As distinguished from American PCV, the British system puts the engine crankcase under engine vacuum. This is done by running a relatively large tube from a breather to the manifold and then providing an orifice to allow metered and filtered air into the crankcase. The theory is good, it's a passive system that can handle great quantities of blowby easily, but any air leaks in the crankcase will increase idle speed, perhaps uncontrollably.
I question just how good the crankcase seals are in dealing with this system? Does air leak past the damper seal? Is a rope seal at the rear going to let air past as well?
I know Rover went to a composite lifter valley pan which is really quite good. Was this because of air leaks?
Are there other places where a 20" vacuum in the crankcase can pull air into the system?
You guys are the best around on technical issues, I'm wondering what your thoughts are about this.
Jim
Quote... an orifice to allow metered and filtered air into the crankcase.
Is it naive of me to ask if orifice size can dictate the amount of vacuum/pressure seen in the crankcase?. Is it possible to size the orifice to get close to neutral or slightly negative vacuum?....then engine sealing becomes less relevant.
I don't think so Graham. The typical orifice will be less than .030" and not much air will get in that way, certainly not enough to offset a 1/2" or larger tube to the manifold. Plus it would make the idle completely uncontrollable. But not a bad thought.
Jim
QuoteI know Rover went to a composite lifter valley pan which is really quite good. Was this because of air leaks?
Sealing may have been a factor, but I expect noise reduction was probably enough reason for Rover.
Jim,
Depending on the seal used you may see a small amount of vacuum pulled down past the dist/cam sensor drive. In this case though, my main concern would be the vacuum constantly sucking the oil away from the upper bushing. Or at least in theory.
Bill
The British or European style PCV system doesn't put the crankcase into a vacuum situation.
The metered orifice is the only piece of the system that sees manifold vacuum.
And as the bleed is stable it doesn't affect the idle speed.
The bleed is used to keep a constant airflow through the crank case.
And draw off any small amounts of blowby that may have occurred under light load.
The larger hose is used to route gross amounts of blowby to the air intake system.
Generally just before the throttlebody, but sometimes into the intake hose or air filter.
Not the other way around as most people think.
I tried putting full manifold vacuum on a small block Chevy once.
It caved in the valve covers and timing cover. And sucked in the valve cover gaskets and manifold end seals.
But not before it destroyed both crank seals.
I think my Dad let me off easy as it was his work truck.
Cheers
Fred
Interesting Fred, I hope I haven't been giving bad advice on these things. I think we need to see some venting diagrams of some Rover vehicles. I know that's a very large area to be applying a vacuum to, and the pan could be a problem area. The front lip seal could let air by but I don't think it would deform. Doubt it'd hurt a rope seal either. Never even thought about the lifter valley tin on the Rover. (not a problem on the 300 or 340) That would be subject to some pretty high forces. 15 psi x width x length. A quick estimate gives about 3/4 ton, yep that would be a problem. So I'm wondering where I came up with this idea? It worked on my 215, maybe I have ventured into an alternate reality?... That would explain some of the little glitches I've been seeing lately...
Take careful note guys, this doesn't happen that often.
Anybody got a diagram handy?
OK then, in practical terms it sounds like on *my* car I need a quick acting vacuum valve on my vent line. (because it is under full vacuum and cannot easily be ingested otherwise due to the Enderlie blower scoop design. Something with a 5/8 or 3/4" bore that closes when vacuum is applied. Maybe some sort of EGT valve or something.
Jim
Jim, diesels use a crankcase depression regulator in the ventilation system - similar to PCV but larger in size - maybe there's an opportunity to use one.
Here's one example:
http://www.ssdieselsupply.com/p56_crankcase_depression_regulator_valve.html
...and some basic operational specs...
http://www.flashoffroad.com/Diesel/CDR/cdrvalve.htm
Jim,
What if you just put an inline PCV valve in the vacuum line and opened the restricted line up to the air cleaner?
You would then have the American style system with a European holiday thrown in.
