precise current control to each actuator is also very trivial and cheap. The compute that controls the valve timing and sequencing is basically free. The firmware is also quite trivial. The biggest cost would be the NRE engineering cost.
V4 seems to mostly kick in during highway driving. I find that just keeping it in the 7th/8th gear in manual mode and holding the left paddle for lowest possible gear every time I need to maneuver is working pretty well.
I usually run z mode off highway which also prevents it. It would be nice to be able to disable it independently but it hasn't bothered me much.
I would be more worried about not activating V4 enough to keep all of the V4 mechanical components functional. it's sometimes detrimental for mechanical actuators to not be exercised for extended periods of time.
A "Mickey Mouse "valve train that made a very success engine was the Penske Ilmore Indy engine. Penske saw a loophole in the Indy engine regulations where they tried to make pushrod engines more competitive. He got Ilmore to design, in secret in PA , what turned out to be a very successful race wining engine. It was a Hemi and used valve springs but a Desmodromic type mechanism, NOT to control the valves but to control the pushrod and rocker arms using cam followers!
Great read for techies with the details, called "Beast"
Highly recommend reading the book....Its a very good read and tells you exactly why they are so good for so long...
Had to look that one up. There must be a little bit of spring action in that arm that hold the valve closed, don't ya think? It needs to exert some pressure to keep the valve seated some something must stretch a little bit somewhere.
Nope. Asked the specific question and Google as usual has the answer!
"No, a desmodromic valve system does not use a spring in the cam follower: instead, it uses a valve closing lever that is driven by a second cam profile. This lever is often L-shaped with a pivot shaft through its apex. The desmodromic valve system also uses a cam lobe and rocker arm to open the valve."
My thought, gas pressure in the compression, power and exhaust strokes keep any potential valve gap closed. On the intake stoke a small amount of exhaust gas may enter but not significant.
I would expect some type of spring action somewhere otherwise as the cam lobe wears, or the rocker wears, the valve is just going to bounce around and rattle more and more when closed. The intake stroke is going to pull the exhaust valve open then the compression stroke is going to push the exhaust valve closed and I expect that's going to be noisy as hell and also wear out the valve seat in short order as the valve is going to be rapping at a high frequency. That would be my guess. Interesting design but I don't see how it has any longevity.
Nope. Asked the specific question and Google as usual has the answer!
"No, a desmodromic valve system does not use a spring in the cam follower: instead, it uses a valve closing lever that is driven by a second cam profile. This lever is often L-shaped with a pivot shaft through its apex. The desmodromic valve system also uses a cam lobe and rocker arm to open the valve."
My thought, gas pressure in the compression, power and exhaust strokes keep any potential valve gap closed. On the intake stoke a small amount of exhaust gas may enter but not significant.
I know. I did not mean a coil spring though. What I meant is that the lever itself - the L shaped thing, must have a tiny bit of flex to it, to create the pressure - else something would eventually fracture - I don't see how it could hold it perfectly closed. Except - you have a good point about the pressure keeping it closed during compression. I have a miniature hit and miss engine that has an intake valve with only a tiny light coil spring. On the intake stroke the downward motion of the piston creates a vacuum that causes it to open and draw the fuel/air. The spring must be weak enough to permit that to happen. On compression, it is of course forced tightly shut. On exhaust, the exhaust valve opens because there is push rod an rocker arm.
I know. I did not mean a coil spring though. What I meant is that the lever itself - the L shaped thing, must have a tiny bit of flex to it, to create the pressure - else something would eventually fracture - I don't see how it could hold it perfectly closed. Except - you have a good point about the pressure keeping it closed during compression. I have a miniature hit and miss engine that has an intake valve with only a tiny light coil spring. On the intake stroke the downward motion of the piston creates a vacuum that causes it to open and draw the fuel/air. The spring must be weak enough to permit that to happen. On compression, it is of course forced tightly shut. On exhaust, the exhaust valve opens because there is push rod an rocker arm.
I worked on a Ducati once decades ago but don't remember the details of their desmodromic valve system, but I don't remember any springs.
You don't need any springs in the system to prevent damage, just the right clearance and correct cam design. Just like adjusting the valve lash on an mechanical (non-hydraulic) design. The cam profile has a ramp to "slowly" take up the clearance and open the valve without undue jerk and decelerate it on closing. Too much clearance and you will miss the ramp and get excessive jerk in the system. And, as mentioned, cylinder pressure will keep the valves fully closed when necessary.
I worked on a Ducati once decades ago but don't remember the details of their desmodromic valve system, but I don't remember any springs.
You don't need any springs in the system to prevent damage, just the right clearance and correct cam design. Just like adjusting the valve lash on an mechanical (non-hydraulic) design. The cam profile has a ramp to "slowly" take up the clearance and open the valve without undue jerk and decelerate it on closing. Too much clearance and you will miss the ramp and get excessive jerk in the system. And, as mentioned, cylinder pressure will keep the valves fully closed when necessary.
I would expect they must employ some way to make valve lash adjustments with some type of rocker arm adjustment not shown in that picture above. Or they just have very tight tolerances that would need to be frequently checked and replace rocker arms if/when they wear too far. And/or replicable cam lobes.
Here is the link to the adjustment procedure for a Ducati. If I read it correclty, there actually are "hairspings" on the closer that help keep it shut at low RPM. So it would appear that there is bit of clearance there, that is taken up by springs at lower RPM, and taken up by (I don't know) momentum and compression pressure at high RPM?
"There are two rocker arms per valve, one for opening and one for closing. Thus on each cam there are four lobes – two for opening, two for closing. The openers are the small lobes on the outside operating the top rockers. The closers are the large lobes on the inside operating the lower rockers. The closing rockers are aided by “helper” hairsprings that close them at rest or at low rpm. You can feel the tension of the helper springs when turning the cams or pushing on the closing rocker. At higher RPM they become pointless, but when the engine is starting or at idle they are needed to maintain sealing with loose closing clearances. No, it's not a good idea to remove them, unless you don't need your bike to idle. "
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