Crankcase Breather Question


Old 06-17-08, 07:39 AM
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Join Date: Jun 2008
Location: E. Central MI
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Crankcase Breather Question

Small, single cylinder, air-cooled 4-cycle engines use a check valve with a hose between it & the carb inlet to recycle crankcase blow-by gasses. The check valve lets the gasses out of the crankcase which appears to create a vacuum there. A continuous vacuum in the crankcase would create even more pressure differential across the piston rings, drawing more blow-by gas into the lower engine & also creating more engine loading on the exhaust & compression strokes. I don't see any air inlets to the crankcase [unlike automotive engines], with or without an inlet check valve, so what prevents the lower engine from always being in a vacuum while it's running. Or would it be better to add a filtered air inlet line to the top of the crankcase [possibly with an inlet check valve]? Thanks.
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Old 06-18-08, 12:24 AM
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No, you want a constant slight vacuum in the crankcase, just as you do in automotive applications. This is exactly what you want, or you'll wind up with oil consumption. When the piston moves downward on the intake stroke, it compresses the air in the crankcase. If it cannot escape, or even worse, if it were pressurized, the pressure in the crankcase would far exceed that in the cylinder during this cycle of engine operation. This would tend to push oil past the rings and valve guides into the cylinder and cause oil consumption and also push oil past oil seals and gaskets anywhere it can. The air that enters the crankcase is almost entirely blowby created during the extreme pressures during combustion. This extra volume of air needs to be able to escape or pressure will develop in the crankcase, so the valved breather allows extra air to be expelled without air being drawn back in when the piston moves back toward TDC.

The vent you see on automotive engines is different. This is because automotive engines actually route manifold vacuum to the crankcase, putting a much higher vacuum on the crankcase area (as much as 20"). If there was not a vent for air to enter, there would be an extreme amount of vacuum in the engine...much more than necessary or beneficial. Small engines don't use manifold vacuum to purge the crankcase, they just allow higher pressures to escape via the checkvalve in the breather.
Old 06-18-08, 01:07 AM
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Ther is realy a # of ways air can get into the crankcase of a small engine.
The constant back and forth motion of the valves and the piston for two, that's always present.Not to mention bad gaskets or oil seals,little cracks in the dip stick holder or cap,Compulsion lines that cause some fuel pumps to work.
or on bad breather itself.
Old 06-18-08, 06:43 AM
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Location: E. Central MI
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OK, thanks repair_guy & cheese.

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