Theoretical gains from weight reduction and aero improvements

[PaleMelanesian]

I see a few points where you lost some efficiency on that route.


1. It looks to me like you could be starting your coasts earlier. For example, at 3 you could have continued to coast all the way to 7 instead of accelerating to 4.

2. You could shut down at 7. That's approximately 25 ml of fuel that accomplished nothing.

3. The acceleration up to 13 was unnecessary. If you started a neutral coast at 12 it would take you all the way home.

[saand]

palemelanesian, thanks for the suggestions.
unfortunately points 1 and 3 couldn't be done because of other cars, i am doing suburban driving with lots of lights and lots of cars around. I would get many people angry if i coasted down as far as i would like to and if i didn't accelerate up to the normal speed from lights.

regarding point 2 i would make a large saving if i could shut the engine down however i am never going to do that because i am worried the car will not restart, i have had issues with this car starting before so unless i have a redundant starting system then i cant do it.

[PaleMelanesian]

If it's a red light anyway, let them be angry in their ignorance. Going faster and then braking is not saving them or you any time. I make a point to not be an obstruction, but if there's no gain to them from going faster and a loss for me (extra fuel), I'll go slower.

Wise choice to not shut down if your car isn't reliable for restarting.

[RobertSmalls]

I haven't fired up MATLAB, but here's some quick figures from Excel.

Your time weighted average speed is 40kph. Your distance-weighted average, however, is 57kph, and your sqrt(distance-weighted average(V²)), which is what the air sees, is 60kph. IOW, if my math is right, your speed profile results in the same total aero drag as a car that cruised the entire 5.7km at 60kph, even though your average speed was 50% lower. (YMMV)

Put another way, you had 2.25 times as much aero drag as you would have if you could have just cruised non-stop at the "average speed" figure the scangauge would read out. I hope everyone here would know enough not to try an aero calc based on "average speed", though.

I used the Excel expression [ =sum(IF(Fuel_rate<0.5,Fuel_rate,0))/sum(Fuel_rate) ] and determined that you spent 31% of your fuel idling! (YMMV, bigtime) I bet you'll think long and hard about a stick shift or a hybrid next time around, so you can install a kill switch.

[RobertSmalls]

Why do in MatLab what you can do in Excel? (answer: It's easier and more accurate in MatLab, but also harder).

I've made the naive assumption that if you're losing more than 0.5m/s², you're braking, otherwise you're doing useful work. This pie chart shows the breakdown (pun!) of braking, rolling, and aero resistance. I've assumed your starting and ending elevation are the same.



So, Saand, it looks like you are dividing your fuel roughly equally between idling, braking, aero, and rolling resistance.

But what about the question in the thread title? For your particular commute, check out the following table:


Get you some better tires and a stick, Saand.

However, this driving profile is MUCH slower than the kind of driving I do, and if you're like me, I suspect you'd be best served by putting a higher priority on aeromods. Naturally, if you brake more than Saand, you'll want to see if you can do some weight reduction.

Does anyone else have speed versus time data representing different driving styles? Perhaps even one with the stereotypical wife behind the wheel, giving both pedals more of a workout than you do?

I've attached the spreadsheet. It assumes a timestep of one second between your data points, but it's otherwise pretty friendly and self-explanatory.

[saand]

Robert looks like some good analysis there,
your assumption of when i was braking is likely fairly accurate as i had no hills to go up in that stretch of travel.
Your excel spread sheet could become a very useful tool for me, thanks very much.

If anyone else wants the ability to datalog so you can analyze your driving with tools like this im sure the mpguino could incorporate datalogging somehow. I can look into methods to do this if there is any interest (I am an electronic engineer for those that don't know)

[MetroMPG]

Saand, what tool are you using to log with? (Sorry if you said it already - I read this thread fairly quickly.)

[NeilBlanchard]

The Illuminati Seven, Dave Cloud's Dolphin, and Allert Jacobs' Honda 125 project all point to the relatively greater importance of aerodynamic drag vs vehicle weight.

Weight does require more energy to accelerate, but it can pertly be regained by coasting -- heavier vehicles have greater potential energy at any given speed. Aerodynamic drag is always a total loss.

[99kleansi]

Just curious at the metioning of a kill switch and restarting your motor several times. The cycle of starting and stoping your engine several times puts quite a few scuffs on bearings and journals causing lots of friction on internals losing power causing the need for more fuel to compensate. In the long run scuffing your cylinder walls causes you have have increased blowby breaking down your oil faster making more frequent oild changes necessary. You also burn more oil the less efficient your cylinder rings become.

[SentraSE-R]

I doubt that starting and stopping a warm engine a few times in a minute causes as much wear as running that engine at 2500 rpm for the same minute.