Sunday, February 11, 2007

BLOW-BY


BLOW-BY and BAND-AIDS

Basics

The cylinders of Otto-cycle engines do not form a perfect seal. The piston rings provide a near perfect seal only during the Power Cycle when the pressure of the combustion process is above a given level. Depending on the fit of the parts and their state of wear, gases and finely divided liquids may cross the piston/ring/cylinder interface in either direction.

Gasses that escape past the piston rings or valves FROM the combustion chamber TO the sump or valve gallery is referred to as ‘blow-by.’ Some amount of blow-by is present in all Otto-cycle IC engines as a by-product of normal operation. The amount of blow-by is determined by a host of factors including but not limited to the number of piston rings, temperature differential across the system of piston, rings and cylinder, the fit of the parts, the, presence of valve stem seals, and the engine’s operating parameters, with more blow-by seen at elevated temperatures and high rpms.

Unless the valves are fitted with suitable stem seals, the intake manifold, exhaust manifold and combustion chamber is NOT isolated from the valve gallery. Blow-by that appears in the valve gallery tends to be extremely hot, easily capable of eroding valve guides and carburizing oil.

The crankcase of all Otto-cycle engines is vented to the atmosphere and meant to operate at atmospheric pressure.

Like all other fluids, the flow of gasses responds to a difference in pressure.

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General

A basic goal of modern engine design is to eliminate blow-by at normal operating temperatures and engine speed. This goal may be attained through the use of shaft- and stem-seals, ‘Total-Seal’ type piston rings, additional piston rings and controlling the normal operating temperatures to within a narrow range.

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All of That and a VW Too

The above should give you some idea why the tree-huggers go zoo when they see an old Volkswagen chugging down the road. (Or flying overhead, too.) The VW engine was designed in the 1930's. It’s crankcase ventilation system consists of pumping the air in around the pulley hub and using a road-draft tube to suck it out, along with whatever it happens to pick up such as water vapor, oil vapor and combustion products.

As Volkswagenwerk AG bored & stroked the basic engine, the spew became worse; so bad they were eventually forced to close the road-draft tube with a flapper valve and use the carburetor as the source of suction needed to provide the pressure differential that ensured a proper flow of ventilation through the crankcase. But unlike modern crankcase ventilation systems, the inlet remained unfiltered and always open.

California’s effort to require Positive Crankcase Ventilation (PCV) on early bugs and buses came to an embarrassed halt after reputable testing laboratories showed the bureaucrat’s solution of add-on valves, hoses and temperature sensors more than DOUBLED the engine’s emissions.

All modern engines are fitted with shaft seals and any air entering the crankcase is filtered. Volkswagen owners who liked to play in the sand quickly discovered the practicality of such features and began fitting their engines with shaft seals, commonly called a ‘sand seal.’

Sealing the inlet to the VW’s crankcase ventilation system dictates the need for an alternative inlet, ideally one that is provided with a filter. After-market retailers provided a number of such devices in which the inlet function was transferred to the valve covers. The stock outlet was left in place. Unfortunately, the purpose of these after-market devices was generally misunderstood by VW owners, most of whom depend almost entirely upon Conventional Wisdom for their automotive information. Most VW owners as well as the ‘technical’ editors of VW-specific magazines ASSUMED the inlet fixtures were a new kind of OUTLET, disabled the stock outlet and ended up even worse off than they were before.

Since the customer is always right, the after-market suppliers merely shrugged their shoulders and began providing a number of shinier and more complex crankcase ventilation fixtures, all of which were eagerly purchased by mechanically naive owners, praised in the magazines and featured at the car shows and then installed incorrectly. Life is strange :-) In the mean time, real mechanics built their own inlet systems or installed a properly plumbed after-market device (there were several good ones) and got on with the race. Most everybody else began blowing smoke in a major way.

(Remember the joke about the idiot carpenter who threw away half the nails he took from his pouch because the point was on the wrong end? Remember how his boss explained that he shouldn’t throw them away because they were for the opposite wall? Keep it in mind as you read the following :-)

The usual cause for disabling the inlet to the VW engine’s crankcase ventilation system was the installation of a sand seal. On flying Volkswagens the most common cause was the installation of the Long-Taper sleeve-type propeller hub developed by Bob Huggins in the early 1960's.

