I also posted this question in the automotive list, but figured some of you could be a big help:
We got a little off-topic today, and a discussion arose about what happens to the temperature of air as it crosses an engine's throttle. For now, assume it's a MPFI, so fuel is not involved. Heywood uses compressible flow equations with a discharge coefficient to calculate mass flow. This suggests that compressible flow equations could also be used to predict the temperature. However, if you look at a fluids textbook, these equations were originally derived using the assumtion of ISENTROPIC flow, which I would think that a throttle is not. Of course, this is why the discharge coefficient term is needed in the first place. But, is it usable to figure out the temperature?
From another standpoint, my thermo book says that "throttling" processes (not really defined) are constant enthalpy, which for air means that the temperature across a throttle is CONSTANT! Thus, unless the discharge coefficient is zero, it doesn't seem like you could EVER use compressible flow equations to calculate the downstream temperature correctly. What am I not understanding?
We got a little off-topic today, and a discussion arose about what happens to the temperature of air as it crosses an engine's throttle. For now, assume it's a MPFI, so fuel is not involved. Heywood uses compressible flow equations with a discharge coefficient to calculate mass flow. This suggests that compressible flow equations could also be used to predict the temperature. However, if you look at a fluids textbook, these equations were originally derived using the assumtion of ISENTROPIC flow, which I would think that a throttle is not. Of course, this is why the discharge coefficient term is needed in the first place. But, is it usable to figure out the temperature?
From another standpoint, my thermo book says that "throttling" processes (not really defined) are constant enthalpy, which for air means that the temperature across a throttle is CONSTANT! Thus, unless the discharge coefficient is zero, it doesn't seem like you could EVER use compressible flow equations to calculate the downstream temperature correctly. What am I not understanding?
