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128 comments on DrumBeat: November 4, 2008

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nelsone on November 4, 2008 - 9:54am Permalink | Subthread | Parent | Comments top

Small engines like 2-strokes on bikes benefit from lower back pressure, but taking the muffler off a car engine can make it run much worse, as the exhaust is "overscavenged."

The same technology that's used on military helicopters to quiet their blades has been sold on dirt bikes to muffle their engines: active cancellation. A microphone and speakers are used to emit a signal that's out of phase with the engine noise, but only in a specified direction.

Active cancellation doesn't absorb any acoustic energy, it just points nulls in the important directions, like in a horizontal plane for a bike, or out in front of a chopper. The overall effect is to increase the noise coming from the source; it's just redirected in less obtrusive directions.

http://www.freepatentsonline.com/EP0725965.html

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Neven911 on November 4, 2008 - 4:06pm Permalink | Subthread | Parent | Comments top

Mufflers are part of the resonant exhaust system that actually increase the volumetric efficiency of ICE, the 'noise' energy component is very small, consider the sound energy of a crowd shouting at a football match is enough to warm a small cup of water. Bigger losses in petrol engines are the "pumping" losses, caused by throttling the air as required to keep the air fuel ratio constant, which is why diesels are inherently more efficient, they only throttle the fuel

Neven

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128 comments on DrumBeat: November 4, 2008

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