Intake Manifold Vacuum Calculation: Flow Engineer Needed

Hello,

With consistent performance optimizations, TDIs already come with decent intake ports from the factory, as impressively demonstrated by the Seat Ibiza Cupra TDI (6L) with its 1.9-liter, 118 kW engine.
While the 96 kW TDI in the 6L model manages with a boost pressure of 1.3 bar and an intake manifold diameter of approximately 20 cm², the Cupra model, from the factory, doubles the intake area to around 40 cm² with only ~0.2 bar more boost pressure, without increasing the rated engine speed – apparently to relieve the (already larger) turbocharger from unnecessary vacuum work.

This also incidentally documents that there is no compelling reason for the typical, narrow intake manifolds used in TDI engines, such as the need to meet intake noise limits.

I would like to include an Excel sheet in a technical article, among other things, about the factory-installed intake throttling of TDIs, to provide a rough calculation of the intake manifold vacuum and the potential for improvement.

I'm attaching a first draft here, and I would appreciate any expert comments and suggestions for improvement.


Off Topic:
My Polo has delivered a quite impressive performance on our vacation trips.
For those interested in details:

Fuel consumption at Autobahn speeds is mostly between 100 and 160 km/h: 4.6 to 4.9 liters (distance of approximately 550 km, with about 20% being country roads) - which is pretty much the same level as my old AFN Golf 3.

The most surprising thing is the (lack of) background noise on the highway, despite having already removed approximately 7 kg of soundproofing material.
There's no typical TDI-style rumbling or booming; the loudest noises are the wind noise or tire noise, depending on the road surface.

In the highest gear (6th gear, 120 km/h at 2000 rpm), the acceleration isn't particularly impressive anymore, but even when fully loaded, it still accelerates noticeably well on any highway incline from around 100 km/h, practically without the driver feeling anything from the engine – except for a distant, deep, almost load-independent hum that barely changes with speed.
If you judge your speed by the sound, you'll always be going much faster than you think – at least if you're used to TDIs in a Golf 3 or Ibiza 6K.

For me, it's a brilliant little car perfect for economical, relaxed, quick, and comfortable travel (even without the usual electronic gadgets!), as long as the road isn't a bumpy track.
For example, the right-hand lanes on the A8 motorway near Pforzheim are completely unusable due to the standard sports suspension -> the only option is to move to the smoother overtaking lane, even if there's nothing to overtake.

On country roads, the handling is hardly fun due to the weight, which is no longer typical for a small car, but the torque of the engine from around 1500 rpm more than compensates for that, in my opinion .
The transmission shifts wonderfully .

Apart from the corrugated metal track behavior and minor quirks, I can generally recommend the strong Polo so far - unfortunately, the transmission has now been "sportified": the 6th gear is now the maximum power = maximum speed gear , and everything below it is spaced more closely.

Hi,
As far as I can tell, the formula for pressure loss dependent on velocity (without friction) is not valid in this form. According to Bernoulli's equation, it should be calculated as ρ/2 * c², where c is the flow velocity and ρ is the density, which can be assumed to be constant in this case.

Generally, this is not particularly relevant. Ultimately, the inlet cross-section at the charger is predetermined. However, a narrow intake nozzle naturally increases frictional losses within the nozzle and pressure loss if the nozzle widens again. However, if the diffuser opening angle is below 15 degrees (if present), this can be disregarded.

christians wrote:

Hi, As far as I can see, the formula for the velocity-dependent pressure loss (without friction) is not valid in this form. According to Bernoulli, it is calculated as ρ/2 * c², where c is the flow velocity and ρ is the density, which can be assumed to be constant in this case.

Hi Christian,

According to the formula, the pressure loss is exactly half of what my calculation shows – apparently because my calculation doesn't include a division by 2.

Then the frictional losses must be much higher than my assumed 30% in order to achieve the measured suppression levels... Do you know of any rule of thumb for this, other than increasing the factor to 160%?

Quote:

In principle, however, this is not really relevant. Ultimately, the inlet cross-section at the charger is predetermined.

...which is usually the narrowest point where the air is accelerated to almost the speed of sound, reaching the limit of the compressor, if my previous research is correct.

Quote:

However, a narrow suction nozzle naturally increases frictional losses within the nozzle due to the higher velocity, as well as pressure loss if the nozzle expands again.

However, if a diffuser is present, an opening angle of less than 15° can be considered negligible. }
What exactly is the diffuser? Is it the expanding intake manifold?

I personally don't consider the approximately 40-50 mbar of vacuum pressure at the engine's maximum power output within my air filter housing to be negligible (measured using LDA via the external pressure hose connection for the engine pneumatics). My intake manifold has an estimated opening angle of less than 15 degrees.
Without the air intake hose (-> hot air intake from the engine compartment), no more vacuum could be detected.

The factory-installed vacuum system, in my opinion, further dilutes the air entering the compressor, thereby reducing the potential airflow through the compressor.

Hi,
Particularly harmful are sharp bends and sudden cross-sectional expansions, such as steps or excessively large angles. Similarly detrimental are poorly rounded tapers, as they temporarily restrict the flow to a cross-section that is narrower than the component's cross-section.
A diffuser is a device that gently expands the cross-sectional area of a flow. In a diffuser, the kinetic energy contained in the velocity is converted back into pressure. Otherwise, this energy would be lost.

christians wrote:

A diffuser is a type of device that gently expands the cross-section. In a diffuser, the energy that is contained in the velocity is converted back into pressure. Otherwise, it is lost.

Hi Chris,

Then it seems my diffuser probably isn't very effective.
Unfortunately, my digital camera is broken, otherwise I would send you a picture.

Is it even possible, under such flow conditions, to roughly and reasonably calculate the negative pressure inside the air filter housing?

Hi,
The only way to calculate this directly is numerically, similar to how Finite Element Analysis is used in mechanics.
While friction coefficients (Zeta values) are available for bends, expansions, etc., these values no longer apply when two such features are located close to each other.
The simplest approach would be to measure the pressure at a few known flow rates and then use a formula to replicate that behavior.

christians wrote:

The easiest approach would be to measure a few pressures at a known flow rate and then replicate that using a formula.

Basically, I've been trying to do something similar with an Excel sheet, but so far it's only based on my own measurements from the Polo and some posts I've seen here...

Okay, I've expanded and rewritten the technical article about the redesign of cooling air ducts for liquid cooling systems.
Initially, I had planned to include the vacuum pressure calculation, but I've temporarily replaced it with an estimated value of 50 mbar of vacuum pressure at Pmax.

For those who have the time and inclination to read a somewhat longer work:
I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.

Correction and suggestion proposals are, as always, welcome .

Hello everyone,
I once took some photos of the intake manifold of the ASZ and BPX engines.
ASZ http://mitglied.lycos.de/monchi1976/hpbimg/sport.jpg
BPX http://mitglied.lycos.de/monchi1976/hpbimg/cupra.jpg
'I installed the 'larger' intake hose last week, but it seems to have made 'no' difference in terms of performance.'
....as you can see, the original intake hose, specifically the ASZ model, is significantly smaller at its inlet, but it widens out at the connection to the filter housing, similar to the Cupra model.
...doesn't this narrowing increase the flow rate? ...and wouldn't the air mass arriving at the filter housing necessarily increase due to a larger diameter of the hose?
Regards,
Monchi.

MONCHI wrote:

...doesn't this narrowing increase the flow rate??...then, wouldn't the air mass arriving at the filter box necessarily increase or decrease depending on a larger diameter of the hose??

"That's correct, but less vacuum pressure is needed to draw in the (practically the same) amount of air. This leads to lower compressor speeds -> technical article."