Cold air to filter

[SpitFire6]

Hi,

 It looks like hose for vacuum service. I guess the best choice. Never seen it with a smooth bore. Rigid would be difficult to support.
Dense air always better.
My 3" vacuum hose is 1 meter long and fitted with a Generic Chinese Air Filter. Looks kak & I guess not good. LOL.

 

[Anglefire]

To be honest, I think temperature changes are fairly insignificant when you compare the density of air with respect to humidity.

Air Temp        Air Pressure    Air Density    Difference    % change from previous 
5'C    Dry Air    1000pa    0.0125245kg/m3        
25'C    Dry Air    1000pa    0.0116843kg/m3    0.0008402    7%
50'C    Dry Air    1000pa    0.0107804kg/m3    0.0009039    8%
75'C    Dry Air    1000pa    0.0100063kg/m3    0.0007741    8%
                                    
Air Temp    RH    Air Pressure    Air Density        
15'C    25%    1000pa    0.0101416kg/m3        
15'C    50%    1000pa    0.0081935kg/m3    0.0019481    24%
15'C    75%    1000pa    0.0062453kg/m3    0.0019482    31%

Of course the reality is a mixture of all of that because for a given outside condition, the air entering the engine will be different as heating it will reduce the RH and the RH going down increased the density, but the rising temperature will have dropped the density.

[Clive]

But from your first table, going from 25 to 75 degrees is a 16% change in density. That will make a significant difference, though with a carb the less dense air will drag less fuel in, so it won't be running massively rich. (different story with injection!)

Never considered humidity. And I would have assumed damp air would be more dense, simply as it contains water vapour. Though on a molecular level water is just over half the mass of an oxygen molecule. 

[NonMember]

No, temperature is not insignificant. That 8% change for 25 degrees is at the lower end of what you get between having the hoses and not having them - under-bonnet temperatures can get well above that. And it's a full 8% more oxygen, meaning 8% more power. The increased density of moist air is mostly water so of no benefit, although water injection was used on some turbo engines but mostly for the cooling effect it achieved.

[Clive]

58 minutes ago, NonMember said:

No, temperature is not insignificant. That 8% change for 25 degrees is at the lower end of what you get between having the hoses and not having them - under-bonnet temperatures can get well above that. And it's a full 8% more oxygen, meaning 8% more power. The increased density of moist air is mostly water so of no benefit, although water injection was used on some turbo engines but mostly for the cooling effect it achieved.

Rob, the figures suggest moist air is LESS dense. That what surprised me initially, until I considered the water is a gas, takes up space etc so displaces oxygen and nitrogen, both heavier than water molecules. 

[Clive]

23 hours ago, JohnD said:

At, say, 6K, a 2L engine draws in air at  2L x 3000 (one aspiration/two revs) = 6000L/min = 360,000L/hour

How fast is the air moving?   A cylinder with volume 360,000L and with a cross sectional area equivalent to those two hoses - can't recall, are they 1 1/4"?  Or 32mm, or 3.2cms,  so 1.6cm radius and  CX area 1.6^2 x Pi = 8cms^2 - will be 360,000,000 (cms^3)/8 cms long = 45,000,000cms = 450,000metres = 450 kilometers.   Per hour. 

In fact, the  velocity profile across a tube is not linear, but maximal in a narrow region at the centre, slowing to near standstill at the walls.    The velocity curve gets blunted if the flow is turbulent, as rough sided walls will promote.   The velocity in the centre will need to be much higher than the mean flow to achieve the full flow delivery.

 

And as I mentioned before, and for the above reason, flow resistance increases markedly as radius decreases.    There are two such tubes on the GT6/Vitesse airbox, so the total flow would need to be half the toptal, or 225mph.     But even faster along each, narrower tube.

John

PS This isn't rocket science!   It's not even quantum chromodynamics!  It's arithmetic!     Please check my working anybody!

John, just ordered a leaf blower/vacuum (the lady wife is getting fed up with "stuff" in the garden) and the airspeed produced by that is 240km/h, broadly the same as teh figures for a vitesse! sort of puts it in perspective. And could be  a useful tool for studying aero if I ever get that far. 

[JohnD]

Clive, it's a tempting thought, but models need faster airflows for the same effects, and Reynold's number makes those non-predictive for full size.

