Hypothetical question - Prius minus the HSD

Discussion in 'Gen 2 Prius Technical Discussion' started by Ophbalance, Mar 11, 2012.

?

Does the HSD matter at any speed?

Poll closed Mar 25, 2012.
  1. Yes

    18 vote(s)
    100.0%
  2. No

    0 vote(s)
    0.0%
  3. I seriously don't care!

    0 vote(s)
    0.0%
  1. qbee42

    qbee42 My other car is a boat

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    Pretty much. I was thinking of pushing it up to speed with a helper vehicle.

    Tom
     
  2. SageBrush

    SageBrush Senior Member

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    All pack use is "net zero."

    You debate partner seems to think the regular Prius is a plug-in
    The question is not "has another car ever matched the Prius?", it is "why doesn't every car on the road today match the Prius?"

    A reasonable answer is "other cars have more power and weigh more." Fine, but then the CRX is out of the running. At this point it becomes an exercise in finding a comparison conventional vehicle on the road today of similar weight and power to a Prius, and then comparing highway fuel economy.

    The answer will still be ambiguous, because the cars have different speeds where the ICE is operating at its most efficient and so different air and transmission frictions come into play.
     
  3. seilerts

    seilerts Battery Curmudgeon

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    If I defeat IMA in the Insight, then I get better mileage on the highway. Since boost/regen is nowhere near 100% efficient, probably more like 50%, whenever the battery supplies energy, it has to be paid for later in some way by the ICE, and there is some energy loss. Prius is a little better than Gen 1 Insight in that regard, but not by much. Tell your friend that he can buy an old Insight, toss away the pack, and get 60 MPG on the highway.

    But yes, from a physics standpoint, a hybrid will do worse MPG at highway speeds than its closest non-hybrid equivalent, if you take into account the dead weight of the pack, and the inefficiency of boost/regen in highway use only. However, for the Prius, the pack and MGs are very necessary to make the transmission work like a CVT, and just about any hybrid will have a driveability issue, namely lack of power for hills and acceleration.
     
  4. SageBrush

    SageBrush Senior Member

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    Here he is on firmer ground, although "hybrid" is too generic. His car still likely loses to a Prius, but the reason is the Prius' low Cd.

    And by the way, a constant speed of x mph is very rarely a constant ICE power output unless the land is completely flat and no traffic is on the road and there is no wind .... Hybrids are *much* more efficient at handling varying power demands than conventional cars
     
  5. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Detractors keep saying "if they made a car like the Prius but with out the HSD" but some of those of us driving a Prius have owned the most fuel efficient cargo moving car available for decades, when THIS decade started, that was a Prius.

    1962 Falcon Station wagon
    1971 Ford Torino Station wagon
    1976 Mazda 808 Mizer Station wagon
    1978 Mazda GLC Station wagon
    1982 Ford Escort Station wagon
    1989 Toyota Corolla Station wagon
    2001 Subaru Forester SUV
    2009 Toyota Prius Liftback

    Can't you just see a Prius v in my future?
     
  6. SageBrush

    SageBrush Senior Member

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    Funny that the detractors never ask themselves *why* a Prius sans HSD is not on the road.
     
  7. usbseawolf2000

    usbseawolf2000 HSD PhD

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    Just present them with Camry 4 cylinder vs. Camry HSD. Which one has higher fuel economy on the highway? EOD.
     
  8. RobH

    RobH Senior Member

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    Non-hybrid vehicles have engines that are large enough to provide good acceleration. The engines are way overpowered in terms of maintaining highway speeds. The result of this overcapacity is that the engines are typically operating at a very inefficient power level.

    The basic reason the Prius gets such good mileage is that it uses a very low HP engine. In addition, the Atkinson cycle engine is more efficient that an otherwise similar Otto cycle engine.

    A Prius with the standard engine, but without the electric motor, might get slightly better highway mileage than with all the electric equipment. But it would accelerate like an underpowered truck. Totally unacceptable in today's market. The electric drive compensates for the deficiencies of the small Atkinson cycle engine. The result is a very efficient drive system that provides normal performance.
     
  9. Dolce_Vita

    Dolce_Vita Member

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    Don't forget the weight of the whole hybrid system. A Prius with it's 76hp ICE and (for example) a 5 speed manual gearbox would be a lot lighter than a normal Prius. This weight loss would compensate somewhat for the lack of power - which would probably make its 0-60mph times go from the stock 11s to about 13.5s, which isnt as slow as it sounds (my car does 0-60 in 13s). It would be interesting to see someone build a Prius minus hsd...
     
  10. usbseawolf2000

    usbseawolf2000 HSD PhD

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    I wouldn't say a lot lighter. Prius c weights about 200 lbs more than 5 doors Yaris 5 speed Manual. They both have 1.5L ICE but Prius c has longer wheelbase and 4 cubic feet more interior volume. Prius c HV battery weights 68 pounds.

    Without the HSD, the car will need a starter, alternator and bigger lead acid battery to start the engine.

