ustolemyname??stevieturbo

iTrader: (1)

While I am not totally familiar with AC systems, im sure that the properties of 'air' are not anywhere near the same as the gas used in AC systems.
You say...A/C Compressor---turbo, A/C Heat exchanger---intercooler, A/C restrictor---Throttle, A/C Expansion chamber---inlet manifold..??? All sound familiar? In an AC system the gas has nowhere to go, after expansion, which will be different. After the air supposedly expands in an engine scenario, it is sucked/blown into the combustion chamber. Not the same. Also the AC gas will have totally different heat/gas/liquid properties than air.
I think that the cooling effect in AC comes from it expanding from a liquid into a gas, and the compressor ccompresses the gas into a liquid, creating heat, which needs dispersing in the radiatior.
Another thought had been using the cooling effect of AC to cool the inlet charge, but the power required to drive the AC pump etc outweighed any gains that would have been created by the cooler charge making it very innefficient.. I did read somwhere that several had tried this approach, but cant remember where.

[Edited by ustolemyname??stevieturbo - 7/3/2002 5:44:14 PM]

submannz

The amount of power loss by running the A/C would be most likely more than the gain you would get.

[Edited by submannz - 7/4/2002 4:09:43 AM]

AndyC_772

iTrader: (2)

A while ago, someone posted the idea of feeding the engine's air intake from the output of the air conditioner, in order to reduce the intake temperature and therefore achieve a denser charge.

It's apparent to me that this wouldn't work (aircon too small and feeble amongst other things), but it set me thinking:

An air conditioner or refrigerator works by compressing air, cooling it back down to (near) ambient temperature with a heat exchanger, then expanding it again. This expansion yields the cooling effect. Taking this cooled air and immediately re-compressing it is a waste of effort.

Now imagine the Scooby's induction system as a big, powerful air conditioner. The compressor is the turbocharger, and the heat exchanger is the intercooler. Run the turbo at a much higher boost pressure than the engine requires, and the air entering the intercooler will be much hotter than usual, so it loses more heat to the atmosphere. Result: very high pressure air at (near) ambient temperature.

Now - and here's the clever (or dumb, you choose!) part:

Use a partially open valve to allow this highly compressed, cool air to expand back to the pressure required by the engine. The intake charge is now at the same pressure as usual, but much colder, because of the cooling effect that takes place when the air expands. It might even be possible to use a little turbine to recover some energy from this expanding air (to run the alternator, perhaps). Bleed off a little, feed it into the cabin and you get aircon for free!

I guess the issue is whether the power gain would offset the extra work required to achieve the 'super-boost' pressure from the turbo. Anyone care to comment?

The total modifications required to the car to run a system like this would amount to a few control valves, reinforced hoses/intercooler, maybe a bigger turbo, and a clever ECU. Open the restrictor valve all the way, reduce the boost from the turbo, and you're back to the original configuration.

Any takers?

Andy.

john banks

Efficiency is the main objection. The turbo would overspeed and run very low efficiency and possibly be damaged.

AndyC_772

iTrader: (2)

Air certainly does have different properties from the refrigerant used in an a/c system. It is, however, possible to use air as a refrigerant, and it works tolerably well; it's just that the CoP (Coefficient of Performance - relationship between the amount of energy fed into the compressor and the amount of heat energy pumped out of the air) is lower than with a (h)CFC refrigerant.

When you compress air, you put energy into it which causes it to get hot. Conversely, expanding it will cause it to cool. This is just conservation of energy.

I do think this scheme could work, although as John points out, a different turbo which is capable of flowing a bit more air at a much higher pressure might be needed. I agree that the question is one of efficiency, whether the power generated by a cooler, denser charge would offset the extra power required to spin the turbo.

Andy.

lokokkee

If I remember my high school physics correctly, over-compressing the intake air will increase the temperature correspondingly and if you then allow it to expand in the intercooler prior to combustion, you are lowering the pressure and temperature back to status quo, no gain, no loss. The idea will only work if you can increase the pressure without raising the temperature, which would be in violation of Charles's and or Boyle's law on gases. May be simplier by just having a bigger intercooler or better airflow over it, as in a FMIC.

The difference between air and aircon's refrigerient is that the latter depends more on latent heat absorption by evaporation, i.e. changing from liquid to gas phase, rather than the mere expansion of air, and hence much more efficient as a coolant volume for volume. As an example, you can bring water to boil in a kettle quite quickly, but it will take a lot more heat energy to turn it all into steam at the same temperature.

Andy.F

Andy's idea neither violates Charles or Boyles law, Compress and heat the air to say 3 bar abs/90C then cool at constant pressure (supplied from turbo) in the intercooler to 40C then futher reduce pressure and temperature via restrictor (expansion valve)to 2 bar abs/10C for engine consumption.
Whether it would actually work or not would depend on being able to make full use of the exhaust gasses which are currently wasted by going down the wastegate hole. If you sized the turbo to use all the exhaust energy it would involve a lot of lag at low rpm.

lokokkee

AndyF, if you increase the boost pressure from 'standard', the temp will have to increase. So if you cool it at constant pressure via the intercooler, keeping the operating environment unchanged (i.e., the airflow and ambient temperature) the cooled air will still necessarily be at a higher temperature than 'standard', unless the steeper temperature gradient can result in a higher heat extraction rate and the IC can bring the charge temp down to 'standard'. Imagine cooling a room from 40 degree C. at a higher pressure to 25 degree with the same aircon compared to say from 35 degree at a lower pressure, it is definitely going to take longer unless you increase the capacity of the aircon. The heat content of the air is proportional to the pressure and temperature.

AndyC_772

iTrader: (2)

The whole point is that the higher temperature of the air entering the intercooler results in a higher rate of heat extraction. If the air temperature entering the intercooler is twice as far above ambient as in the unmodified case, then heat will be removed from it at (almost) twice the rate, provided that the supply of cooling air remains adequate. It is this additional heat removal that ultimately leads to the cooler charge.

Effective what we're doing is allowing the existing intercooler to achieve a greater reduction in intake temperature, by ensuring that there's a larger temperature gradient between the air flowing through it and the cooling air flowing over the fins.

This differs from having a larger intercooler in the first place, because the intake charge temperature is no longer limited to ambient or above - when the air is allowed to expand again it can fall below ambient. There could even be an issue with freezing

Andy.

[Edited by AndyC_772 - 7/5/2002 8:47:39 AM]

lokokkee

Andy, the limiting factor according to your assumption is the adequate supply of air to the IC. If the IC is 100% efficient, the charge temp after passing through the IC will be the ambient temp of the outside airflow, regardless of inlet temp. I have seen somewhere in other threads that the IC charge temp rises at higher rpm, thus indicating that IC cooling is a limiting factor.