Advantage...Prius

Discussion in 'Gen 3 Prius Fuel Economy' started by qdllc, Apr 4, 2016.

  1. qdllc

    qdllc Senior Member

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    I learned something interesting on a recent trip to Florida for a funeral.

    Unlike traditional ICE vehicles, the Prius doesn't appear to suffer a significant loss of MPGs for going faster on the highways (e.g., 75 instead of 55).

    A traditional ICE vehicle has to rev the engine based on its velocity because there is a level of physical connection between the engine and drive train. So, going faster in top gear = higher RPMs = more gas consumed.

    In contrast, it appears that the Prius employs only the amount of output power needed to maintain the existing velocity. So, at 70 mph, you do use a bit more gas to get up to 70, but once at 70 you only use the fuel needed to maintain that speed (which may not be noticeably more than maintaining 55). You don't burn more gas because the hybrid system MUST run faster to match the higher speed.

    Even coming home with the Prius was loaded with stuff we were bringing back from Florida, there didn't seem to be any real loss of MPGs.
     
  2. bisco

    bisco cookie crumbler

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    good observation, thanks! it would be very revealing to see high speed mpg graphs of hybrids vs gassers.
     
  3. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    Bob should be along shortly, with a graph to shatter your thesis, lol. Just for warm up: there's no free lunch; mileage drops dramatically as speeds go up. You'll do a lot better on secondary highways, say around 80 kmh (50 mph).
     
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  4. bisco

    bisco cookie crumbler

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    but it's a comparison with gassers, not against itself over varying speeds.

    unfortunately, without a prius gasser, there isn't a perfect comparison. maybe camry vs hycam.
     
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  5. ForestBeekeeper

    ForestBeekeeper Active Member

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    The difference in mpg, for us, between 60mph and 80mph is minimal.

    The same goes for having one passenger, vs four and all their luggage. Even holding it at 80mph there is not a huge difference in mpg.
     
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  6. Tideland Prius

    Tideland Prius Moderator of the North
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    The lower drag due to its aerodynamic shape, the low rolling resistance tires and the Power Split Device's ability to keep the engine at a low rpm will all contribute to the lower highway mpg compared to a regular car. (though CVT-equipped cars are getting close). But physics is physics and I've seen a 8 mpg drop from a steady 50mph to 70mph on the Gen 2 and 7 mpg on the Gen 3.
     
  7. fuzzy1

    fuzzy1 Senior Member

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    Bob's thesis-shattering graph is right here:
    Updated MPG vs MPH chart | PriusChat

    Uncle Wayne over at CleanMPG.com has very similar graphs. Here is one for the 17" wheel version:
    2014 Toyota Prius Package 5 Review | CleanMPG
     
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  8. WolfpackBill

    WolfpackBill Active Member

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    I have numbers very similar to Wayne's post. I know I get around 50-55mpg going 60mph with many ups and downs. Then somewhere in the low to mid-40's mpg when I go a long road trip going around 70mph.
     
  9. bwilson4web

    bwilson4web i3 and Prime

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    What is harder to quantify is the engine 'operating line' and Brake Specific Fuel Consumption (BSFC). On a constant-speed, flat, no-wind, 65-75F day, the Gen-3 engine will easily generate the power needed to cruise at high speeds. We have measured that as the engine rpm increases for more power, the EGR valve setting increases. The exhaust gas cools the combustion temperature allowing a leaner, more efficient engine. But the EGR valve opening peaks at 3,200 rpm.

    In our Gen-3 Prius above 3,200 rpm the car moves towards a different strategy to avoid excessive exhaust gas heat . . . fuel enrichment. One of the Gen-4 features is an increased EGR capacity. It is very likely that this provides a significant part of the 40%, record breaking Gen-4 engine efficiency. It would also slightly reduce the peak HP which is found in the Gen-4 specs.

    As for the OP's original statement, you are absolutely correct that the limited gearing of ordinary cars means the engine is always mismatched to the load. Worse, even the automatic transmissions have been poor at shifting to gears to maximize BSFC at peak efficiency. So they spin the engine faster and partially close the throttle plate so the engine is working against itself. They just can not compete.

