Topic: Backpressure

[Chrisboden]

I've often heard that all IC engines require backpressure, yet, for the life of me I can't understand why.

An IC engine (and this is true for ALL IC engines, from your lawnmower to your bike to the jet engines on a 777), is, at it's most basic level, an air pump. Engines run on Air, not Gas. All engines use the basic Suck-Squeeze-Bang-Blow system and all of them see improved operations through an increase in air and efficiency.

Now, how in the hell can backpressure be a requirement?

And doesn't a tuned/scavenging exhaust kind of defeat the whole argument that backpressure is a requirement? In a tuned exhaust multiple pipes are tuned in respect to the resonant frequency of the blasts of air exiting the engine, so that the inertial forces of the passing air can be used to suck out the air of the following cylinder, this creates a NEGATIVE backpressure!

Chris "That damn kid who's too smart for his own good" Boden

[kghost]

See Valve overlap.

But you are correct that less is better. Some engines are engineered to use back pressure in combination with the valve overlap. Emissions play a role in this.

[screwtheman]

I was under the impression that a little back pressure is a good thing to promote scavenging. But only in normally aspirated applications. I always picture it as the same effect as blowing into an empty plastic bottle and it blows back at you when you uncover the bottle mouth. So when the exhaust valve opens, the exhaust gasses are forced out at such a rate that it will aid in pulling the intake charge when the intake valve opens (when overlap begins). But that's all entirely misinformed speculation.

Paul "More tall than he is smart" Reynolds 

Just wanted to note that I'm talking about inertia scavenging as opposed to wave scavenging.

[amattel]

...All engines use the basic Suck-Squeeze-Bang-Blow system ...

Chris "That damn kid who's too smart for his own good" Boden

Shouldn't that go Squeeze-Suck-BLow-Bang?

[MetalHead550]

  Suck,Blow,Squeeze,Bang 

[oldbiker]

The last two replies depend on what sort of model you're working on

[Chrisboden]

http://www.rolls-royce.com/education/schools/journey02/index.html

[Patrick]

"Blow" is just a euphemism and is not appropriate. As to the rest of the discussion, huh?