Here is a post I copied from the NCRS Corvette discussion board a few years ago. This guy (Duke Williams) that posted the PCV 101 gave a history of the system. He is a retired mechanical automotive engineer that did his grad work on engine designs. Anyway, he has the credentials to back up his post.
Doug
Crankcase ventilation 101 (new thread)
Crankcase ventilation 101 (new thread)
Duke Williams NCRS #22045 <Send E-Mail> -- Monday, 27 March 2000, at 1:52 a.m.
Well, the old thread on this subject was becoming somewhat confused and convoluted, so I'm taking the liberty to start a new one. I think this somewhat arcane piece of automotive technology is important for enthusiasts to understand.
Let us begin by going back to the prewar period. This was before motor oils had decent detergent and dispersant additives and sludging was a problem in both automotive and aircraft engines. Research eventually indicted the crankcase blowby gas (which is primarily unburned fuel/air mixture, not exhaust gas) because the heavy fraction of the unburned fuel was condensing in the crankcase, which accelerated sludge formation. This led to a fair amount of engineering effort to purge blowby gas quicky before it had time to condense, and Rob posted an interesting anecdote on this from his Ford documentation. One method was to attempt to sweep the crankcase with a supply of fresh air to quickly purge the crankcase blowby gas. Thus was born the "road draft tube" to (hopefully) use the slight vacuum generated to pull air in the crankcase, and as Dale reports the inlet on the breather cap was positioned in such a way that the fan forced a bit of air into the engine through a strategically placed "breather", which was often incorporated into the oil filler cap
In the mid-fifties research, primarily by GM, indicted automotive hydrocarbon emissions as a cause of photochemical smog. It was also known that about 20 percent of automotive HC emissions at the time came from expelled crankcase vapors. Thus, first in California (1962 or perhaps even as early as 1961) and nationwide in 1963, authorities required "closed" crankcase ventilation systems, where the crankcase vapor was drawn back into the engine and consumed by combustion. These systems were the first emission control devices, but they had a positive benefit on more than just emissions. Fuel economy was increased, and by using manifold vacuum (which exists even at WOT) a positive pressure differential was assured to continuously scavenge crankcase vapors with fresh air.
One problem had to be overcome. Crankcase vapors are flamable, and a carburetor backfire (not uncommon) can propogate wherever there is a combustible mixture. , So "closed" crankcase systems had to incorporate an anti-backfire valve to prevent potential crankcase explosions. It was relatively simple to incorporate this feature into a metering valve. So for the first time ever, a known quantity of air could be flushed through the engine to effectively scavenge the crankcase of blowby gas. Thus was born the term "Positive Crankcase Ventilation" or PCV.
The basic architecture soon became cast in concrete. They were typically designed to draw filtered air from the air cleaner, circulate it through the crankcase and then to a manifold vacuum source, typically at the base of the carburetor, with the ubiquitous "PCV valve" somewhere between the crankcase outlet and vacuum source. This architecture prevailed for years, even well into the electronic fuel injection era; however, at some point some bright, probably young, engineer thought: With fuel injection, there is no combustible mixture above the injectors so why do we need an anti-backfire device. Further, he surmised that the oil additives of today and tight, low blowby engines of the modern era really don't need to have the crankcase scavenged, so why do we need all this extra hardware. Thus was born the simple expedient of merely venting the crankcase to the throttle body (upstream of the butterfly, perhaps with a small orifice to the downside). Rather than using manifold vacuum, and inch or two of water pressure buildup in the crankcase was enough to force the blowby gas out, but not enough to damage gaskets or affect power and economy, and modern motor oil additives have made sludging a thing of the past. And, there is always a slight vacuum downstream of the air cleaner, so there is always a pressure differential to keep the blowby flowing one way from the crankcase into the intake air tract. As typically implemented, these modern systems have a simple liquid vapor separator in the cam cover and a line to a convenient place in the inlet pumbing downstrean of the air filter. The nameless engineer who developed this new crankcase ventilation philosophy and architecture has saved the automotive industry MILLIONS, but probably didn't receive more than his regular salary. Not all manufacturers have adopted this new simple architecture. All I can say about this is the answer given by a grisled "old major" who had recently returned from a tour in Viet Nam flying C-47 "Puff the Magic Dragon" gunships to to an iconoclastic young lieutenant: "Williams, traditions die hard."
