A/C cooled air charge, has it been tried?

Discussion in 'Diesel Engines' started by CDK, May 26, 2012.

  1. CDK
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    CDK retired engineer

    My wife complains about the icy air whenever the air con in my Kia Sorento tries to reach the set temperature on hot days. And she has a point, because sometimes the side windows show frost when the car is parked.

    It made me think about using an A/C unit to reduce the intake air temperature in both normally aspiring or turbo charged diesels. Lowering the temperature by 30-40 C. increases the air density and allows more fuel to be injected.
    I know the air con uses a few HP, but my gut feeling says there must be a considerable net gain.

    Has it been tried?
    1 person likes this.
  2. baeckmo
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    baeckmo Hydrodynamics

    Yes it has, ....sort of. If you install an expander or turbine before the engine, you will have a reduced inlet temp (and unfortunately a reduced inlet pressure as well). There were some attempts to optimize this for Brayton cycle engines (~gas turbines), but they failed. The temp reduction did not compensate for the losses in other parts of the engine cycle. Putting a heat pump in front of the engine mans that you would have to reject low temperature heat somewhere, instead of using it in the engine process.

    It's only a cooling of the compression phase that can be done effectively IRL.

    I guess the solution to your comfort problem would be to find a better position for the temp sensor in the AC system; it should be placed closer to the cold air nozzle. There it would be easier to make it "consider" the air mix effects.
  3. CatBuilder

    CatBuilder Previous Member

    I used to have the same problem in one of my vehicles. Check to see if the Kia allows you to use air conditioning and heat at the same time.

    If you put the "red/blue" temperature selector about 1/4 of the way from blue to red, you can knock down the icy cold feeling and still get adequate cooling.

    Seems silly to suggest, but being from a cold climate, I had never fully understood this until spending some time in FL.

    It also works great to keep the inside of the vehicle dry and comfortable on damp, foggy nights with just a little bit more heat applied.

    It seems wasteful, but of course the AC uses very little power and you have the compressor running anyway. Adding in some heat that would be lost to the radiator is no big deal.
  4. CDK
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    CDK retired engineer

    I mentioned the Kia only to illustrate the capability of the AC. That made me think of using it to improve the performance of a diesel engine; not the one in my vehicle, it is powerful enough.

    It is quite logical that a large, black vehicle parked in the sun, needs a lot of very cold air to achieve a comfortable temperature. I've explained to my wife that there is a little Korean embedded in the AC control unit. He takes his task very seriously and does what is necessary to get the temperature I've selected: full duty cycle for the compressor, fan speed at maximum, reducing both in small steps when the desired temp is within range. I can easily switch to manual, but the Korean does his job quite well.

    Turbo charged diesels usually have an air charger cooler or inter-cooler to lower the intake temperature. It uses ambient air, which is not a very effective way to do that, especially on a hot day. Using the evaporator of an AC unit might be a way to further increase the engine's peak output. The turbine waste gate should be set to a higher pressure to compensate for the cooling effect.
    In theory, the air mass density can increase by 15-20%, boosting the engine output by almost the same percentage. Or it can be used to increase torque at lower rpm.
  5. groper
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    groper Senior Member

    you need to remember that a large engine "breathes" a hell of alot of air, much more than is passed by the small evaporator fan in your AC unit... a typical after cooler or intercooler (depending on where your from) has a similar surface area to the engine radiator to get a reasonable job of cooling the charge air down after its been compressed (and heated up in the process) by the turbo. Id hazard a guess that a very large AC system would be required to cool this volume of air substantially below what ambient air will given sufficient surface area...

    Its no secret... cooling the charge air gives significant improvements to power output.

    But on a boat, why use an AC system when you can do the same with practically zero energy penalty by using free energy from the sea, running a seawater to air heat exchanger/intercooler?

    Drag racers even go to extreme length as running dry ice intercoolers... they only need the cooling for 10 secs or less so it works for them... injecting NOS cools the charge air also with the added bonus of extra oxygen at the same time. Diesel / gas injection systems have a similar effect, as the LPG expands in the inlet manifold it cools the charge air with it...
  6. Frosty

    Frosty Previous Member

    Its not bloody frost its condensation.
  7. CDK
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    CDK retired engineer

    Thank you Groper, that is useful information, especially the drag racer part.

    A typical small diesel with 2 ltr. displacement needs approx. 600 CFM at 3000 rpm. I don't know how that compares with the evaporator fan in the car, but it certainly isn't small because last year there was a rat's nest in it with a litter of at least 8, judging from the amount of meat loaf I had to clean out.
    But I will try to measure the air flow at full speed.....
  8. Frosty

    Frosty Previous Member

    A 600cfm Holley will feed a 454 inch chevy. I dont think they make a bigger carb.
  9. mydauphin
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    mydauphin Senior Member

    You have discovered the intercooler. But who said intercooler are ambient air cooled only.
    When I was a kid, we made air boxes that we put either dry ice or plain ice to chill air coming into carb. Water injection also did similar things. Problem was frosting in carbs, which we solved by keeping carb hot with stock water passage under carb.

