Air Intake

[Dicky]

Did a bit of a service yesterday and cleaned the throttle valve amongst other things. This may seem daft to the more savey amongst you but I've only just realised that the air intake scoop, the one that pushes into the front of the air filter box, is completely closed off when the bonnet is closed.

Air is obviously available via the front grill although not directly into the air scoop. I had thought the air was in effect rammed into the intake when driving but no. So what's all the fuss about getting more air into the engine to improve performance. It's seems as though the air intake is dependant solely on the vacuum produced in the plenum ?

[KyleR]

I think the point of the snorkel in the 350 is to make cooler air available to the engine that hasn't passed through a hot radiator. There are air ducts you can buy and fit right in front of the snorkel to allow air a direct flow into the filter.

[brillomaster]

ram air effect is pretty minimal, its still only atmospheric pressure.

 

bit of rough maths here... if you hold your hand flat out the window at 70mph, you probably need to apply a force of ~80newtons to keep it steady. say your hand is 8" by 3", that has a total area of 24 sq inches. 80newtons is about 18 lbs, so, pressure from wind is 18/24 = 0.75psi.

 

now bear in mind that even small turbos are operating at at least 8psi, which is 10 times as much pressure as a 70mph wind can provide. its clear how minimal the ram air effect is! (that said, 0.75psi blowing on the side of a 30ft by 30ft house for example, will exert a force of 97,000lbs, which could well blow the house down)

 

the amount of air drawn into cylinders will be mainly decided by the volume of the cylinders, the compression ratio of the engine, and the engine speed. ie a 3 litre engine will consume roughly 50% more air than a 2 litre engine, and an engine that revs to 9000rpm will consume 50% more air than one that revs to 6000rpm.

 

a 3 litre engine will consume roughly 3 litres of air every air every 2 engine revolutions (in a 4 stroke engine) so at 6000rpm, the engine will be consuming 9000 litres of air a minute, or 150 litres a second. maybe. i might be way off, i made that last bit up.

[brillomaster]

hmm, just checked, im out by a factor of 10!

 

wind pressure at 70mph is 12.5 pounds per square foot, which is 0.087 psi. which is 100 times less than a turbo producing 8psi. or in other words, diddly squat.

[Gus-350z]

The other question is whether or not the cooler air (with higher density) makes much difference...

 

Just did a quick back of the envelope calculation and make the difference in air density between temperatures of 30C and 10C (not sure how accurate these are, but bare with me)

 

The difference in air density (I make out) to be 0.082kg/m^3 (just using ideal gas law)

 

Going on Brillo's number of 150ltrs per second this means an extra 0.012kg of air per second which (again I may be wrong here with my calcs)

 

This is 0.088 moles of oxygen... Apparently combustion (this is where the calculation is going to become quite inaccurate) releases roughly 418kJ of energy per mole of oxygen, so in 1 second we release 36784 more joules which corresponds to 36784 watts which is 49bhp!

 

So IN THEORY if the air coming into the engine is 10 degrees C and not 30 degrees C, we should get about 50 more bhp (at 6000rpm) (although these calculations are extremely rough, this is a rough order of magnitude) I doubt the temperature of the air will vary that much, this is for ideal combustion (which we do not have) etc etc so maybe 10 more bhp at most from reducing the intake air temperatures as such

 

But as Brillo said - ram effect is negligible, and do not expect any real noticable power increase from the cooler air, as realistically it's probably already cool enough at the point of intake

 

Although saying that, in my mini I did used to notice it being a bit more "sprightly" on a cold morning, but when you're only dealing with 30bhp to start off with even a couple more will be noticable in a car weighing in at about 600kg

Edited May 24, 2016 by Gus-350z

[brillomaster]

cooler air is definitely noticeable when you're boosted. and i always get better milage when its warm, which i suspect is due to less dense air, therefore reduced ability to burn fuel.

[davey_83]

the air intake isnt going to run out of air, so i wouldnt worry too much

[Gus-350z]

the air intake isnt going to run out of air, so i wouldnt worry too much

 

^^ this at the end of the day

[Dicky]

ram air effect is pretty minimal, its still only atmospheric pressure.

 

bit of rough maths here... if you hold your hand flat out the window at 70mph, you probably need to apply a force of ~80newtons to keep it steady. say your hand is 8" by 3", that has a total area of 24 sq inches. 80newtons is about 18 lbs, so, pressure from wind is 18/24 = 0.75psi.

 

now bear in mind that even small turbos are operating at at least 8psi, which is 10 times as much pressure as a 70mph wind can provide. its clear how minimal the ram air effect is! (that said, 0.75psi blowing on the side of a 30ft by 30ft house for example, will exert a force of 97,000lbs, which could well blow the house down)

 

the amount of air drawn into cylinders will be mainly decided by the volume of the cylinders, the compression ratio of the engine, and the engine speed. ie a 3 litre engine will consume roughly 50% more air than a 2 litre engine, and an engine that revs to 9000rpm will consume 50% more air than one that revs to 6000rpm.

