2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jetta TDI

Discussion in 'Volkswagen' started by xcel, Apr 16, 2014.

  1. SI_Prius

    SI_Prius Well-Known Member

    Re : 2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jetta TDI

    I like playing with numbers and I would like to to discuss those BSFC numbers a little further. Sorry for the metric units ;)

    - The TDI is supposedly achieving peak efficiency of 190 g/kWh so 1 kg of fuel gives you 5.2 kWh and 1 liter of diesel (0.840 kg/lit) gives you 4.368 kWh
    - Atkinsonized Accord petrol engine gives you 214 g/kWh or 4.6 kWh/kg or only 3.5 kWh/l (0.750 kg/l)

    The difference per volume of fuel is HUGE on paper, diesel engine is supposed to get 25% more energy out the same volume of fuel, therefore a diesel engine should get you 25% more MPG on cc on the highway than a hybrid because of the engine alone. Then you have drive train loses that are bigger on the hybrid because of energy transferred to generator and el. motor than a normal TDI with the stick, we are talking about another 3-5% easily.

    Then I look at the Accord and Passat TDI speed vs fuel economy and Accord is better at the 50 MPH and at 70 mph Accord is only 12% behind in MPG.

    Where did that 28+% boost of the TDI MPG go?

    I can tell you: The 2.0 TDI is achieving it's peak efficiency at higher load, I would guess that around 50 kW (250Nm @ 2,000 rpm), that kind of power will get you 100 MPH on the straight easily, but at that speed engine on a MT will not rev at 2,000 rpm. Atkinson petrol engine on the other hand will have it's sweet spot considerably lower (12-30 kW?) and you are using that power a lot more than 50 kW. Even when you are hypermiling I seriously doubt that you are getting in this high region, it's just not efficient driving, or am I wrong? Under lighter load a hybrid petrol engine must be at least 5% more efficient than diesel engine to achieve those final numbers at 60 MPH, because of less energy per volume, remember BSFC number is per mass not volume.
     
  2. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Re: Re : 2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jetta T

    Pulse and Glide. ;)

    And I did 80 mpg on my 10 mile commute this morning in my 27/33 rated Fit. What would the 32/45 TDI give me?
     
  3. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Andrew:

    100. :p ;)

    SI Prius, as you have somewhat surmised, where the Accord HEV and most hybrids falls apart is high speeds with higher loads. Set CC at 75 mph in the AH while crossing the desert and 40 is gone. Do the same with the Passat TDI and 40 + is a gimme. Yet one is rated at 45 and the more efficient is 43.

    Wayne
     
  4. RedylC94

    RedylC94 Well-Known Member

    Another way to look at that diesel-hybrid comparison is that hybrids have tricks which overcome their less efficient engine and tranmission at low speeds, but those tricks don't apply at high speeds. There's no real mystery about it.
     
  5. tribosessive

    tribosessive Well-Known Member

    I can't wait for xcel to work his hypermiling magic on this car. A Golf or wagon sounds good. I will also look at the 1.8T.
     
  6. seftonm

    seftonm Veteran Staff Member

    Hybrids have often had aerodynamic enhancements and lower rolling resistance tires, which help on the highway.
     
  7. RedylC94

    RedylC94 Well-Known Member

    Yes, that's much of why the diesel advantage at 70 mph was less than the theoretical 28% SI Prius calculated.
     
  8. phoebeisis

    phoebeisis Well-Known Member

    Wayne
    Will it actually make up the 15% diesel fuel premium?
    To match the Prius it would need to get maybe 65mpg at 60 mph?

    If this new VW is driven 60 mph-what does it deliver?
    Usually 45 EPA is maybe -55mpg at 60 mph-

    And the AT-gets about the same hy EPA as the MT-?

    The diesel fuel premium-is absolute POISON for small car FE delivered at a reasonable price-poison-usually makes them pointless relative to the Prius
    Except of course the Prius is a bit of a DUD- fun wise-yeah typical dull Toyota
     
  9. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Charlie:

    Here in IL and especially in California, diesel costs less by as much as $0.20 per gallon. Beating the $0.60 diesel premium this past winter would have been very tough to bare for a TD driver.

    Too bad as it takes less energy to refine diesel (assuming lower cost too)?

