Where do the MPGs go? in town/highway consumption breakdown

[pfunk678]

This info taken from a Transportation Research Board study found here:
onlinepubs.trb.org/onlinepubs/sr/sr286.pdf (page 40) Thanks, tasdrouille for posting the link!

In Town driving results in a 68% loss of efficiency due to engine and drive loss. The remaining 31% of fuel goes to:
idling (17%)
braking (6%)
rolling resistance (4%)
aero resistance (3%
accessories (2%)

This 32% is the fuel consumption we have control over.* Of this 32%:

Idling (53%)
Braking (19%)
RR (13%)
Aero (9%)
Accessories (6%)

As I'm sure everyone here is well aware that significant gains in mpg can be made by reducing idling and braking in town, but what was eyepopping to me was the highway numbers from this study.

Highway driving results in 74% engine and driveline loss. The remaining 26% is broken down into:

Aero resistance (11%)
Rolling resistance (7%)
Idling (4%)
Accessories (2%)
Braking (2%)

Of the 26% we can readily control: *

Aero (42%) !
RR (27%)
Idling (15%)
Accessories (8%)
Braking (8%)


After reading this information I arrived at several conclusions:

1. Hypermilers drive in such a way that there is great focus on reducing idling & braking. In short, we get more highway-like results from in-town driving.
2. The lowest hanging fruit to harvest in highway driving (or hypermiling) is to reduce aero (it is clearly the biggest chunk)
3. Idling & Braking are driving behaviors. They are decisions you make while driving
4. RR, Accessory usage, and Aero are mostly determined by how you configure your car in your driveway. (air dams, low RR tires, removed electrical fixture, etc) but some of this is determined by driving behavior (drafting, choosing to drive in dry/wet conditions, using a/c).
5. The * earlier in the post are due to my ignorance/inability to grasp BSFC concepts. I'm sure that loss due to engine efficiency can be influenced by understanding and exploiting it. This would change how big a piece of the pie all the numbers listed above are.

My intent in posting this was to help provide a context and framework for hypermiling and to understand where the biggest gains can come from and how much gain can come from mods and how much can come from adjusting the "nut behind the wheel". I welcome your feedback/corrections on my calculations and thoughts. Thanks!

[Thymeclock]

Originally Posted by pfunk678

After reading this information I arrived at several conclusions:

1. Hypermilers drive in such a way that there is great focus on reducing idling & braking. In short, we get more highway-like results from in-town driving.

3. Idling & Braking are driving behaviors. They are decisions you make while driving

Um, no.

95% of my driving is local and idling is unavoidable (except at those traffic lights that I know are red for more that one minute long where I can shut the engine off momentarily for the duration). It has had no measurable effect upon my MPG, despite the wishful thinking of doing it.

Braking is unavoidable when you have a stop sign at nearly every corner in a residential neighborhood due to mommies petitioning the local politicos to erect more and more stop signs. Once a stop sign is erected, it is never removed. Braking for stop signs and traffic lights is not subject to "decision" making and is NOT optional. (The amount of fuel saved is negligible, and stopping is cheaper than paying fines for violations incurred.)

[chrisgerman1983]

Thanks you for posting this Breaking may be unavoidable but depending on the route, it can be drastically reduced. I have no stop signs on my way to work. Most days I have been able to get to the shop without coming to a complete stop and without touching the skinny pedal

[Fat Charlie]

I like some of my stop signs. I get to take my glide all the way down and bump as part of the stop.

[SentraSE-R]

Originally Posted by Thymeclock

Um, no.

95% of my driving is local and idling is unavoidable (except at those traffic lights that I know are red for more that one minute long where I can shut the engine off momentarily for the duration). It has had no measurable effect upon my MPG, despite the wishful thinking of doing it.

Braking is unavoidable when you have a stop sign at nearly every corner in a residential neighborhood due to mommies petitioning the local politicos to erect more and more stop signs. Once a stop sign is erected, it is never removed. Braking for stop signs and traffic lights is not subject to "decision" making and is NOT optional. (The amount of fuel saved is negligible, and stopping is cheaper than paying fines for violations incurred.)

??? I'm not idling when I'm 3 blocks away from and approaching lights, much less when I'm sitting at them. Why is your idling situation at traffic lights "unavoidable" when mine isn't?

Stopping at stop signs is unavoidable. Braking at them often is avoidable. You don't know the difference?

[PaleMelanesian]

How you approach stops is a decision you make. If you do it right, you can roll up to the point and stop exactly where you want, without braking. If you do have to brake, you're doing it wrong.

Idling at a red light? With a gauge you can see exactly how much fuel that consumes. With an efficient car like mine, every minute idling costs about 1.5 cents. A minute is about the average stop light cycle. If I have 10 lights on my commute, that's 10 minutes each way, or 20 round trip. That's 30 cents per day of fuel needlessly consumed. That's $1.5 per work week or $78 per year. A guzzler like an SUV will double or triple that.

If you're not seeing any difference, you're doing it wrong or measuring it wrong.

[Thymeclock]

Originally Posted by SentraSE-R

??? I'm not idling when I'm 3 blocks away from and approaching lights, much less when I'm sitting at them.

Neither am I idling then. The car is in gear and in motion. What's your point?

Why is your idling situation at traffic lights "unavoidable" when mine isn't?

Probably because I encounter much more traffic on roads here where I am than you do.

Stopping at stop signs is unavoidable. Braking at them often is avoidable. You don't know the difference?

Your last comment is unnecessarily provocative. One cannot always coast entirely to a stop without braking - especially when you are not the only car on the road.

[Thymeclock]

If you're not seeing any difference, you're doing it wrong or measuring it wrong.

That's purely an assumption.

Just out of curiosity, how many times in the course of a day do you shut off and restart your car at traffic lights?

[Diesel_Dave]

For those of us who are more visual, I've copied their charts:

Urban:


Highway:

[ProDarwin]

Originally Posted by Thymeclock

One cannot always coast entirely to a stop without braking - especially when you are not the only car on the road.

Agreed. Some on this forum don't seem to experience this

Of the 11 stops on my commute to work, there are 0 I could coast to a stop at without enraging the occupants of cars behind me. 10 of the 11 would cause traffic jams/backups if I were to coast to a stop.