 

a 3 litre engine will consume roughly 3 litres of air every air every 2 engine revolutions (in a 4 stroke engine) so at 6000rpm, the engine will be consuming 9000 litres of air a minute, or 150 litres a second. maybe. i might be way off, i made that last bit up.

 

WOW..my brain hurts.

Very interesting though. So just to be sure I understand..a hand will produce 0.75psi and there are 14 lbs in one stone, so Newtons foot, being 12" long, will increase to 13" if stroked 4 times a minute. YEP I THINK IVE GOT ALL THAT. ha ha

[Dicky]

Just had another daft though while thinking about air density. What if you dropped a plastic bag of ice cubes into the bottom of the air filter box (so you don't obstruct air flow to much) would that then increase performance until they melted?

[brillomaster]

fractionally... after all the new M4 sprays cold water directly into the cylinder heads to cool the air down to make more power i think...

 

and a lot of turbos use water to air intercoolers to cool the intake temps.

 

although just sticking ice in the air filter box wont work so well as air is a terrible conductor of heat, you'd need a proper heat exchange system.... like air conditioning...

 

now theres an idea, plum the nice cold air conditioned air into the intake for the engine...

[Dicky]

fractionally... after all the new M4 sprays cold water directly into the cylinder heads to cool the air down to make more power i think...

 

and a lot of turbos use water to air intercoolers to cool the intake temps.

 

although just sticking ice in the air filter box wont work so well as air is a terrible conductor of heat, you'd need a proper heat exchange system.... like air conditioning...

 

now theres an idea, plum the nice cold air conditioned air into the intake for the engine...

 

Now your talking

[Richf]

 

 

although just sticking ice in the air filter box wont work so well as air is a terrible conductor of heat, you'd need a proper heat exchange system.... like air conditioning...

 

now theres an idea, plum the nice cold air conditioned air into the intake for the engine...

 

http://fiinterchillers.com/

[brillomaster]

interesting... so that system uses the A/C to cool the water in a water to air intercooler?

 

ultimately, its using more fuel to make more power. but its using fuel to drive the a/c, which in turn increases the engines ability to burn fuel by cooling the intake charge. which is all very well and good if just want more power, but don't care about gas mileage (AKA being American).

 

 

[KyleR]

I haven't the foggiest what you'e all on about, I'm away

[brillomaster]

oh here you go, this is essentially putting ice in the airbox... except its dry ice, and looks to be for drag racing, and you need to keep refilling the icebox.

 

http://www.dryiceintercoolers.com/

 

 

[Richf]

interesting... so that system uses the A/C to cool the water in a water to air intercooler?

 

ultimately, its using more fuel to make more power. but its using fuel to drive the a/c, which in turn increases the engines ability to burn fuel by cooling the intake charge. which is all very well and good if just want more power, but don't care about gas mileage (AKA being American).

 

Wouldnt have a great impact on the economy at all , not that that is a prime consideration running a large capacity supercharged engine

 

Its something I have been looking into a fair bit recently

[Dicky]

Thanks for all this interesting input. I don't really have any intention of using ice, even though it's crossed my mind a few times when driving in cold weather, the cars plenty fast enough as it is.

Cleaning the throttle valve just made me wonder, because Ive never really thought about it much and had always assumed the air was forced in when driving at speed. Obviously not, so I stand corrected.

 

[brillomaster]

interesting... so that system uses the A/C to cool the water in a water to air intercooler?

 

ultimately, its using more fuel to make more power. but its using fuel to drive the a/c, which in turn increases the engines ability to burn fuel by cooling the intake charge. which is all very well and good if just want more power, but don't care about gas mileage (AKA being American).

 

Wouldnt have a great impact on the economy at all , not that that is a prime consideration running a large capacity supercharged engine

 

Its something I have been looking into a fair bit recently

 

I spose it makes sense if you already have a water to air heat exchange you can plumb into, beats refilling the tank with ice every so often.

 

but back on topic (ish) is an engine actually any more efficient at turning fuel into power if the intake temps are lower?

[Gus-350z]

oh here you go, this is essentially putting ice in the airbox... except its dry ice, and looks to be for drag racing, and you need to keep refilling the icebox.

 

http://www.dryiceintercoolers.com/

 

Why not go one step further and plumb in some liquid nitrogen Could start using superconducting hardware then which will help with fuel economy

[Gus-350z]

interesting... so that system uses the A/C to cool the water in a water to air intercooler?

 

ultimately, its using more fuel to make more power. but its using fuel to drive the a/c, which in turn increases the engines ability to burn fuel by cooling the intake charge. which is all very well and good if just want more power, but don't care about gas mileage (AKA being American).

 

Wouldnt have a great impact on the economy at all , not that that is a prime consideration running a large capacity supercharged engine

 

Its something I have been looking into a fair bit recently

 

I spose it makes sense if you already have a water to air heat exchange you can plumb into, beats refilling the tank with ice every so often.

 

but back on topic (ish) is an engine actually any more efficient at turning fuel into power if the intake temps are lower?