So to answer the question: Does a positive crankcase ventilation system need a fresh air inlet, the answer is "not really", but it really depends on the epoch of your system design. If it was designed in the fifties or sixties, prevailing engineering philosophy said yes. By the eighties the answer was no, why bother, why spend the money.
Going back to Dale's original question about Chevrolet replacing the "breather" oil filler cap with a sealed one to placate customers who complained of oil misting in the engine compartment, it probably wasn't the most elegant solution, but the customers stopped complaining and engines usually didn't sludge up until long after the warranty expired. Your comments and/or questions are invited.
Duke
Re: Crankcase ventilation 101 (new thread)
Rob Brainard <Send E-Mail> -- Monday, 27 March 2000, at 6:46 p.m.
Duke, Well said!! I agree and stated in my post, I don't think GM really cared about the crankcase ventilation of a few Corvettes just that the customers were off their back. Everything else you said should put this question to bed. Thanks for the education.
Re: Crankcase ventilation 101 (new thread)
Doug Flaten <Send E-Mail> -- Monday, 27 March 2000, at 7:08 p.m.
I have an inline 6 on my Mustang that has the vented cap at one end and the PCV valve at the other end of the valve cover. It seems to me this is probably a poorly designed PCV system since the fresh air would likely short circuit out the other end of the cover and provide very little scavenging of crankcase gases. It would keep the gases from going to atmosphere but would do little else in the way of reducing combustion products from accumulating in the crankcase. Based on your comments, maybe this is one of the reasons that the oil in that car seems to get dirty so quickly.
Re: Crankcase ventilation 101 (new thread)
dale pearman <Send E-Mail> -- Monday, 27 March 2000, at 8:56 p.m.
I think my original question is finally answered, if I could only remember what it was! Thank you Duke for a throughly enlightening disertation on this truly profound subject. The engineer who saved the auto industry millions was not young and promising however. He was old, bald, nearsighted and very hard of hearing!
Glad you brought this up, Doug
Duke Williams NCRS #22045 <Send E-Mail> -- Monday, 27 March 2000, at 9:02 p.m.
It was getting late when I wrote my post last night so I tried to be brief. First let me address the vented oil cap. On page 6-8 of my '63 Corvette Shop Manual there is a brief, but good write-up on the '63 Chevrolet crankcase vent systems. It describes two types. The first is called "closed" and uses a vented oil filler cap. The second is called "positive" and aspirates from a connection to the air cleaner. Of course, both had a valve communicating with manifold vacuum. All Corvette engines were of the positive type for '63, and I think the vented breather cap quickly became a thing of the past because there was a possibillity that high blowby from engine wear could overcome the draw through the valve and vent out the breather cap. Drawing the inlet air from the air cleaner would be more likely to ingest any backflow vapors.
Your point on the relative location of the inlet and outlet on the Mustang six is well taken. The inlet air is likely to short circuit and not ventilate the crankcase well. The design of and EFFECTIVE PCV system is somewhat of an art. Some designs are elegant. Others less so. The system in my '63 draws air through the oil fill pipe to the lifter valley and the outlet is at the back of the valley, so the crankcase is not well ventilated, but at least it doesn't ingest oil. In the seventies, Chevy V-8s generally drew fresh air from the air cleaner to one valve cover and pulled it out the opposite cover. This guaranteed that air was circulated through the crankcase and it ventilated the rocker boxes too. An excellent design.
Inline engines are problematic in this respect because unless the inlet or outlet circuit communicates directly with the crankcase, it can be short circuited. Chevy engineers attempted to do this with my Cosworth Vega by having a fresh air inlet (from the air cleaner) at the base of the cylinder case, but oil slosh under cornering and high rev windage forces oil up into the air cleaner, so I finally redesigned the system using the new age EFI model that I described. I use SAE 30 CF-4 heavy duty diesel engine oil with big doses of detergent and dispersant and change it annually (1500 miles) so I'm not worried about sludging.
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