Cheers
Fred
Graham, thanks for the suggestion, I'll look into that. I was also considering a vacuum controlled heater valve: http://www.ebay.com/itm/Four-Seasons-74857-HVAC-Heater-Control-Valve-VACUUM-CLOSES-5-8-X-5-8-/390569679970
which is closed by vacuum. How much vacuum I don't know.
Fred, the air cleaner is inside the Enderlie scoop, which is at manifold pressure except during boost, as the throttle body is at the scoop inlet. There is no place to plumb the vent line. The alternative would be an open vent or a road tube, neither of which is a very good option.
The other thing is, at idle there should be some draw through the valve, but it should be a controlled amount.
Jim
Most, if not all PCV's meter air at high vacuum levels. Bigger engines meter more and visa-versa. So choosing one from a similar sized engine should get you there. The vent line could simply go to a small K+N that is made for the purpose. Or they also make valve cover mounted ones as well. Whatever the case the vent needs to be filtered as it would be drawing air most of the time.
Cheers
Fred
I'd rather not just leave an open vent line, even filtered as it will spew oil vapor on WOT which will settle all around and make everything oily.
I just looked at photos of the Rover and it seems pretty clear that the large vent line from the flame trap goes to a bung on the throttle body which is upstream of the throttle shaft. The opposite valve cover has a filtered orifice.
http://c7716.r16.cf2.rackcdn.com/RN-Forum/users/TSR53/D90-engine-pcv-vent.jpg
So, assuming this is the intended configuration at least for that year (3.9L), it would seem that the system is almost entirely passive. The only pressure drop applied would be from the inlet air going through the main air filter and through the airflow meter. Clearly any airflow through the orifice would be minimal.
At WOT of course blowby would be directly routed to the throttle body and a small amount would escape through the orifice.
But it does not appear there is any vacuum applied to the system anywhere.
Jim
That vented orifice on the left valve cover is intended as a vacuum break. It's meant to prevent oil being trapped in that valve cover.
The system isn't really passive per se. The vacuum connection is off the T on the vent line. It runs through a restriction into the lower 1/2 of the plenum.
Your situation is a little unique. You can't run a PCV under the blower because it will see boost and pressurize the crankcase.
And you can't vent at the air filter because it is under vacuum.
So I would still suggest a filtered vent of some sort and running a positive crankcase ventilation valve up to the air filter. Even at WOT there will be some negative pressure there. And if the flow is insufficient enough to cause blowback through the vent then run two PCV's
Or run the PCV line through a venturi at the air filter. That would provide a linear pressure drop directly related to engine load.
I have a couple more ideas if those don't tickle your fancy, but I've got a scotchy scotch waiting so I've gotta go.
Cheers
Fred
Fred, I see the "T" you refer to, are you saying this goes to vacuum through an orifice? About the only way that really makes sense is if there was something to close off the line to the throttle body so it could pull vapors from the crankcase. Obviously there is still quite a bit about this system I am still are not familiar with. I don't quite buy the vacuum break explanation for the filtered orifice. There are no shortages of windage passages into the rocker box and I can't see them being blocked with engine oil. I do see it as an inlet for filtered air for PCV flowthrough, but to make that work some level of vacuum has to be applied. That can't really happen if it is escaping through the throttle body. I would really like to understand what's going on here.
I picked up that 4 seasons heater valve #74857 at AutoZone for about 9 bucks. It is a plunger type valve and has a positive seal. Plumbed it into the large vent line between the scoop and the Rover flame trap pulling the vacuum on the upper side of the valve towards the diaphragm so that the vacuum would pull it closed instead of open. Plumbed the diaphragm into a manifold vacuum port. It stays closed at idle but will open at WOT. That shuts off the high vacuum signal to the crankcase but should vent the blowby under hard acceleration. Looks pretty tidy too. The only real question is at what level of vacuum does it begin to open?
I considered the CDR but Diesels have no throttle plates and therefore do not develop the vacuum of gasoline engines so I went this route.
