How do you select an engine

Discussion in 'Diesel Engines' started by Mik the stick, Dec 23, 2012.

  1. Mik the stick
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    Mik the stick Senior Member

    I was looking at Lugger engines on there site. Take the L1064A, it has 3 ratings at High output Medium output and continuous. I thought this was one engine but the spec says 2000rpm for 270ft/lbs torque and 115hp continuous. Medium duty can go to 2200rpm for 398ft/lbs and 125hp. medium duty is rated at 2000rpm for 328ftlbs and 125hp, I just don't understand Is this TWO versions Of the same engine tuned differently.
     
  2. michael pierzga
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    michael pierzga Senior Member

    For marine use ,Duty cycle is how many hours per year you anticipate. Continuous would be shipping or generators diesel running 24 hours per day, huge run time.

    Propulsion engines for cruising would be medium duty. You can do a bit of googling and pull up duty cycles. I believe medium is 2000 hrs per year. Think of a fishing boat.

    High output would only be suitable for a sport type boat.

    http://marine.cat.com/cda/layout?m=319656&x=7&id=914324
     
  3. jonr
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    jonr Senior Member

    More HP from the same engine means more wear and shorter life. So it's about expectations and warranties.
     
  4. tunnels

    tunnels Previous Member

    Bigger is better !!
    If the max brake hp out put of a engine is 100hp for a pleasure boat take 20% off that !!!
    For continuous running 24 hours a day etc or a work boat like fishing etc take 30% off or even more !!!.
    Bigger motors using less hp last long time, use less fuel ,and are more reliable !!

    I have a 14- foot boat had a 70 hp out board its used to work hard from the moment it went into gear !! , now i have on the same boat i have a 115 hp motor its capable of over 60 mph but at 2500 to 3000 rpms its hardly using much hp at all and cruises reall easy and is hardly working and is more economical than the 70 that used to have to work its wee heart out !! The extra power is there if i want but i hardly ever use it , where as the 70 was wide open all the time and was a thirsty little beast .
    Never underpower a boat bigger is better !!:D
     
  5. Steve W
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    Steve W Senior Member

    Tunnels, what type of 14ft boat do you have that needs 70hp? this seems like an awful lot of power for a little boat let alone 115hp.
     
  6. tunnels

    tunnels Previous Member

    Just a little old semi cabin glass boat i use for fishing and mucking about in . The guy that had it before me used to go fishing a lot and at some time had been a ski boat !.The old 70 was a bad installation and motor was to high so i rebuilt the whole of the transom with step back and fittered a 25 in shaft motor instead of the 20 like it had !!. :D:p
    Yip the 115 hp is exciting thats for sure . It jumps on the plane like a jet ski when you push the throttle down quickly !!.
     
  7. jonr
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    jonr Senior Member

    > Bigger motors ... use less fuel

    Up to a point, then smaller uses less fuel.
     
  8. tunnels

    tunnels Previous Member

    smaller may use less fuel only if its working in proportion . If the smaller motor is wide open and working to its max they wont last long thats for sure !!
    Bigger is better than smaller always !
    Just remember a boat motor the moment it goes into gear is climbing a hill and the faster you go the steeper the hill!!!
    There is never any down hill !!, well almost never !! or any over run unless you can surf down the face of a wave !! then you will deffinetly be thankfull for lots of horse power .:eek:
     
  9. MikeJohns
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    MikeJohns Senior Member

    People commonly tend to find that confusing. The engines for the different ratings are identical. It's what you ask the engine to do that determines how you work it and it's rating. That relates to the propeller pitch and diameter and the rate the engine turns it and the speed of the water inflow to the prop.

    The prop power requirement is ( or should be otherwise it is lugging or overwheeled) always less than the engine is capable of producing up to the point where at a certain RPM it meets the engine max possible power curve. The system is designed for it's max rating at this intersection and we specify pitch diameter and reduction ratio to match the vessel resistance at a required speed. The power at the prop is basically the power the engine develops even though it's power curve shows a much greater power through most of the range (reserve power). The rating is determined by where we design the intersection of the max power of the engine and the max demand of the prop. It's then up to the operator not to exceed that RPM.

