There was a lively discussion of this topic a while ago in another forum. It makes a big difference if you're talking about mechanical supercharging versus turbocharging. Here is the link.
Well, with typical turbocharged applications, pressure in the exhaust ports will vary from slightly below to significantly above the pressure in the intake ports. At the end of the exhaust cycle (where cylinder pressure is at its lowest while the intake valve is still closed), the pressure in the cylinder will generally remain at or above the pressure in the exhaust port. So if you have say, 30psi in the intake of a turbocharged engine, I would expect the minimum cylinder pressure while the intake is still closed to have a lower bound in the vicinity of 30 psi.
To get rigorous about this, we need to sum all the pressure forces acting on the valve. First, the intake port pressure, acting on the area of the valve seat ID, minus the valve stem cross-sectional area. Second, the pressure under the valve cover (hopefully near atmospheric), acting on the valve stem cross-sectional area. Third, the pressure in the cylinder, acting on the valve seat OD. So the way it looks to me, on a turbocharged engine, like an NA engine, the pressure forces acting on the intake valve will always be helping it stay closed, when it actually is closed, i.e. everywhere except during induction stroke.
That said, I can see the potential for trouble on the exhaust side of a turbocharged engine, if exhaust port pressure gets out of hand relative to intake port pressure.
"the pressure forces acting on the intake valve will always be helping it stay closed,"
I see how you're saying the compression stroke and the exhaust stroke will have gasses in the chamber holding the valve closed. The problem occurs when I want 259 degrees of duration on a intake valve on 30psi of boost with 50lbs of seat pressure on a 2" intake valve. The valve will begin to float and not close on that pressurized air and my valve open duration will change as the valve spring does not have enough force to close till the piston begins to pressure the chamber. Also the piston will force some of the charge back through the intake port till that valve closes as the change of inertia takes place. Since supercharged and turbo engines run little overlap it's important to maintain cam duration. Even when the chamber is at near vacuum.
"The problem occurs when I want 259 degrees of duration on a intake valve on 30psi of boost with 50lbs of seat pressure on a 2" intake valve"
Is this a mechanically supercharged or turbocharged engine?
Actually with the camshafts used in these applications and the spring pressures required to make everything function properly, the boost pressure doesn't hardly come into being a factor.
That's pretty much where my thought process has led me.
On a highly boosted mechanically supercharged application it would be worth doing the calculations to ensure there is a net force holding the intake valve closed during the relevant part of the cycle.
Assuming a mechanically supercharged street driven engine having relatively low exhaust system backpressure for a streetable exhaust system, and looking at the portion of the cycle late in the exhaust stroke prior to intentional intake valve opening, there is potential for premature intake valve opening due to pressure in the cylinder dropping below boost pressure. I'm not convinced this is a real issue, but let's do some calculations to see if this is likely based on some assumed conditions pulled out of the air. Lets assume cylinder pressure = 5 psig (probably low), boost pressure = 15 psig (high for a mechanically supercharged engine), intake valve seat outer diameter = 2", intake valve seat inner dia = 1.875" (i.e. seat width = 1/16"), valve stem diameter = 0.3125", and pressure under the valve cover = 0 psig.
Force acting on the back of the valve (i.e. the part exposed to the intake port):
Net area = 3.1416*((1.875"/2)^2 - (0.3125/2)^2) = 2.76in2
Force = 15 * 2.76 = approx. 40 lb (trying to open the valve)
Force acting on the face of the valve (the part exposed to the cylinder):
Net area = 3.1416*(2"/2)^2 = 3.14in2
Force = 5 * 3.14 = approximately 15 lb. pushing the valve closed
Force acting on the valve stem (the part sticking out of the valve guide = 0 lb since the assumption was 0 psig pressure under the valve cover
So there is a net pressure force of approximately 25 lb trying to open the valve. Typical closed valve spring pressures of a performance engine are in the 100 lb range, so I conclude by this analysis there is no issue.
Something else to take into consideration is the RPM at which the valve train would normally start to bounce/ become unstable. If your valve were to go "unstable" at "X" RPM, then having 25 pounds of pressure to force the valve open would be equivalent to removing 25 pounds of spring pressure. That's not something that a sane man would WANT to do.
In addition to this, consider that that the fluid flow of the air charge passing through the port could be related to a pair of skis skimming across water. With the increased velocity/flow past the valve, the back-side of the valve can easily flutter as it attempts to close. Objects in motion tend to stay in motion.
I gave some thought to mechanical and fluid dynamics in relation to this question, and don't see any significant interaction with boost pressure insofar as intake valve closure is concerned.
Keeping our focus on the intake valve, valve bounce is a phenomenon of the valve closing event. At that point in the cycle the piston will be travelling upward, and any remaining positive pressure differential from port to cylinder, will quickly reverse and become negative, acting to close, not open the valve.
I didn't follow your point about the valve fluttering. Can you produce data or an estimate of the velocity past the valve near IVC, and relate that to forces that would cause the valve to flutter?
I recall a thread on another Forum where there was a guy using a highly modified six cylinder diesel engine for tractor pulling.
This engine had a five inch bore with an intake valve just short of three inches in diameter. It used two compounded turbochargers running at around 120psi of boost pressure.
Similar to how the impact of a bullet is measured in pounds while it only weighs a few ounces, the reciprocating motions of valve train components are measured the same way. I work for Ferrea Racing Components in sales/tech, and have this conversation quite often. We use the term "flutter" in place of bounce since a valve will bounce a few times and not simply once. If you've ever seen a Spintron video of this occur, it would be simple to understand. I'll try to host a video of this in action sometime soon.
A great example of the importance of proper spring pressure/weight would be where we had recommended to a customer to increase his spring pressure in order to accomplish what he was attempting to run. His reaction was that more spring pressure would be counter-productive and consume more horsepower. He took faith in our recommendation, tested it on an engine dyno and called us back ecstatic that he had GAINED 27 horsepower at the high-end by keeping the valve train components under proper control. This case happened to be a circle track engine without boost.
I understand your thought about the piston creating positive pressure against the valve helping to close it, but keep in mind that the pressure has to build up from a negative to a positive and on it's way to becoming positive pressure, the valve may be bouncing back open and the piston can easily push the air charge that is attempting to continue to enter the cylinder right back out the port. This leads to a loss in momentum for the next charge cycle in addition to losing a small portion of the charge that HAD entered the cylinder.
On MOST boosted engines the dyno graphs will carry out further than a naturally aspirated engine would, but it will eventually fall out as well. The way I explain it to most is that if you feel that your cam-timing events are critical, then having appropriate spring pressures is even more critical. The exhaust valve (because of exhaust pressure, especially on a turbocharged engine) will pollute the intake charge via too much overlap when it bounces back open.
REDlines, since we are talking about positive delta P from the port to the cylinder at IVC, I would like to see your analysis or evidence showing that this delta P (at IVC) is significantly greater for a typical supercharged engine than for an normally aspirated engine.
The point I am trying to make is, people have been talking in this thread about boost pressure opening or keeping open the intake valve, but have not yet produced a convincing argument with real or estimated data showing that this actually occurs or that there is any practical difference between an NA and a typical boosted engine in this regard.