    Frosty in most performance cars, we used 600-650 cfm on a 350 and 700-800 on a large block. But it 600+hp engines the carbs were over 1000cfms. Figure 200cfm per 100 hp.

    Diesel non-turbo figure about 1.6 that, with turbo, 2 times that.

    What I have seen diesel guys do, is inject propane , into diesel. Serves to cool intake and fuel, it is also not that explosive, so it won't blow engine. Nitros does not work that good on diesels.
  10. IMP-ish
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    IMP-ish powerboater

    They make an 850 cfm Holley 4150 http://www.holley.com/0-80443.asp
  11. WestVanHan
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    WestVanHan Not a Senior Member

    I thought of this when I was a kid like 25 years ago.

    One thing I read in recent years is that if there is a leak and it is burned in the engine,the refrigerant creates a very deadly gas- being R12 or 134 I don't know.

    Also in recent years an auto manufacturer (Ford?) was looking into it-so far nothing.

    I guess if using propane as a refrigerant,it wouldn't be a horrible event.
  12. Frosty

    Frosty Previous Member

    Hp is not exactly relative to engine capacity, a 350 ci is 5.4 liters. Thats nearly 3 times the size of a golf engine

    A vw golf engine is NOT a high performance flow engine and runs at less than half the RPM which is equal to air flow.

    Using your figures
    600 cfm - 1/2 =300

    ( 300/ 3)= 100 cfm + x2 for turbo =200 cfm
  13. CDK
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    CDK retired engineer

    That was R12 (Freon), combustion turns it into phosgene, a powerful chemical warfare gas. At the time that was a good reason not to pursue the matter, but nowadays there are safe refrigerants and very efficient compressors. With propane or isopropane an evaporator leak is harmless.
    A modern diesel with digital injection already has an air mass sensor that governs the maximum amount of fuel to be injected. If refrigeration fails, the engine power drops and a warning light informs the driver why.
  14. CWTeebs
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    CWTeebs AnomalyGenerator

    Is your goal to increase power or increase plant thermal efficiency? In a steam plant you've gotta be careful about dropping the temperature too much because of cold corrosion (usually bypass until the plant comes up to temp).

  15. brehm62
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    brehm62 Junior Member

    You are talking about two different things.

    During WWII aircraft would fly at high altitude and experience tremendous loss of power. Superchargers and turbochargers were used to get the manifold pressure back up to 75% of sea level (what you would normally see at about 7,000 ft). There is plenty of cold high velocity ambient air available when you are cruising at 250 mph at 25,000 ft so using an intercooler is not much of a problem.

    But when you start talking about using a supercharger on a drag racer you are talking about something very different. Boosting the air pressure above sea level has the same effect as increasing the compression ratio. During the compression stroke you get adiabatic heating and this will cause pre-detonation without a high octane fuel. The only difference between adiabatic heating inside the cylinder and that which takes place in a supercharger is that theoretically you can put an intercooler in between and reduce the temperature. But that's only in theory. When the velocity of a drag racer reaches 200 mph there is plenty of free stream air available to cool an intercooler. Unfortunately, adiabatic heating occurs just as soon as engine rpm increases and the engine will have been destroyed long before the vehicle reaches 200 mph. The solution is to pre-load the cooler so that it can provide cooling without high velocity free stream air. It's interesting that dry ice was mentioned because dry ice doesn't really work very well during the race. This is because expanding CO2 will push the solid dry ice away from the metal surface. This means that almost all cooling from dry ice comes from the cold mass of the exchanger itself. You run into a similar problem with liquid nitrogen cooling where boil off of the nitrogen again pushes the cold liquid away from the surface (Leidenfrost effect).

    In other words, drag race engines do not use cooling to increase power; they use cooling to prevent detonation caused by adiabatic heating. The increase in power comes from the increase in compression ratio. When you start talking about using an air conditioner to cool the incoming air you have to ask why you would do it. Can you get more power by having a cooler air charge? Yes, this is because the maximum outgoing temperature is limited by the softening point of the exhaust valves. And power produced is based on the difference between incoming temperature and outgoing temperature (Carnot efficiency). However, it can also be shown that the increase in Carnot efficiency is net negative. In other words it will require more power to reduce the temperature than the power gained.

    I've noticed a few other exaggerations. There is some cooling effect from evaporating fuel but it is tiny, not enough to significantly change power output. This is also true of water injection. I think people get these confused with back end effects of increasing compression ratio. For example, if you keep increasing the compression ratio then at some point you will burn the exhaust valves. You can fix this by using hollow valves filed with sodium or by using titanium valves. The other way of fixing it would be by reducing the exhaust gas temperature. If you run the mixture rich then you produce more carbon monoxide. The chemical reaction to produce carbon monoxide rather than carbon dioxide absorbs energy and this cools the gas. Water has a very high specific heat which means that it takes more energy to raise the temperature of water by 1 degree than it does to raise the temperature of almost any other substance. Water requires more energy to warm up so it absorbs more energy and results in cooler exhaust. Again, these things do not directly increase energy output; they are ways of dealing with the negative effects of increasing compression ratio.
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