Interesting Exhaust Dynamics? 1

[Methusalah]

Hiya

Anyone got any views on this?

We have a 2.0 ltre supercharged engine producing 0.35 bar of boost pressure.

On the dyno we are seeing an exhaust back pressure of 0.5bar before the cat (a blocked cat is suspected), but what is interesting is that the back pressure measured at any point along the exhaust system after the cat is -0.2 bar depression/vacuum.

I know that it's possible to create a depression in the exhaust by placing the tail pipe in a low pressure location when the car is moving at speed, but this is static on the dyno.

Anyone got any idea what may be causing this please?

 

[patprimmer]

Maybe measurement errors.

Regards

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[Methusalah]

Deffo not measurement errors. Validated several times in different locations along the exaust system.

 

[SomptingGuy]

Static pressure perhaps? The gas is moving pretty fast in the pipes. If your total pressure is ~1 Bar, what speed would give a static pressure of 0.8 Bar?

 

[Fabrico]

Deffo an error somewhere. Otherwise air would be rushing into both ends of the exhaust system.

 

[CCycle]

Time to get acquainted with Bernoulli's Theorem. The gas in your pipe is moving. The energy in that gas is the sum of the static head (mgh) and the Kinetic energy (1/2mv^2).

One way of thinking about it is that when that moving gas comes out the end of the pipe and impacts the static surrounding gas, the pressure needed to stop that gas is the 0.2 bar you are missing.

Another thing you can do is orient your probe such that the open end directly faces the oncoming gas. This should, mostly at least, convert the kinetic portion to pressure and you read the total energy in pressure. This would give you the pressure you are expecting.

 

[GregLocock]

You'd need rather a high exhaust flow to get 0.4 bar of head. almost 600 mph!

Still, it sounded like a good technical answer.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

600 mph exhaust? Hmm...afterburner propulsion!

 

[NickE]

Are you using pitot tube to measure pressure, or a port on the wall of the pipe?

 

[Methusalah]

Hi, it's measured by pito tube

 

[unclesyd]

Could you describe your setup and equipment like the pitot tube dimensions, pressure measuring device, etc.