Why mileage gets worse in winter

Discussion in 'Gen 3 Prius Fuel Economy' started by cwerdna, Oct 30, 2011.

  1. walter Lee

    walter Lee Hypermiling Padawan

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    Agreed. Fwiw - from what I can tell even the dealerships don't see many of them. However, if you do the research you'll see that some people have posted on the internet experimental data on EBH usage. There's some electric energy cost vs MPG benefit analysis, reliability data, system setup, installation tips, and useage warnings as well on EBH.
     
  2. a priori

    a priori Canonus Curiosus

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    I wonder if there aren't more people who used them on Gen II vs Gen III. The new recirculation system added with the Gen III brought the temp of the ICE up faster and provided cabin heat faster. These are the two main reasons for using an EBH. I had one with my 2007, but I haven't gotten one for my 2010. I would put one in if it were dropped on my front porch, but I don't feel the same need for it that felt one those cold mornings in '07, '08 & '09.
     
  3. FrankTiger

    FrankTiger Member

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    Hi everyone [​IMG]

    This post is in the Technical Discussion but I believe it also fits here.
    In this post I showed you how I measure the energy flow within our GenIII Prius. I have been recording the necessary data since December 2011 so I can plot the different energy flows in warm and cold days in my usual trip described here.

    Just to refresh my usual trip, that descends 28.1m (92.2ft) of elevation, this is the scheme:


    [​IMG]

    And this is the scheme of the power flow I measure within our Prius:


    [​IMG]


    In the colder days, the mean ambient temperature of the four trips I averaged here was 3.2ºC (37.8ºF). The speed was 42.8Km/h (26.6mph) and the fuel consumption was 3.80l/100Km (61.9mpg). The energy flow in kilojoules was:

    [​IMG]

    You may notice that the ICE efficiency in these cold days was 6860/21028 = 32.62%

    The arrow that goes into the ICE represents the (135kj) energy drained by the ICE when is turning with injection cut-off. This energy goes to pumping air and internal friction.

    In the warmer days, the mean ambient temperature of the six trips I averaged here was 23.5ºC (74.3ºF). The speed was 43.6Km/h (27.1mph) and the fuel consumption was 3.23l/100Km (72.8mpg). The energy flow in kilojoules was:

    [​IMG]

    The ICE efficiency in these warm days was 5612/17878 = 32.39% just a bit lower than in colder days.


    We can also display the differences of the energy flow between the cold days Vs the warm days and see where the energy goes to.

    [​IMG]

    We see that the final destination of the 3150kj of chemical energy used in cold days Vs warm days is heat. Two thirds of this heat is generated in the ICE (as always) and of the remaining third, some 65% is lost in PSD and transmission friction, some 30% is lost in heat due to tires friction and some 9% is lost in heat in the inverters. Those numbers do not total 100% because the lower aerodynamic drag (due to lower speed) and the lower hybrid battery current adds the extra 4%

    From my data, it seems that the main manageable culprit of the lower mpg during winter is the PSD and transmission friction. Tire friction is physically linked with temperature, so it seems impossible to overcome unless tire pressure is dynamically changed. Inverter and ICE efficiency do not change with temperature, but both systems have higher loads and also absolute losses in winter due to the increased friction of PSD, transmission and tires.

    I hope you like the post.

    Big hugs from Frank
     
  4. F8L

    F8L Protecting Habitat & AG Lands

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    Thanks, Frank!
     
  5. SteveWlf

    SteveWlf Old-on-Hold

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    I am new to the Prius (family) and currently driving a 2002 classic. I started driving this last early Nov., here in the Pac NW USA. The weather was moderately warm yet and our Rainy Season hadn't started yet, so road were mostly dry. Under these conditions, I was getting a 48 mpg on steady and relatively level freeway driving. As a Prius Newbie I was driving conservative, using the recommended procedures for getting the best mileage. Typical speed were consistant with the traffic and 60 to 70 mph limits on the same 100 mile stretch of highway. I used the cruise control flat areas and a feathered throttle on the slight uphill grades and allow for some regen on the down hill. Always an eye on the fuel consumption numbers. I feel I was getting optimum mpg.

    And then the seasonal rains began. The next couple trips, I found it difficult to get over 42 mpg. At first I thought it might be some winter blend of gas but, eventually I found that the water on the road surface and in the air was adding signficate resistance. To confirm the road surface theory, I noticed if I drove in the right lane that had been worn down by the heavy trucks, there would be a constant puddle of water in this low area. Mileage in the right lane was reduced by a several mpg over what I could get by moving over to the left, less worn lane. Now I might point out that I did this experiment over a dozen or so miles on flat highway and while it was between rain showers and no significate wind.

