Engine exhaust flow rate

[EngJW]

I've been looking for a method to get an approximate value for the exhaust flow rate of an automotive engine. We have an empirical equation, and I also looked at some muffler websites. All the methods found are basically this: Calculate the air flow into the engine based on displacement and assumed volumetric efficiency, then assume an exhaust temperature and use the perfect gas law to calculate the new volume.

I have a problem believing this. Air goes in but a mixture of gasses comes out, the gasses have different properties and those must change at the higher temperature, etc. Also a muffler sized by this method came out much larger than one that was just taken from a similar size engine and tested ok for back pressure and emissions.

Short of buying a simulation program, are there any other empirical methods?

 

[IRstuff]

Or, you can calculate the MASS flow rate, which is not dependent on temperature. Similar basic assumptions about displacement, efficiency, plus stoichiometry gives you the mass, which, for the purposes of this type of calculation, is invariant over temperature, once you've pick the inlet temperature and gas density there.

TTFN

FAQ731-376

 

[SomptingGuy]

EngJW,

The mixture of gasses coming out has a very similar average molecular weight to air (unless you are running really rich), so the calc you describe is sensible (R in the ideal gas equation depends on molecular weight).

Don't forget to add in the mass of fuel though.

- Steve