    The bottom line is that, HSD is not just electric motors with a battery pack added on. HSD united the gas engine, electric propulsion and electric transmission into one integrated package.
     
  11. miscrms

    miscrms Plug Envious Member

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    I'm not sure there is any point arguing with someone who is not listening. I just did a 350 mile weekend trip from Phoenix to Tucson with a family of four plus all the luggage a family of four needs for a weekend. Speed averaged 70-75 mph all the way down and back. Final result was 54.5 mpg. I'd love to know what non-hybrid vehicle could carry a similar load at a similar speed in similar comfort and safety and return the same result. A diesel Jetta might get close, though the 2012 6sp is still only rated 42mpg.

    Its not the battery/motors that make it efficient on the highway. Its the battery/motors that allows it to use an engine that's that efficient on the highway. Not only would it be a real dog without the electric assist, its city mileage would probably suck because you'd spend so much time with your foot on the floor trying to get it to move. The battery/motors not only let you accelerate at a reasonable pace and recapture braking energy, but keeps the engine operating near peak efficiency under nearly all load conditions rather than just the low rpm/high load most vehicles need to operate at peak efficiency. Figure 11 on page 9 says a lot IMHO.
    http://www.transportation.anl.gov/pdfs/HV/454.pdf
     
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  12. miscrms

    miscrms Plug Envious Member

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    Ok, so we can't build a non-HSD Prius very easily, but we can model one. Time for some fun with math. ;)

    Here is my basic method, please feel free to point out any inconsistencies, as vehicle/engine dynamics are certainly not my field of expertise.

    1. Start with the BSFC fuel consumption maps for various engines posted here:
    Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

    2. Use the EVcalc to estimate hp required for a steady state cruising Prius at 60mph and 80mph.
    EV Calculator

    3. For BSFC maps w/o power curves, use this site to pick a favorable spot on map with appropriate power output at correct rpm ratio for 60/80 mph:
    WEN Technology - Calculators

    Example Calculation:

    2004 Prius cruising at 60mph:
    From EV Calc stock Prius power required = 19.256 HP
    19.256 HP = 14.365 kW
    From BSFC map, estimate 235 g/kWh at this power output
    235g/kWh / 60 mph * 14.365 kW = 56.263 g/mi
    2754.7 g/gallon / 56.263 g/mi = 48.96 mpg

    Close enough for government work :)

    Using this method we can then experiment with substituting other engines into the Prius and calculate the estimated mpg at 60 and 80 mph. Assumptions made for each case are listed at the end.

    Case ---------------------- 60mph -- 80 mph
    2004 Prius Stock ---------- 49 mpg -- 36 mpg
    Prius No HSD -------------- 50 mpg -- 37 mpg
    G2 Prius 1.8HSD ----------- 53 mpg -- 38 mpg
    G2 Prius Corolla 1.8L ----- 41 mpg -- 30 mpg
    G2 Prius Saturn 1.9L DOHC - 41 mpg -- 33 mpg
    G2 Prius Geo 1.0L --------- 47 mpg -- 30 mpg
    G2 Prius Honda 1.0L Lean -- 57 mpg -- 34 mpg
    G2 Prius VW 1.9TDI -------- 50 mpg -- 40 mpg ** diesel mpg
    G2 Prius VW 1.9TDI -------- 45 mpge - 36 mpge * gas equiv

    Based on that I'd say the stock G2 and G3 Prius design stacks up very well particularly considering that this sort of high speed steady state operation is the least favorable state for a hybrid. The HSD equipped vehicles gain 10-30% more mpg compared to the similarly sized 1.8 and 1.9L gasoline engines. Dropping the HSD from the 1.5L Atkinson engine only yields ~2% improvement, while a substantial loss in city mileage and pickup would be expected. The Geo Metro engine is only off by ~4% at 60, but is 20% off at 80 as it struggles to put out enough power as well as being dirtier and much slower. The Honda lean burn 1.0L is excellent at 60mph, picking up 16% but is again struggling to keep up at 80mph and will likely have much greater emissions. The older "dirty" TDI also does quite well, but is even to 8% worse when you account for the additional energy content of the diesel fuel, and of course emissions will be much higher.

    So, to summarize IMHO the 1.5L and 1.8L Atkinson cycle engines in the Prius are a significant improvement over typical small engines, and are as good or better than some of the most efficient vehicle engines on the planet. And all in their "least efficient" operating mode. For all other driving conditions they do even better when paired with the HSD.

    Assumptions:
    Stock 2004 Prius 2890 lbs, Cd = 0.26, A = 24 sqft
    Prius - HSD = 200 lb weight reduction
    Corolla 1.8L = - 200 lbs
    Saturn 1.9L ODHC = -200 lbs, 60mph = 2000rpm/66NM, 80mph = 2700rpm/93NM
    Geo 1.0L = -300lbs, 60mph = 2700rpm/48NM, 80mpg = 3700rpm/89NM
    Honda Insight 1.0L = -300lbs, 2100rpm/62NM, 2800rpm/89NM
    VW 1.9L TDI = no weight difference, 1 gal diesel = 1.115 gal gasoline
     
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  13. miscrms

    miscrms Plug Envious Member

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    And just for kicks....