That leaves the drawthrough aspect to consider, and that could be accomplished with a metered orifice from the manifold to the valve cover. Something to think about. As this arrangement has allowed my idle to come down to a very reasonable 600-800 rpm (before tuning), I can afford to bleed some air and have some leeway in orifice size. The restriction in the small inlet filter on the opposite valve cover can be removed (enlarged to the 1/4" NPT nipple ID). I think that will work, and will allow some blowby to escape if the valve does not open early enough. I'll need to fit a hose barb to the left valve cover that has the flame trap. I'll try to get a photo tomorrow.
But back to the OEM Rover system. I had missed the "T" in the vent line. Is it possible that something in the valve body blocks off the vent line when the throttle closes? One of the primary claims of the PCV system from the very start was that it pulled fresh air through the engine and kept combustion byproducts from accumulating inside. In order to do that isn't something needed a bit more than a gentle suggestion that the air move along? So far that's really all I'm seeing.
Jim
Here is a picture of the T piece Jim.
w01331630345apa.jpg
The brass insert is the orifice and it connects directly to the manifold plenum.
No magic occurs at the throttle body, just a straight through passage in front of the butterfly.
A slight pressure drop is maintained by the natural resistance provided by the air filter and its plumbing. As well as the inlet airhorn on the filter housing. The Air horn serves two purposes, one is to muffle intake noise. The second and more important is to provide a pressure differential for the breather system.
The Vacuum break on the left valve cover only existed for a few years and was deleted when valve stem seals were introduced.
My understanding is that oil was building up in the valve cover and causing high hydrocarbon emissions.
This is highlighted by the fact that on teardown the left cylinder intake valves are always carbon fouled and the right is usually clean.
The use of the vacuum heater valve is very clever but I'm worried about it's ability to tolerate oil, fuel and combustion gasses. And where does the blowby go when the valve is closed?
What you have created really is a complex version of the lowly PCV valve. Under high vacuum it's closed (and meters a small amount of air). Under low vacuum/high flow situations it's wide open. So why not use a normal PCV valve?
And to everyone watching this is not an argument, it's an exchange of ideas between what I consider friends. (I received a couple of comments)
Cheers
Fred
Just sounds like friendly brainstorming to me.
Right, no argument going on here in case anybody is wondering, Fred and I are the best of friends and I value his expertise.
So a gentle suggestion that the crankcase gasses move on along it is, eh? Then perhaps this is what distinguishes it from the American approach.
OK. The reason for not simply sticking a PCV valve in the large vent line is because I am not aware of a PCV valve which is that large. (5/8" to 3/4"). It needs to be that large because as we have seen in other examples (Carl for instance) smaller than that can lead to massive oil consumption as the engine wears and blowby increases. Also, the traditional PCV valve closes down under two conditions. The easily understood high vacuum of idle, but also at high flow rates under excessive blowby. Which is not a problem with a large vent line to the air cleaner bypassing it, but if you raise the spring rate of the valve to keep it open during blowby you also keep it open under higher vacuum and that mode of operation is probably not compatible with my system. The part of the heater valve of questionable durability is the sealing o-ring. And possibly the diaphragm. Is it viton? Is it buna-n? Nobody knows. But if it fails it is a common part, inexpensive, and easily replaced. It could in fact be regularly changed at oil changes if needed. You know, change the oil, change the PCV valve.
Here is an image:
IMG_0007.JPG
The vent on the other valve cover is the original tube with orifice. Turns out I simply slipped a sleeve of tubing over it and pushed on a small filter element. I will be thinking seriously about perhaps enlarging that to a 1/4" NPT pipe stub.
That "T" gives me some Idea of the size orifice which might be suitable in the valve bypass line. One advantage of discrete circuits is the ability to selectively vary each piece. One disadvantage is more tubes and lines running around. Perhaps I should just weld or braze a bung into the Rover flame trap.
Jim
Another though just struck me! I tried to duck but it still hit me square in the head.
You could run the vent line, or all of the crankcase gasses for that matter. Through a checkvalve into the low pressure area of your header collector. The collector always has a negative pressure differential and the check valves (common ones) are available all the way up to 3/4" Almost every AIR system made uses a version of these valves so they are cheap and easily found.
