Is this a stupid question? Compression effects in the crankcase

It’s pretty easy to understand that when the piston goes up toward the cylinder head, the air and fuel in the cylinder will be compressed. But what about the other side of the piston? I mean what goes up has to go down. When the piston goes down, does it compress the air in the crankcase too? If so, why doesn’t it blow out the seals, like the oil pan gasket or front and rear crank seals? I have a feeling this is a stupid question … lol … But I’d still like to know.

As long as there are a number of cylinders those going down are balanced by those going up. Not the case on singles and some twin cylinder motorcycles.

It’s not a stupid question. Any pressure in the crankcase is vented through the PCV valve or breather tubes into the air cleaner or intake tract; all depending upon engine type. If a PCV does stick closed engine oil can be forced out; usually past the front and rear crank seals.
That’s why if crank seal leakage exists it’s always a good idea to check the PCV first.

In the old days, cars used road draft tubes which were nothing more than pipes from the valve lifter galley and which vented underneath the car. At speed, the air flowing over the end of the tube would help to draw crankcase vapors out and into the atmosphere.
It’s not difficult to see why road draft tubes disappeared.

+1 to ok4450’s explanation.

…and, I can well recall the stench from the road draft tubes of older cars when they were idling at a stop light. Is it any wonder that the first mandatory emissions control device (in 1963, IIRC) was the PCV system?

PCV is to handle the blowby. The air moved by the piston is offset by other pistons.

And 2 stroke engines use the crankcase pressure from the reciprocating pistons to pump air into the combustion chamber.

2 cylinder ‘boxer’ engines have the biggest issue with crankcase compression. Here’s how it was handled in the Citroen 2CV:

“As long as there are a number of cylinders those going down are balanced by those going up.”

Even in this case there is a variation in crankcase pressure that varies at twice the crank rate (2nd harmonic).
Because the pistons do not move in a symmetrical, sinusoidal fashion, due to the sideways component of the connecting rod motion.

Hadn’t heard of that, thanks. Another issue with high rpm motors (say, a 4 cylinder 1000 cc motorcycle) is the difficulty moving those crankcase gasses between cylinders quickly. It’s another drag on the motor, they try to minimize those losses.

Off topic… HOW did you get that GIF to embed in the post @texases ?? Cool GIF, btw

@Mustangman - I found the gif on Google, right clicked on it, selected ‘properties’, copied the URL, and then pasted the URL in the box you get when you click on the ‘image’ button in the ‘Leave a Comment’ box.

Even one cylinder lawnmower engines have. some type of positive crankcase ventilation these days. I remember the old days when the lawn mower engines were vented right to the atmosphere. I can remember the smoke from the crankcase coming from around the flywheel.

As early as 1960 the Briggs & Stratton 3hp lawn mower engines had a vent tube from the tappet cover to the carburetor intake.

The 1965 Rambler that I bought in 1965 was the first car I owned that had positive crankcase ventilation. I was talking to the service manager about why it was needed and he told me it was for smog control that we,didn’t have smog here in east central Indiana. His,response was,“Works pretty good, doesn’t it?”

@“Rod Knox” I was thinking about the Lauson engines we had on a,lawn mower and a rototiller back in the early 1950s. There,wasn’t. a problem of crankcase ventilation with the 2 stroke LawnBoy we bought in 1955, but it made up for it with the 16:1_fuel mixture.

It seemed to me that crankcase ventilation was a significant factor in lengthening the service life of automobile engines. Removing the blowby gasses from the crankcase reduced the build up of shellac, gum and sludge.

I dont know if Diesels have a pcv or not(fairly recent industrail diesels didnt)the problem is the diesel can actually run on its own blowby.I know some fairly new caterpillar excavators I have looked at have road draft tubes.

@texases … & to all, thanks for the explanation and cool animation. Just curious, in the animation I can see how the one-way valve allows the compressed crank air to escape, and why that’s particularly necessary in that engine architecture, but there doesn’t seem to be any path in the animation for the air to get back in during the suction part of the cycle. Is it coming back in via the oil filler tube?

IIRC (and I may not), isn’t it a “hot rod hack” to use the smog pump to evacuate the crankcase, to make slightly more HP at higher RPMs? I know that the pistons, on net, balance each other out–but apparently it takes enough time that it’s an impediment to high revs.

George - I think they were happy to not have air come back in, like meanjoe said.

I think they oversimplified the illustration If it operated as drawn, a vacuum would develop every time the pistons went to TDC. There has to be something left out of the illustration. The crankcase must be allowed to breath in.