    Bob Wilson
     
  10. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    The ramifications of EGR go completely over my head, but I do recall in the initial Toyota press releases they kept mentioning it, implying it mattered.
     
  11. bwilson4web

    bwilson4web i3 and Prime

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    • dilutes fuel-air - with less oxygen, the flame burns cooler
    • cooler flame - reduces NOx and protects the catalytic converter from heat damage
    • allows leaner fuel-air mixture - less fuel burned, more efficient engine
    • throttle can be opened more - the engine breathes easier without 'pumping losses' (i.e., think cocktail straw effect)
    • slightly reduces peak HP - so the engine has lost a few but not so many affect performance
    Sometimes I'm not very clear about why it is important. Perhaps Google might help find some better descriptions.

    Bob Wilson
     
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  12. Former Member 68813

    Former Member 68813 Senior Member

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    yes, unless you consider CVT.

    why would that be? in my case mpg dramatically dives over 75 mph.

    yes, this is exactly what toyota said (without hedging).
     
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  13. tv4fish

    tv4fish Member

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    The difference in mpg, for us, between 60mph and 80mph is minimal.
    The same goes for having one passenger, vs four and all their luggage. Even holding it at 80mph there is not a huge difference in mpg.


    That is not the case for my 2010 - nor was it that way for my 2005. A difference of 20 mph always made at least a 5 mpg difference.
     
  14. fotomoto

    fotomoto Senior Member

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    One trip does not make a trend. For example, the OP may have traveled in favorable weather conditions on both legs; especially the northern outbound leg could have had a tailwind the whole way from the traditional gulf breezes.
     
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  15. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    80 mph, roughly 130 kmh? Oy veh: I get antsy if the posted limit is 100, tend to stay right and around 90, 80 if no one's looking.
     
  16. tpenny67

    tpenny67 Active Member

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    I've found that the length of the trip confounds MPG measurements, as MPG gradually increases over time as the transmission, tires, etc, all warm up which reduces pumping losses, rolling resistance, etc. It can take an hour of driving to reach peak efficiency depending on initial conditions and the speed you drive.

    For example, I was experimenting with my ScanGauge on a recent 3 hour trip in our F-150. At the start, we were getting 18-ish MPG doing about 60mph (more traffic, lower speed limits), then 17-18 doing 70-75 (less traffic, higher speed limits) during part of the second hour, and finally 20-21 with the cruise control set at 63 in the last hour.

    I've found the Prius to have an even more dramatic MPG drop with speed (relatively speaking). On the other hand, it can probably do 80+ and still get double the mileage of the F-150.
     
  17. CR94

    CR94 Senior Member

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    All true, except the part about pumping losses. They should remain very nearly constant with CVT, and increase with fixed gearing---although obviously not enough to overcome the other effects of warmup.
     
  18. Tideland Prius

    Tideland Prius Moderator of the North
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    Fun fact, over 130km/h, the ECO light in the HSI disappears and the 1.8 litre engine switches from efficiency mode to power mode (not to be confused with PWR Mode). i.e., it focuses more on max horsepower. I think it's mostly for autobahn driving so it doesn't feel like the Prius is running out of steam.
     
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  19. tpenny67

    tpenny67 Active Member

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    Oops, confusing choice of terms. I was referring to the losses pumping fluids, particularly those outside the engine where they don't warm up as fast (transmission, etc). Not to be confused with the loss due to pumping air through the engine, which is the canonical pumping loss. Way back when I had a manual transmission that actually used gear oil there was on obvious difference in shifting between starting out on a cold morning and after a couple hours on the highway.

    Which reminds me of the time I tried topping off that car's engine with 10w-40 on a cold winter morning on vacation in NH. Turned the bottle upside down and nothing came out until I gave it a squeeze and that extruded a nice cylinder shaped piece of near-solid oil, sort of like dispensing toothpaste.
     
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  20. Bay Stater

    Bay Stater Senior Member

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    That's funny! The engine must have had metal grinding on metal through the warm up period. :ROFLMAO:
     
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