    Prius is still a go to vehicle due to not only its all-around fuel efficiency on Regular unleaded but its low TCO because of somewhat low up front and minimal maintenance costs. It is just that it needs to be updated in the very worst way with better seating and driver control ergonomics, better chassis dynamics, much better safety, and quite a few more $'s spent on the interior bits including both touch points and appearance plus a massive upgrade of the infotainment system(s). 2015 is coming soon which should help that situation.

    Driving some of the excellent super TDs from Germany as of late, Toyota has to provide their A-Game on the redesign if they do not want to get passed. Driving a Prius across the country is ok, driving an ES 300h would be my idea of the best Hybrid offering to date for that drive. Really nice. Driving any of the VW TDI super diesels and very expensive super super diesels (Audi TDIs, MB and BMW TDs) across is a whole lot easier and much more enjoyable however. They just never strain if that helps describe the situation?

    I wish more could drive the various technologies side by side. The Asian's have refined the hybrid to world class around town super cars and pretty good open road vehicles. The German's have had the TD ace up their sleeve for decades and they have finally figured out how to bring it to the states with low NVH, huge performance, and the always welcome huge fuel economy in vehicles most would love to drive. Just that I cannot afford to drive them as a personal vehicle. :(

    Wayne
     
  10. SI_Prius

    SI_Prius Well-Known Member

    Re : Re: 2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jetta T

    I would like to see a source of that info and particularly how much less energy and don't forget the de sulphur process.

    There is limited amount of energy in a barrel of crude, but with refining it you end up with much more volume of end products (like popcorn). Gasoline just gets bigger bump in volume than diesel. In the end no mater how you put 50 MPG of diesel is not the same as 50 MPG petrol. If you would measure consumption by mass (kg, lbs) you would end up with a lot more fair results.

    There is a specific point in using more diesel in US, that is true and I support you on the diesel propaganda. That is because you use it so little, but the exact opposite is true in Europe where diesel to petrol ratio is at 7:3, no refinery is able to produce that ratio (even more energy and effort is taken to refine diesel), so luckily for Europe they can buy your American excess diesel and sell you their excess petrol. :Banane35:
     
  11. Trollbait

    Trollbait Well-Known Member

    How far is your daily commute? What is the average speed? How many stops?
    The average speed for the city test is 21.2mph with 23 stops over 11 miles.
    The highway cycle average speed is 48.3mph with no stops.
    http://www.fueleconomy.gov/feg/fe_test_schedules.shtml

    Yes, an Interstate in stop and go traffic isn't highway miles, but a suburban road with few lights and near constant moving traffic isn't city either. The city and highway test cycles, along with the split used for combined numbers come from driving patterns in the 1950s. The city is basically driving along the streets in the heart of L.A. from that time.

    While those driving in the heart of the city, or in regular gridlock, meet those conditions, many aren't at such extreme. Meanwhile, the highway test isn't at the opposite end of the extreme for modern driving. It is an issue of perception. People hear highway and think interstate, and anything not on what a modern person calls a highway falls into city.

    Any road that allows 55+mph speeds for extended stretches isn't a highway from the perspective of the test. Driving on county roads is closer to the highway test, and the roads most take between their housing development and the mall or work aren't as rough as the city test.

    43/57 split was selected for the MOVES program and GREET models because it more accurately reflects conditions outside the lab. Maybe the 55/45 split did so at one time, but is there any proof that it is so of the current driving population, or is it used for the combined number on the window sticker because that is what is used for figuring CAFE numbers?
     
  12. Trollbait

    Trollbait Well-Known Member

    Re: Re : Re: 2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jet

    I don't know the energy difference for the end products, but refining gasoline is more involved than diesel.

    Distilling off gasoline will take less energy than for seperating out the diesel. Unfortunately for us, there are two big issues with this straight gasoline. Its octane is only in the 60s, and, the major issue, the amount of straight gas recovered will meet about 20% of our needs. So we need to make more by cracking the larger molecules in petroleum into smaller ones. Then we reform the small ones from the cracking into medium sized ones in order to get an acceptable octane.

    Diesel on the other hand is pretty much just a straight distillation. Sulfur removal adds to the process cost, but that needs to be done with gasoline also. It isn't naturally sulfur free.

    We aren't lucky that Europe is buying our diesel. European demand is part of why diesel prices here have gone up.
     
  13. wxman

    wxman Well-Known Member

    According to Argonne National Laboratory's GREET model (GREET1_2013), diesel fuel (ULSD) processing and refining in the U.S. is 81.2% efficient (including desulfurization) and gasoline processing and refining is 76.5% efficient.

