Breathing easy !!

Discussion in 'Diesel Engines' started by tunnels, May 16, 2012.

  1. tunnels

    tunnels Previous Member

    How much fresh air does a diesel engine need to breath easy ??
    Its always a deep concern of mine that air intakes in some boats is to small !!
    Is there a general rule of thumb so to speak of engine room wants , needs and must have's ????
    What about turbos do they need more air than non turbo'ed ??
    At the end of the day and all is shut down and quiet, air in and air out ,vents and thermosyphen ??
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Engine HP divided by 3 as a minimum in square inches for vent. If forced (blowers) CFM (cu. ft. per minute) 2.75 times HP minus 90. Of course more and bigger is better, just ask your wife.

    So a 350, turning 250 HP needs 84 square inches of vent area. If equipped with a blower you'll want to flow close to 600 CFM. Interestingly enough if using a carbureted engine or TBI, just use the CFM rating for the carb or TBI as the flow rate. They'll be about the same.

    The best answer is the manufacture's recommendations.
  3. tunnels

    tunnels Previous Member

    I just handed what you sent to the factory manager and is checking . We had a boat in for some repairs and service and its on the sea doing trials but the motors are down on hp so after checking to see if the airflow into the engine room was ok Now i am told the vlolvo person changed some settings the ec units . why we dont know !!
    So thank you for the information !!:D
  4. CDK
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    CDK retired engineer

    The basic air requirement for a NA-diesel is engine capacity times rpm, for a turbo charged engine that must be multiplied by the charge factor, usually 1.3 to 2.0.
    So a 5 liter NA diesel, running at 4000 rpm needs 20 cu.meters (600 CFM) of fresh air each minute just for the combustion at full throttle.

    A rule of thumb for the vent aperture is not very reliable because the shape also matters. One large hole facing the engine's air intake performs better than a long, narrow gap. An intake grid will also cause higher resistance.

    In this example, a vent of 1 sq.ft. will have an air velocity of 600 ft/min for the combustion air, which is quite acceptable. I would provide some extra air to keep the engine room cool, but that requires also an exit vent and/or forced ventilation.
  5. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    We have used the "Det Norske Veritas Rules For Construction and Certification of Vessels less than 15 metes", for many years. In the section covering "Additional requirements for commercial vessels; installation and machinery", you find the following:

    Engine compartment
    The engine compartments air intake for natural ventilation is to satisfy the engines air requirement in accordance with the supplier's recommendation, however, not less than 7 cm2/kW.

    The exhaust area for engine compartment ventilation shall be at least 2 cm2/kW.

    I did some temp measurements on a couple of alu Fifi catamarans a "hot" scandinavian summer (yup, everything is relative...) and found that the venting capacity (with areas as per above) was more than adequate, even for pretty cramped engine bays. There was some condensation on uninsulated hull parts, indicating that there was, in fact, a slight "overventilation" with an alu hull. So, for a GRP hull, it might be just correct.

    Engines were turbocharged and aftercooled high performance diesels, water temperatures ~20 C, air temp ~23 C.
    Last edited: May 17, 2012
  6. souljour2000
    Joined: Aug 2009
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    souljour2000 Senior Member

    The diesel on my boat is a MX25 (25hp) Universal/Kubota. It is enough power hopefully...will have to boat weighs alot (18,000+) though "skinny", but I want to make sure that little engine is getting enough air to be able to get the most power and good burn I can this thread is interesting....the engine is in the salon floor at the foot of the companionway so it seems I should check that it it fairly isolated it seems...I am still getting aquainted with ALL her systems having only spent a total of maybe 25 hours aboard her thus yet...I will have to re-visit this thread after I get a better look at her...good info guys..
  7. tunnels

    tunnels Previous Member

    thank you all !!

    A boat builder designer friend of mine had designed a tunnel boat that had twin vee 8 diesels and when the boat went in the water and on a couple of test runs the motors just didnt quite meke the grade and there were lots people scratching there heads untill the diesel machanic unclipped the engine covers and wanted to be in the engine room while the boat was going! its went like a cut cat and no problems were found so the covers were replaced and behold the performance was down so the covers were rasied and away it went again . :idea:ahah it wasent getting enough air so vents and scoops were made and so much air was going in the floor hatchs were lifting and had to be cliped down !!

    Another story i remember as well was my father worked for catapillar and a fishig boat had a intermitant problem of performance was down and the motor would smoke . everything was checked and rechecked over and over again so my dad went to sea for a week and lived with the engine !! One wet rainy day and the motor was perfect not a thing wrong . The next couple of days were fine and the motor was playing up again.
    The motor air intake was in the wall of the side cabin where wet weather get was hanging and one coat was sucked up against the intake and partually blocking the air supply !!, removed the coat and the problem went with it
    Simple things that cause intermitant problems are the hardest to locate .
    Thank you all who replied and i hope as always other people have read and are a little wiser as i am !!
    i was alwas told 1 square inch of unrestricted air intake for each horse power !! and its what i have stuck to over the years !! :D:p
  8. powerabout
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    powerabout Senior Member

    Dont forget about the temperature rise, thats reducing horespower as well if you dont have forced ventilation.
    Measure the inlet temp and the engine manufacturere will have a graph showing the drop
  9. DaEdster
    Joined: Jan 2012
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    DaEdster Boat builder

    That is a big hole in the side of the hull! Why haven't I seen these size holes in hulls? Any idea how they 'hide' this vent?
  10. CDK
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    CDK retired engineer

    I fully agree that it is big, but there are ways to disguise it.

    The Norwegians who built my boat in 1980 cut two 6 inch holes in the top of the transom and covered them with a square black grille. They made similar holes behind the benches on the aft deck with white grilles. And because they still weren't convinced that it was enough they left an opening of 2 sq. ft. above the bulkhead so the engines can draw cabin air.

    When I did an installation job on a Swedish sailing yacht at first I couldn't find any air vents at all. The hinged wooden seats rested on spacers so there was a 1/2" gap of at least 8 ft. long.
  11. DaEdster
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    DaEdster Boat builder

    Cheers, and the noise? Does travel via air gaps..... Mmmmmmm.....
  12. CDK
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    CDK retired engineer

    Yes, the coin has two sides.
    But fortunately noise doesn't travel well around corners, especially when they are padded. In my opinion providing enough air is more important than reducing noise. I would rather accept a few decibels more than smother the engine.
    And large vents in the transom do not increase the sound level on board, at least not much.
  13. mydauphin
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    mydauphin Senior Member

    Noise travels through the structure, vibrations in solid or as sound waves in air. If your opening is slightly smaller and curved, sound waves will have harder time traveling out. make opening toward rear of boat and unobstructed and noise will bother the guy behind you more.

    Make sure engine room walls and ceiling are isolated/insulated, you get more noise vibration there.
  14. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    Not nit picking CDK... but thats twice as much.
    For every 2 revolutions,one is intake,other is exhaust.
    So 2.5 litres of intake (per revolution) x 4000 rpm is 10,000 litres/10 cubic meters a minute.

    My fetish is to have the quietest boat possible- the best thing to go after is isolating the engine room as much as possible.

    Nothing like going along at 9 knots and the only thing to hear is the waves slapping on the hull..

  15. CDK
    Joined: Aug 2007
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    CDK retired engineer

    You are correct WestVanHan, I exaggerated things a bit.

    With a 5 ltr. diesel running in your engine room I think you'll hear a lot more than just the waves slapping!
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