The usual cause for disabling the outlet of the VW engine’s crankcase ventilation system was the installation of an after-market air-cleaner or dual carbs, in each case having no provision for the outlet hose. For flying Volkswagens the most common reason for destroying the crankcase ventilation system is because most people didn’t even know the Volkswagen HAS a crankcase ventilation system (!) (Must be for the other wall, right? :-)

The punch line is that once the crankcase ventilation system had been disabled Volkswagens began blowing their oil overboard. The cause of such behavior differs slightly between rolling and flying Volkswagens but the end result is the same. And of course, since the PERCEIVED problem was ‘blowing oil overboard’ the obvious solution was some kind of vapor separator; an oil recovery system. Which as you’ve probably guessed, the after-market retailers were quick to provide, along with boxes of nails for the Opposite Wall :-)

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Wheat/Chaff, Men/Boys, Fact/Fiction, Oil/Vapor

One of the funniest lectures I ever heard in my entire life was a VW ‘expert’ telling a bunch of people that if your 1600cc engine was turning 4600 rpm, then it was producing exactly 53 horsepower. No exceptions. God Has Spoken.

Here’s the Real World version: The amount of power produced by your engine at ANY rpm is a function of it’s volumetric efficiency, which to save time you make think of as the position of the throttle. Throttle wide open? Then the cylinder is going to draw in a larger charge than if the throttle were barely cracked. Volumetric Efficiency defines the ratio between the maximum possible charge (100%) and how much the cylinder actually manages to suck in. The actual amount is sometimes referred to as the Effective Volumetric Efficiency or EVE. (I’ll get to ADAM, Seth and the boys in a minute :-)

(Have trouble getting a grip on this concept? Think about rolling down the road, lightly loaded, no wind, doing a steady 30 mph. (Do this on a chassis dyno, it will tell you that you’re putting out between seven and ten horsepower.) Then a Hill comes along (dreaded object for any VW owner). If you want to keep doing 30 mph you gotta keep pushing down on the accelerator pedal. If the hill is steep enough you’ll soon find the pedal flat to the floor. Your temperatures are starting to head for the red. The throttle is WIDE OPEN and you are only doing 30 mph. The engine’s rpm has NOT changed... but the engine is producing the maximum amount of power for those conditions. How much is that in horsepower? I donno... 25, thirty... around there. Truth is, horsepower isn’t what you should be concerned with; you should be looking at your head temps and your manifold pressure. But one thing I can guarantee you: If you just sit there, foot flat to the floor, watching your speed decay, you’re going to trash the engine. (And yes, Virginia, you can do exactly the same thing in your airyplane :-)

EVE for the air cooled Volkswagen ranges from about 10% at an idle to about 60%. (And that may help you understand why I’ve spent so many years trying to improve the volumetric efficiency of this particular power plant.)

You need to understand this because the problem of blowing oil is related to Maximum Output. The tricky bit is that Maximum Output may occur at less than 3,000 rpm in a flying Volkswagen but over 6,000 rpm in one with wheels. And if you really believe in equal power for equal rpm, in horsepower instead of thrust and the Tooth Faiery instead of slipping the kid a buck, you may as well toss this aside right now because nothing that follows will make any sense to you.

Maximum torque occurs at the point of peak volumetric efficiency. You may consider the former as the product of the latter. Peak volumetric efficiency occurs when the chamber is filled as full as possible under the existing circumstances, you light the fire and are rewarded with a specific impulse of the greatest possible magnitude and duration; lotta fuel means lotta fire; fire means heat; heat means pressure and the leg-bone is connected to the knee-bone.

Still with me? If so, you will see that the VW on wheels is blowing oil because of the high rpm, peaking temps and so forth. He’s a long, long ways away from his maximum volumetric efficiency but has managed to reach maximum output relative to rpm. The flying VW is rev-limited by the prop but the engine has reached its maximum output relative to that particular rpm. His volumetric efficiency is higher, as is his blow-by. And at that point his engine temps are liable to be well ABOVE anything you’ll ever see in a vehicle. (Why? Because John Thorpe is dead. The most popular of the flying Volkswagens are nothing more dune buggy engines with a fan on the nose, except they lack the dune buggy’s cooling system. The configuration of those engines as well as the public statements of the people selling them makes it painfully clear that they don’t know very much about aircraft engines, either in building them or cooling them, which John Thorpe did and taught to the rest of us.)

What it boils down to is a pair of engines lacking a proper crankcase ventilation system. One of the engines is maxed out for rpm, hotter than it should be, thrashing most of its liquid oil into hot vapor. It’s got some blow-by but it ain’t all that serious because the effective volumetric efficiency is right down near the bottom of the scale, not because the throttle is closed but because of the inertial mass of the fuel/air charge; at high rpms the cylinder doesn’t have enough TIME to suck in a big charge.