    Gasoline will be required to be "ultra-low sulfur" in the U.S. by 2017, so the gap may actually widen when that happens.
     
  14. Trollbait

    Trollbait Well-Known Member

    The energy going in then is close for both fuels.
    45.9MJ/L for ULSD, and 44.7MJ/L for gasoline.
     
  15. WriConsult

    WriConsult Super Moderator

    Actually, an average of 21.2mph with 23 stops over 11 miles sounds exactly like average suburban traffic to me. No way am I going to average 21.2mph in the city unless it's late at night. 11 miles of surface-street driving in any city I'm familiar with (I don't mean downtown core, just the former streetcar suburbs of a municipality at the center of a typical metropolitan area) is a pretty long trip and would involve a lot more than 23 stops. Not to mention lower-than-EPA-city mpg for most drivers. The sprawl of L.A. with its massive car-oriented boulevards might have higher speeds, but L.A. is an outlier. ;)

    I agree, it is an issue of perception. People hear city and think downtown urban core, but that's not really what the current EPA schedule is intended to do.

    Right. Which, if anything, is an argument for biasing an combined figure even more heavily in favor of the "city" number.

    Maybe it would be better if we got away from the misleading "city" and "highway" labels, and adopted "local" and "rural". I think if you look at it that way -- the so-called "city" figure should be an estimate for the median of commuting and local errand-running, and the so-called "highway" figure (as you point out) an estimate for lower-speed driving on rural highways, I think we would find that 55/45 is still an accurate estimate of how average Americans' miles are split between the daily grind and roadtrips.
     
  16. SI_Prius

    SI_Prius Well-Known Member

    Re: Re : Re: 2014 NYIAS: The Most Fuel Efficient Automobile in the US Is? 2015 VW Jet

    And when US demand goes up, it will suddenly become cheaper?
     
  17. SI_Prius

    SI_Prius Well-Known Member

    There are very different numbers around the web, the best way to know how much energy difference is between gasoline and diesel is to weight them both, because per weight the energy content is very comparable.

    But the numbers that I have found on wikipedia are:
    Gasoline: 32 – 34.8 MJ/L at 76.5% efficiency that gives us 41.8 - 45.4 MJ/L
    Diesel: 38.6 - 40 MJ/L at 81.2% efficiency = 47.5 - 49.2 MJ/L

    Is there a candidate to weight the same amount of fuel? :)

    And let me repeat: the refinery that maximizes one fuel will make less efficiency on that fuel, the above efficiency numbers are true for the US but they are most definitely not true for Europe.
     
  18. wxman

    wxman Well-Known Member

    There was a fairly recent GPS-based study of driving profiles in Southern California. According to that study, median driving intensity was between the EPA highway cycle and US06 cycle (http://www.erc.wisc.edu/documents/symp09-Freitag.pdf - slide #24). "Urban" type driving actually does not appear to make up a very great percentage of driving based on that graphic.

    It still doesn't appear that the current 55% city/45% highway is representative of average drive cycle in the U.S. and EPA needs to update that cycle mix (at least be internally consistent with its MOVES model).
     
  19. SI_Prius

    SI_Prius Well-Known Member

    Ha ha, there is a Prius on that slide and there it says real world hybrid driving consumption 6.7 l/100 km. They did get the diesel right at 5.8 l/100 km
     
  20. WriConsult

    WriConsult Super Moderator

    I still think 55% "local" (mixed urban/suburban driving) and 45% "rural" driving is pretty accurate, if we follow reasonable definitions of those words and don't get hung up on unrepresentative "city" and "highway" labels. "City" vs. "highway" is not a realistic way to characterize people's driving patterns. My point in bringing all this up was not to get into a huge debate about the average mix of driving, but to point out that 0%/100% is NOT representative of people's average driving mix.

    I think we can all agree that neither EPA cycle is representative of as large a slice of real world driving as it should be. The average person drives faster on the "highway" than the EPA "highway" cycle takes into account. And the average driver, more suburban than urban, spends a considerable amount of time idling at stoplights and in stop-and-go/slow-and-go traffic, which the EPA "city" cycle does not take into account at all. Calling that "city" driving would be misleading anyway: in my experience American suburbs - where most Americans now live - are heavily congested these days, an uncomfortable truth for a lot of people.
     

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