PCV and evap system 1

[GroovySupra]

I'm trying to understand how the PCV and evap systems aren't just big vacuum leaks. When the PCV valve opens it lets air flow through the crankcase and into the intake manifold, same with the evap purge valve but it pulls air through the charcoal canister. Both of these systems let air into the intake manifold after the throttle body and MAF. I'm especially confused about how this works with forced induction.

Thanks for any help!

 

[Lou Scannon]

Well, the idea about returning the blowby gas, mainly air, after the MAF is that it's already been metered, so you don't want to meter it again.
After the throttle body is not an issue from a metering aspect, but is useful to give a helpful delta P to ensure the flow goes in the desired direction, most of the time. Typically, a PCV valve is not only a restriction, but a check valve, to ensure that the flow does not go in the wrong direction.I haven't worked on evap systems, but I understand they are highly controlled via algorithms and actuators that function to quantify the loading of the carbon canister, and purge it under favorable conditions, while avoiding detrimental effects on air/fuel ratio control.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[TugboatEng]

Forced induction crankcase ventilation systems use the air filter to generate a pressure drop to draw out crankcase gasses. There is no PCV valve.

There is no carbon canister for the crankcase ventilation. Usually it's some type of coalescing filter.

 

[GroovySupra]

Okay this is definitely helpful, for further details I'll talk about what I'm working on. I have an 87 Toyota supra turbo with a 7MGTE, I replaced the mechanical throttle body with a Bosch drive by wire unit and I'm using a haltech ECU. The original throttle body had a nipple for the OEM PCV system and I believe the PCV valve is built into the throttle body. Switching to the new throttle body I have to re-route the PCV system. Currently the the only parts driven by engine vacuum are the brake booster, fuel pressure regulator, and blow off valve. All these parts have sealed diaphragms and don't allow a constant flow. The reason I'm confused about PCV and EVAP is that these allow constant flow into the intake manifold when their valves are open. I now understand that the PCV valve is not only a check valve but also controls flow depending on the delta P (more flow at low vacuum and less flow at high vacuum. I guess this just means it is a controlled "vacuum leak" that the ECU will have to account for during idle and partial throttle. I'm still not sure how I'm going to control the EVAP system. I guess I can have the ECU open a purge valve when the engine conditions are correct.