    Late, when I came into some rain showers, still no wind, the mileage dropped in left lane by another 2 mpg and the water now being deeper in the tire tracks on the right lane, my mileage went down to less than 40 mpg. I should also be noted that my highway speed was reduced by about 5 to 10 mph during the rainy runs.

    I also notice that if I was in the truck worn lanes and following a car or truck at the normal safe distance, there was an effect on the mileage since, I theorize, the vehicle ahead and displaced the surface water long enough for me to take this advantage.

    I know it is possible to "draft" behind a large truck and gain as much as 5 mpg gain, but this is not recommended. Brave truck drivers and some crazy driver have used this method over the years but again, not recommended.

    Steve
     
  6. bwilson4web

    bwilson4web i3 and Prime

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    Originally posted January 2010, I've edited my post to reorder the test cases and added standard units:

    Blocking the fender, air inlet reduces the amount of drag creating air that goes into the engine compartment. This drag effect becomes more severe with high-density, cold air. So I wanted to test the original configuration and two blocking approaches: recessed and conformal with the front bumper.

    TEST A - 51.6 MPG @70 mph (4.56 L/100 km @112 km/hr)

    [​IMG]
    This is the baseline, air inlet.

    TEST B - 53.3 MPG @70 mph (4.41 L/100 km @112 km/hr)

    [​IMG]
    The recessed block leaves a 'parachute' that spills air out the edges. This air carries away energy, increases drag. However this is consistent with ordinary, air inlet blocking. The conformal block avoids the air spillage out the corners and provides what should be the minimum, drag.

    TEST C - 53.6 MPG @70 mph (4.39 L/100 km @112 km/hr)

    [​IMG]
    The conformal cover has no significant lips that allows air to slow down and then spill back into the slip-stream. The 'hand hold' cutout is leftover from the cardboard box used to make the block and given its location, no air would spill out.

    Protocol and test conditions
    • 57F (14C), dry pavement, humid, altitude ~650 ft (~200 m)
    • two runs, West and East, south wind at ~10 mph (16 km/hr)
    • I-565 Jordan Lane Huntsville AL to Mooresville AL
    • cruise control maintained speed, GPS calibrated
    • does not include acceleration to speed or deceleration at turn arounds
    • +13 miles (20.8 km) each leg, added together for total performance
    • 2010 Prius (VZW30), 8,500 miles ODO, OEM tires, 45 psi (310 kPa)
    • ICE and transmission oil changed at 5,000 miles
    • Shell 87, E10
    • Warmed up at least 20 minutes before first run
    • Disposable, cardboard shields held with duct tape
    Test notes:
    1. The test temperatures were close to a 'standard day' so it should be fairly easy to calculate the effects of air-temperature on vehicle drag and performance.
    2. Although I have both ScanGauge II and AutoEnginuity, I used the engine fans to determine if it was getting too hot. The engine radiator fans did not come on during these tests.
    3. To estimate the temperature effects on mileage, divide it by the percentage air density change. For example, unblocked air inlet 51.6 MPG @59F (15C) and 70 mph (112 km/h) should be 51.6/97% ~=53.2 MPG @77F (25C). The partial block MPG improvement, 51.6 MPG to 53.3 MPG is roughly equal to raising the effective temperature ~18F (~10C.)
    C F % drag
    1 35 95 94%
    2 30 86 95%
    3 25 77 97%
    4 20 68 98%
    5 15 59 100%
    6 10 50 102%
    7 5 41 104%
    8 0 32 106%
    9 -5 23 107%
    10 -10 14 109%
    11 -15 5 112%
    12 -20 -4 114%
    13 -25 -13 116%

    Source: Density of air - Wikipedia, the free encyclopedia

    Bob Wilson
     
    #86 bwilson4web, Dec 15, 2012
    Last edited: Jun 9, 2014
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  7. priusCpilot

    priusCpilot Active Member

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    Wow good data posted here. So then whats the best way to keep the PSD and transmission hot?
     
  8. Scott M

    Scott M New Member

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    Great read!
     
  9. SeniorMoment

    SeniorMoment New Member

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    My personal experience suggests fuel is a big factor at least in climates such as Western Washington where the typical winter daytime temperatures are between 43 and 55 degrees. In order to comply with emissions standards urban areas typically are required to add at least 10% ethanol during winter. When we bought our 2007 Prius we quickly noticed a 10% drop in fuel economy when winter fuel blending was in effect. Then the State of Washington required year around use of ethanol and that 10% fuel economy drop became year around. In other words adding 10% ethanol lowered our fuel economy by 10%.

