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Thread: Nissan's new LMP1 engine - 10hp/kg engine

  1. #21
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    Also only 75% of the cylinders!

    It's bigger cylinder capacity than my bike (1.2l twin) and revs a lot lower.

    I also wonder if they are counting the turbo in the weight!

  2. #22
    Corvette Enthusiast Kchrpm's Avatar
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    I was assuming that the 80 lbs was in reference to the amount of the engine that was being held by Mr Executive up there.

    Again, your bike doesn't have a turbo. Most applications I've seen with turbochargers have relatively low revs, in exchange for a big wall of torque once the turbo hits. It's not because they can't rev higher as much they make more power when they're tuned to lower revs for that boost sweet spot. I'm assuming Greg could give a better (aka actual) technical explanation.
    Get that weak shit off my track

  3. #23
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    Higher revs also = more fuel...

  4. #24
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    Many turbos are an addition to an existing design, so they don't tend to change the revs.
    A small high revving engine can have a turbo added without having to drop revs (but might need octane and inter cooling)

    And turbo means more fuel. Basically more power means more fuel!
    I'm not sure if turbo is any more efficient than higher revs.
    It's generally cheaper, but I would have guessed it was heavier.
    Adding a turbo adds a fair chunk of weight.
    Going higher revs usually means lighter internals, just that's it's not a cheap way to go.

  5. #25
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    I'm thinking the lower revs are for reliability and to a certain extent helping reduce the weight by lowering the same stresses that degrade reliability and/or necessitate increased strength.

    I do also think the high revs comes back to more fuel due to increased internal losses. Since they're on race gas, the power is made up by increasing boost. I don't know if there are air restrictors in that class anyway.
    Last edited by LHutton; February 5th, 2014 at 07:55 AM.

  6. #26
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    I want two of them!

  7. #27
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    More Power = more fuel yes. I remember reading somewhere that the increase in RPM requires more fuel than turboing. I don't remember exactly, and I certainly don't have the mathematical ability to support or prove it. Perhaps someone else does?

    But as already stated, higher revs = increased cost at an almost exponential level due to the requirements for higher tolerances for the increased load as RPMs increase.

  8. #28
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    This specific fuel consumption against revs plot (y axis is probably effectively throttle opening) is similar to what I've seen before:


    It shows that there tends to be a best efficiency at fairly low revs. I think this is because the slower the burning fuel-air mixture expands, the more mechanical work can be extracted from it, so higher revs are inherently less efficient for a given power.

  9. #29
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    That graph is for a diesel engine. The issue there is that diesel just doesn't burn quickly enough to extract the work at higher rpm. With petrol engines it's more the case that friction increases at high rpm. I'm no expert but I've often heard it said by people who are that there's a big difference between operating under 8000rpm and operating over it.

    Turbocharging is always more fuel efficient than high rpm because it involves recovering waste energy from exhaust gas at a relatively minor parasitic expense. Whether to increase boost or increase rpm is bound to be more of a balancing act, depending on the fuel you're using and what rpm and boost you're already at. The power load from combustion, which is increased by boost tends to counter the inertial load from rpm. The power load is also compressive, whereas the inertial load is tensile. Tensile loads are more of a problem because they induce fatigue failure - most materials are far stronger in compression.

    Pe is the BMEP (Brake Mean Effective Pressure) in the cylinder over a cycle, which will change with boost and throttle opening. The great thing is that you can double the BMEP with turbocharging whilst only increasing the peak cylinder pressure by a small fraction. Increasing rpm, on the other hand, increases tensile load by the rpm increase squared. So achieving the same power via rpm increases damaging tensile forces by far more than extra boost increases peak pressure and increasing tensile loads only increases bearing friction. Pumping losses also increase with the square of rpm.

    4-stroke Power = BMEP x Length of Stroke x Bore Area x Number of Cylinders x rpm/2

  10. #30
    Corvette Enthusiast Kchrpm's Avatar
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    That was an awesome explanation, thanks.
    Get that weak shit off my track

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