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

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

  1. CWTeebs
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    CWTeebs AnomalyGenerator

    Ships do use charge air coolers, it's just normally cooled with jacket water. My guess is that you can use AC for this, at best it will increase your power at the cost of decreased plant thermal efficiency, because you're spending energy on a compression process (ironically similar to Brayton cycle).

    My understanding was that loss of power at high elevations was decrease in the mass flow variable, but the context I learned this in was the thermal analysis of power generation plants (e.g. a why power plant in the Rockies is less efficient than a similar plant at sea level).

    Now I'm looking for my thermo and marine engineering books...you guys got me thinking, damn you!
     
  2. CDK
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    CDK retired engineer

    I'm merely playing with an idea. If the response points in the right direction, I'll invest some time and money in a prototype or test setup.
    If it has already been tested and rejected because of legitimate technical issues, I'll forget I brought it up.
     
  3. CWTeebs
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    CWTeebs AnomalyGenerator

    To be blunt, my guess is it's already been tried and rejected (it's ultimately not a great idea IMHO).
     
    Last edited: May 27, 2012
  4. Yellowjacket
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    Yellowjacket Senior Member

    The cooler the charge, the less work it takes to compress the gas. In addition, the volume of the charge is increased, and finally the lower temperature at the time the charge is ignited, the potential for detonation is reduced. All of these things are good, but you can't get something for nothing, and the amount of work it takes to cool the charge is more than you can get back out from the engine cycle.

    A point about water injection. Water is very good at suppressing detonation. Not only does vaporizing the water in the manifold reduce the temperature of the incoming charge, but the vaporized water in the cylinder boils during compression and this reduces the charge temperature and this reduces the tendency to detonate. In addition, during combustion, the water helps control flame speed and prevents the charge from detonating.

    Water/alcohol injection was used very successfully in fighter engines in WWII, the alcohol was mixed in to prevent the mixture from freezing at high altitude as well as richen the mixture to reduce combustion temperatures.

    I am surprised that some auto companines haven't used water injection to boost fuel mileage and prevent detonation in turbocharged or high performance engines. It's actually a very cost effective way to allow higher compression ratios and improve overall engine efficiency. At high power or under boost, you would inject, but at low power you don't want or need it. Oldsmobile had a turbocharged small v8 in the 60's that used water injection and there were problems with corrosion and control issues. With modern control systems, as well as with plastic manifolds, and more corrosion resistant materials water injection could substantially reduce fuel consumption.

    In some of the testing I saw in the early 70's, the total power of the engine running much higher compression ratios was the same as the lower compression engine, but the amount of fuel was reduced by 50%, and the cooling required from the engine was also substantially reduced.
     
  5. CDK
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    CDK retired engineer

    I've considered only small diesel engines. Detonation is no issue there because there is no fuel present in the compression stroke. Of course you get nothing for free, but if sacrificing a few HP can increase the engine output by 20% it is worth it. And without the inter-cooler the engine could be much more compact.
     
  6. FAST FRED
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    FAST FRED Senior Member

    An intake air cooler need not be fed with engine temperature coolant.

    Most of the water we operate in is far cooler than 180F , and the supply is unlimited.

    So the question then becomes is the intake air flow slow down and disruption from an intercooler compensated for by cooling the intake a bit?

    Perhaps simply engineering a better engine room vent system might lower the intake temps , with no engine air flow disruption.

    FF
     
  7. Yellowjacket
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    Yellowjacket Senior Member

    Most if not all intercoolers use coolant that isn't pre-heated water from the cooling system since the turbocharger outlet temp is close to coolant temp. If water from around the boat is used first in the intercooler it can then be used in the cooling system, but intercooling is very important to a turbocharged engine running at high power. Autos can get away with limited inercooling since they don't run at high boost for a sustained period of time. They can have some themal mass in the intercooler that will allow high boost for a while and then they would cut back the boost after a minute or so. That approach won't work on a boat where you run high power all the time. Most marine diesel installations use cool water in the intercooler first, and then, if they don't dump it, they use it in cooling the engine.

