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Post by Scoutpilot on Mar 9, 2015 6:28:09 GMT -5
In a properly set up and functioning air flow system, the air is drawn, by vacuum pressure, into the oil bath air filter and cleared of the larger particulates. That clean air is drawn to the carburetor and some of it into the oil fill tube. That air flows down and through the crankcase collecting the moisture, volatiles, and particulates in the motor. Manifold vacuum keeps pulling that air through the PCV and it is combined with the fuel/air mixture, on its way to the cylinders, in the intake manifold. |
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gmcjr
KJRT
Posts: 932
First Name: Gary
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Post by gmcjr on Mar 10, 2015 21:52:58 GMT -5
Thanks for explaining a often misunderstood subject!!
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athawk11
Full Professor
Full Professor
Posts: 298
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Post by athawk11 on Apr 8, 2015 11:36:38 GMT -5
This question is based on the PCV question posed at the 2A page. You know which one. The guy that’s redesigning the L-134 to push 130hp.
The PCV closes when RPMs increase. The vacuum drops during increased RPMs. When the high speed circuit on the Carter kicks in, is there less need for air, and more need for fuel? Is this how the high speed circuit is designed? Less air…more fuel, thus no need for the air being drawn in by the closing PCV?
I’m just trying to understand the relationship between the PCV closing and the Carter kicking into the high speed circuit.
Tim
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Post by Scoutpilot on Apr 8, 2015 15:33:40 GMT -5
OK. I see it's time for PCV 401.
From the late 19th century through the early 20th, blow-by gases were allowed to be exhausted into the atmosphere. Bearing and valve designs generally made little to no provision for keeping oil or waste gases contained. Sealed bearings and valve covers were for special applications only. Gaskets and shaft seals were meant to limit the loss of oil, but they were usually not expected to entirely prevent it. On internal combustion engines, the hydrocarbon-rich blow-by gases would diffuse through the oil in the seals and gaskets into the atmosphere. Engines with high amounts of blow-by (usually worn out engines, or those not well built to begin with) would leak profusely. One could also add to the category engines manufactured in haste to supply a war effort.
The first real attempt at crankcase ventilation was the road draft tube, which is a pipe running from a high location on or near the crankcase (such as the side of the engine block, or the valve cover on an overhead valve engine) down to an open end facing down and located in the vehicle's slipstream. When the vehicle is moving, airflow across the open end of the tube creates a draft that pulls gases out of the crankcase. The high location of the engine end of the pipe minimizes liquid oil loss. The road draft tube, though simple, has shortcomings: it does not function when the vehicle is moving too slowly to create a draft, so MB's and GPW's tended to suffer rapid buildup of engine sludge due to poor crankcase ventilation. A design change was incorporated going from a flat open end to a tapered tip with the taper aimed toward the rear of the Jeep such that a higher vacuum pressure could be generated and draw more vapors out. But it was short-lived. This created objectionable odors. Moreover, the draft tube could become clogged with snow or ice, in which case crankcase pressure would build and cause oil leaks and gasket failure.
As a result of these problems encountered by the military, a different type of crankcase ventilation had to be invented to allow tank engines to operate during deep fording operations, where the normal draft tube ventilator would have allowed water to enter the crankcase and destroy the engine. The Closed PCV system and its control valve were invented to meet this need and The Jeep became the first car/truck to benefit from the design. As soon as the system was proven Willys and Ford incorporated it into the production line.
The purpose of the PCV valve is to meter the flow of the vapor from the crankcase to the intake manifold. This is necessary in order to provide proper ventilation for the crankcase, while not upsetting the fuel/air mixture for combustion. An air inlet path to the crankcase was necessary. Starting at the Air Cleaner, air was drawn in by vacuum created by the pistons and flowed through the Cleaner and out through the Crossover Tube to an adjunct rubber tube which connected to the oil filler tube allowing fresh air to be swept through the crankcase to clear out the blow-by gases. The greater volume of air from the cleaner went straight to the carburetor. Vapor is introduced into the intake manifold and is carried with the fuel/air mixture into the combustion chambers where it is burned. The vapors drawn in to the manifold do not alter the fuel/air mixture in any appreciable amount. The fuel/air mixture is produced as a result of vacuum air flow through the carburetor barrel and the fuel is metered by the draw of the vacuum.
