Ram Air - forced induction?

[towcester]

Originally posted by paul_f
RAM air doesn't really work. The increae in power in massively offset by the increase in wind resistance. If it worked then all car manufacturers would use it.

The only thing for inlet that does help is variable length, and only a very few engines have this (high revving hondas etc)

Paul

??????????????

paul, how do you work this out ? how does wind resistance increase ?

if i cut a hole in the front of my bus and put a duct in that allows some air through(albeit into the engine) then wind resistance has reduced.

if the pressure at the throttle body increases so that there is positive pressure (and the german charts suggest a 100% increase in cfm-though what this relates to in psi is highly debatable), even if its only by a few psi this is will make a noticable HP increase when combined with the efficiency of cooled charge for something completely non mechanical for about £20 worth of duct and gaffer tape.

put your hand out of the window at 60+mph and try and hold it flat... pipe that pressure into the engine for peanuts and it's got to help.

if ram air didn't work then none of the performance cars would do it and the bonnet/nacr scoop would not exist... IMHO

[Neil H.A.B]

Would it be a good idea to have the air coming into the front of the bus being duvcted through a chamber that has cold water sitting in the bottom, just to cool the air as it passes over?

Just a thought...

[towcester]

could run it through a charge cooler on the way in, the pressure increase might work out greater than the velocity reduction of the charge cooler ?

BTW - i'm just guessing on all this to see if i can work out what the actual figures are. my engine's basically a big airpump and i'm convinced forced induction is the way to go so i'm trying to figure every angle on this one

[Neil H.A.B]

Ooh, charge coolers - I start to go a little bit hazy in my knowledge there! I was jsut thinking about a method of passing air over a cool surface!

[**ANDY**]

Originally posted by paul_f
RAM air doesn't really work. The increae in power in massively offset by the increase in wind resistance. If it worked then all car manufacturers would use it.

The only thing for inlet that does help is variable length, and only a very few engines have this (high revving hondas etc)

Paul

The car makers do try to get ram air effect, just look under the bonnet of most modern cars.

The vtec honda's are messing with valve lift not variable length. Loads of modern car use the variable lift but do not use it the way honda does. Loads of cars are using varialble manifolds.

[paul_f]

Originally posted by towcester
??????????????

paul, how do you work this out ? how does wind resistance increase ?



Its something we discussed at work and a guy found some technical papers about why it doesn't work. I can only remember sketch details however here goes

However you have kind of answered the question for yourself :


put your hand out of the window at 60+mph and try and hold it flat... pipe that pressure into the engine for peanuts and it's got to help.

if ram air didn't work then none of the performance cars would do it and the bonnet/nacr scoop would not exist... IMHO


So holding out your arm you feel this pressure. However your engine also feels this pressure, which is why if you fit an empty roof rack to your car or drive with the window open the fuel consumption increases by around 15% (window open increases drag due to pressure differentials)

Now if you drive in a bus with safaris with reasonable door and window seals. Drive along with the side window open you get a lot of buffeting - a lot of volume of air is passing through the bus and out of the window. If you then shut this window the amount of buffeting decreases - this is due to a pressure increase within the bus.

So the amount of torque the engine will require to keep at a constant speed will increase.

You need to be a fluid dynamicist to fully understand how air passes over a body, but it travels over smooth surfaces better than it does over voids where you can get pressure build ups. There a few race cars now with holes at the rear of the front wings - to prevent pressure build up in these areas.

Now with a ram air system you will get a pressure increase within the tubing, and therefore your manifold pressure will increase and therefore the power will rise.

But you would also get an increase in drag due to the badly explained reasons above.

Now as your pipe lengths would be long this would cause even more of a problem.

The ram air systems on formula 1 cars a few years ago were carefully designed to minimise drag but maximise pressure. There is no way you could do this on your bus unless you have around around £10000 a day to do testing in a wind tunnel.

Most performance cars with grills, or naca ducts or whatever are just there for the boy racer appeal, these sort of things are added by the design department not the engine teams.

To have a correctly working ram air design you would have to have very close working with the design teams and the engine teams and the integration teams which really isn't going to happen. On a bike it is more likely though.

It is very difficult to perform ram air tests anywhere than in a car on the road, all engine calibration work has to take place on engine dynos and then chassis dynoed.

Cold air in the engine bay is another matter though, and when you have a engine with a high output cooling it becomes a problem so the more air you get in the better you can cool the engine.



if i cut a hole in the front of my bus and put a duct in that allows some air through(albeit into the engine) then wind resistance has reduced.


It won't always go into the engine, you would reach a certain pressure and then hit a point where the pressure increases too high


even if its only by a few psi this is will make a noticable HP increase when combined with the efficiency of cooled charge for something completely non mechanical for about £20 worth of duct and gaffer tape.

Try it but the only improvement will be in your mind - unless you manage to prove me otherwise on the strip

[paul_f]

Originally posted by **ANDY**
The car makers do try to get ram air effect, just look under the bonnet of most modern cars.

They are trying to get cold air not ram air.

