Catch Cans - Questions about why/how
Discussion
I know what they are but what I don't know is:
why there is fluctuating pressure in the crankcase
why does 'blow by' affect crank case pressure and deposits
why do these deposits end up in the intake when normal engine oil doesn't, is it because they are 'vapour'?
on turbo charged cars this apparently gets input before the turbo and so can affect the turbo
why do some cars have them and others not
why there is fluctuating pressure in the crankcase
why does 'blow by' affect crank case pressure and deposits
why do these deposits end up in the intake when normal engine oil doesn't, is it because they are 'vapour'?
on turbo charged cars this apparently gets input before the turbo and so can affect the turbo
why do some cars have them and others not
The basics are-
there's always some blow by, and pressure in the crank case, you need to get rid of that pressure (but not spray oil everywhere) and there's pollutants in these gasses, so the current method is to feed those gasses back through and into the engine to burn again.
That means that there's an oily spray and mist that gets put through the intake. Oil separators are usually used to try and scavenge as much oil from this and dump it into the sump, rather than the intake.
Crank ventilation valves are usually fitted now which help stop a situation where high vacuum at the manifold can start sucking oil all through the pipework, eg it's not just the overpressure that goes through, it can actively get sucked through as well.
A catch can is like Heath Robinson oil separator, the crank case vapours go into a can, the oil hopefully stays at the bottom, and just the gasses go out the other side. They usually don't have drains on them, just a pipe along the side to show how much oil is collecting.
Most modern engines have a better fully sealed system that self drains into the sump.
there's always some blow by, and pressure in the crank case, you need to get rid of that pressure (but not spray oil everywhere) and there's pollutants in these gasses, so the current method is to feed those gasses back through and into the engine to burn again.
That means that there's an oily spray and mist that gets put through the intake. Oil separators are usually used to try and scavenge as much oil from this and dump it into the sump, rather than the intake.
Crank ventilation valves are usually fitted now which help stop a situation where high vacuum at the manifold can start sucking oil all through the pipework, eg it's not just the overpressure that goes through, it can actively get sucked through as well.
A catch can is like Heath Robinson oil separator, the crank case vapours go into a can, the oil hopefully stays at the bottom, and just the gasses go out the other side. They usually don't have drains on them, just a pipe along the side to show how much oil is collecting.
Most modern engines have a better fully sealed system that self drains into the sump.
TheEnd said:
Most modern engines have a better fully sealed system that self drains into the sump.
You can't (by definition) have a fully sealed system when you have gasses from blow-by being continuously added. Modern engines use an oil separator to remove as much oil as possible from the fumes. Any oil removed drains back to the sump but the fumes are fed via the PCV valve into the intake manifold.The vast majority of crankcase gasses comes from blowby, which is leakage from the combustion chamber, past the piston rings. This is high velocity gas stream into each bore space under each piston. As the pistons pump up and down those gases get pushed back and forth between cylinders. They also entrain the oil being thrown off the crank bearings.
This mist of combustion products And oil must be vented from the crankcase, and in modern engines the intake depression is used to keep the crankcase below ambient pressure.
The rotating and reciprocating actions result in a highly turbulent flow, but the net flow is always outwards
This mist of combustion products And oil must be vented from the crankcase, and in modern engines the intake depression is used to keep the crankcase below ambient pressure.
The rotating and reciprocating actions result in a highly turbulent flow, but the net flow is always outwards
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