    The pressure loss in a liquid to air intercooler is relatively low. Modern designs are capable of a pressure loss of less than 5%, and the technology is there to only lose 2% if you want to get sexy. Considering that the temperature drop of a highly effective intercooler can drop the charge temperature to well under 150F, it's always going to be more effective to employ an intercooler.

    And engine room venting doesn't have much to do with it. The compression of the air in the turbocharger makes the heat. While where you start is important, the temperature of compression is where the heat comes from, and that's just a property of the process. Find an on-line compressor calculator and punch in 1.5 atm and look for the temperature rise. At 4 atm the temperature at the compressor discharge is going to be over 400 degrees.
     
  8. groper
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    groper Senior Member

    Agree with nearly everything you said...

    The cooler charge air (however it is obtained) is denser, thus effectively increases the quantity of air the engine is moving - which directly relates to HP produced. So yes, drag racing engines do use cooling to prevent detonation from the compressed (and heated) charge air AND increase power from a given capacity engine. So engine preservation / longevity is not the only reason to cool the air, but ALSO maximize the power for a given capacity engine. i think you were essentially saying the same thing via teh compression ratio...

    You point on the carnot efficiency is the main point however, whatever power you take from the fly wheel to spin a AC system to cool the charge air will always be a net loss in total power gained - it has to be, or we would have invented free energy for eternal world peace in the process...

    However, sea water is free, so your best plan is to use a good seawater to air intercooler that brings your charge air temps as low as possible, for the densest charge air your engine can breathe. Remember at 1 bar boost pressure, assuming you cool the charge air back down to ambient, it is moving double the air the same naturally aspirated engine of the same capacity would be... but due to volumetric efficiency losses, you typically see around about a 60% increase in power instead of double the power. So a good intercooler, is well worth spending the money on if you need the extra power...
     
  9. Carioca
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    Carioca Junior Member

    CDK, a similar idea crossed my mind, after working in a construction site for a gasturbine-powered electrical generating plant.

    Indeed, chillers - fed from the generated electrical power - cool down the intake air for the turbines, in order to augment output power, by 20 to 30 % (net gain). Not sure though, if plant efficiency is affected and how ( pos or neg).

    This plant was located in a hot, sultry equatorial region and the designers mentioned that to operate without chillers would be simply out of the question.

    One of the chaps on site added that it is not unusual to chill intake air on (large) diesel-powered air compressors that work deep down in mining installations - for the same reason (maximise power output).
     
  10. Yellowjacket
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    Yellowjacket Senior Member

    Chillers are generally described as systems that take waste heat and use it to provide cooling. If one were to use the waste heat from a turbine to drive a chiller, it would be beneficial to the cycle, and the overall system efficiency would improve. Unfortunately, there's a huge amount of air entering a turbine and the chiller required to cool that much air would likely cost more than the gas turbine itself. Not sure, but the cost, and maintenance of a chiller probably would preclude its use for cooling the inlet air of a turbine.

    Power generation gas turbines generally use inlet air misting to use evaporative cooling to decrease the inlet air temperature as well as add mass flow to the engine. The water in the air also turns to steam in the compressor, and this cools the air and reduces the amount of work necessary for the compression process, since cool air takes less work to compress than hot air. Basically this is simply water injection, to the cycle, and the amount of power that results is substantial. Inlet water injection has been used on gas turbines since the 1950's to augment takeoff power in aircraft like the B52. Water injection works on gas turbines, but you have to make sure the water is clean, because the boiling of the water will leave deposits in the engine and will crud things up if you don't use clean water.
     
  11. Carioca
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    Carioca Junior Member

    The electrical generating plant in question was equipped with lagged piping between the chiller facility and gas-turbine (aero-derived) units to provide the requisite cooling for the intake air. If I recall correctly, the temp. set-point for the ingested air was around 20ºC, as the designers were not unfamiliar with the 'law of diminshing returns' and performed their calculations up front.

