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rohl4th (Automotive) (OP)
14 Jan 05 17:58
Does anyone know of a good design source for crankcase breather systems on small IC engines?
I've looked at the SAE website and found very little info.
icrman (Automotive)
14 Jan 05 20:47
You can baffle it and pcv valve it just like the bigger ones. Most older ones are vented at the valve cover.
apb2005 (Automotive)
17 Jan 05 17:15
Breathers can be a bit of a black art.  Do you have any particular issues you need to resolve?
magnograil (Aeronautics)
24 Mar 05 20:15
I believe the reason for the Kaw BSB and NASCAR connection to the exhaust is to use the exhaust pulses to pull down the crankcase pressure. Less air in the cases is less air that has to be pushed around by the pistons. NASCAR probably also uses it for the flaming exhaust entertainment value.
The new Kaw/Suz 250/450 four stroke dirt bikes have a timed breather from the primary. The old British twins had a cam driven timed breather from the timing chest. Road use engines have to route the fumes into the airbox for emissions reasons. It is usually taken from the valve area because by the time the vapors have moved up through the drain holes most of the oil mist has collected inside the engine. The Saab 900 turbo motor has a large velocity drop box inside the valve cover to capture more of the mist.


Rob45 (Automotive)
29 Mar 05 17:01
The reason old Brit bikes and such had timed breathers was because both pistons moved together,  producing rather large variation in crankcase volume with time!

This suggests that in devising a crankcase vent system, your umber-one concern will be the type engine,  since that is what governs how much "breathing" the crankcase nees to do.
Size you vent(s) accordingly,  and be sure to place them where they won't blow oil out.
magnograil (Aeronautics)
29 Mar 05 18:32
The old Brit vertical twins had poor crankcase seals so anything that helped keep the oil in was tried. However, they had very little more change in crankcase volume than the modern 500-600cc singles. You need a crankcase breather to get rid of the blow by gases.
Venting to the airbox dilutes the fresh air entering the cylinder so you lose power there. A vent to the atmosphere will require a catch tank which is added weight. Even a system connected to the exhaust will require some sort of oil separator and return line otherwise you will be black flagged for smoking.
How much horsepower gain is worthwhile? High speed engines have the internals smoothed to help the air/oil blown around by the pistons move between cylinders. Pumping down the cases by exhaust pressure reduces the amount of air the pistons have to move.

ntsqd (Mechanical)
30 Mar 05 22:45
Drag racers have used the check valves found on GM Air Injection systems to pull a vacuum on the crankcase. They typically weld a piece of pipe, w/ male threads, at an angle pointed downstream onto the header collector. A crucial detail is a gap or slot at the base of the pipe to bleed off some of the vacuum signal.

Pulling a vacuum in the crankcase has appearently been shown to increase ring seal. Not sure if that is true of any ring, or if they are using some special design. Drag racers started out using "Smog" pumps to pull this vacuum, but there are now purpose built pumps.
Dry Sump Pumps are also known for their scavange section's  ability to pull a significant vacuum on the crankcase.
evelrod (Automotive)
31 Mar 05 18:29
Yep, the my old British twins had pistons that went up and down together, but the breather was on the DRY SUMP tank!

Also my crankshaft seals were just fine.  I had lots of other leaks as the Brit stuff split their cases vertically and they liked to use oil bath primary.

Now, of course--- and it is a fine British distinction for sure--- there is a difference between a "leak" and a "seep".

Rod
EHudson (Mechanical)
5 Aug 05 12:24
apb2005 had it about right - It's a black art.

I had an experimental engine on the dyno and it pumped oil out of the small hose fitting exiting the breather.  The lab techs put a small hose on it and ran it to a catch can and it stopped pumping.  Just the length of hose will affect performance.

Small single cylinder engines are best run with crankcase vacuum so that the seals have an easier life.

You need a check valve.  Reed valves, disc valves and elastomeric umbrella valves are all solutions.  a tiny drain back hole to return any errant oil from the wrong side of the breather is customary.  Crankcase gases are expelled on the down stroke of the piston and a vacuum is created on the up stroke.

Locate the breather where it is not likely to be splashed with oil continuously by the crank assembly.

Breathers tend to become less effective as engine speed increases.  Typically the internal crankcase pressure at idle might be on the order of 14 - 16 inches of water below ambient.  At 3600 this might drop well into the single digits.  It is tough to get a breather that works above 6000 rpm using a check valve.  

If you are unlucky in your design, the pressure will be positive and you have some tuning to do.  As mentioned the hose restriction is one thing to play with.  The spring rate on you check valve is another.  

These days, everyone sends the breather hose to the air intake to avoid fouling the atmosphere with the oily mist.

Hope this helps a bit.
tbuelna (Aerospace)
7 Aug 05 6:29
With current production engines, separating oil and air at the crankcase breather is becoming a serious issue, due to emissions regulations.  In the old days, with PCV valves, the crankcase vapors were routed to the intake manifold and burned in the engine. Nowdays, this is unacceptable, as it causes high HC emissions.  So separating oil vapor at the crank breather is very important.

Of course maintaining a slightly negative crankcase pressure helps with ring sealing and minimizes oil migration past the piston rings.

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