Positive Crankcase Ventilation

[Wesley]

I was doing some thinking on future design decisions, and came to the topic of PCV systems. It seems, from search and observation, that most teams simply vent their crankcase to a tank at atm. pressure.

In my understanding, there are 3 types of PCV systems.

1. Pan-evac systems - in which exhaust flow by an orfice in the header collector creates a pressure differential and pulls a vacuum on the crankcase. Impractical if not impossible on an FSAE car due to muffler requirements creating backpressure that would actually pressurize the crankcase with exhaust gasses.

2. Manifold intake systems - utilize engine vacuum to pull the same - ineffective and potentially harmful on FSAE vehicles, both because of the restricted air intake, and decrease in effective anti-knock characteristics (octane.) Diluting the already-limited oxygen with oil vapors seems undesireable. Also brings up plenum volume/turbulence issues.

3. Vacuum pump - Either by dry sump or by an additional vacuum pump scavenging the crank case, a partial vacuum is created, removing oil-contaminating blow-by and reducing crankcase pumping losses. Searches yield that others estimate the HP gain of a 12-14"Hg vac. to be ~4HP on a wet sump system. This seems to be the only viable option for an FSAE car, given restrictions, but the question I pose to you is: is there a vacuum pump with a low enough current draw and high enough capacity (and low enough weight) to justify the possible power gains?

Obviously dyno testing with various pumps would be the way to go - but before I jumped off into that endeavor and wasting time I thought I'd pick the collective brain and see what your thoughts were on a PCV system. Your thoughts?

[Wesley]

I was doing some thinking on future design decisions, and came to the topic of PCV systems. It seems, from search and observation, that most teams simply vent their crankcase to a tank at atm. pressure.

In my understanding, there are 3 types of PCV systems.

1. Pan-evac systems - in which exhaust flow by an orfice in the header collector creates a pressure differential and pulls a vacuum on the crankcase. Impractical if not impossible on an FSAE car due to muffler requirements creating backpressure that would actually pressurize the crankcase with exhaust gasses.

2. Manifold intake systems - utilize engine vacuum to pull the same - ineffective and potentially harmful on FSAE vehicles, both because of the restricted air intake, and decrease in effective anti-knock characteristics (octane.) Diluting the already-limited oxygen with oil vapors seems undesireable. Also brings up plenum volume/turbulence issues.

3. Vacuum pump - Either by dry sump or by an additional vacuum pump scavenging the crank case, a partial vacuum is created, removing oil-contaminating blow-by and reducing crankcase pumping losses. Searches yield that others estimate the HP gain of a 12-14"Hg vac. to be ~4HP on a wet sump system. This seems to be the only viable option for an FSAE car, given restrictions, but the question I pose to you is: is there a vacuum pump with a low enough current draw and high enough capacity (and low enough weight) to justify the possible power gains?

Obviously dyno testing with various pumps would be the way to go - but before I jumped off into that endeavor and wasting time I thought I'd pick the collective brain and see what your thoughts were on a PCV system. Your thoughts?

[VFR750R]

number 1 is fixed by putting the header orifice after the muffler since the flow rate is equal into and out of the muffler. The problem is it is engine speed dependent and at idle where most teams want vacuum to control turbo oil consumption there isn't any.
Would work good for an NA system though, and with a proper merge collector can pull substaintial vacuum even with a muffler.

[Biggy72]

We're trying #1 out this year so we'll see how it works once we get the new car running.

[RawePower]

The only effective electric crankcase vacuum pumps that I'm aware of are made by Star Machine:

http://www.starvacuumpumps.com/viewPart.php?id=STR.04.06.000

However, these are intended for larger engines (500+ c.i.), making much more power. They'd probably be way overkill and too heavy for an FSAE car, once you add in the 24V battery to operate it. Not to mention, it's probably cost prohibitive.

In the drag racing world, electric smog pumps off of late model GM products are somewhat popular for this purpose. But, the one on my drag car only pulls ~3 inches of vacuum at idle. It's not light either. It only functions to pull out the methanol vapor from piston blow-by.

[Darryl S]

With respect to option 1, if you are pulling the blowby gasses and the oil droplets suspended on it into the exhaust, wouldnt you be burning the oil droplets and creating alot of smoke? And wouldnt it be a risky thing to do since the rules say that you shouldnt be pouring anything out onto the track or risk disqualification?

[Mike Flitcraft]

If your engine is in good shape, then no, you won't. Most of the draggers I see with smoke is jet black from running rich on the carb, burning oil is a rarity.

[Wesley]

Thats a possibility if you vented from somewhere close to oil level, and didn't put a baffle in front of your PCV pickup. Most stock systems are baffled to prevent that.

Think of it this way - if you put it into the intake, which is what most stock systems do, you'd be burning the oil just the same. But you could also run it to a catch can and use the exhaust extraction to pull a vacuum on the can.

Thanks for the replies, everyone. Keep the suggestions coming.