Might work for localised tests though.    I squirted air from my comppressor past a cardboard mockeup, full-sized, to test the extractor ducts in the bonnet sides of Old Blue, similar to those on a LeMans Spitfire.

I demonstrated to my own satisfaction that  sloped floor to the vent was more effective than just a hole.    Even better was the tripping lip at the front of te hole, but I didn't take any pics!

[NonMember]

1 hour ago, clive said:

Rob, the figures suggest moist air is LESS dense.

Oops, wasn't reading the figures closely enough!

[Anglefire]

Ok, I'll do it another way.

at 15'C and 75% RH (Not unreasonable) the density of air is 0.0062453kg/m3

That same air heated to 75'C now has an RH of 3% (Its not gone below dew point which is still 10'C, so no vapour has been removed)

It now has a density of 0.0053295kg/m3

That's a difference of less than 1%

Do the same thing with 15'C air at 50% and heat it to 75'C and the dew point is now -6 so the air density has changed and id now denser but by less than 1%

As I say, I don't think its significant most of the time and there must be other reasons for the cold air theory.

(I used https://www.omnicalculator.com/physics/air-density to get the raw data)

[Clive]

0.0062453/0.0053295= 1.172, so surely a 17% change in density?

But I think there are some aspects missing from all these figures. What we want/need is the oxygen density of the air under these conditions, that is all the engine cares about. And the change in RH with temperature seems to negate much of the air density idea??  Also I am interested in how the RH changes so fast. Is it that warm air can hold more water, so the RH is a comparison of water in the air against what it could hold as a maximum? In other words it is not an absolute.

Excuse my ramblings, but I do find this interesting. I have spent too much time at home recently! And now my head is starting to hurt.

[NonMember]

Clive is right, RH is not a useful measure, and the ratio is high for your example. It's also similar to the ratio based on temperature alone.

This just feels right to me, because in 30 years of working in automotive and motorsport engine control I have only once seen anyone ask for a humidity sensor, whereas inlet air temperature has been fitted to every ECU I've ever worked on. Even the really cheap ones.

[JohnD]

On humidity.

Consider this.    In a mixed gas, that includes gases and vapours, each exerts a Partial Pressure, that is proportional to its concentration.     For instance in normal dry air, 21% Oxygen, 78% Nitrogen, the rest is mainly argon and CO2; the partial pressures are in the same proportion, in standard sea level pressure of 760mm of Mercury (mmHg) so 160 (mmHg) of Oxygen.   Add water vapour, which at, say, 25C has a vapour pressure of 23 mmHg, as 0.7% of the air.    The air cannot have a greater pressure than 760, so will contain a little less oxygen.

John

[Anglefire]

Rh is indeed a relative measure so simply changing the air temperature from low to high will reduce the humidity.  
going the other way then the amount of water vapour in the air remains the same - until it gets to dew point, which is the point the air can no longer hold the amount of water that is in it so it falls out as liquid (rain!) 
 

I can do the sums to get the amount of water in the air. 
 

there is a lot more to this than I suggest but is interesting as Clive says. 
 

given forced air engines nearly all (modern ones 100%) have intercoolers to reduce the charge air  Temperature then there has to be something to it - though those temperatures are significantly higher than normally aspirated engines  

I will get some loggers setup in the engine bay when this is all over and see if there is anything significant going on   
 

 

 

[SpitFire6]

v/t = V/T

(273°K=0°C. 300°K=27°C)
10L/273 = x/300. 10L/273 = 10.98L/300
Therefore, 10L of air at 0°C is equal to ~11L at 27°C
Amount of O² in the air,(volume x 0.21)(0.21 = 21% O²). So for 10L of air, you would have 2.1liters of O² (by volume). For 11 liters of air, you would have 2.31 liters of O^2.

 ΔT 27° is 10% more Oxygen?

The calculation is correct. Not sure I should post but here you go.

 

[Guest]

I think the OP was only asking what diameter hose he needed to obtain to finish the job !!

[Pete Lewis]

50mm with a  jubly clip if sink waste doenst appeal then pond pump hose is generally smooth inside and looks original in the bargain  approx £7 mtr of ebay

some garden centres are open if they sell produce 

Pete

[Anglefire]

2 hours ago, classiclife said:

I think the OP was only asking what diameter hose he needed to obtain to finish the job !!