The other engine is maxed out for torque, running way over in the red, producing enormous quantities of blow-by, the combination of which has thrashed most of its liquid oil into hot vapor.

So now you want to separate the oil from the vapor.

Good luck :-)

You CAN separate oil vapor from air and I’ll describe the usual methods in a minute but the whole idea behind everything written up to this point was to help you understand that you’re buying a dead horse. Vapor separation AT THIS LEVEL is dealing with the symptom rather than the problem. What you should be doing is addressing the root problem, which is to PREVENT the vaporization of your oil. But the fact you’re here to begin with is good evidence that you are not mechanically adept; that you’ve probably bought an engine that came with the problems BUILT IN. And if you are not mechanically adept, when it comes to engines you are literally at the mercy of others; a victim-in-waiting with legions of slick hucksters eager to screw you out of your last buck. And your very life, in many cases.

‘Nuff of that; you won’t believe it until it happens, by which time it will be too late. So let’s go sort the wheat from the chaff. Or whatever.

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Oil vapor is a generic term applied to everything from smoke to rain. True vapor, which is like smoke, responds best to condensation; chill it, the stuff turns back into liquid oil. Oil that has been divided into minuscule particles is still liquid oil. It may be hot and it may respond to cooling but so long as it is ALREADY a liquid the best strategy is to use its greater mass to cause it to coalesce into a FILM of liquid oil that you may then collect using gravity, centrifugal force, wipers (!!) or whateverthell you got.

So whatcha got? Can you drive a centrifugal separator? Prolly not.

If what you got is a bug, bus or airplane, the tactics you can apply to the problem are limited. When Porsche ran into this problem in the late 1950's (i.e., high revs resulting in excessive oil loss through vaporization) they added MORE OIL. Then they bit the bullet and put a vapor separator on the front of the blower housing. Hot weather, they still blew a lot of oil overboard but so long as they won their share of races nobody gave a shit. (You gotta be a Real Man to drive a sports car, right? :-)

The separator Porsche used was the column-type, similar to the one shown in the drawing (OIL_SEPARATOR_01). (As with most of the other drawings it is in .dc file format; download the free demo software to view it.) Mounting the separator on the front of the blower housing kept the thing reasonably cool. As the particulate oil collected on the baffles, it cooled and served as a cool-surfaced collector for the vaporized oil. End result was to reduce the oil loss by about 75%.

The outlet of the vapor separator must go to an area of low pressure relative to the inlet. On a carbureted engine the most logical low-pressure source is above the carburetor. If the vehicle is moving at a fairly high speed you can use a road draft tube; at higher speeds you can rig a venturi in the slip stream.

The oil separator should have the largest possible exterior surface in order to facilitate cooling of the captured oil. Fins would be a good idea. In an airplane you should consider an air blast tube.

Vapor separation occurs at ever level within the system. The plumbing runs to the inlet ports should have a constant downward angle toward the source. I’ve found half-inch or larger 3003 tubing to be the best stuff for the inlet plumbing runs. Hose makes suitable connectors and flex fittings. The liquid oil return line should use regular hose fittings.

The diameter of the column is up to the builder, as is the number of baffles. To fabricate the thing I simply cut a series of angled slots in opposite sides of the tubing. The baffles are trimmed to match the contour of the tube then welded in place.

The idea here is to force the vapor to turn a lot of corners. Oil, either as a true vapor or a suspended particle, has a mass several MILLION times that of a molecule of air. The air doesn’t even notice the corners, other than to spend a bit more time getting from Inlet to Outlet. The oil however sees the baffles as virtual dead-ends and can’t help but hit the wall. And that’s what you want. Once the oil hits the wall, you got it. Gravity takes over, the oil heads downhill, finds the liquid oil outlet and ends up back in the sump. You want to maintain an adequate head on the return line. Remember, this whole mess got started because the sump was allowed to get above atmospheric pressure. If you keep an adequate head on the return line there may be enough pressure in the sump to prevent the return of the liquid oil.

