Exhaust cfm

[BlueZ31]

Just a quick question im hoping you guys can answer for me.

If an engine (this is just for example, not an actual figure) ingests 410cfm of air at its peak rev's, would it also dump that same 410cfm amount into the exhaust or would the added fuel+combustion process dump a greater amount into the exhaust than the 410cfm it ingested? ( i know this is probably a dumb question and could be calculated somehow, so im sorry if im wasting anyones time with this post)

 

And does anyone know the average velocity(in mph) of exhaust gas and if its more dense than normal outside air? Im working on a experiment for a class im taking and i need to know these figures to get something right and i hope someone can lend a hand...i know that hot air tends to be less dense but the addition of combusted fuel by products leads me to believe it would be more dense, and since its hot it would also move much more rapidly having a greater impact force on, say, the turbine wheels of a turbocharger...

 

im just trying to prove the practicality of a air driven compressor (similar to a centrifugal supercharger) for a class im taking in community college and whether or not you could pump enough air into the turbine side of a turbocharger to spool it to some or any boost and how quickly it would respond. Im working with a air pump (relatively compact unit) that can send 430cfm at 230mph using a 2 3/4 tube and its pretty tough and not hindered much by backpressure (its similar to the kind they use to fill giant mattresses or houses you commonly see at family events and such so its built to fight through backpressure)

 

we're dispelling the electric turbo/supercharger myths and seeing if there is a true non- vehicle engine driven way to produce boost to any extent at all or if its just a wild-goose chase.... i know of Thomas knights work but it only seems practical for small (very) engines and defeats its own purpose with the huge weight addition it brings..

 

My last post was regarding this same subject but i may have worded it confusingly so i decided to make it a little better in the hopes of someone responding sooner (or at all, ha..)

 

i also have a larger unit that flows 1,700cfm but uses a 6'' diameter pipe, and making the tube small enough to adapt to a t25 turbo (2 3/4'' inlet on the turbine side) would cause a drop in cfm flow but probably give it a higher velocity...ugh..im putting to much thought into this rediculous project, especially this late at night..hope you guys can help!

 

My last thought would be to adapt the turbine side of a diesel turbo (which im assuming is larger since the motor is large=higher exhaust volume) to the compressor side of a small turbo, like that of a 90's eclipse or similar, in the hopes that the larger turbine blades would use more of the incoming pump air and exert more force on the compressor side...hmmmm...

[Z2000]

Much more cfm out. If it werent anymore out then we could just use air and no fuel to run an engine. the combustion process multiplys the exit cfm by about 5x.

 

The exhaust charge is much less dense then the air you breathe.

 

Exhaust velocity varies like crazy from exhaust valve seat (where it sees the highest velocities) to the tail pipe (slowest section) exit. Average is probably 280 fps but totaly depending on application.

 

Good luck with your project!

[BlueZ31]

Hmm...from what ive figured id need about 1000 cfm to (theoreticaly) spool it based on 410cfm/4 (# of cylinders)x5 (amount of gas generated by combustion) x 2 (number of cylinders dumping into exhaust at any given time) but this is confusing to me since i assumed that if each cylinder had (for example) 150cfm exhaust valves thats how much it would exit no matter what, meaning that it would dump about 300cfm consistently

 

But since exhaust gas is less dense it would occupy more space but it would have less impact force per square inch other than the fact that its highly heated im thinking that colder denser outside air at 230mph would have a greater force on the turbine wheels....i guess the only true way to figure this out would be to hook it up ( i need to find a cheap but good used turbo) and see for my self...how would i measure boost from the turbo when not hooked up to the engine?

[Z2000]

yes exhaust is far less dense. dont forget that when the exhaust valve opens there is 80-100psi exiting the cylinder. so 150 cmf on a flowbench measured at 1 psi means nothing to the real world pressures the exhaust system really sees.

[BlueZ31]

wow, 80-100psi? then again a relatively large piston forcing waste gas through a much smaller opening like the exhaust valve would create such tremendous force, i didnt think of it like that...

 

But maybe theres another alternative, such as a fluid? (i know air can be viewed as such, but generally speaking) such as water/coolant (to prevent rust) im sure theres an electric pump up to this task, ill just have to do some more research....

 

Again thanks for the tip!