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

 

[VEBill]

Could it be as simple as: PV=nRT ?

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.

 

[SomptingGuy]

Yeah, it's the secondary processes I'm struggling with. There is no wall heat transfer, I shut that off.

Steve

 

[VEBill]

More guessing:

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.

 

[SomptingGuy]

I think there are maybe two thermal events starting at the same time and the final behaviour is something like superposition. It's more interesting than steady-state simulation for sure.

Steve

 

[VEBill]

"...returns smoothly to ambient (in about 1 or 2 seconds)..."

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.

 

[SomptingGuy]

I'm wondering if the 1D discretization is affecting the behaviour. That's what's in common between my two different flow solvers. Think I'll try 2D or 3D for the same geometry when I'm back at work tomorrow.

Steve

 

[Compositepro]

Here is my guess. At full throttle you have high air flow, high fuel flow, and low manifold vacuum (high pressure). If you snap the throttle closed, you will immediately have a very high vacuum (low pressure) due to the engine still being at high speed and trying to pump a large volume of air through the now tiny throttle opening. The air in the manifold will cool due to expansion but also due to the flashing of the liquid fuel that is wetting the walls of the manifold. This is rapidly ingested into the engine and replaced with warm fresh air and the manifold vacuum decreases (higher pressure) as the engine slows to idle and pumps less air. There is often a spring loaded valve on throttle plates to prevent very high vacuum when the throttle plate snaps closed. Does your throttle have this?

 

[SomptingGuy]

Thanks for your thoughts 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

 

[Compositepro]

Can you show a plot of temperature and pressure vs time? The temperature change in your simulation must be due only change in pressure in your case. The instant drop would be due to adiabatic expansion where work is extracted from the gas during expansion. There should be a pressure recovery as the engine slows. The expansion of gas through the throttle plate does not extract work from the gas and results in less temperature drop. This is what I vaguely remember about reversible versus irreversible gas expansion.

 

[SomptingGuy]

I'll see if I can produce some useful plots tomorrow. My engine speed doesn't drop - it's an imposed boundary condition, not a solution variable. In fact I can get the same behaviour with a constant volumetric exit velocity to represent the engine (removes pulsation effects). My aim is to isolate the plenum's behaviour to understand it.

Steve

 

[Compositepro]

I wonder how your simulation can be accurate if speed does not drop when you snap close the throttle (a massive flywheel, perhaps), but my suggestion in the previous post should still apply, except that you will not see a pressure increase due to engine slowing.

 

[SomptingGuy]

Accurate? I'm only interested in the airpath at the moment. It's normal and common for (gas dynamic) engine simulations to have their crank speeds imposed in this way for steady-state work. Infinite flywheel if you like, with a given initial speed.

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

 

[BigClive]

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?

 

[SomptingGuy]

Here's my understanding now:

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

 

[Lou Scannon]

What happens with a sudden increase in throttle area? (in your simulation)

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

There is something wrong with your simulation. Pressure drop across the throttle is the sole source of any temperature drop. You should be getting a similar pressure drop under the final steady state conditions and consequently a similar temperature drop as air expands across the throttle.

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

je suis charlie

 

[SomptingGuy]

I get the opposite effect 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

 

[Compositepro]

Gruntguru, there are two distinctly different types of expansion happening in this case. The expansion of the air that is already in the manifold is doing work while expanding by pressing against a piston (or the air moving toward the piston). This results in cooling due to energy being extracted (isenthalpic).
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.

 

[Lou Scannon]

This results in less cooling, and for some gases may actually result in heating.

In other words, ideal gas laws cannot be assumed?
What assumptions are made by SomptingGuy's simulations?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz