Theoretical gains from weight reduction and aero improvements
 

Hi All,
There are some modifications which are very hard to test due to the small amount of improvement expected. This weekend i have been attempting to put some numbers to these.

I have seen several threads where people have asked what improvement will i get from weight reduction.

I have used info from the efficiency tool page and some standard equations to find the theoretical improvement based on weight reduction
for anyone interested the method can be found in the following link
Theoretical Weight Energy - EcoModder
feel free to point out mistakes if you see any

For anyone that wants the actual numbers they are in the wiki for weight reduction mod but here is the summary
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Theoretical calculations
For 50kg weight reduction
each acceleration from 0 to 60km/h uses 1.27 ml or 0.00034 gal less fuel

Using this info, if you typically do this acceleration change (0 to 60kph) 30 times for every 100km distance that will use 38ml every 100km

If you are achieving 6 liters per 100km that gives a 0.636% efficiency advantage
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There are may assumptions that feed into this final number however it is a guide for anyone interested in weight reduction



I also have done a similar analysis for aerodynamic modifications, the equations are very simplified though so they are only valid for a few modifications which involve a simple shape in the main air flow path. The perfect candidate for this is the antenna delete so i have written up the method for calculation of the theoretical improvement of antenna deletion

The method is explained in the wiki here
Antenna Efficiency Calculations - EcoModder


I have also estimated efficiency improvements from removal of electrical loads
I have not written up the full method of these calculations however it is only slightly different from the aerodynamic method as everything is converted to watts
final info can be found here
Electrical Loads - EcoModder


So hopefully this info helps some people and hopefully there aren't any errors in the calculations but please speak up if you find any.

cool, the back seats are going then.

are you assuming use of brakes or coast down in your weight reduction calcs?

I did a real-world back-to-back 150 mile round-trip drive with and without 200 kg of additional (3 adult) passengers. The trip was from Bryce Canyon National Park (8000' elevation) to Hell's Backbone Grill in Boulder, UT (6700' elevation). The 1300' elevation drop doesn't give a good picture of the multiple uphills and downhills, including a couple of 12 and 14% grades that had me geared down to second gear. The trip takes you on a hogback with 1500' drops on both sides - one of those helicopter photo-ops for sports car advertisements.

The first trip was on a Monday afternoon and evening, driving four of us to dinner. I got about 50 mpg on the way to dinner, and 42.7 mpg for the round-trip. Conditions weren't ideal, as I had to run the AC occasionally on the drive out, and drove home with the lights on. I probably took a 5% hit on mileage that trip.

Two days later, I repeated the drive alone, under near-ideal conditions. I averaged 140 mpg for the first dozen miles, an amazing 71.9 mpg for the first 48 miles to Escalante, and 55 mpg to the restaurant. Overall, I averaged 47.5 mpg for the low weight round-trip. My mpg difference for the two trips is about 10%. Obviously, there's too much experimental error to come to any conclusions, but I got 170 people miles/gallon taking us to and from dinner, and had a gorgeous scenic drive on the repeat trip.

bestclimb, the calcs assume braking. The calcs look at a change from 0 to 60 kph and assume no coast down which is probably fairly close to how "normal" drivers drive. However hypermillers are likely to see stop lights far ahead and take the power off and maybe even coast in neutral in which case the example calcs are probably not realistic for them.
If you coast down and you want to calculate the theoretical efficiency gain of weight reduction you can go through the calculation process again but just evaluate the change in speed from breaking only as this is the wasted energy your actually interested in.
Note: There is another thing hypermillers may be interested in, more weight in the car means longer gliding for pulse and glide driving therefore the calculations in these pages do not account for benefits from longer gliding.

SentraSE-R thanks for posting your trip info, the data is very interesting. Even though there is too much error in the numbers sounds like a great trip and at the very least it shows weight reduction works well for you.

Not to burst your bubble but I have a strong feeling your calcs are off without looking over the mathematics.

Yes the amount of energy in gasoline is high, but there a many more variables than just simple physics. Kinetic friction, rolling resistances, drag/aero, inertia, etc. Yes it may take just a tiny bit of energy to accelerate mass to 60kph, But thats neglecting everything else acting on the mass.

the rotating mass, aero drag and what not will not change with a reduction in weight (perhaps a small bit as it comes up off the springs, I have it on good authority that 5 angles can dance on a pin head) The rolling resistance and friction in the drive line will be slightly less with reduced mass.

I know that the plural of anecdote is not data. I have noted that my mileage is often less with other people in the car. I think it is because my coasts carry farther and end up having to use more brake. It is also just as likely that I am not paying as much attention while carrying on a conversation while driving.

zerohour, agreed. the calcs are only a guide they will never be a perfect substitute for real test data so until we have test data the calcs might be a good guide for some people. With any simulation or calculations people using them need to know what assumptions have been used and what information isn't accounted for which is why i put the process of calculating into the wiki rather than giving a set of numbers.

Having said all that a general sanity check on the calculation results suggests they are a good guess. I have not seen anyone give proper test data on here for weight reduction implying that the improvements are small. From what i have seen anything above 5% is easy to see, anything below 1% we dont usually see test results for because its too hard to see the result from the noise. I have also reduced weight myself and not noticed any improvement over the noise

Regarding the calculations lacking factors effecting efficiency, if anyone can identify anything major im happy to modify the calcs to take them into account. However kinetic friction will be a small effect as it is a small weight change on a small contributor to efficiency, weight has 0 effect on aero drag, weight has 0 effect on inertia (for the purpose of these calculations).

Having scene students make bad assumptions over the years always makes me bit weary of results pulled from "simple" math by plugging numbers into equations. Take no offense on that one :-) There's enough energy in a gumball to power the world's demand for electricity, if you think can split the atoms apart.

As to your request it goes back to alot of variables. It's going to differ on the same car based on even simple things like auto/manual trans, and final drive ratio. Every engine has a different rpm when it operates most efficiently, and depending on your setup, the amount of power produced at the sweet spot and the gray area around it are going to have positive or non-noticeable effects from weight reduction. If the car is underpowered for the sweet spot of the motor, say an old jeep motor from WWII at 40HP trying to drive down the road. Yeah on level road, minimum road the 40hp will get you to 50-60mph, but an escort of similar mass with 110hp will do faster and use less gas. And how FE and mass plays into that situation: If you add 4 soldiers to the jeep and 4 people to the escort, which one struggles more to move down the road? Obviously the Jeep. If the power/weight becomes extreme if you need to run the engine under more of a load, and thus more fuel.

Now if you have a vehicle, say something like a v6 or v8 pickup, the amount of power and torque produced on the lower-end rpm yields a power/weight excessive enough that removing weight will not have an effect. But if you take something with a smaller engine that produces 60bhp at 1500-2000rpm range then the power/weight gains could be made with weight reduction.

I remember a few years ago there was a video where someone started cutting apart a small sedan (maybe Nissan Altima?) and the zero to sixty time went from 10 sec to 5-6second range. Obviously they did a bit of damage to aero without a hood, roof, doors, etc BUT the point is that the weight reduction made the car faster to get up to speed. And that helps alot depending on where you're driving as well. If the commute is alot of stop and go with stop signs every block or few miles on back road, then the potential to gain some FE by getting to speed sooner. And that will be true for the pedal to the floor or a conservative feather-light throttle.

Hopefully that helps a bit.