    A Toyota 1.8L HSD in a 2003 Honda Insight, assuming 300 lbs weight increase:
    60mph: 65mpg
    80 mph: 48mpg

    HP/lbs of a Civic Si, fuel economy of the original Insight. The CR-Z Honda should have built?
     
  14. Ophbalance

    Ophbalance Member

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    Well, the other party gave up the fight and moved on. Two other folks on the board defended the position I put forth, one stating that "did you ever stop to consider the engineers at toyota may know more than you or me?" and the other pointed out "you realize that no road is truly flat?". Though, the second poster would rather not have hybrids and use twice the fuel due to the ecological impact of rare earth mining. Dude is a bit lost on that one, being that he's posting in a high performance computing site... and every computer that I know of uses its fair share of rare earth stuffs.

    @miscrms I actually see between 55-60 MPG at a steady speed of 60 MPH over any significant distance. Heck, given the 70-80F temps right now, and the cruise set at 65 MPH, I'm still pulling at least 55 MPG most days. That that's with 40 mile jaunts (one way) on I40. I also do about 15 (one way) miles on surface streets. Coming out of Durham though is only about 1.5 miles of surface streets then straight on I40.
     
  15. miscrms

    miscrms Plug Envious Member

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    Agreed, absolute accuracy is very tricky in these sorts of calculations, particularly with this simple a model. However the relative performance for a given vehicle platform should be in the ballpark.
     
  16. usnavystgc

    usnavystgc Die Hard DIYer and Ebike enthusiast.

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    Woowwww!!! Remember miscrms, I'm from KY!!!! :)
     
  17. lamebums

    lamebums Member

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    There's certain situations where a Prius isn't a good buy. Mainly, if your drive is 90% highway, you'd be better served with a small 4-cylinder engine mated with a 5 speed manual transmission.

    All through college I drove a 2000 Echo with the 5 speed - and I was largely on the highway. Applying a few fuel-saving techniques meant I could easily pull 50 MPG, closer to 60 in warmer temperatures and some more radical driving techniques. Not a lot of $ saved if I buy a Prius in that situation, and certainly not enough to justify getting rid of a perfectly good vehicle.

    Of course, times and situations change - just before I sold the Echo I was getting closer to 35 MPG, because of much more city driving. The fuel savings could add up in a hurry there, especially since I can get a heck of a lot more than 35 in the city in a Prius. (The real impetus to get rid of it - it needed work that outweighed the value of getting it fixed. Otherwise I'd still be driving it.)
     
  18. Ophbalance

    Ophbalance Member

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    Oh c'mon Austin, that's not even a slightly fair comparison. If you want to throw out that example, then a better comparison would be to pit the Echo vs what Wayne pulled in the Chicago to NYC run.
     
  19. lamebums

    lamebums Member

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    Hi Ophbalance--

    If I recall correctly Wayne pulled something like between 65-75 in the Prius? Since I don't remember exactly, I'll split the difference and say he got 70 MPG (unless you can prove me otherwise). I once drove from Cincinnati to Philadelphia and managed 56 MPG, although I had to fill up about 30-ish miles before I arrived. Although not exactly the same route that Wayne took, I'll compare since both of us drove through the mountains of PA. I'll just assume both of us drove 600 miles for sake of this calculation.

    If I extrapolate at $4/per gallon, at 70 MPG vs. 56 MPG I'm only looking at $42.85 in fuel for the Echo versus $34.28 for a Prius - a savings of a grand total of $8 over six hundred miles? Cool, but that's barely enough for a decent meal at a fast food place. Plus I doubt I was nearly as radical as Wayne in my driving - if I'd hypermiled like him I could have easily pushed 60+, narrowing that cost difference even further.

    To stretch this extrapolation even further, a fuel savings of $8 per 600 miles would, (at 70 v. 56 MPG) over a hypothetical 200,000 mile vehicle life, save you $2,667 in fuel.

    I'm not disputing that the Prius can, nine times out of ten, get far better fuel economy. And certainly, the fuel savings will add up over time. But there are some situations - such as when someone's almost entirely on the highway - where the extra $12,000+ for a Prius vs. a basic economy vehicle simply can't be justified. My own individual situation changed - and I changed vehicles accordingly. But until I was getting less than 35 in the Echo due to lots of city driving, I couldn't justify the higher cost.
     
  20. nutmeg

    nutmeg just rollin'

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    I changed my 4cyl 5speed econobox (mazda3) to the prius not for milage, but so my crew in the back seat had somewhere to put their legs, I could fit my drums in the back without having to fold everything down, and I get the cool glass cockpit "digital dash". (one of three things a car needs for me to consider it cool; the other two being pop-up headlights and t-top roof)

    but going from ~35 to ~55 mpg was also nice
     
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