I have a homework problem where exhaust is traveling through an exhaust system (assumed to be air for simplicity) from and engine and then released into the atmosphere. The exhaust is at a temperature of 250F. I need to find the pressure at the beginning of the exhaust system.

I've been able to set this up and created an equation to solve for the pressure. I assumed that the pressure of the atmosphere where it it exits is 0 atm. But the problem is that the pressure depends on the density of the exhaust (air). I was only given temperature of the exhaust in the problem and I need a second property to determine density. Is there another way I can determine density? I am also given the volumetric flow rate and the area of the piping system

  • $\begingroup$ "I assumed that the pressure of the atmosphere where it it exits is 0 atm" You know, the unit "atmosphere" mean the normal atmosphere pressure. So the exit pressure is 1 atm since the gas is "released into the atmosphere". $\endgroup$
    – Yrogirg
    Sep 9, 2012 at 15:58
  • $\begingroup$ oh, it turns out you won't need the pressure at the exhaust. $\endgroup$
    – Yrogirg
    Sep 9, 2012 at 16:10
  • $\begingroup$ I assumed that the pressure of the atmosphere where it exits is 0 gauge pressure. I am not using absolute pressure. And yes I do need the pressure at the beginning of the exhaust system.... that't what I am solving for $\endgroup$ Sep 9, 2012 at 21:58
  • $\begingroup$ I've edited the answer. $\endgroup$
    – Yrogirg
    Sep 10, 2012 at 4:22

3 Answers 3


The key's is the Bernoulli's equation for the compressible flow:

$$\frac{v^2}{2} + \frac{p}{\rho} + u = \text{const}$$

$u$ is internal energy per unit mass, or using enthalpy $h$ per unit mass:

$$\frac{v^2}{2} + h = \text{const}$$

The other equation to find $v$ you'll get from the definition of the volumetric flow.

You have two equations to solve the system. I let it to you to find the $h$ for the ideal gas, you'll find that it depends only on the temperature.

So you'll find the temperature in the engine. To find the pressure in the engine you'll need also the density. Here mass conservation comes in hand:

$$\rho v S = \rho Q = const$$

where $Q = vS$ is the volumetric flow rate. All you need is the density at the exhaust which you find form the ideal gas law, knowing the exhaust temperature and pressure (1 atm or 0 atm gauge).


Maybe I am missing something, but it looks like you can do the following: 1) determine the gas velocity based on volumetric flow and pipe size; 2) write down the Bernoulli equation; 3) write down the equation of state for the (ideal) gas. As a result, you'll have two equations for pressure and density.

  • $\begingroup$ akhmeteli: Could you please tell me if "T" is in Celsius? $\endgroup$
    – user43300
    Mar 26, 2014 at 17:51
  • $\begingroup$ @veronica: For example, in the ideal gas law $PV=nRT$, $T$ is in Kelvin. $\endgroup$
    – akhmeteli
    Mar 26, 2014 at 23:35

I am a exhaust system studier!

Exhaust gas density could be approximated by $r = 352.5/(T+273)$, in $Kg/m^3$. Where $T$ is the exhaust gas temperature after the turbo in °C.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.