Power/fuel economy correlation

Discussion in 'Trucks [ Eighteen Wheelers ]' started by JReding, Nov 20, 2014.

  1. JReding

    JReding Road Train Member

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    Okay, folks. Looking for opinions from those that know, including any service techs who may be here: what are the facts regarding power and fuel economy? My boss is of the belief that increased power=lower fuel economy. I am among those that believe, within reasonable parameters, dialing up your engine's power can lead to better fuel economy, while dialing it down can decrease fuel economy (the harder an engine has to work, the more fuel it consumes: e.g. climbing a hill, same engine, one at 350 HP vs. one at 450 HP. The lower horsepower driver is forced to downshift more often, consuming more fuel. Thoughts?
     
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  3. Helowrenchturn2

    Helowrenchturn2 Medium Load Member

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    I too believe more power will get more mpg. If the driver knows how to use it. With great power comes great responsibility lol
     
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  4. mattbnr

    mattbnr Road Train Member

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    Anything over 450 hp isn't needed for general freight. Now heavy haul you could go up to 600+ but be reasonable with the throttle.
     
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  5. mattbnr

    mattbnr Road Train Member

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    There's a reason they can fuel go go juice. The more you dump in the motor the more hp the less fuel economy.
     
  6. Tmtbob

    Tmtbob Medium Load Member

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    On your example of the 350 vs 450. The 350 will be working harder under the same conditions using more fuel. Put those engines side by side in identical situations and they start to pull down a little. The 350 keeps fueling till its wide open throttle just trying to keep up with the 450 that's running at half throttle. The more you have to work it to keep moving the more it will drink. Not to mention the wear the lower hp engines take because they have to work harder.
    That's not to say the more horse you have you are guaranteed better mileage. Take someone who drives with his foot to the floor all the time and put him behind the 450 driving the same as he did the 350 and you will get close to the same results but he will show up faster. When he learns how to handle more power he will get better mileage and show up faster.
    I prefer bigger engines than that and never had to explain it with fleet truck engines but I just pretended we were talking 550 to 750 and used your numbers. Me personally couldn't live with a stock 550. They can put out so much more and I don't mind the work it takes to get it.
     
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  7. JohnP3

    JohnP3 Road Train Member

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    If you take an engine that is made to run at 350 HP and boost it to 450, you will lose fuel economy as the engine is not made to run that HP, if you run an engine above 1550 you are losing fuel economy.
    I worked on a lot of N14's that ran 144,000 lbs over two mountain ranges they bought them as 460 HP to get the better cam for the Jakes and then reduced the HP to 430, the one owner had 6 units all running 11hour shifts 2 drivers two loads a day, he left the last three as 460's for a month they burnt about 7 gallons more fuel a day and gained 5 minutes a trip. There are two hard pulls on was 55 minutes at a 95% power factor all the trucks pulled the same gear, the only increase was topping the hills.
    If you have an engine that is made to handle 500, and is de-rated to 350 it may not be as be as efficient as if it was a 450, the hard parts of an engine, the compression ratio, cam, and the turbo all make a difference.
    Then there is will the transmission, clutch, driveline, rear gears and axles, handle it.
    Most modern higher HP engines have set in the program a torque reduction spec when the truck is in the lower gears
     
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  8. Pool6710

    Pool6710 Medium Load Member

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    It's the low nox in the tune that does it. But my 600hp vs my 425 still gets almost 1mpg better fuel on 600.
     
  9. SmokinCAT

    SmokinCAT Road Train Member

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    While it may take more fuel to make more power the applied throttle in a truck with 600hp is going to be quite less than a truck with lower HP, the possibility is there that you will be less likely to need to use full throttle to accomplish the same job as a lower HP truck, but it puts more on the driver.
     
  10. JReding

    JReding Road Train Member

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    Thanks for the feedback so far! Here's our situation, which I've mentioned only briefly in another thread: we have 6-2014 Freightliner Cascadias, leased from Penske. They have the Detroit DD13's, which, from what I've read from Detroit's website, are rated up to 500 HP. Ours apparently are supposed to be set for 420 HP, but based on our driving experience, we drivers agree that the best they are producing is perhaps around 325 HP (we lose speed going up a 3% grade, pulling an empty set of 32's!) I'm not sure about the rest, but I can assume the others drive the same as I do, especially since it is our boss's request: shifting between 1300-1500 RPM, driving fairly conservatively, and not going past 60 MPH. Next time I'm in Penske, I'm thinking of asking them to plug my truck in to see where it is set at, to be sure.
     
  11. Cowpie1

    Cowpie1 Road Train Member

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    [h=4]I had it explained to me this way, and it makes sense, at least it plays out this way in my experience also.

    Truck #1:[/h]
    • An ISX equipped tractor going up a grade at 1600rpm has a fuel burn rate of approximately 24 gal/hr or 0.40 gal/min.
    • If it takes the tractor 30 seconds to climb this grade, 0.20 gallons of fuel are burnt during this heavy load situation.
    [h=4]Truck #2:[/h]
    • The same ISX equipped tractor, with higher hp and torque rating, going up that same grade at 1600 rpm will have a burn rate of approximately 26.5 gal/hr or 0.44 gal/min.
    • However, it only takes the tractor 20 seconds to climb the same grade, which correlates to .13 gallons of fuel burnt meaning .07 gallons of fuel were actually saved under the same heavy load situation.


     
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