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Old 03-17-2020, 06:08 PM   #11 (permalink)
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Right, but the curiosity is in fleet average, or typical figures.

The reason I'm curious is people have the idea that it's the weight of vehicles that causes poor fuel economy, and it doesn't seem like much of a factor to me. That got me wondering what percent of fuel is spent accelerating/braking compared to rolling and aero drag taking into consideration that some people drive very heavy vehicles almost exclusively in stop and go traffic, and others might drive small cars almost exclusively on the freeway, and everything in between.

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Old 03-18-2020, 01:17 AM   #12 (permalink)
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Again, I think it's highly dependent on where you live and drive. If you live in a crowded city area, hell yes weight is a problem. My stock MR2 Spyder could get 25+ mpg with its ancient cheapo engine driving between stop signs or in traffic, while my FR-S with its extra 600lbs would do noticeably worse. That was like half my driving, so I would be pretty happy with 30mpg.

If you drive mostly on the freeway for long distances, then aero is very important for obvious reasons. Do the people who drive these long distances use a large portion of the whole fleet's fuel? I dunno...probably?

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Old 03-18-2020, 03:15 AM   #13 (permalink)
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I don't think people are getting the question.

As an aside, I get better fuel economy in gridlock traffic than steady freeway cruising at 70 MPH in my Acura. The point isn't if I could get better economy not being a sheeple, but how the herd as a whole does.
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Old 03-18-2020, 05:08 AM   #14 (permalink)
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If you divide by the engine efficiency you will get the numbers you want:

https://www.fueleconomy.gov/feg/atv.shtml
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Old 03-18-2020, 05:00 PM   #15 (permalink)
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typical

Quote:
Originally Posted by redpoint5 View Post
Just curious if there is any ballpark figure out there about what percent of motive energy is spent for the average vehicle to:

Accelerate
Overcome rolling resistance
Overcome aero resistance

By motive energy, I mean not factoring in engine (in)efficiency, drivetrain losses, accessories, etc.

My guess is that acceleration accounts for less than 5% of motive energy as an average, perhaps much less than that even. Obviously this figure would be higher for those that spend more time in stop and go traffic, and less for steady state highway cruising. As an average though, what do you think it would be?
oilpan 4 is correct,you've got to parse it out for urban,highway,mixed,level,mountainous and there's also a consideration for curve resistance.SAE had a lot of literature on this back in the late 70s,early 80s.Hucho's textbooks will show at least the European models used to break it out.
In town it's all about inertia and momentum.You get the best efficiency at city speeds,but the stop and go destroys all that,and it's the reason for hybrids.
Out of the city limits,and out of gridlock,it's all about aerodynamics.
In his second edition,Hucho commented,'With a medium-size European car,aerodynamic drag accounts for nearly 80% of the total road resistance at 100km/h (62-mph).' And considering that one can legally drive at 85-mph between Austin and San Antonio,Texas,it's even more of the total,as the power varies as the cube of the velocity.
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Old 03-18-2020, 05:15 PM   #16 (permalink)
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...and then, after a sufficient sample size has been tested, the average % of energy expenditure on braking is what?

Perhaps it's not easy to obtain a reasonable estimate, and there isn't a defined purpose to know what that average is.
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Old 03-21-2020, 03:25 PM   #17 (permalink)
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braking

General Motors Laboratory,in 1981,published that 50% of a cars energy was expended in acceleration,and 67% of that energy was lost to braking (33.5% of total energy).
Stop-and-go driving consumed up to 100% more fuel than in highway driving.
With 25-gallons of gas,you could get:
Ten 40-mile trips
Sixty 4-mile trips
Ninety 2-mile trips
or one hundred 1-mile trips
In: Transportation Research Board Special Report 286,Tires and Passenger Vehicle Fuel Economy,Informing Consumers,Improving Performance,National Research Council of the National Academies,copyright 2006,ISBN 0-309-09421-6,153-pages:
For EPA City cycle,23.07% energy= Aero,30.77% energy = rolling-resistance,52.84% energy = braking.
For EPA HWY cycle: Aero = 55%,Rolling-Resistance = 35%,Braking = 10%
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Old 03-25-2020, 10:33 AM   #18 (permalink)
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There's a new Engineering Explained video out which bears some relation to this question:

https://www.youtube.com/watch?featur...Rw&app=desktop

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Old 03-25-2020, 12:35 PM   #19 (permalink)
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There's a new Engineering Explained video out which bears some relation to this question:

https://www.youtube.com/watch?featur...Rw&app=desktop
Watched the video. Thanks! Jason needs to be commended for ferreting out the details.
SAE test protocols would have included continuous meteorological measurements and normalizing data to standard atmosphere. Temperature variability as related to rolling resistance would have to be accounted for. Topographic effects, A-B, back-to-back testing . Curves on the 'test track' would not be allowed. Climate control would not be allowed.(I have a note which suggests that a heat-pump-sourced AC unit absorbs 1.6 kW).The 18-inch wheels are an unknown quantity. Only a wind tunnel would tell, unless Tesla has made a claim as to their added efficiency.
What Jason and CAR and DRIVER has done, does reflect the 'real world', in terms of what the average motorist would probably be willing to put up with ,when compiling road trip data.
One telling road test, I believe,by MOTOR TREND, involved a Model S, with a BMW 5-series shadowing behind, at exactly the same conditions. On a Btu-basis, when the cars were 're-filled', the Tesla ended up with a brake-specific-fuel-consumption-equivalency equal to around 1/3rd that of the Bimmer., on the order of BSFC 0.138-pounds/bhp-hour. This is where the battery/inverter/motor/and planetary gearset really shine. I think Jason Fenske has used 'insane efficiency' to describe Tesla's performance. Quite fitting!
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Old 03-31-2020, 10:29 PM   #20 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
Just curious if there is any ballpark figure out there about what percent of motive energy is spent for the average vehicle to:

Accelerate
Overcome rolling resistance
Overcome aero resistance

By motive energy, I mean not factoring in engine (in)efficiency, drivetrain losses, accessories, etc.

My guess is that acceleration accounts for less than 5% of motive energy as an average, perhaps much less than that even. Obviously this figure would be higher for those that spend more time in stop and go traffic, and less for steady state highway cruising. As an average though, what do you think it would be?
this


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