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Toyota Prius, Volkswagen Golf, Diesel, Hybrid Cars, Hatchback, Sedan
#393 of 791 Re: Toyota Prius vs VW Golf TDI [pf_flyer]
Sep 17, 2009 (8:05 am)
I have an interesting comparison between the Prius and the Golf TDI. Well, interesting to me, but unlikely to sway anyone in their purchase decision. It is a comparison of steady-state driving efficiency. I had found this website: http://www.epa.gov/otaq/tcldata.htm which lists the a-b-c coefficients for all 2009 vehicles. the abc's are used to calculate the roadload (force) at a given speed (v) by a + b*v + c*v^2.
When I calculated this for all vehicles in the spreadsheet at 70 mph, the Prius stood out as having the lowest force (hence least work performed over a given distance) of any of the cars, at 110 pounds. Interestingly, the Chevy Cobalt was second at 118 pounds, along with Honda Civic and a couple of Hyundai / Kia. The Golf's (Rabbit's) force was 137 pounds, OK but not spetacular, and 25% greater than the Prius'. Kudos to the Prius for being efficient of form. It's doing the least amount of work to transport four people. But does that mean it is more fuel efficient than the Golf? Not necessarily.
Fuel efficiency is a measure of work output / energy input. It is a fraction, up to 1.0. It is a measure of the efficiency of the powertrain, really. In this case, the SI engine with CVT and regeneration vs. the diesel with manual (or automated manual) gearbox. Here's where talking MPGs falls apart if you are trying to compare powertrain efficiency. MPG doesn't tell you how much work has been done (miles isn't work, it is just distance), or how much energy was used (gallons of what?). More information is needed. Fortunately, for a steady state, like 70 mph, we can calculate the force from the abc's. We can also look up the amount of energy in a gallon of fuel (gasoline, diesel) here: http://www.afdc.energy.gov/afdc/fuels/properties.html
So efficiency = work / available fuel. Work = force * distance, so efficiency = force * MPG / LHV of fuel. For a steady 70 mph on flat ground, the only thing I don't know is how many MPGs each vehicle gets. Let me assume that the '09 Prius gets 52 MPG and the Golf gets 46 MPG at 70 mph. Someone may have better figures, but I think these are reasonable from http://www.fueleconomy.gov/mpg/MPG.do?action=browseList.
The fuel efficiency of the Prius is then 33.4% and the Golf is 33.3%. I don't think you could get any closer than that. Again, kudos to the Prius, because matching the efficiency of a diesel with an SI engine is hard to do. However, using this analysis, you can calculate that any diesel, whether Golf or 7000 pound Cummins pickup is over 30% efficient at this steady speed. So Kudos to diesel technology, regardless of what sheetmetal it is surrounded by, for being efficient. Most SI powered vehicles are about 25% efficient at this speed.
In the city, any hybrid will be more efficient than a diesel because of regeneration and stop/start, unless the diesel also has those technologies.
Final thought, if Volkswagen could get the roadload (abc's) of the Golf to be the same as the Prius, it would get 10% better MPGs than the Prius because there is 10% more energy in a gallon of diesel than gasoline.
#394 of 791 Re: Toyota Prius vs VW Golf TDI [104wb]
Sep 17, 2009 (8:27 am)
There is a (net effect) flaw in your analysis and you have alluded to it. The hybrid (gasser) gains its "perceived" mpg advantage by the % time it is OFF ; that is techno speak for off. The practical percentage is 20%. That is one of the real utilities of the so called hybrid advantage, another being the "engine off during idle" feature which is not excluded to diesel. Some European diesels in fact do have it. (BMW 1 series TDI) As for the difference in energy content between gasoline and diesel, why would you want to further refine it out of the natural consquences of the refinery process of 46% gasoline to 23% diesel? This is almost nonsensical. So given that natural consequences, a HUGE technological break through would be; being able to refine 100% of either RUG to PUG or D2. This has not happened in least 100 years. (there are other reasons for this but we are just talking fuel mileage here) So as a result, D2 will retain the 20-40% advantage. So while 50-52 mpg in a Prius is great, what would be wrong with a diesel Prius 70/73 mpg?
