Tests for fuel consumption 1

[dicer]

Is there anyone that has done tests for fuel consumption at vairous speeds in a variety of automobiles and engines?

I am especially interested in case where the results are counter to what is expected. Meaning improved economy at higher speeds than at lower speeds.

I would like to see some figures on various vehicles HP requirements at various speeds, not calculated but actual measured.

 

[evelrod]

Haven't a clue about street mileage, never concerned me.
However when it comes to race cars...I have done several test sessions to determine fuel mileage in several SCCA and IMSA race cars over the years. Three that I have posted in these forums before come to mind mostly because if you plot them you will find that the graph is linear...for what that is worth. Thanks to Robert R. for that little tid bit.

1963 Mini Cooper 1380cc, ~136hp, 7 gal/hr.
1966 Lotus Cortins 1594cc DOHC, ~185hp, 9 gal/hr.
Tube frame Pantara (IMSA GT)5000cc, ~500hp, 24gal/hr.

Lotus and Pantara were tested at RIR in the early 80's.
Mini was tested at Willow Springs in June, 2004.

Rod

 

[GregLocock]

http://en.wikipedia.org/wiki/Image:Fuel_economy_vs_speed_1997.png

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

see some unexpected trends around 50 to 60 mph. Do you think they correlate with gear change points.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

These are now fairly old cars. Are they carby or EFI

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

I think they'd all be EFI, I'm pretty sure Lotus for example dropped all carburetted engines in the late 80s.

No, I'm pretty sure al the cars would be in top by 30 mph.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Dtuck]

After a certain point around 55 mph for most average cars, wind resistance becomes the dominant factor as to why fuel
economy goes down. Pick-up trucks for obvious aerodynamic reasons start going down hill a long time before a prius would. The point at which a vehicle starts to lose out depends on its design. How high its mileage can get to start with depends on the engine, transmission, gearing, etc. See

http://www.electrimetric.blogspot.com/2008/07/speed-vs-mpg-aka-55-in-left-lane.html

as a good reference with a graph.

 

[patprimmer]

If these were measured at constant speed then speed increased and settled at constant speed again so as to remove inertia from all but the reciprocating parts from the equation would expect to see fuel consumption decrease initially as BSFC increased, then drop off as wind drag overcame BSFC increases until the next gear change, then the process repeat for each gear.

As the gear ratio and speed got higher, I would expect the aero to overtake BSFC as the dominant factor.

I would expect great variations from model to mode, depending on variations in rolling resistance, aero resistance, gear ratios and BSFC sweet spots at the required throttle opening to maintain steady state speed, but I would have expected to see optimum MPG at a lower speed and I would expect to see smooth curves on the graph.

Some of those lines look more like a saw blade across an area that represents one gear only. Not at all what I would expect from a car with EFI. Surely the BSFC does not jump around that much.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drwebb]

Here's the source with additional details from the above referenced plot:

http://cta.ornl.gov/data/tedb27/Edition27_Chapter04.pdf

 

[GregLocock]

Great find - so most of the apparent complexity in that graph is due to curve fitting, not real detail.

None the less several cars do have a couple of peak efficinecy speeds.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dicer]

Good info DrWebb.

So notice how the 97 Celica gets better MPH at 65 than at 55. And best is at 60, kinda blows some holes in some popular ideas about fuel economy. Thanks again.

 

[SomptingGuy]

Yeah, that is a huge dataset. Thanks.

- Steve

 

[msquared48]

As I have commented in another thread, my 2007 Prius gets better mileage on the highway (55 - 75 mph) than in city (0 - 50 mph). Been that way since I bought it 20 months ago. I figure 45 City and 55 Highway, average for me is 50. Go figure...

Mike McCann
MMC Engineering

 

[BrianPetersen]

There was a comment before suggesting that they would probably all be in top gear by 30 mph. With transmissions that have a tall overdrive top gear, that's not necessarily true, and if the car has an automatic transmission, even if it is in top gear, the torque converter is probably not locked by 30 mph.

My dad's VW DSG (6-speed automated-manual with no torque converter) is in 4th at 30 mph / 50 km/h constant speed cruise. At constant speed cruise, it's not in 6th until around 70 km/h. I would expect a couple of peaks and valleys in the consumption curve in that speed range.

 

[70AARCUDA]

...back in 1999 I wrote to GM asking "...at what highway speed will my new '99 3.1L V6 Malibu with automatic transmission achieve its 'best' fuel economy?"

...I was hoping GM would reply with something like: "...55 mph on straight and level roadway..." or something similar, but after a LONG wait, I instead got the terse non-answer: "...depends on your vehicle, weight and driving habits."

...one would "think" the GM engineers would have an "a prioria" design goal for such a major performance criteria...apparently not?!?

 

[ivymike]

I'm sure they had goals for similar and related criteria, although perhaps not for that quantity specifically. I don't see what benefit it would be to them to share that information with you. I can think of a few scenarios where a detailed response would become a thorn in their side... like when you went to the dealer to complain that you weren't geting the fuel economy that the engineers said you should.

 

[patprimmer]

Also opposition marketing may turn it against them if they could spin an advantage to themselves

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.