Re: How do i make the air colder using dry ice

Salis,

Thanks for writing.  So you want to chill the intake air on your formula 
car?  You are correct in assuming that a cooler air charge can help the 
performance of your vehicle.  First, since cooler air is denser, you can 
ingest more oxygen per cylinder event, mix it with more fuel, and achieve 
improved engine output.  Also, the cooler air will help reduce the engine's 
sensitivity to knock, allowing you to possible run a more aggressive and 
better performing spark advance.  But actually cooling your air charge can 
be difficult.  

You said that you would like to target a -100C air charge using dry ice to 
cool the air.  First of all, dry ice surface temperature is only about 
-75C, so there's no way you'll be able to reach -100.  Even so, when your 
intake air gets that cold, it may actually start causing you new problems 
rather than just giving you a cool, dense charge.  Air that cold could 
definitely hurt the fuel's ability to vaporize in the intake, and you will 
definitely ingest more liquid fuel, which may not even combust, which will 
hurt your performance.  You could always just use more fuel, but this will 
hurt your fuel economy, and that can be important during races as well.  

There are also some different considerations depending on whether you are 
using a carburetor or fuel injection.  With a fuel injection system, 
hopefully you will have some method of either measuring air mass or inlet 
air temperature in order to determine the actual air mass (which will 
change with temperature) in order to deliver the correct amount of fuel.  
With a carburetor, your fuel delivery is not as easy to control, and you 
may end up either running lean with the cold air or rich when you're not 
cooling.  Also, the problem of fuel vaporization could become more apparent 
with a carbureted system which usually depends on a high intake manifold 
temperature in order to vaporize the fuel, and you're likely to have a 
cooler intake manifold if you're air charge is that cold.

Now that I've given you a couple of considerations for your air cooling 
idea, let's see if I can answer your question, how to actually performing 
the task of cooling the air.  You're obviously going to need some sort of 
heat exchanger in your intake system.  This could be a simple air to 
dry-ice heat exchange, or you could make a radiator-type heat exchanger.  
What you'll probably want to do is determine your engine air flow 
requirements (a simple calculation based on engine displacement and speed) 
and calculate how much heat you need to extract from that mass of air to 
achieve your desired air temperature.  Then you can calculate what kind of 
surface area of dry ice to air that you will need to achieve that 
temperature drop.  You're likely to find that it will take a lot of dry ice 
to significantly drop your inlet air temperature.  In your question you 
asked, "once it has reached a certain temperature how do I maintain it?"  
Basically, you can't.  The engine is constantly ingesting fresh air, and it 
will constantly need to be cooled if you want to continue to have cool air 
in your intake.  

You could also try to develop a radiator-type heat exchanger (which will 
give you greater heat-exchange area and increase your cooling capacity), 
but now you have to find a way to continually cool the radiator.  Dry ice 
may not work too well as the working fluid in the radiator, and you'll 
probably have some type of pump, which would be driven by the engine, and 
of course cost performance.  

Now maybe you're starting to see why inlet air cooling is not very common 
in cars.  There are definitely a lot of things to consider, and it's not as 
easy as it sounds.  But you never know, I've haven't gone through the 
calculations myself to see how much dry ice you would need to drop the air 
temperature enough to be helpful.  You may want to look into books on heat 
transfer to help you with some of the details.  If you're not a mechanical 
engineer, hopefully you can find one on your team who may have a book on 
heat transfer.  Let me know if you have any other questions.  Good luck on 
your car.  

Your Mad Scientist,
Mike Scannell