#395 of 791 Re: Toyota Prius vs VW Golf TDI [ruking1]
Sep 17, 2009 (8:45 am)
I don't understand what you mean by % off time as it pertains to my 70mph steady state analysis. Engine off time? Like running off of batteries? Does the Prius do that? Even if it did, that's still part of the overall powertrain efficiency. All of the energy stored in the batteries was at some point generated from the gasoline. Draw a box around the Prius. Gasoline in, work out. What happens inside the box (energy management) is part of the powertrain efficiency. Now a Plug in is more compolex because it has two energy sources.
I don't understand what you meean by refining out natural consequences. Diesel has 10% more energy than gasoline. That's a good thing for vehicle range, A diesel Prius would get higher mileage. That would also be a good thing.
#396 of 791 Re: Toyota Prius vs VW Golf TDI [104wb]
Sep 17, 2009 (8:55 am)
Pretty easy, from a practical point of view.
I should state my TDI's real world consumption 70 mph, to be in excess of 59 mpg.
I say in excess (inxs) because I can easily get 59 mpg 75 mpg with bursts to 80/85 mpg.
So at a steady 70 mpg...(I swag) 60-65 mpg. This would be what, 15 mph better than the Prius? Not bad for the aerodynamics of a brick, eh? Of course this probably would be dismissed out of hand by the Prius folks?
Sep 17, 2009 (10:06 am)
I think if you took a prius and a golf tdi and rolled them into a ball you'd have a really fun to drive, super efficient automobile. Call it the Priolf or the Golfius.
#398 of 791 Re: . [thammer62]
Sep 17, 2009 (10:18 am)
I would say the perceptions of unreliability and unresponsive to dismal customer/repair service are major boat anchors that VW has to shed. So for example if you get a statistically average VW and have great independent shop support, those are not real issues anymore.
Indeed if what I read is true, good Toyota dealer repair service is about as rare as hens teeth.
#399 of 791 Re: Toyota Prius vs VW Golf TDI [ruking1]
Sep 17, 2009 (4:31 pm)
> Of course this probably would be dismissed out of hand by the Prius folks?
Since you can't actually buy it anymore, nor will you be able to in the future, dismissing is easy. It's hard to take the comparison seriously when the vehicle doesn't even meet minimum emission criteria and it forces you to shift gears yourself.
Show me an automatic shifting diesel-fueled vehicle that is at least as clean as a common new non-hybrid gasser that delivers an average (city & highway combined) of 50 MPG, then reminder yourself that Prius is even cleaner.
Face it, non-hybrid diesel simply doesn't cut the mustard.
#401 of 791 Re: Toyota Prius vs VW Golf TDI [john1701a]
Sep 17, 2009 (5:03 pm)
..."Face it, non-hybrid diesel simply doesn't cut the mustard. "...
Face it, a 70 mpg Polo beats 50 mpg hybrid gasser by 40%.
Interesting that you are of the burning more is actually better than burning less point of view. And you wonder why America actually burns more??? I would not have a clue why...........
Of course 12 to 17 k cheaper drawfs what Toyota asks for the Prius.
I'd try to keep it out of the country too, if I had the adversarial attitude that EPA, CARBS and seemingly you have also.
Really in terms of competition ,the Camry hybrid more matches the Jetta in hp (147 hp vs VW TDI 140 hp. Torque (the real motivator) is somewhat mismatched 138# ft vs TDI's 236 #ft. Also the EPA is 33c/34h/34 comb vs TDI 29c/40h/33 comb.
Camry is app $2,800 more MSRP.
#402 of 791 Re: Toyota Prius vs VW Golf TDI [ruking1]
Sep 17, 2009 (5:55 pm)
Of course 12 to 17 k cheaper drawfs what Toyota asks for the Prius.
A Polo diesel for $10,000? Bring it over, I'll buy one!!