How does positive crank ventilation work?

Answer 1

The question should really be "How does positive crankcase ventilation work?" You can't ventilate a crank, it's either solid (roller bearing crank) or the only spaces are oilways.

Positive crankcase ventilation, PCV, is used to prevent a build-up of combustion gases in the crankcase. The piston rings prevent most combustion gases getting down the sides of the pistons and into the crankcase but a little does sneak past. This is called 'blow by' and increases as the engine wears. If there was no ventilation, pressure would build in the crankcase and cause oil leaks and other trouble. In early engines the crankcase was just vented to atmoshere via an oil trap so combustion gas fumes and some oil vapour escaped into the air. Later, to prevent this, the crankcase was vented instead to a low pressure part of the inlet system. The engine then consumes its own fumes. Originally this was just a flow restricting hole to the inle manifold but complication has crept in over the years. We have seen one-way valves, 'gulp valves' and similar all intended to hold the crankcase pressure just below atmospheric and get the engine to consume it own fumes without upsetting fuelling accuracy.

On diesel engines and supercharged engines a pressure lower than atmospheric can be created by putting a venturi in the the inlet. On a conventional gasoline engine the inlet is always below atmospheric pressure.

Mark

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