    A side benefit of this scheme was that it also precipitated water (brine) out of the ingested air - the site was close to the ocean - thereby ensuring less contaminants would encounter the turbines´blades.



    By far the most inefficient operating scenario for an airplane´s gas-turbine propulsion units (fan jets etc) - specifice fuel consumption - is during the taxi-ing to and fro between the runway and passenger terminal, which coupled with the need for a burst of power for take-off comprises the principal reason for making recourse to expedients as you correctly point out above.
     
  12. MechaNik
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    MechaNik Senior Member

    The limiting factors in todays MARINE diesels is engine wear and exhaust emissions.

    Take a look at some specs on a series of engine and you may note that the higher power output units offer better specific fuel consumption, poorer emissions and a disproportionate drop in Time before rebuilds. Some of this seems to many people counter intuitive but it is the same with many things.

    Several methods to reduce emissions also results in poorer fuel cons, and methods to reduce wear give poorer emissions.

    The use of Jacket cooling water with after coolers has made for better engines with much greater operating conditions. Consider that many modern diesels no longer bare penalties for operating at 45 degrees C ambient temp ie MTU.
     
  13. CDK
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    CDK retired engineer

    For the type of engine I had in mind it doesn't work.
    I've done some measuring and calculating and discovered that my assumptions about automotive AC units were wrong. The one in my Kia is capable of supplying all the combustion air a small diesel needs and it can also lower the air temperature dramatically, but it cannot do both at the same time. So a substantial decrease in temperature can only be obtained with the fan at low speed.

    Also, only the short term performance is impressive, but to sustain that it needs a 60 mph airstream. When parked, the 2 electric fans cannot cope with the amount of heat generated in the condenser and the overall capacity is reduced by more than 75%.

    My guess is that a much larger unit would be needed. The present one consumes approx. 4 HP and could only marginally improve engine performance.
     
  14. mydauphin
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    mydauphin Senior Member

    I remember many years ago a study regarding engine performance and efficiency, it stated that maximum performance/efficiency was provided by cool dry air. The cooler and drier the better. So a hot humid day is worst than dry cold day. So simply cooling air is not enough unless you maintain it humidity also. I work on a yacht with turbines and it had a system to wash air coming into turbine with fresh water to eliminate salt from air. It also cooled engine, but used several baffles to remove water droplets.

    The question is this, much air means more power, but to get more power you need more fuel. So you are producing a little more power at a specific rpm. But is it worth all the trouble of additional gear and maintenance. Working on jet boat I did learn the importance of where you place intake of air on boat. I believe providing plenty of clean, salt free air is far more important than a couple of degrees of intake air cooling. That said, keeping turbo and air manifold cooler is probably a good idea, otherwise you get hot air anyway.
     

  15. PAR
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    PAR Yacht Designer/Builder

    I take it they still are using ox drawn carts in your part of the world Frosty, so not knowing a 350 is a 5.7 liter and typically use a 500 to 650 CFM carb in street applications is normal. It's also likely you've never seen a 4500 series Holly Dominator, like the two I have that were mounted as a pair of 1,150 CFM's on top of a 671 blown 500+ CID Chevy. A typical street engine of 5 to 6 liters needs about 600 CFM, depending on it's cam. The 1,150 Holly's are basically just slow leaks, from a garden hose into the engine, with an atomizer and control plate to improve efficiency and throttle response. You can physically watch the fuel gauge go down with full throttle blasts.

    I've used dry ice, but not to cool air, but the fuel. Still seen on some limited HP bracket racers, these "cool cans" offer a modest gain for little weight, no moving parts and a trunk load of ice from the local pharmacy, just before arriving at the track is all you need.

    Brehm62 has it pretty well and though several things have been attempted, most of the "solutions" robbed more than they produced (again as pointed out).
     
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