180 mile commute, will aero mods help me?

Discussion in 'Gen 2 Prius Accessories & Modifications' started by oil_burner, Nov 15, 2017 at 12:39 AM.

  1. oil_burner

    oil_burner Junior Member

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    I have started a new job that requires 180mile round trip commute each day. It is 100% highway mileage, I pull out from my driveway, half a mile later I am on the highway. Gas has just jumped in price and I am looking for any fuel savings I can get.

    I drive at higher speeds, usually 85mph and I do climb a long grade. I know slowing down will give me better mileage but I work for 12 hours, plus an hour commute, I just want to be home quickly at the end of the day.

    Right now I've taped off both grilles entirely and it doesn't seem to have helped fuel economy noticeably. Even though I can see my coolant temperatures are staying around 95-97C compared to without the blocking they are around 86C. Also MG1 and MG2 temperatures will get to around 55C compared to without blocking they stay around 35C. I'm running 0-20W oil and leaving the block heater plugged in at night. Running AC is a requirement to keep the windshield from fogging in the winter.

    Should I go down the route of wheel covers? Removing the rear wiper? How much MPG increase can I expect to get?
     
  2. SFO

    SFO Active Member

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    Only real way to find out would be to test and then test again.

    What kind of aero mods did you have in mind?

    What is your current MPG?

    Not sure what your hourly rate is or what price you put on down time, that might be the real math when all is said and tested.

    Edit: grammar
     
    #2 SFO, Nov 15, 2017 at 1:32 AM
    Last edited: Nov 15, 2017 at 11:13 AM
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  3. 05PreeUs

    05PreeUs Active Member

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    Aero is the biggest factor above 40 MPH, very considerable above 55 MPH and HUGE at your 85 MPH speeds! At those high speeds and given the distance, you should see an immediate and measurable increase with almost any drag reduction you can achieve.

    The grille vents really only need to be open below ~40 MPH or so, the smooth wheel covers will help as well, but the frontal area is the pink elephant. Lowering the vehicle would likely bring the single biggest improvements and not impact cooling (engine, brakes, HSD) at all.
     
  4. Kenny94945

    Kenny94945 Active Member

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    The answer is yes, aero will help.

    Yet question is $ return on investment.
    Only testing will tell what increase per gallon you'll get, yet over 180 miles each day this could be a valid number for the $ROI.

    My take,
    removing rear wiper: insignificant vs the need to use.
    side view mirrors: significant
    flat hub caps retain: significate
    modify under carriage fairing to flat bottom: significant
    covering front air intake: significant (I'd try a $100 bra instead of tape) but, versus vehicle AC and coolant needs.
    lowering the car: significant vs. driveways.
    tire pressures: variable wear vs mpg If spreadsheeting maybe figure 10K mileage reduce in tire life.

    So again, cost of doing the above mods verse you fuel savings.
    Low cost recap: lower the car, increase tire pressure, add bra.

    Good luck and your analysis would be of interest to me and perhaps other forum members.
     
  5. Kevin_Denver

    Kevin_Denver Active Member

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    I wouldn't completely block off the front grill. I would leave at least a couple of rows of the bottom grill open to allow cool air to come in, in the wintertime. The bottom part of the radiator also is used to cool the inverter, and you definitely want to keep it cool; a replacement will run in the thousands of dollars. The top grill can be completely blocked. The engine water temperature should run at 82-87C in normal conditions. If it hits 95C, the radiator fan comes on. I think it's okay if the car sees 95C at the top of a long hill, but it shouldn't be regularly running at this temperature, which will shorten your engine life, especially with thinner 0W-20 oil. If you see it spike over 100C at any point, you've definitely got the radiator too blocked off. I wonder if having the radiator fan run constantly is worsening your fuel economy due to its fairly large power draw?

    I have the top grill completely blocked off and the bottom grill 50% blocked off currently for wintertime. The car gets a little better fuel economy.

