Thursday, January 7, 1999

The Crankcase Vacuum System (CVS)

By: The ARC R&D Team Date: January 7, 1999
Re: Crankcase pressure management

For as many years as I can remember, crankcase ventilation went like this:
If you were blowing oil and having gasket and seal problems, you just added another line from your motor to the catch can. I have seen as many as 6 lines running from a motor, but the problem was still there.
In case you didn’t know it, we were just adding insult to injury.

Viewed from the crankcase, the single cylinder motor is an excellent air compressor. The more air you take in, the more you have to push out somewhere. Consequently the more air that is being passed through the crankcase, the more oil that will follow the air out.

All small engine manufactures have done a fair job in their crankcase ventilation system, but it only works up to about 3,000 rpm.

NASCAR and the Drag Racing industry have long used a dry sump oiling system that also creates a negative pressure (or partial vacuum) in the crankcase.

A multi-cylinder engine is a little simpler to deal with because one piston going up is canceling the pressure of the other one coming down. The basic thing you are dealing with here is called blow-by.

Without getting into a long technical and complex discussion on the subject, let me just make this statement. The rings on the piston are designed to work at their maximum with pressure from the top and vacuum from the bottom.

Common sense will also tell us that the piston will function much better and develop more horsepower if it is being pushed down in a negative pressure environment. In fact, with vacuum in the crankcase, the piston is actually being sucked down the cylinder wall.

Any positive pressure in the crankcase will allow a certain amount of oil to get passed the rings and contaminate the fuel charge. This can and will reduce horsepower.

Our Crankcase Vacuum System is very complex in design and every hole, groove, passage and vent are critical to its successful operation. Even the length and size of the tubing used in the catch can model for Kart racing are critical.

While the design is very complex, the operation is very simple to explain.

Without the CVS, the tappet room (valve spring area) is continually being flooded with oil and, contrary to popular opinion, this volume of oil is NOT being caused by the length or design of the dipper on the connecting rod - the cam gear is the culprit.
A little side note right here on horsepower. If the tappet room is flooded with oil and the valve guides are a little on the sloppy side, oil can easily be sucked by the valve stem and contaminate the fuel charge.

Our CVS works like this:
As oil and air are being pushed up to the tappet room, the first baffle is atomizing the oil and lubricating the valve stem with a mist. The second baffle is now starting to separate the air from the oil.
The 2 check valve discs are sensing the blow-by of each down stroke of the piston, regardless of how minute the amount, and are opening and closing on each stroke.

When the piston is at the top of the stroke, we will have our maximum vacuum. When it nears the bottom, vacuum gives way to the amount of blow-by pressure from the rings. The best calculations we can come up with is that we have vacuum 95% of the time and little or no pressure 5% of the time.

As the mist of oil and air move through the CVS, we are continuing to separate the two in our maze of holes, grooves and passages.

In the final step of this unique process, we are now using the vacuum in the crankcase to pull the oil, which is heavier, back into the crankcase, and the oil free air is vented.

While the operation is simple, the R&D on this project has been the most intense of any project we have ever undertaken at ARC Racing. The Dyno testing has been extensive, not only by us, but other engine builders as well along with track testing that has been on going for months.