Something like this.
images.jpg
Or even one of the electronic ones. (could be run by your megasquirt.)
Cheers
Fred
"checkvalve into the low pressure area of your header collector"
AlternativeCrankcaseVentilation.jpg
(Coincidentally, I took this snapshot just last weekend.)
"Or even one of the electronic ones. (could be run by your megasquirt.)" That's an interesting idea. I suppose you'd plumb some sort of MAP sensor to the crankcase to facilitate optimizing valve timing? Wouldn't it need to register both positive and negative values? Can you get sufficient data from existing sensors?
I heard that those collector vacuum things don't work so well at part throttle and with mufflers,etc. Hey, just what I heard so I'm only passing it along. I don't know anyone with real life experience with them.
Guess I could plumb it into the bung for my EGT... only when MS3 goes in I sort of planned on dual O2 sensors.
Jim
Those are really meant for open exhaust applications, as in drag cars.
True, the drag racers use that system but it's been around much longer than that.
The OEM's have been using it off and on since the 50's.
Even on a restricted exhaust system there is still a pretty healthy pressure differential there.
On an open collector the check valve serves as a flame trap to prevent crankcase fires.
On a muffled system it is needed for that as well as to filter out the unwanted positive pressure waves.
Hook one up with a vacuum gauge some time and you will be surprised at the vacuum level.
The more I think about it though the more I like the idea of electronic control.
The MSQ can easily be configured to run a valve into the blower hat. Something like the Ford ISC valve. A simple RPM switch would get it done.
Or if you really want to get all fancy a PWM output would give you full control. All you need to do is anticipate the crankcase pressure.
All the needed sensors are already in place.
Cheers
Fred
It's an intriguing thought. But I do already feel somewhat overburdened with electronic controls and I'm not sure of the advantages.
OTOH, currently a low vacuum signal opens the valve. This indicates that it will function somewhat like the old vacuum dashpot in that if the engine dies at idle it will let more air in. Whether that will keep it alive then depends on the fuel map but it can be set to go rich at base idle speed if the manifold pressure goes up (vacuum goes down). If that is done successfully this represents a positive result.
At WOT the valve is open. Inlet pressure is high, (The valve diaphragm really should be plumbed to the blower inlet, not manifold pressure, but perhaps light positive pressure will not hurt it. Certainly it will drive the valve open more quickly under boost.) and full flow is achieved for blowby. The amount of additional fresh air ingested will be determined by the inlet breather restriction. The crankcase will not be placed under significant vacuum as the vacuum signal applied across the system will be equal to the K&N filter element restriction and throttle body restriction so fresh air flow will not be a large factor.
At some value of manifold pressure the valve will crack and then progressively open wider as manifold pressure increases (vacuum drops towards zero). I do not currently know what that value is, but idle vacuum is 20", so that number will represent significant acceleration, power output, and therefore blowby. The fuel map can be tuned to account for this easily using the O2 sensor, the only remaining question I see is this: Will there be any point in the operating curve where blowby builds up before the valve begins to open? Worst case scenario, build up of blowby at high speed cruise in top gear, possibly on an incline. I believe the restricted bypass line can be sized to account for that, with the inlet restriction sized to relieve any additional pressure. Or it may not be a factor.
The restricted bypass line will handle fresh air circulation through the engine under vaccuum conditions, much like the traditional PCV valve does now.
It may sound complicated but it really isn't, since separation of the bypass line makes orifice sizing easier.
So what did I miss?
Incidentally, the heater valve is made of fiber reinforced plastic and should not see temperatures any higher than in its intended application. Considering it draws from the blower inlet it should in fact run noticeably cooler.
Jim
I Don't think that you missed anything Jim.
Your system is clever and thoughtful.
I do agree part throttle will be an issue. But you're going to need some seat time for that
So put on your rubber party pants and take her for a spin!
Cheers
Fred
Rubber pants, blown 340 LBC, and the "Dragon", kinda ...flow together ? Enjoy, roverman.