    It may help to understand that with a diesel engine the throttle doesn't directly alter the fuel burned, rather it sets the speed the engine is to run at. That speed is maintained by a govener ( electronic or mechanical) which leads to more or less fuel injected to meet the speed set by the operator.
    When the boat is slowed by wind or waves more thrust is required and the engine has to work harder to turn the prop at a given speed and so more fuel is burned and you may also want a higher RPM to get the same vessel speed. There should be reserve power (or the difference between the engine power curve and the prop demand curve at a given RPM) at times of high demand such as going to weather or towing a disabled boat the engine will run at a much higher fuel usage and it's then that an improperly designed system can exceed the chosen usage rating. This is particularly an issue with work boats that are run hard.

    So to recap, it's a combination of the design and the operation that determines the rating and nothing to do with any differences in engines.

    I hope that helps.
     
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  10. FAST FRED
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    FAST FRED Senior Member

    "Bigger motors using less hp last long time, use less fuel ,and are more reliable !!

    Bigger is better than smaller always !"


    Not so, on a Diesel,,an underloaded gas engine will survive fairly well, but an underloaded diesel dies a rapid death.

    Run a large engine , basically at idle, and death by burnishing the cylinder bore and seizing the rings in solid carbon results. Slobbering is what its called.

    A fish boat with 650 HP , running just the hyd pumps to recover a trawl , 40 Hp will usually have long term problems.

    For a yacht cruiser the simple concept of 80% RATED power at 90% of rated RPM will usually create the longest engine life and lowest fuel burn per HP.

    IF the 24/7 rating is used , many times the medium rating works for higher cruise when near fuel stops .

    And the "performance" rating will do to race to beat the proverbial storm to port.

    Perhaps a couple of hours every few years.
     
  11. jonr
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    jonr Senior Member

    > 80% RATED power at 90% of rated RPM will usually create the ... lowest fuel burn per HP

    It varies so much that it's worth checking or testing, but I see more like 70% of rated RPM and even less for minimum wear.
     
  12. MikeJohns
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    MikeJohns Senior Member

    Fred
    Not so much of an issue with modern engines, with modern materials/construction, we are seeing 20,000 hrs + now on modern heavy built engines like the John Deere 6068 that spend half their time driving nothing more than a hydraulic pot hauler. They get a burn at 60 - 70% going to and from the fishing areas and then they just burble around for a day or two and power home again. So the bulk of their hours are at very low power levels.
    The Detroits ( as an example of older engine design ) didn't like that sort of use and didn't make anything near the same hours before a pull down.

    Modern designs and emission standards see the rings sealing very well at low power levels. Some of the modern low reving engines are getting a lot more reliable, and with much longer periods between rebuilds than we ever had before.

    The big issue today is designing the installation for a max economy point.
     
  13. Frosty

    Frosty Previous Member

    We have bashed this to death many times before. With modern oils and perfect fuel injection and variable timing the days of diesel underloading are gone.

    Fuel in an engine is merely to light the oxygen, many fuels can be used.

    A diesel produces power relative to fuel consumption.

    5 galls per hour per 100HP used..... petrol 7....... 2 strokes 10.

    Small engines is false economy.
     
  14. FAST FRED
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    FAST FRED Senior Member

    All is a trade off, EG. using square cut rings will help engine life at minor loads ,

    but will not be as efficient with high power and good turbo boost.

    The owner must make a realistic decision about how hard the engine will be run , and then decide on the trade off between ultimate engine life and the best operating cost per hour.

    For commercials this is important , for most 200 hour a year boats , much of the "efficiency " optimization is simply mental ************ , tho great fun.
     

  15. michael pierzga
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    michael pierzga Senior Member

    Also consider the human factor. On this boat cruising at 1300 rpm the boat is quite, normal conversation can take place...at 1800 rpm its a whole different world of noise, vibration, stress and crew fatigue.

    An engine that can achieve your goals at low rpm is a beautiful thing
     
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