Supercharger theory
Turbo Mini Forums

The way i see it is that by simple thermodynamics the temperature of air increases when under compression.

When a working fluid moves from a high pressure to a low pressure it rapidly expands and in doing so absorbs heat. (This is why the deodrant can or other aerosol goes cold when used for more than a few seconds)

By the time the air has reached the supercharger and been compressed it will have increased in temp significantly. The air would then enter the engine at said temp. On injecting the fuel (via direct injection) which would instantly vapourise it would absorb heat from the air but this thermal energy remains in the fuel and in the cylinder no matter how well mixed the air and fuel are.

If the fuel was injected prior to compression then wouldnt it have the same effect? the nett thermal energy from the air and fuel in to the engine is exactly the same but the charger has to work a little differently to compress the fuel as well as the air.

Having said that if the fuel is compressed in the charger then would it expand more in the inlet and cylinder thus removing more energy from the air? but surely then the thermal energy is still carried in the fuel which is then forced in to the cylinder anyway so once again the thermal energy in is exactly the same! Shit im confusing myself now!

Edited by Andy500 on 26th Mar, 2009.

Thinking about it, i guess the fuel would have an overall cooling effect to intake temperatures. As the fuel would absorb heat energy which would be converted to pressure.....something along the lines of p1/p2=v1/v2=t1/t2

Andy, you touched on it in the post above.

What you get at the port on the head is the average temperature of both the fuel and the air, whether the fuel is added before or after the charger, no 'heat' has been removed. You have simply reduced the air temp and increased the fuel temp.

A high pressure gas going through a huge pressure drop goes through an endothermal process so the higher the initial pressure, the more cooling you will get.

Compressed gasses are stored energy. As a gas is compressed, its temperature will rise. If the compressed gas was not allowed to cool and then returned to its original pressure, the temperature would be the same as it was before it was compressed. Now then. If you compress the gas and allow it to cool to room temperature, then reduce its pressure, the gas needs to absorb heat energy from its suroundings in order to do so, or, its temperature drops. this is the basic theory of intercooling, you have removed heat energy.

without an intercooler, the engine still needs air and fuel, if you do not remove any heat energy from it, what goes in pretty much is what comes out, well you actualy get a little more heat coming out due to the work done by the compressor and heat soak.

What we as engine builders are interested in is cooling the inlet charge sufficiently to increase its density therefore pack more oxygen and fuel into the cylinder, however, its primeraly to prevent the engine from suffering detonation caused by elevated inlet charge temperatures caused by compressing the inlet charge in the first instance, the bonus is that you get more of a bang in the cylinder

Edited by Sprocket on 27th Mar, 2009.

There is an element of Le Chatelier's principle at work.

Consider Fuel(l) <=>Fuel(g).

Under normal cirumstances, there will be an equilbrium between the 2 states. Altering the P T or V will by the old PV=nRT (at best) will cause the fuel to evaporate under reduced pressure and absorb heat. When this pressure is increased, the thermal energy of the charge is greater and more heat has to be got rid of. There is also more Fuel(g) taking up space that air could occupy.

With a suck-through blower, where there is a pressure drop between the blower and the carb (overrun/idle) there will be fuel evaporation, charge cooling which will cause heat to flow from the surroundings into the pipe linking the carb and the blower.

It means very little, but you could lag it to prevent the dT causing a net heat flow into the charge.

The amount of fuel evaporating will be small due to the short amount of time it has to do so. Most of the cooling will be to the pressure drop.

The added bonus of suck through with such a crap blower such as the rootes, is that the rotors will be wet and seal better, increasing the efficiency due to less recirculation, particularly at higher dP's.

Bugger off, I'm getting there.

Ah, just had a thought.

Fuel is a sub cooled liquid. As it boils and changes state from a liquid to a vapour, it has to absorb heat as latent heat. This is the stored energy bit. so there will be a beneficial cooling effect, perhaps the chagrer may run a little cooler with the fuel added before it. The only down side I can see with that is that you are using the fuel to cool the charger, rather than allowing the charger to reject its own heat from its casing. aslo I can see a negative aspect to the fuel in the charger itself, and that is erossion and wear of the rotors.

Edited by Sprocket on 27th Mar, 2009.

little bit from wiki

http://en.wikipedia.org/wiki/Supercharger


In drag racing applications where large volumes of fuel are injected with that hot air, vaporizing the fuel absorbs the heat. This functions as a kind of liquid after cooler system and goes a long way to negating the inefficiency of the roots design in that application.

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Interesting stuff. I don't understand the science behind it, but my (very simple) understanding of this is that the air that is fed into the carb and then of through the suck through s/c with the fuel needs to be as cool as possible before getting mixed with the fuel?

So having the air filter right down at the front of the car in the airflow will help slightly with efficiency??

A more active discuaaion than I was expecting. Still not convinced I now know the answer.

But it seems that the fuel will aid cooling by making the blower more efficient. However, any direct cooling between carb and blower is likely to only have benefits at idle.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

In my opinion, wherever there is a pressure drop between the rotors and the carb, you want to prevent heat ingress. (Part throttle settings). You also want to maximise heat loss between the blower and the head.

I'd go as far to say that you don't want the fuel evaporating in the carb-blower pipe as when comressed on the other side, it will not "re-condense" but will take up place as vapour which will add to the dP across the blower.

Although the pipe feels cold, this is not a good thing as there will be a net flow of heat into the pipe. If you are concerned with every last hp (you wouldn't be using an Eaton Blower anyway) this may allow a tiny fraction more advance..... but it's a bit of a waste of time really.

Besides, if you were concerned about performance, you also wouldn't be considering suck through. (eaton themselves have mentioned that the rotor coatings are not compatible with fuel).

Suck through is a vintage lash up and whilst you can get about 150hp/L using a marshall cabin blower and avgas, a decent compressor and blow through would achieve better results with the same fuel.

I like blowers and a shed job is to "shorrocks it" with a blower you got off ebay.

anyway..... :)

Bugger off, I'm getting there.

I like te look of those G Lader jobs, they look the ticket, but can you get one small enough

the g laders are a bit fraggle though, seem quite pricey too.

get a rotrex, 10 times better:)

internationally known as "big" swede

The G40 Polo used a G-ladder, and that was 1300 iirc.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I used a Ladder to get in my loft yesterday..

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

fat birds use a g ladder to get their knickers on!

and the knickers are G ropes!