    The service tech at the dealership explained the reason. He said that because the ethanol added fuel has less energy it takes the Prius engine more running time to recharge the main battery each time that is needed, which is a big part of the lower fuel economy. So in this case, a measure intended to help the environment actually increases the number of gallons of gas burned annually with respect to at least our 2007 Prius and probably all later ones as well.
     
  10. gliderman

    gliderman Active Member

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    I think the ICE run time is the same. But since the fuel has less energy, it takes more fuel to get the same amount of power from the engine.
     
  11. SeniorMoment

    SeniorMoment New Member

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    Callie has made some respectable points, but I believe all but the snow are marginal contributions, but they may reduce fuel economy by more than the 10% loss we experience in the New England and central Northern states.

    1. Certainly what gasoline you use can make a small difference, but states set the standards for gasoline in their state along with whether or not fuel pumps need to compensate to accurately deliver fuel year around. My state does not require that, but the last state we lived in did. The volume of fuel does vary with temperature, but that is somewhat mitigated by storing the fuel tanks underground where the ground temperature more closely reflects the annual average temperature, unless the fuel was just recently delivered after sitting in a fuel delivery truck at atmospheric temperature.

    2a. Colder air is slightly more dense, but density only affects the partial pressure of oxygen and the car's computer and sensors are supposed to compensate for that in determining the amount of fuel to inject. Intake air in modern cars is also normally preheated by running it over the engine before combining it with fuel. The engine also quickly heats up. After all the entire purpose of the cooling systems (fan, pumps and radiator, as well as air flow) is to keep the car from overheating. Only the thermostat and in a Prius maybe the flow control value assembly work to retain heat. All heat generated by a combustion engine is waste heat, except what is diverted to heat the interior.

    2b. We all live under high pressure with the weight of the atmosphere above us equal to 14.7 pounds at sea level per square inch. Why that doesn't crush us is because it is all around us and our bodies have enough pressure to compensate. The air does get lighter the higher the altitude but not that many people live at altitudes about 6,00o feet, and again the computer is supposed to compensate using the oxygen sensor to adjust the fuel air mixture to be appropriate for the altitude and that has been an EPA requirement now for many years. Colorado in particular lobbied heavily for that requirement.

    A bigger factors is almost certainly the amount of wind, since winter storms can pack a lot of wind, and having been in a blizzard myself it is not fun and also forces cars to drive slower and perhaps even in lower gears that do burn more fuel. The Colorado Rockies stadium is bigger but the driving reason was that Denver is a regional hub for sports and itself fuel of sports fanatics. The Denver Broncos for example always filled the stadium even when they added thousands of seats to it, so baseball stadium planners had to consider that, and remember that although distance records are set in the Colorado Rockies stadium because of less air pressure, such things as speed of the ball, etc. are measured in units as small as hundreds of a second or feet. The Baseball Commissioner rules set the actual size of the diamond on the field.

    2c. Water temperature rise quickly in a Prius and only vehicles used for exceptionally short trips will be running at too low a temperature on the average trip. A bigger factor I understand is likely to be the main battery has a small heater to keep it from losing too much current storage capacity in the winter. Again the fuel injector calculations do take temperature into account. In fact the number one failure item I understand on a Prius is sensors because there are so many of them. People in really cold states and those living at high enough altitudes and low enough temperatures also usually run a radiator fluid heater or dip stick heater to aid in starting up in the winter. In fact, a student at the college in Gunnison, CO, told me that if you didn't run your car everyday long enough to heat it up in the student parking lot you would not be able to start it until winter was over.

    Also keep in mind that modern cars, including the Prius have less friction than old cars due to cold temperatures because they used multiple viscosity oil which starts out at a viscosity of only 5 (or 10) and increases to 30 (or more, depending on the oil used) which get thicker as they get hotter, which compensates for the change in engine friction.

    3. Remember all heat generated by the engine (and the battery discharging) is waste heat, which is which is why the main battery is vented to keep it from overheating. My Prius in fact has 4 water pumps to help dissipate the heat faster. However when the car is running only off the battery, I sometime get curious as to whether the heater is electric or whether the intervals between recharges is small enough nothing special has to be done. I just have never been curious enough to look it up.

