Inertia vs aero drag vs rolling resistance in urban and highway driving

[Logic]

So where is the energy in fuel used in urban driving vs highway driving?
How much of each type of driving is done on average?

ie: On average; is vehicle's weight more important than it's aerodynamic drag, because it spends most of it's time doing urban commutes?
Is that why most car manufacturers haven't bothered flat undersides and boat tails etc?
Because the extra weight (and inconvenience) they add actually increase the fuel consumption of their cars, on average..?

Then, to muddy the waters: What effect does the energy recuperation during regenerative braking actually have in EVs?

My initial research points to the overcoming of inertia during acceleration as being the main consumer of fuel in cars:

Fundamentals of Fuel Consumption
National Academies of Sciences, Engineering, and Medicine. 2011. Assessment of Fuel Economy Technologies for Light-Duty Vehicles. Washington, DC: The National Academies Press. https://doi.org/10.17226/12924.

"...To illustrate the dependence of tractive and braking energy on vehicle parameters, Sovran and Blaser (2006) used the following three sets of parameters. Fundamentally the energy needed by the vehicle is a function of the rolling resistance, the mass, and the aerodynamic drag times frontal area. By combining the last three into the results shown in Table 2.4, Sovran and Blaser (2006) covered the entire fleet in 2004. The “high” vehicle has a high rolling resistance, and high aerodynamic drag relative to its mass. This would be typical of a truck or an SUV. The “low” vehicle requires low tractive energy and would be typical for a future vehicle. These three vehicles cover the entire spectrum in vehicle design.

The data shown in Table 2.5 were calculated using these values. The low vehicle has a tractive energy requirement that is roughly two-thirds that of the high vehicle. It should also be noted that as the vehicle design becomes more efficient (i.e., the low vehicle), the fraction of energy required to overcome the inertia increases. As expected, for both driving schedules the normalized tractive energy, ETR /MS, decreases with reduced rolling and aerodynamic resistances. What is more significant, however, is that at each level, the actual tractive energy is strongly dependent on vehicle mass, through its influence on the rolling and inertia components. This gives mass reduction high priority in efforts to reduce vehicle fuel consumption.

The numbers for "low" vehicle:
In the UDDS (urban) driving cycle: (98% of driving)
Rolling Resistance (%): 19
Aerodynamic Drag (%): 14
Inertia (%): 68

In the HWFET (highway) driving cycle: (2% of driving)
Rolling Resistance (%): 29
Aerodynamic Drag (%): 47
Inertia (%): 24

https://nap.nationalacademies.org/re...4/chapter/4#20


For EV's and recuperation from regen I have found this so far:

The amount of energy an EV recovers through recuperation depends on a few key things. Heavier vehicles may consume more energy to accelerate, but their momentum helps them generate more electricity during braking.
Similarly, electric motors with higher power ratings are better at converting braking energy back into electricity.
Additionally, the best time to maximize recuperation is in city driving.
Constant stopping and starting gives the system lots of opportunities to recapture energy. Recuperation is less effective on highways with their consistent speeds and limited braking.

To illuminate these factors, the German automobile club ADAC conducted a fascinating experiment.
They tested three EVs - the nimble Dacia Spring, the luxurious BMW i7, and the best-selling Tesla Model Y Long Range - on an uphill and downhill route.
Not surprisingly, the hefty BMW i7 came out on top in downhill energy recovery.
However, the lighter Dacia Spring turned out to be the most efficient overall during the mountainous journey.
This underscores the delicate balance between energy consumption and recuperation – while heavier EVs may recover more, they also need more power to move in the first place.

Beyond mountainous routes, how do EVs fare in regular driving? Data from Green NCAP reveals that - on average, an EV recaptures around 22% of its driving energy through recuperation. Standout EVs like the Nio ET7 and Hyundai Ioniq 6 boast impressive figures of 31% and 29%, respectively.

While recuperation partially offsets the energy penalty of heavier EVs, it emphasizes the potential of lightweight construction.
Innovative materials and designs hold the key to maximizing EV efficiency.
Reducing a vehicle's weight is similar to giving your EV a continuous energy boost, both when accelerating and while using recuperation.
https://www.arenaev.com/road_test_re...-news-3326.php

So when you boil it down; light weight wins out over adding weight to improve aerodynamics for the average car, doing the average commute?

That would explain why manufacturers don't generally bother with the added weight (and inconvenience) of flat undersides and (unparkable) boat tails etc..?

I'd like to keep this about the science/research rather than 'knee jerk' opinion, so plz do link 'the research' when posting.
I'll post more links either way as I find them.

[freebeard]

Were you expecting support or dissent, or is this just online storage of your notes?

If only there were some way to hack the parameters:


Negative Cd.

[Logic]

Originally Posted by freebeard

Were you expecting support or dissent, or is this just online storage of your notes?

If only there were some way to hack the parameters:


Negative Cd.