 

If the intake temps are lower, the air is denser.. denser air -> more O2 -> more O2 = more efficient combustion so yes

 

Although colder engine temps would hinder the combustion so it's a weigh off.. I'd imagine you don't want the intake air to cool your combustion chamber too much

Edited May 24, 2016 by Gus-350z

[brillomaster]

mmmm liquid nitrogen is pretty darn cool... that would certainly be a good way of lowering intake temps! however there may be a limit... probably don't want ice forming

[Gus-350z]

A fairly sizable (sorry) but interesting post taken from a physics forum

 

 

There are a lot factors involved here, but as internal combustion engines follow the Otto or Diesel cycle, those cycles (which are less efficient than the Carnot cycle) are the right model to follow, rather than the Carnot cycle itself.

 

Let's say we reduce the inlet air temperature from 300 Kelvin to 270 Kelvin (90%) while keeping compression ratio and RPM the same.

Now, the engine ingests the same volume of air, but greater mass. This means that a greater mass of fuel will be needed and greater power will be produced.

The compression step remains the same, that is, if before it doubled the temperature from 300K to 600K, it will now double the temperature from 270K to 540K. The combustion chamber pressure before ignition will also be the same.

 

The burning of the fuel will now increase the temperature by a fixed amount, so in the first case it will reach 600+2000=2600K and in the second case it will reach 540+2000=2540K.

 

Expansion will now half the temperature (assuming reversibility and neglecting the minor change in composition of the gas during combustion) so the exhaust temperatures are 1300K and 1270K.

 

What we can see from this theoretical internal combustion engine cycle is that the efficiency of both compression and expansion is determined only by the compression ratio. But also note that a significant proportion of the energy generated in the expansion is needed to drive the compression. The proportion is marginally less at lower temperature (due to the fixed temperature increase provided by combustion) so the engine will theoretically run slightly more efficiently at lower temperature.

 

For identical efficiency, we would need the fuel to increase the temperature by 2000K in the 300K ambient case and 1800K in the 270K ambient case. But as the temperature increase from combustion is independent of the ambient temperature, the lower ambient temperature gives higher efficiency, at least in theory.

There are many practical factors that may affect this analysis. The first is that my exhaust temperatures are extremely high. I can assure you that a temperature increase of 2000K is typical for complete combustion of hydrocarbon in air Reciprocating internal combustion engines do run at close to complete combustion of all the air ingested, so it would appear that there are significant losses to the cooling system during the combustion process.

 

My own experience is more with industrial gas turbines, which have proportionally much less loss to cooling systems, and an exhaust temperature arount 800K. Gas turbines never burn sufficient fuel to use all the oxygen in the inlet air, because the expansion turbine would melt. If suitable materials were available they would use all the oxygen, which would make them a lot more efficient.

 

The next thing to consider is the valve timing. It will be apparent that if we have the same ratio on the compression and expansion stroke, we have effectively the same mass of gas entering the engine at 270-300K and leaving it at a higher temperature. The higher temperature means there is a higher pressure at the end of the expansion stroke, and if the exhaust valve opens too early, about 2-3 bar absolute (1-2 bar gauge) of pressure will be wasted. Although there are from time to time suggestions of ways to improve this situation, including closing the inlet valve late (to reduce compression) and opening the exhaust valve late (to increase the expansion) this is not a common pattern of operation. One reason is that when there is only 1-2 bar gauge pressure left, the amount of friction means that not much shaft power can be usefully extracted. However modern engines are often able to vary their valve timing and this complicates real-world analysis considerably.

 

Thirdly, as per the above analysis an engine will produce more power in colder conditions due to the greater mass of air ingested. If more power is produced and friction stays the same, we would expect a small gain in engine efficiency, too.

 

Finally, let us consider turbocharged engines. Here the pressure in the exhaust gas which would otherwise be wasted is used to drive a turbine, which compresses the air at the engine inlet. The primary motive for this is to increase power by increasing the mass flow into the engine. The work of compression heats the inlet air, so an intercooler is installed between the turbocharger and the engine. The purpose of this is to cool the air back down to ambient to further increase the mass flow of air into the engine.

 

In conclusion, colder conditions produce a measurable increase in engine power (otherwise intercoolers would not be installed on turbo engines.) According to a theoretical cycle analysis they also produce an increase in efficency, though this is likely too small to be measurable, and may be negated or reversed by other factors.

[Gus-350z]

mmmm liquid nitrogen is pretty darn cool... that would certainly be a good way of lowering intake temps! however there may be a limit... probably don't want ice forming

 

Absolutely, that's a problem with planes I believe where icing restricts (and can eventually block) the intake.. which is bad when you're 30,000 feet above the ground

 

My initial idea would be to just have a sleeve around the intake/intercooler which could be kept at a good low temp using bursts of liquid nitrogen... only because it'd probably be "easier" to plumb in and would probably last longer than dry ice

[brillomaster]

you could just set up some something that sprayed liquid nitrogen onto a front mount intercooler... much like the waterspray button on early impreza turbos, but a lot colder