The effectiveness of the vapor separator is a function of its internal surface area, the number of baffles, the pressure differential and the temperature. To get more length you may have to lay the thing down. The tricky bit here is that if you place it too close to horizontal you will defeat the purpose of the baffles, turning them into oil traps. The thing will fill up with liquid oil, reducing the interior volume and you’ll commence blowing oil overboard again. So think it out, especially if the thing is going airborne. Not only must it be functional, it must be able to withstand whatever acceleration you plan to impose on your butt. (Hint: Go for at least eight g’s; you can do that much on a bad landing without even trying :-)

Like most other crankcase ventilation systems the one found on the early air cooled engines is a superb bit of engineering. (Indeed, just about everything on the basic VW engine reflects the results of evolutionary refinement during the production of twenty-two MILLION engines over more than half a century of use. ) The ratio of inlet to outlet accurately reflects accepted standards for such systems and is very similar to the equation applied to aircraft engine cooling systems. When you modify such a system, or when you add a vapor separator, you must pay the keenest attention to maintaining an adequate pressure differential across the system or device. The basic rule is to keep the outlet larger and at a lower pressure than the inlet. Temperature, the length of your plumbing runs, and a host of other factors will effect the outcome, as does where and how the thing is mounted. The point here is that what works for me may not for thee. Tinker with it. You’re the Mechanic in Charge.

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Couple of concluding remarks for the Instant Experts:

The use of a synthetic lubricant addresses only the heat-related side of the equation, in that synthetics typically have a higher boiling temperature. Excessive blow-by, itself an artifact of elevated temperature, and any rpm above an idle (when the engine is hot) is more than enough to give you a oil ‘vapor’ consisting of finely divided particles.

We’re talking fog here, okay? Ever seen a real pea-souper? According to NOAA the densest fog on the American continent is the so-called ‘Tule Fog’ that occurs in the Central Valley of California. And fog is water vapor, right? So how dense is dense? About 900 particles per cubic centimeter. (How bigz a centimeter? About... that big.)

So that’s water. And naturally occurring fog. (You can make a denser suspension using ultrasonics. Very tricky, kinda like cold steam.)

So what about OIL? Well... according to the U.S.Army’s kemical corpse, using simple procedures and light oils you can produce colloidal suspensions as dense as 4000 particles per cc. How? Same way you do with your VW engine: Just heat & stir :-)

So what’s the major factor, heat or rpm?

Heat. Oh, there’s a strong linkage but if you solve the heat problem a lot of the down-stream effects simply don’t occur.

But my 2180 ALWAYS RUNS COOL.

More happy horseshit. If you’ve followed the instrumentation procedures advocated by Great Plains or John Monnet you’re measuring the temperature of the CRANKCASE rather than the oil it contains, and the temperature of the SPARK PLUG rather than the cylinder head.

Volkswagen knew what it was doing when it instrumented its industrial engines and measured CHT for its EFI systems. Measured at the spark plug your ‘cht’ could be as much as 150* F too low, compared to the measurement point recommended (and used) by Volkswagen, which is a specially cast lug on later model heads although they provided a Service Note explaining how to attach the CHT sensor to the lower exhaust stud on early model heads.

Same problem with the oil temp. If you just screw the sensor into a hole in the side of the crankcase, that’s the temperature you’re going to get. Volkswagen poked the sensor into the core of the stream of oil being sucked into the oil pump. On average, it reads nearly 100* F more than the temperature of the crankcase. And of course, the interior temps of the valve gallery runs about 100 degrees hotter than the average oil temperature.

This is another case of nails for the opposite wall. Wanna sell a kid a junker? Just diddle the speedo so it reads about ten miles per hour faster.

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Bottom Line Time

Blowing oil? Then find out why. There are three main reasons: Excessive blow-by at the rings. Excessive blow-by at the EXHAUST VALVE STEM. Improper sizing of the inlet-to-outlet ratio of your crankcase ventilation system.

A leak-down test will detect the first cause. The wiggle test will detect the second. Direct inspection will detect the third, assuming you know what you’re looking for, which is the TOTAL RESTRICTION offered by the outlet of the ventilation system. You could be running hose that is 3/4" in diameter, which should be more than enough. But if that hose is too long or if it has too many bends, the sum of its restrictions may cause the engine to ‘see’ only a tiny outlet.

Tiny outlet, the velocity goes up. When the velocity goes up so does its energy density, meaning it’s now strong enough to suspend & transport oil droplets of significant size, meaning you’re going to be blowing oil despite having a big hose.

The stock VW crankcase comes with a very effective oil separator built-in. Pull the dynamo tower and you’re looking at it. You can improve its effectiveness by stuffing the space under the dynamo tower with coarse metal mesh, such as a bronze or copper ‘Chore Girl’ pot scrubber. Not real handy as an oil-filler port since all new oil has to filter its way down through an inch of pot-scrubber but it works a treat at keeping the blow-by dry.

In the attached drawings the top of the column is often made to accept a removable valve-cover vent, like you see on an old Chevrolet Six. The cap contains a wire ‘filter’ that can be washed.

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