Blow-by gases and vapor should be removed at about the same rate they enter the crankcase. Since blow-by is minimal at idle and increases during high speed operation, the PCV valve must control the flow of vapor accordingly. The PCV valve is designed to compensate for the engine ventilation needs at varying engine speeds. It is operated by manifold vacuum, which increases or decreases as engine speeds and loads change.
For example, at low or idle engine speeds manifold vacuum is high. This pulls the plunger to the extreme forward position, or manifold end of the valve. Due to the shape of the plunger, vapor flow is reduced to a minimum. The low rate of the flow is adequate for ventilation purposes and will not upset the fuel/air mixture ratio.
At high speeds manifold vacuum is decreased. The plunger is only drawn to a point about midway in the housing. This allows a maximum flow of vapor. Since the engine needs more fuel/air mixture at high speeds, the introduction of more vapor does not significantly affect performance. In the event of a backfire, pressure from the intake manifold forces the plunger to the closed or engine-off position. This prevents the backfire flame from reaching the crankcase and exploding the combustible vapor.
A neglected PCV system will soon fail to function and the result can be expensive as well as troublesome for the Jeep owner. If the crankcase is not adequately ventilated, the motor oil will become contaminated and heavy sludge accumulations will begin to form. Internal parts, not protected by the motor oil, will begin to rust and/or corrode due to the water and acids that will become trapped within the crankcase.
If the PCV system is not functioning properly, the flow of crankcase vapor into the intake manifold will not be properly metered. This, in turn, will upset the fuel/air mixture for combustion and can cause rough idling or even stalling of the engine. Furthermore, intake and exhaust valves, in addition to spark plugs, may eventually be burned and rendered useless, prematurely affecting performance and requiring expensive repairs. To assure trouble-free performance of the PCV system and, in turn, the engine and vehicle, routine maintenance of the PCV system is absolutely recommended and required. The spring inside is calibrated for the correct vacuum pressure over the power range of the of the motor. Over time the spring weakens and will remain closed longer under less vacuum pressure and will result in blow-back which appears in the Crossover tube and in extreme cases will cause oil to be blown out of the Oil Filler Tube.
Many Jeep owners think that if a PCV valve rattles when shaken that it is okay. Wrong! Just because it rattles doesn't mean its calibrated spring is metering correctly. I recommend one cleaning and then replacement.
All hoses or tubes used in the PCV system should be cleaned and inspected. If any cracks or breaks are noticed in the hose, it should also be replaced. All hose connections should be inspected to assure an air-tight seal. Proper servicing of the PCV valve system will help reduce overall vehicle emissions and make your car run smoothly all the time.
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Post by Scoutpilot on Apr 8, 2015 16:12:15 GMT -5
This question is based on the PCV question posed at the 2A page. You know which one. The guy that’s redesigning the L-134 to push 130hp. The PCV closes when RPMs increase. The vacuum drops during increased RPMs. When the high speed circuit on the Carter kicks in, is there less need for air, and more need for fuel? Is this how the high speed circuit is designed? Less air…more fuel, thus no need for the air being drawn in by the closing PCV? I’m just trying to understand the relationship between the PCV closing and the Carter kicking into the high speed circuit. Tim |
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Post by Hawk on Apr 10, 2015 9:52:20 GMT -5
Nicely stated. On other sites, I had read your explanations of the importance of this system, and the repercussions when neglected. Just trying to stay in the game.
Tim
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Post by Scoutpilot on Apr 10, 2015 10:43:42 GMT -5
I know you, just like all the rest of us, have been climbing that learning curve. You are a lot further along than you think.
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