    4. Snow. Snow obviously affects mileage, so I don't dispute that at all, but it is hard to know whether the cumulative mass of the snow is why, loss of highway traction, and/or cars slowing down to avoid slipping on ice or sliding into other cars. Impaired visibility is also a factor in a safe driving speed, and few if any cars function more efficiently at low speeds. I am uncertain if electricity is used in the front window defrost, but it is used and a tiny drain on the car's power for heating the rear window glass. A bigger draw is the heater/ac blower fan, but in general that is used year around so wouldn't lower winter mpg.

    5. Daylight is a factor for people who live in Northern USA latitudes, because the winters are both darker and the nights longer. This often means unless one direction of a commute to work is in darkness. The headlights do use a lot of power as anyone who has left them on in a car with no automatic shut off learned early in their driving career, heat though to the extent it is supplied by the engine is waste heat, which means it is an undesirable bi-product of internal combustion, but conquering it as a problem requires perfecting a ceramic engine, which has yet to have been achieved. Also the brighter, non-OEM headlight bulbs draw more current (and burn out sooner, although the extra light they put on the roadway is worth it for those who do a lot of night driving on unlighted highways).

    6. A factor not mentioned is that generally speaking cold batteries provide less current capacity, and I assume but do not know for certain this is also true of the Prius main battery, and if it is that means the engine will have to run more often to recharge the battery for lack of enough storage capacity. This however is only conjecture on my part.

    7. I believe the biggest factor is the use of ethanol in winter fuel, because when my states started requiring year around use of ethanol my winter fuel economy became by summer fuel economy, except when we travel through other states which have not imposed that requirement. In those states our fuel economy increased even going 70-80 mph.
     
  12. markabele

    markabele owner of PiP, then Leaf, then Model 3

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    Colder batteries are definitely a part of the issue. Talk to anyone that has a PiP or EV and they will tell you how much their range goes down during the winter.
     
  13. Corwyn

    Corwyn Energy Curmudgeon

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    This is quite simply not possible (if we have any knowledge about how fuel is burned in engines). Your claim that adding 1/10 of a gallon of ethanol to 9/10 of a gallon of gas provides the same energy as the 9/10 of a gallon of gasoline alone would mean that the ethanol was inert in the reaction, and should be coming out the tailpipe. Otherwise those 7600 BTUs have to go somewhere. And those race cars that drive on ethanol should be awfully slow...

    The difference should be around 3.3%.
     
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  14. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    ^ The way I look at it: if adding 10% of something results in a 10% drop in fuel economy, you're going to have a revolt on your hands, sooner or later...

    Or there should be!
     
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  15. hyper blue

    hyper blue New Member

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    I was averaging about 54 mpg on my 2010 (gen III) last summer, courtesy of hypermiling, pumped and taped tires and a K&N filtercharger. Then in October I switched to Mobil one 0-20 AFE, stopped using a/c and my mileage went to 60 mpg (all mpgs calculated by Prius, so perhaps 3% optimistic). Now in December (Washington, DC) I use a block heater and winter gas and am averaging 59.5 MPG. I am hoping to beat 60 MPG in the spring when we get summer gas back. Is anyone selling precut lower grill blocks?
     
  16. markabele

    markabele owner of PiP, then Leaf, then Model 3

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    What is taping tires?
     
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  17. pfour2131

    pfour2131 New Member

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    thanks for the links!
     
  18. BadAssPrius

    BadAssPrius I got to make my Prius stand out all on its own!

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    when i sitting in my car with the heat on every so often the engine turns on to charge the battery to keep my heat going. So that how my MPG get worse.
     
  19. Andre V.

    Andre V. New Member

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  20. qdllc

    qdllc Senior Member

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    My 2 cents....

    I know that ethanol hurts mileage. It may make your gas "cheaper," but the numbers don't like....you get better power and MPG from 100% pure gas than any ethanol blend because it doesn't burn as efficiently. Maybe a "flexfuel" vehicle does better, but ethanol is "snake oil" for dealing with the energy crisis issue.

    So, you get lower MPG on an ethanol blend than on pure gas in any situation where all other variables remain the same.

    Also, fuel is mixed based on time of the year and location/elevation. Winter gas is different from summer gas. This plays a role. However, I find my mileage "hurt" in winter or summer depends on the vehicle and heat/AC use. A standard ICE vehicle might do better in winter because the "heat" is free while a Prius needs to run the ICE as needed to keep the coolant hot enough to provide heat. Likewise, the Prius might do better in summer (even with AC) because the AC is 100% electric and the ICE doesn't have to run to generate heat for the cabin. In winter, the AC is still needed to defrost the cabin...even if not so much as it's used in summer.

    I find spring/fall is the "sweet spot" for energy use. Outside air is good enough for comfort so neither side works that hard.
     
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