    I would try inflating your front tires to maximum sidewall pressure, and your rears to maximum sidewall pressure minus 2-3psi if you haven't already. A mod that will get you better fuel economy is lower rolling resistance tries (there's a thread on here for this).

    The aero mods Kenny mentioned will help (have you seen ecomodder.com?). The car has coeffecient of drag of .26. It's reasonable with mods to think you can get this down to .24 (unless you want to build a full boat tail! Google Aero Civic if you haven't already). This will net you about 8% better fuel economy.

    A simple change that will likely yield better fuel economy with only a little change in driving habit is to keep your engine rpm under 3800 while driving (use a Scanguage or other tool). The Prius engine above 4k is not very efficient. Driving up the mountains in Colorado I use this strategy to maximize fuel economy at the expense of only a few minutes over a couple hour drive.
     
  6. 05PreeUs

    05PreeUs Active Member

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    Any modern internal combustion engine will operate perfectly safely at any temperature up to 100*C and most over that by a significant amount (221*F/105*C) without any real risk. All the engine management software I have worked with will log overtemp at something greater than 106*C/223*F, even then you will not "boil over" as long as the system holds pressure. At about 111*C, you lose cooling system integrity in most cases and engine damage is nearly certain.

    In point of fact, the Prius thermostat is not "full open" until 95*C (203*F) and most OEMs choose the thermostat full open temp as the fan_on temperature as well (give or take a degree C).

    If you can manage a method to block maybe 80% of the air from the upper and 50% from the lower that was easy to remove/install, you could monitor temps for a couple drives and see if either approach 100*C/212*F. If not, success!

    From my reading, MG1, MG2 and inverter temps are the critical ones, there is no inverter coolant temperature sensor. The MGs will be happy below 111*C and inverter below 50*C (40*C is better). Most reports are that both inverter temps should be within 20*F of ambient under normal conditions. Since Prii operate world-wide in ambient temps as high as 130*F, without any apparent damage, I would set 140*F as my high limit.

    Having said all that, the formula for aero drag is pretty simple and since atmospheric conditions are not in question here, and for comparison sake, assuming they are constants: Fd = 0.5*v^2

    Therefore Fd @ 70 MPH = 0.5*70^2 = 0.5*4900 = 2450 and at 85 MPH > 0.5*85^2 = 0.5*7225 = 3613

    In other words, increasing the velocity from 70mph to 85mph increases drag 47% and therefore reduces fuel consumption by approximately 28%.

    WAY more than you can hope to get back using grill blocking, mirror removal or chassis lowering!
     
  7. Prodigyplace

    Prodigyplace Senior Member

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    I believe removing the side view mirrors would make the car illegal to drive on US roads.
     
  8. Lucifer

    Lucifer Active Member

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    If you drove at 50mph and wanted to save gas, slow down to 35mph, but you are driving at 85mph, with an 06’, ha
    So oil change every 5000, keep the tires fresh and hard, if you want to get better mpg’s get a tesla
     
  9. bisco

    bisco cookie crumbler

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    i have always thought that a boat tail shape at the rear would have an enormous effect.
     
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  10. 05PreeUs

    05PreeUs Active Member

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    Actually, a Tesla is not the most efficient vehicle per mile.
     
  11. Prodigyplace

    Prodigyplace Senior Member

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    They said mpg's not efficiency.
    A Tesla will get infinite mpg's since for any distance traveled it uses 0 gallons of gasoline :D.
     
  12. 69shovlhed

    69shovlhed Surly tree hugger

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    got to have lrr tires first. ecopias are good for that but are noisy and have lame traction. inflate to 40psi. lower the car if you can; some areas have ridiculous speed bumps and poorly graded driveways so you'd beat your front bumper apart. run the a/c intermittently.
     
  13. oil_burner

    oil_burner Junior Member

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    I run ecopias in the summer but right now I am running winter tires, no way around that. Nor can I lower the vehicle since I do have to drive down a gravel driveway for about 2 miles and need the clearance to get over cattle guards and ruts.

    Does anyone have actual figures on those pizza pan hubcaps or rear wheel skirts?
     
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