Blow-by gases

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Learn about ‘Blow- by’ gases...

Blow-by gases are present with all internal combustion engines.

A healthy engine will normally have a Blow-by of 1% of its swept volume (flow in LPM – Litre per minute).

Blow-by is the gas that enters to engine’s crankcase during Power Stroke and  Compression Stroke. This gas is composed of unburnt fuel,  air and combustion by-products.

 

BLOW-BY FLOW PATHS

Blow-by happens because perfect sealing between the Cylinder wall (Cylinder Bore) and Piston is not possible. The tangential load pressing the Piston Rings into the bore wall create a good seal, but still there are gaps, and gases get through them and enters the Crankcase. Refer the figures to see the gaps.

The Blow-by gases enters the crankcase, if they are left unchecked it will eventually build up pressure and can cause the oil to be blown past the crankshaft and camshaft seals !!

Some engine design have a Open Breathing System where the Blow-by gases are breathed out to atmosphere and some design have a Closed Breathing System where the Blow-by gases re-enter into the air intake system.

If engine is put under full load during starting (when the piston temperature is yet to reach its operating temperature) it will cause accelerated wear and tear of the Cylinder Bore. Worn out cylinder leads to increased blow-by and loss of power. It is always advisable to give sufficient time to warm-up before the engine is put under load.