---------------------------------------------------------- Engines-List Digest Archive --- Total Messages Posted Sun 06/29/03: 4 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 02:11 AM - Re: cooling air requirement (James R. Cunningham) 2. 05:21 AM - (Gary Casey) 3. 08:49 AM - Re: cooling air requirement (James R. Cunningham) 4. 08:52 AM - Re: (James R. Cunningham) ________________________________ Message 1 _____________________________________ Time: 02:11:33 AM PST US From: "James R. Cunningham" Subject: Re: Engines-List: cooling air requirement --> Engines-List message posted by: "James R. Cunningham" Gary, you made a good post with this info. Thanks. Some comments are inserted below. Gary Casey wrote: > The calculations for cooling air requirements aren't too difficult if you're > only looking for a rough order of magnitude answer. And not too difficult if you are looking for a pretty close estimate on an engine-specific answer. > A 100-hp engine will dissipate about 25hp through cooling. The specific engine that I used to make my calculations was rejecting about 45% of power as waste heat for the two input conditions that I set up. > Some goes into the oil, so let's say > the cylinder cooling requirement is 20 hp. Don't hold me to this, but I think Lycoming uses a maximum of 15% of the heat into the oil. I can look it up if anyone is interested. In the meantime, your number is 20%, which should be slightly conservative for a Lycoming and perhaps more accurate for some other engines. > You could probably assume the > air temperature rise across the engine is 30 degrees: This you can't do. You need to calculate the temperature rise across the face for the specific flight conditions. It will typically range from 30 to about 150 degrees F (Lycoming uses a constant 150 degrees). It is usually well over 30 and usually well under 150. > To absorb 850 BTU/min of heat you need 1,650 CFM air > flow (850 X 14 / 7.2). Problem is it takes a significant pressure to force > that much air around the cylinders - a few inches of water maybe. Pressure drop is specific to each engine and varies substantially with cylinder size and fin design. For the specific engine and input conditions I used in my example, the pressure drop across the face ranged from 25 to 43 psf. Well less than an inch of water. Your mileage may vary with other engines. > Normal ventilating fans are rated at zero pressure drop (blowing into free air) and > may not be able to flow nearly their rated flow with a significant pressure > drop. Well said. > The best suggestion I have heard is simply to build a reasonably > large plenum that will capture propeller blast and use that, watching the > cylinder head temperatures. It might be worth computing the flow available before trying that. > A very effective cooling method would be to add > water mist to the air, but the steam cooling that will result is probably > very uneven and that would worry me. Me too. JimC ________________________________ Message 2 _____________________________________ Time: 05:21:24 AM PST US From: "Gary Casey" Subject: Engines-List: --> Engines-List message posted by: "Gary Casey" Previously I said: <> I was in error as I was using the rule of thumb of 20% of the power goes into cooling. Actually, it is more like 20% of the INPUT power. Of the heat energy from the fuel a very rough estimate is that 50% goes out the exhaust, 25% into useful work (hp out the crank) and 20% out the cooling system. The remaining 5% into the oil. Gary Casey ________________________________ Message 3 _____________________________________ Time: 08:49:28 AM PST US From: "James R. Cunningham" Subject: Re: Engines-List: cooling air requirement --> Engines-List message posted by: "James R. Cunningham" James R. Cunningham wrote: Gary said: > > Problem is it takes a significant pressure to force > > that much air around the cylinders - a few inches of water maybe. I said: > Pressure drop is specific to each engine and varies substantially with > cylinder size and fin design. For the specific engine and input > conditions I used in my example, the pressure drop across the face > ranged from 25 to 43 psf. Well less than an inch of water. Your mileage may vary with other engines. Oops. Wish I'd said more so I could take that back too. When I converted to inches of water, I omitted the multiplication by 12. For the engine and conditions I used, the pressure drop ranged from about 3.5 to about 8 inches. Gary, you were spot on when you said, "a few inches of water". All the best, Jim ________________________________ Message 4 _____________________________________ Time: 08:52:31 AM PST US From: "James R. Cunningham" Subject: Re: Engines-List: --> Engines-List message posted by: "James R. Cunningham" Welcome to the club. And I was in error when I converted psf to inches of water. You just made me feel better, knowing I'm not alone. I'd buy the estimate of 5% into the oil (but I'd design the oil cooling system to allow for more, and would include a cowl flap on the oil cooler outlet). Jim Gary Casey wrote: > > --> Engines-List message posted by: "Gary Casey" > > Previously I said: > < dissipate about 25hp through cooling. Some goes into the oil, so let's say > the cylinder cooling requirement is 20 hp.>> > > I was in error as I was using the rule of thumb of 20% of the power goes > into cooling. Actually, it is more like 20% of the INPUT power. Of the > heat energy from the fuel a very rough estimate is that 50% goes out the > exhaust, 25% into useful work (hp out the crank) and 20% out the cooling > system. The remaining 5% into the oil. > > Gary Casey