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| Crankcase bypass valve cuts emissions | |
February 2006 |
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An innovative bypass valve for crankcase ventilation from Mann & Hummel for cars and trucks ensures that the pressure drop in the oil separation system does not lead to undesirable noxious emissions or engine damage. DaimlerChrysler is installing the valve in a petrol engine and Audi in V8 Otto engines. When designing an oil separator design engineers have to pay attention to the pressure drop of the system. To avoid extra pollution such as oil mist or hydrocarbons from the crankcase, legislators demand vacuum pressure for the crankcase in all conditions – from engine idling through to full load. Normally this is not a problem since the required vacuum pressure is built up in the intake system by the intake of fresh air. The pressure drop of the oil separator system can, however, reduce the available vacuum pressure. Since the pressure drop increases by the square of the volumetric flow of the blow-by gas, an overpressure in the crankcase may occur when vacuum pressure is too low on the intake side of the engine. This causes noxious emissions to escape into the air and damages seals and gaskets, and needs protection by the bypass valve. A second reason to use such a system is to do with the return flow of the separated oil. Systems with discontinuous return oil flow do not pose a problem because the separated oil is stored temporarily during driving in a small tank. After the engine has been switched off a non-return stop-valve opens and the oil can flow back into the sump. Separation occurs differently in a continuous return oil flow. Here the separated oil is constantly drained off through a tube or a hose which leads from the separator to under the oil level in the sump. Here the effect is to act as a siphon: an oil column occurs which prevents gas from the crankcase accessing the clean gas area of the oil separator. If the pressure drop in the oil separator system increases too much, the engine sucks the oil out of the sump. This results in engine damage. The new bypass valve prevents this happening. If the pressure drop of the oil separator exceeds a specific amount the valve opens and a bypass occurs. One part of the blow-by gas is now not released via the oil separator to the engine intake system but flows uncleaned directly to the intake. As a result the pressure drop is limited and the engine is protected.
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