And your point is what exactly 😂😂

[PeteH]

Hi

Haven't actually researched this in any depth. But I understood that part of the reason IC engines "appeared" more efficient in cold/damp conditions was 1) lower temperature of the Air Mass. and 2) higher density, as a result?. And as a bonus the molecular structure of the entrained moisture was H2O. (ie; 1 3rd Oxygen)?.

Probably all bollocks, I Know?.

P.s. Does Advanced Member, relate to my age?.

[Clive]

The oxygen in water is useless. Water is extremely inert, so won't take part in combustion. In some ways it will inhibit, as water is a product of combustion, so will push the equilibrium back. (but will be so very very tiny an effect) Worse is the water vapour has displaced some oxygen in the air.

Carburettors, to a certain degree, self calibrate for different air density. Denser air will "drag" more fuel into the engine. (which is why PI and some EFI cars struggle going over the Alps. At 3000m the air is about 35% less dense. Carb cars don't really suffer too badly, PI cars are a different matter. My injected spitfire was an issue, so we carried a laptop and changed settings as we went up/down! now fitted barometric correction. EFI also uses intake air temp sensors. And of course, OEM uses MAF, mass air flow, so does everything in one go.)

[NonMember]

Carburettors tend to run rich at altitude, which isn't too much of a problem. Speed-density EFI systems generally run lean, although the PI system is technically vacuum controlled and may go very rich. I've not driven one over the Alps myself. If your EFI is speed-throttle then I'd expect to to go rich without a BAP sensor to compensate. If you have a MAF sensor (not all OEM systems use these, by the way - a lot are MAP-based speed-density) then I think they're largely self-compensating, but they need to know intake air temperature to calibrate the hot wire.

[Pete Lewis]

so for the olde school  ..it goes better on a damp /foggy /wet day ...is down to air density or moisture content that gives the va room  a kick 

 back in the 60s  i made a copper coil round the exhaust to generate some steam/vapour  into the air intake to get an extra horse .  it never worked but the ideas were about back then 

Pete

 

[Colin Lindsay]

27 minutes ago, Pete Lewis said:

so for the olde school  ..it goes better on a damp /foggy /wet day ...is down to air density or moisture content that gives the va room  a kick 

 back in the 60s  i made a copper coil round the exhaust to generate some steam/vapour  into the air intake to get an extra horse .  it never worked but the ideas were about back then 

Pete

 

There WAS a device (I think - no joke - that it was called a 'bomb') back in the 1950s / 60s which you fitted to your car engine and it injected water into the mixture, apparently giving better performance and better fuel economy - and all from water, as the ad claimed.

Now I have to go looking for the ad...

Of course, there are all sorts of modern gimmicks too...

  

 

 

[DerekS]

Back in the early 70's in West Cumbria I met a guy who was a mechanical engineer, he had a Vauxhall Velox or Cresta fitted with a home-made set-up. I can't remember any details other than a copper heat exchanger on the exhaust manifold. Presumably it had some valve or whatever to regulate it. He swore it made a noticeable difference...

[Colin Lindsay]

 

The concept has been around a long time; I wonder how many were just gimmicks, like the unleaded fuel lozenge thingies?

[Clive]

1 hour ago, NonMember said:

Carburettors tend to run rich at altitude, which isn't too much of a problem. Speed-density EFI systems generally run lean, although the PI system is technically vacuum controlled and may go very rich. I've not driven one over the Alps myself. If your EFI is speed-throttle then I'd expect to to go rich without a BAP sensor to compensate. If you have a MAF sensor (not all OEM systems use these, by the way - a lot are MAP-based speed-density) then I think they're largely self-compensating, but they need to know intake air temperature to calibrate the hot wire.

Mine uses Alpha-N (throttle position sensor for load, MAP sensor not ideal with 4 throttle bodies)

When I bought the ECU, there was little choice (at sensible prices) for one that would control cams, but it never occured to me to check it had barometric correction, so did 2 10CR's by fudging injector size and relying on wideband control. Then bought a MAP sensor. Took 10 minutes, literally, to plug in and set up. But miffed as it should have been in the ecu.

My point is that unlike EFI, the change in air density goes some way to mitigating the reduction in oxygen concentration. Not perfect, maybe half way?? (not sure on the maths!)