If 98% of car travel is urban, is it weight saving or aero mods (that add weight) that I/we should prioritize..?

So far it does not look as if aero mods that add weight are worth it unless the majority of your travel is on highways.

That negative Cd thing:
On forums where it could be helpful; that gets no traction either.

[freebeard]

Fair enough.

How about those Moon disks. Screw them onto a set of alloys and they increase weight; replace a set of 1956 Plymouth Fury spinners and they reduce weight?

[Ecky]

Aero is generally easier to realize than weight savings. 20% drag reduction is relatively achievable on most vehicles, but that's 800lbs out of the typical modern car.

[freebeard]

With weight reduction you get better 60ft times.

60ft times are an important part of the urban cycle (width of an intersection)

[Logic]

Originally Posted by freebeard

Fair enough.

How about those Moon disks. Screw them onto a set of alloys and they increase weight; replace a set of 1956 Plymouth Fury spinners and they reduce weight?

Now those are the kind of details I'd like to see here!

ie: This aero mod vs weight saving and that aero mod vs weight saving etc.

My feeling is that with the top half of the wheel moving against the wind, moon disks and light rear wheel covers are a worthwhile aero mod..?
Especially if they are light and replace OEM hubcaps.

(On the Lotus I have machined a place for flush 'moon-ish disks' but into the mags, on the lathe. (reduced weight)
But with a brake cooling gap around the periphery.
The 'moon disks' are circles cut from 2.5 mm aluminum plate.
I'm not liking the look of my initial 'more aero' 2 (2.5mm) bolts affixing design, so will add fake bolts (heads) for each of the mag's 'spokes'.
The sort of bolt head is undecided as yet.
I'm thinking Black Oval head (hexagonal) as a more aero compromise between Countersunk and and Socked-Cap which I like the look of)

[Logic]

Originally Posted by Ecky

Aero is generally easier to realize than weight savings. 20% drag reduction is relatively achievable on most vehicles,

Hmmm... that's debatable Ecky.
Shaving off excess bolt lengths and getting rid off excess metal (Drilling for lightness) in various brackets etc adds up and is easy to tackle in bite size chunks without the car being out of commission.

Shooting yourself in the foot with heavy and impractical boat tails and such just because 'that's easy to do low hanging fruit' is what I am ...er... weighing up.
That's the reason for this post.
That makes it unpopular but facts are facts...
(Can't be less popular than the the Boric Acid thing! )

Originally Posted by Ecky

but that's 800lbs out of the typical modern car.

Er...? I'm not understanding this bit?
800lbs of added weight specifically 'for aero' in a typical car??

[freebeard]

The sort of bolt head is undecided as yet.

duckduckgo.com/?q=button+head+allen+bolt&iax=images

Three or four 'real' bolts minimum.

[Ecky]

Originally Posted by Logic

Hmmm... that's debatable Ecky.
Shaving off excess bolt lengths and getting rid off excess metal (Drilling for lightness) in various brackets etc adds up and is easy to tackle in bite size chunks without the car being out of commission.

Shooting yourself in the foot with heavy and impractical boat tails and such just because 'that's easy to do low hanging fruit' is what I am ...er... weighing up.
That's the reason for this post.
That makes it unpopular but facts are facts...
(Can't be less popular than the the Boric Acid thing! )


Er...? I'm not understanding this bit?
800lbs of added weight specifically 'for aero' in a typical car??

How I meant it was, the typical new vehicle weighs near 2 tons. 20% from that is 800lbs, or ~400 kilos. That's a lot of weight to remove, and, having seen some cars go on extreme weight loss diets, I'm of the opinion it is wildly out of reach. A percent of weight loss may be more useful in most driving scenarios than a percent of aero, but a percent of aero is a lot easier to achieve.

As an example, one forum member here did an extreme weight loss diet in a 2004 Saturn Ion. It started at ~26xx and ended (if I recall correctly) at 23xx. To get there, he removed the interior, removed all seats but the driver seat, lightweight battery, cut the support webbing out of the hood and trunk, cut the crash beams out of the doors, and took a hole saw to and made swiss cheese of every bit of sheet metal that didn't face the outside. He cut the floorpan out of the car behind the rear seats and replaced it with coroplast. Door latches deleted. Window hardware deleted. Speakers and "unnecessary" lights deleted. Exhaust deleted. Power steering, engine balance shafts, A/C deleted. You name it, he cut it out. I believe he didn't quite reach 15% weight savings.

I'm not advocating everyone build boat tails, but MetroMPG found a boat tail (alone) on his Insight (an already very slippery car) to improve ABA fuel economy by 10% on the highway. That suggests a near 20% improvement in Cd from that piece alone. I guess you can get at least half that from a small rear kamm or well placed spoiler, front grille block, smooth underbody, smooth wheel covers and rear spats - most of which I could put together in an afternoon with $50 in materials, and which would not alter the function of the car in any way.

Simply put, weight savings are great (I do them too), but they're much harder and more expensive.