Transient intake manifold temperature

[SomptingGuy]

My engine simulations (different flow solvers, same geometry) are both showing a transient dip in the manifold air temperature after a sudden reduction in throttle angle. No fuel involved, just gas dynamics. My engine runs at a fixed speed (1000 rev/min) on near full throttle. I then snap the throttle to nearly closed, with no other changes. My manifold temperature drops by ~50K and then returns smoothly to ambient (in about 1 or 2 seconds). The pressure drops instantly and stays low. Is this something that's known about? It seems like it should be one of those undergraduate thermo problems:

- Constant downstream (volume) velocity outflow from a plenum
- Transient throttling upstream
- Plenum temperature does???

My two flow solvers are about as different as they could be, so I don't suspect an error in the equations being solved.

Steve

 

[Compositepro]

Well, PV=NRT has three, not two variables that are changing, and assumes that no work is done. So it is not relevant to this problem.

An ideal gas will not change temperature while expanding through an orifice. The Joules-Thompson effect is due to deviation of gasses from ideal gas behavior. To be clear, I am relearning some of this as we go here, so I find it rather educational, myself.

 

[SomptingGuy]

What assumptions?
Ideal gas, PV=nRT. The molecular weight of the gas is constant. There are no phase changes.

(...I made a 3D model of this. The animations were much more interesting, but the core behaviour was the same, if pressure and temperature at any specific location were plotted vs time)

Steve

 

[j2bprometheus]

StomptingGuy-

My recommendation: re-run the baseline case with this one difference: make the engine displacement 20 % smaller.

Predicted result: the temperature drop will be smaller and the duration shorter.

 

[RodRico]

SomptingGuy,

Here's my stab at it off the top of my head...

Since you've stated the boundary conditions are fixed and there's no heat transfer occurring, the only thing affecting heat will be T=pV/mR.
R is obviously constant, so we can strike R and look at T=pV/m.
The plenum is fixed volume and the throttle plate is always in the air flow, so we can strike V and look at T=p/m.
Closing the throttle doesn't instantaneously change the mass in the plenum, so we can strike m and look at T=p

Your problem relates to p (pressure).

Plot what's happening in both pressure and temperature, and I bet you'll see them moving in tandem.

As for why this is happening, I think...
Any interruption in steady state flow creates a momentary change in pressure.
If the flow can be sustained when the throttle plate is closed, then pressure will quickly return to steady state.
If the flow can not be sustained when the throttle plate is closed, then a new steady state condition with a persistent change in pressure will result.

Rod

 

[SomptingGuy]

I found quite a good analysis of the problem in this paper:

(P60/N56) Adaptive transient engine air charge estimation using a mass state observer.
P. Schaal, B. Mason, K. Ebrahimi, M. Cary

Presented here: pmc2016_conference_programme

(Can't find the conference proceedings online anywhere, but my employer has a copy)

Steve

 

[IRstuff]

Went looking for that and came across the following, the 2nd is a thesis from one of the authors, I think...:

Adaptive Air Charge Estimation for Turbocharged Diesel Engines without Exhaust Gas Recirculation

Observer-based engine air charge characterisation: rapid, observer-assisted engine air charge characterisation using a dynamic dual-ramp testing method

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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