## Transient intake manifold temperature

## Transient intake manifold temperature

(OP)

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.

- 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

## RE: Transient intake manifold temperature

Closing throttle, resulting pressure drop ("vacuum"), so commensurate temperature drop. All very dynamic.

The temperature recovery would [be] caused by other secondary processes.

Maybe. Guessing.

## RE: Transient intake manifold temperature

Steve

## RE: Transient intake manifold temperature

Even with the throttle closed, there must still be some air entering to support idle. That air would arrive carrying ambient heat, so the steady state (dynamic changes are finished) would see the temperature rise back to ambient.

The timing itself could provide some supporting or refuting evidence.

If I understand the PV=nRT equation (?), the steady state doesn't continuously generate a temperature drop.

Pure Speculation Alert.

## RE: Transient intake manifold temperature

Steve

## RE: Transient intake manifold temperature

How does this timing (duration) relate to anything, anything at all?

Is the plot linear, or exponential?

Does it match the idle air fill rate? If you double the idle air flow, does this duration halve?

Try varying other parameters to see if any of them change the timing, and how.

The Axis of Time is a very powerful weapon.

## RE: Transient intake manifold temperature

Steve

## RE: Transient intake manifold temperature

## RE: Transient intake manifold temperature

compositepro.I am trying to understand some simulation, not measured responses. There is no fuelling and the speed is held constant (I can easily do this in a simulation, of course). The throttle is just an idealised area restriction, which changes instantaneously. I even removed all heat transfer.

Steve

## RE: Transient intake manifold temperature

## RE: Transient intake manifold temperature

Steve

## RE: Transient intake manifold temperature

## RE: Transient intake manifold temperature

The plenum's transient temperature dip was first identified (and not understood) when this engine was the plant in a full vehicle model, where the engine speed was solved for. I've simplified it back so it's a pure flow problem to explain.

Steve

## RE: Transient intake manifold temperature

Isn't VE's original suggestion the most likely? Suddenly drop the pressure in the manifold and the temp will drop due to the laws relating to gas expansion.

What type of engine is this?

## RE: Transient intake manifold temperature

We are just seeing basic emptying & filling behaviour here. Transient behaviour of a plenum, not the effect of pressure waves. The engine is pulling mass out of the plenum at a given volumetric rate and the throttled intake is supplying it. When the throttle area suddenly decreases, the engine still pulls mass from the plenum, rapidly cooling it. The throttle's rate of supply of more warm air is limited by the pressure gradient across the new area. As the gradient is establishing itself, the temperature rises back to steady-state value, that of the ambient.

The dip is basically the result of two competing boundary conditions. The shape of it (depth, duration) is controlled by the volume of the plenum, the speed of the engine and the restriction of the throttle.

Steve

## RE: Transient intake manifold temperature

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Transient intake manifold temperature

What is happening to plenum pressure during the time period shown in your chart?

je suis charlie

## RE: Transient intake manifold temperature

hemi. Different shape (magnitude & time constants), but in general: it goes up; it goes down again. I'm just swapping the before & after diameters here, so the plenum conditions (pressures) just before the throttle transient aren't the same.Steve

## RE: Transient intake manifold temperature

The air that expands as it passes through the throttle expands without doing any work in a non-isenthalpic process. This results in less cooling, and for some gases may actually result in heating.

## RE: Transient intake manifold temperature

What assumptions are made by

SomptingGuy's simulations?"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Transient intake manifold temperature

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.

## RE: Transient intake manifold temperature

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

## RE: Transient intake manifold temperature

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.

## RE: Transient intake manifold temperature

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

## RE: Transient intake manifold temperature

(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

## RE: Transient intake manifold temperature

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)

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