Engine efficiency formula?
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    Engine efficiency formula?

    When I was looking to buy a new tractor, one of the decisions that came up was whether to buy an E model versus an M model. Same horsepower rating for the tractors but different engines. Nearly identical displacement but 5 cylinder versus 3 cylinder.

    One comparison tool was a basic formula that would calculate a relative efficiency number with which to compare different engines. Went something like Hp x Rpm / constant = efficiency value

    Question... ? Does anyone know this equation?

    Thanks.

    AKfish
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    dzldanz's Avatar
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    Quote Originally Posted by AKfish View Post
    When I was looking to buy a new tractor, one of the decisions that came up was whether to buy an E model versus an M model. Same horsepower rating for the tractors but different engines. Nearly identical displacement but 5 cylinder versus 3 cylinder.

    One comparison tool was a basic formula that would calculate a relative efficiency number with which to compare different engines. Went something like Hp x Rpm / constant = efficiency value

    Question... ? Does anyone know this equation?

    Thanks.

    AKfish

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    HydroHarold's Avatar
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    Quote Originally Posted by AKfish View Post
    When I was looking to buy a new tractor, one of the decisions that came up was whether to buy an E model versus an M model. Same horsepower rating for the tractors but different engines. Nearly identical displacement but 5 cylinder versus 3 cylinder.

    One comparison tool was a basic formula that would calculate a relative efficiency number with which to compare different engines. Went something like Hp x Rpm / constant = efficiency value

    Question... ? Does anyone know this equation?

    Thanks.

    AKfish

    Sounds like a job for Fizxmannnnnnnnnnnnnn! He's probably in a phone booth somewhere changing into his tights and lab coat right at this moment!

    Interesting!!!!
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    Keeper of the GTT Cookies dieselshadow's Avatar
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    Simple or hard?

    Simple. HP (as measured on something repeatable like a dyno) divided by fuel burn rate or BSFC (Brake Specific Fuel Consumption). In a nutshell, how much fuel are you burning for the HP produced.


    Hard.... Measure ambient air temp, air temp and flow coming through radiator (heat loss), exhaust flow and temp vs intake temp, HP produced, fuel flow. All measured exactly. Then you can find how much heat is being lost to the atmosphere. The more heat lost through the exhaust and radiator for fuel burned and HP produced, the less efficient the engine is. Ideally you want as little heat lost, this means you are converting more of the fuel's energy to work instead of heat. It's very complicated and you'd need a lot of measuring equipment and lots of calculations.


    I recommend the simple version.
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    It was a simple ratio formula or index. For example, 2 engines are rated @ 65Hp but one achieves that power level @ 2,400rpms and the other engine @ 2,200rpms. Obviously, the latter engine is more "efficient" - with other variables more or less equal; i.e. fuel consumption, etc.

    I recall the discussion on TBN. But, after searching for a number of hours the past couple days, no luck dredging it up.

    Interesting to re-read some of the old posts from Zebrafive and Arlen and Jer; as well as some of my own... Forgot that I had signed the papers with Shane on a 5065M. He must have talked me out of it at the last minute!

    AKfish
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    dzldanz's Avatar
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    As mentioned before by ?
    Torque at the wheels is what it's about
    Nuff said

    Current: 3320 HST cab with 59" front blower, Hydraulic chute deflector, Vertical Exhaust, Front Fenders,
    Grooved and loaded Front and Rear R-4's
    External Mirrors, Jensen Bluetooth Radio / Headset. Front Hydraulics
    Whelen warning lights, JD366 front blade
    Gone: X595 with heated cozy cab,stainless vertical exhaust

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    MattF's Avatar
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    OK, I do these equations for a living but I'm a bit confused about what you are looking for.

    AKfish
    It was a simple ratio formula or index. For example, 2 engines are rated @ 65Hp but one achieves that power level @ 2,400rpms and the other engine @ 2,200rpms. Obviously, the latter engine is more "efficient" - with other variables more or less equal; i.e. fuel consumption, etc.
    Based on this statement I think you're looking for BMEP, or brake mean effective pressure.

    One of the many variations of this metric is BMEP=Power*# of revs per cycle (2 for 4 stroke, 1 for 2 stroke)/(total displacement volume * speed).

    I use BMEP=Torque*2*pi*# revs per cycle/Displacement volume, which is exactly the same thing... I just get to measure torque directly w/ a dyno as opposed to picking a value off a power/speed curve.

    This isn't actually a measure of efficiency, its a normalizing method for comparing two different engines. It's hard to compare a small displacement, 4-cyl engine running at high rpms and boosted to say a naturally aspirated big V8, lugging. The V8 may make a boatload of torque and horsepower more than the other smaller engine... but which is the getting more out of the hardware, i.e. the "better" engine design? BMEP normalizes output per unit displacement of the engine. Units of BMEP are in pressure, so you get an output in psi, bar, kPa, etc... The higher the BMEP, the more you are getting from the displacement you have available to use. Typically, modern small displacement, boosted SI engines win compared to big naturally aspirated SI engines, for example. It works to compare SI engines to diesel and viceversa... interesting comparisons... Each has its strengths.

    A measure of HP/liter is an approximate of this metric. For SI, 120 to 130hp/liter is world class these days. It used to be 100hp/liter - we're getting better.

    Unit conversions can be a bit tricky if you stick w/ English units... use metric (SI to be precise) and save the headaches. Actually, it looks like 150.8*Torque(ft*lbs)/displacement (in^3) works for 4 stroke, 75.4 for 2 stroke = PSI, which describes your initial recalled form of the equation very well.

    Am I anywhere close to what you were thinking of? There are (several) other metrics, but most are a bit more involved than a simple ratio as well as require measurements of values that aren't easy to come by/find published.

    Matt
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    Hmmmmmmm, wonder what the price of rice is in China, today ? . Dan
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