eli5: Where does the vacuum come from in the engine crack case?

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When I open the oil filler cap, there is a vacuum. The engine is trying to suck the oil cap back.

Why?

Where does this vacuum come from?

In: 1

There is a valve, called the positive crankcase ventilation (pcv) valve. This valve pulls gasses out of the crankcase and into the engine to be burned. This is better than letting them loose into the air. It’s powered by engine vacuum, the same thing that draws air and fuel into the cylinders.

Extrapolating on /u/WRSaunders ‘s answer, the PCV system connects the engine crankcase to the intake. There are generally two air hoses that make it all work:

* one that connects the crankcase to the intake on the *upstream* side of the throttle valve, known as the **breather hose**, and

* another that connects the crankcase to the intake manifold *downstream* of the throttle valve, often known as the **PCV hose**

Downstream of the throttle valve will be a lower pressure relative to upstream, so this induces air flow through the breather hose into the crankcase, through the crankcase, and back out the PCV hose and into the intake manifold. The crankcase gases are thus reintroduced into the combustion air stream and just get burnt off.

There is a device called PCV *valve* connected to the PCV hose, which is basically just a spring-loaded pressure regulator that ensures at idle—when vacuum pressure downstream of the throttle is highest—the air flow through the crankcase is choked. Otherwise the crankcase pressure would become significantly negative and could be strong enough to suck oil into intake manifold. As the throttle opens the vacuum pressure in the manifold decreases (or rather the absolute pressure rises), and the PCV valve will open to ensure the crankcase is still ventilated.

Older engines didn’t bother with this arrangement and any crankcase gases caused by blow-by—the unburned air-fuel mixture making it past the piston rings or valves stems—were simply vented to atmosphere. This was a significant source of pollution.