[towcester]

Originally posted by paul_f
if i cut a hole in the front of my bus and put a duct in that allows some air through(albeit into the engine) then wind resistance has reduced.


It won't always go into the engine, you would reach a certain pressure and then hit a point where the pressure increases too high
surely this will be boost at the manifold then ?

Try it but the only improvement will be in your mind - unless you manage to prove me otherwise on the strip

you raise some very good points, as i said earlier I don't know about this stuff but am trying to find the 'truth' of it all.

from some recent reading ive been going through that delves into it fairly well (find it here)

it looks like this (sorry if i summarise this wrong..)

atmostpheric pressure (1bar) = 14.65psi. when your N/A engine starts running it creates MAP with a theoretical max of 14.65 psi (at some point in the rev range) but due to practical inefficiencies usually operates up to 2.0psi below 14.65 psi (atmos pressure) but as the revs climb this NEGATIVE pressure at the manifold increases. a ram air system has two qualities, cfm & psi. given that air is feeding from the highest pressure point of the front of the car (say +0.4psi @50mph) and supplies sufficient cfm (not a problem) then the manifold pressure returns to 14.65psi (1bar)

there is no theoretical boost beyond atmos pressure here but the actual increase at the manifold could be as high as 2psi...

(some figures i've seen state up to +0.176 psi possible from standard RAM system (ignoring (as you rightly point out very clever fluid dynamic designing to create pressure multiplyers in the system)

so, if my engine develops 200bhp with manifold pressure of say 13.65psi (only a 1psi loss)
then i'm developing 14.65HP per psi

so if i can flow enough air through the ram system to get back to normal atmospheric pressure (+1psi) then i've gained 14.65HP. if i can get +ve pressure at the manifold of 0.175psi then i've added another 2,5hp to the figure.

if i add in the cooled charge effect (apparently between 1.5-2bhp for every 6-10 degree drop in charge temp)
-in the bus the air is drawn straight in from the engine bay (and it gets WARM in there) from the ECU temp monitor in the engine bay i could be looking at 20degrees or more then i can add another 3-4bhp to the equation.

so
14.65 HP for return to 1bar at manifold by increased cfm
+ 2.5 HP for 0.176psi increase in MAP
+3HP increase for cooled charge =

20.15hp increase !
for the cost of a length of duct and some gaffer tape...


as a BIG caveat, i agree totally with paul about the problems of actually designing this to work efficiently at all, and the figures above are fairly loose and based on the outline of very few other sources BUT! i think i understand where the power MIGHT come from. based on this i think i might be doing some more investigation

revisiting this, if i guessed my engine made 220hp and lost 2psi at the manifold (17.4 hp /psi) then the ram system could add 34.8HP even without making 'any' boost at the manifold ! (oh, forgot to mention this all doesn't start to happen until 45+mph) one additional thing though, as map pressure drops at gear changes the ram system is supposed to compensate even more... ??

[towcester]

http://www.vararam.com/images/tech/sebring01.jpg

a quick pic of some performance cars with grills, naca ducts or whatever that are just there for the boy racer appeal, these sort of things have been added by the design department not the engine teams.

[towcester]

read an interesting article here from a mini tuning/racing forum that knocks the ram air thing completely...

BUT

here are two telling paragraphs from it..
---
Anybody that has stuck his or her hand out the window of a moving car has felt the seemingly immense pressure it creates against your hand. Essentially there are two factors that affect this 'ram-air' pressure - air speed and density. The pressure changes in proportion to density, but changes at the SQUARE of speed. This means that when a ram-induction equipped car doubles it's speed, the ram-pressure generated increase by a factor of four. Intense! Now the downer. There is a formula to illustrate/assess this, but take it from me - a vehicle traveling at 100mph under ideal conditions only generates 0.177psi of ram effect pressure. ThatÌs 1.22% more than normal atmospheric pressure. You therefore need to be doing well in excess of 200mph to achieve 1psi of pressure. Not the territory of your common or garden Mini. Nor even your super-tuned one, come to that (as, brick, a, aerodynamic - rearrange to form a common phrase associated with Minis). So that's the 'hand-out-the-window' syndrome explained.
---
Air Density increases as temperature drops - roughly speaking 3.3 deg C will see a change of around 1% in density. Production engine intake charge temperatures are regulated to about 80-85 deg C for maximum all round efficiency. Feeding cooler air in from outside the under-bonnet environment can see that dropped to around half that. That's an increase of air density in excess of 10%, which gives a similar effect to around nearly 2psi of supercharging pressure.

what this seems to say to me is that i underestimated HUGELY the performance gains from cooling the charge (by my calculations a 2psi effect means 28 HP more !)

and also the writer agrees with the +ve pressure boost (but assumes that the engine always works at 100% efficiency) and doesn't look at the HP/psi figure...


just think about that hp/psi figure i was feeling a bit sceptical about it but if i were to run my 200hp engine with a turbo at 10psi boost i would be looking to make 300-350 bhp (apparently) so maybe the calculation holds true in real life!