Prius fuel economy at higher speeds tested

Interesting. My display shows 26 mpg at around 107.

 

Those results look a little odd to me. I just got back from a 300-mile trip of almost entirely interstate driving (70 mph) and averaged almost 49 mpg. Their chart seems to indicate they were only getting 33 mpg at that speed.

 

<div class='quotetop'>QUOTE(brandon @ May 22 2006, 06:18 AM) [snapback]259169[/snapback]</div>

Too much variability in real driving. The report is as reliable as EPA, which is horribly unreliable - they just use a different measurement scheme which makes the Prius seem low.

REAL driving includes the following:
Terrain changes (uphill, downhill, flats)
Stops and starts (either local driving or traffic conditions), or just general speed changes (coasting downhill)
Environmental factors (temperature and humidity which may effect the MPG)

For example, I admittedly drive around 70 on the freeways, and sometimes faster. However, according to the gauge in my old BMW, my average speed over the past 7 years is only 42 MPH. Makes a big difference on my fuel economy. My Prius is getting around 43.5 (which I'd like to get up, but only had it for a few weeks), while the lifetime on my BMW was 23.1.

 

Just because they got worse results than people usually do, that doesn't mean this test is unreliable. The point is to compare the cars, and all of their cars were equally low-mileage. Obviously a Prius doesn't get only 30 mpg at 80 mph, but likewise a Golf TDI does not get only 36 mpg at 80 mph. They were obviously driving the cars hard, harder than just cruising at a fixed speed like it sounds like.

 

<div class='quotetop'>QUOTE(Potential Buyer @ May 22 2006, 08:52 PM) [snapback]259467[/snapback]</div>

Hi Potential Buyer,

Probably the difference is due to the closed coarse driving. In the curves the differential heats up more and there is more tire friction.

Diesel has 113% the fuel energy of Gasoline, at least in the USA on average. So, 30 mpg in a Prius is equivalent to a Diesel doing 34 mpg. Which gives the TDI only a 2 mpg advantage. Wonder what a Prius with a Turbo exhaust heat recovery system (true Miller cycle engine) would perform at?

 

This actually confirms my own personal experience: at virtually any speed on the highway, a diesel is more fuel-efficient than a Prius. This is logical because you don't really get a big advantage from the hybrid system, a diesel engine is inherently more efficient and diesel has a higher energy density.

On the other hand, for stop-and-go traffic, the Prius is definetely more efficient. This clearly shows that the regenerative braking is working very well.

Now, if only we could have a diesel hybrid with particle filter, ureum injection system for the NOx, and the HSD system...

 

Vtie --
Isn't gasoline HHV greater than diesel fuel?

 

<_< May last two "road trips" of 265 miles & 340 miles, cruising at 70-75 MPH (with bursts to 80+ to pass slower traffic), I achieved 54.5 MPG and 49.8 MPG. The lower MPG resulted from rather heavy crosswinds. Around Dallas metroplex, stop & go driving, I usually run 42-45 MPG (calculated).

Can't complain. That's twice the MPGs of our '96 Camry 4-cyl.

 

<div class='quotetop'>QUOTE(Vagabond @ May 22 2006, 04:29 AM) [snapback]259151[/snapback]</div>

Hey.... not bad!

 

<div class='quotetop'>QUOTE(judibob @ May 23 2006, 04:47 PM) [snapback]259695[/snapback]</div>

That depends - if you measure it per weight (kg) or per volume (liter).
Per weight, gasoline has a higher HHV:
Gasoline: 46.7 MJ/kg
Diesel: 45.9 MJ/kg

However, diesel has a higher density: approx 850g/liter versus 720 g/liter for gasoline. Overall, this means that diesel has a higher energetic content per liter:
Gasoline: 33.6 MJ/liter
Diesel: 39.7 MJ/liter

But people measure fuel consumptions using volumes (liter, gallon), and not masses. From a scientific perspective, this is wrong, since only the mass tells something intrinsic about the amount of "stuff" you have. For example, volumes slightly change with temperature. This is taken to the extreme if you look at gas, such as LPG. Volumes don't say anything here unless you also mention the pressure and temperature.

The difference in density also means that it takes more crude oil to come to the same amount of diesel as for gasoline, a fact most diesel fans stubbornly chose to ignore...

Ironically, if fuel consumption would be measured by correctly, by mass (kg), the consumption of diesel and gasoline engines would be much closer to each other...

 

37 mpg at 60mph? I smell a conversion problem. I get 37mpg going 65mph up a hill and accelerating.

 

Ahhh - OK, thanks for that. I tend always to think in terms of unit mass, and not volume.
Thanks for that clarification, and I didn't realize the relatively large difference in density in these two fuels. This may also explain some of the misgivings about bio-fuel? Would these vegetable oils & the like have lower density than conventional diesel fuel? Also lower HHV? I know this is true for ethanol, which causes a drop in MPG.
But, this forces me to get out my old fuels & lubes material, as wouldn't you just use a bit less gasoline than diesel fuel to produce the same energy needed for vehicle movement? I know, this gets into the differences between the Otto & diesel cycles, and I'll need to review that material as well - very interesting, but too much water under the bridge to remember clearly. Not really prepared to get into this deep debate, but I appreciate the clarification on the volumetric energy content of the fuels.

<div class='quotetop'>QUOTE(vtie @ May 24 2006, 05:02 AM) [snapback]260240[/snapback]</div>

 

<div class='quotetop'>QUOTE(vtie @ May 24 2006, 05:02 AM) [snapback]260240[/snapback]</div>

So the conclusion would be that we should be measuring miles per barrel, not miles per gallon...

 

<div class='quotetop'>QUOTE(vtie @ May 24 2006, 02:02 AM) [snapback]260240[/snapback]</div>

Yep. A a barrel of crude oil produces many products but a given barrel produces 2x the amount of gasoline as it does diesel. So arguably, a diesel car that gets 2x the miles of per gal. of refined fuel compared to gasoline version hasn't saved any oil. See:
http://experts.about.com/q/Energy-Industry...il-refining.htm http://api-ec.api.org/filelibrary/oilfacts_rgb.pdf

 

<div class='quotetop'>QUOTE(clett @ Jun 21 2006, 08:06 AM) [snapback]274653[/snapback]</div>

I guess someone is making ethanol out of trash.... thats something we have in abundant supply without having to create it "like corn".
If they just don't get stinkin greedy and try to make more than they deserve, it will be wonderful to make power from our trash.

Probem is.... they treat it like a comodity and milk it for all its worth and charge as much as the market will bear.
thats fair for business, but chokes the purpose for going to ethanol "cheaper fuel" without dependency on others.

I think I'll change that old saying that "necessity is the mother if invention"... it todays age, we are so fat and comfy, its more like "greed is the mother of invention".. and unfortunaltey they kill the goose that lays the golden egg.

there have been many a wonderful invention never get off th ground because the greed monster choked it out before it could get up and going.