Theoretical gains from weight reduction and aero improvements

[cr45]

Firstly, well done for tackling this topic.

I have studied your "Theoretical Weight Energy" wiki and there is however a few errors in it.

1/ You have written the following
********
Energy of momentum
E = 0.5 * W * V * V
E is given in joules

Where:
W is the weight in kg
V is velocity im meters per second
**************
That equation is actually the kinetic energy equation and not the momentum equation.
The kinetic energy equation is the correct equation to use though. As the KE equation uses mass as a parameter, might I suggest writing the equation as KE = 0.5*m*v*v.

2/ You give the units for weight as kG. However, strictly speaking kG is a unit of mass.

Anyway, I certainly think that your approach can lead to ways of improving fuel economy.

[Frank Lee]

Theater students do math?

I'd say just plug yer numbers and yer what-if numbers into the EM performance calculator and save the headaches. The equations in the calculator have been proven.

[saand]

cr45 thanks for picking up some naming errors i appreciate your time in looking through the calculations, they are fixed now.

I also agree with frank lee, the EM performance calculator generates more information and it is a valuable tool for theoretically evaluating some modifications. The methods and equations for my methods in the wiki and the EM performance calculator are likely the same and i have also checked the results of the wiki calculator against the EM performance calculator and they match closely.

zerohour, it might be worth taking this conversation off this thread so we dont clutter up the thread (we can start a different thread something like "problems with assumptions for theoretical calculations").
Note the EM performance calculator i believe makes all the same assumptions when it is calculating weight influence (someone please correct me if you actually know the equations used in that calculator)
Anyway regarding your last comment
firstly i take no offense at your comments and i am happy to have the discussion about the calculations in the hopes that any errors will be found and others can use the calcs as a useful tool.
secondly regarding the
yes agreed every car is different, the calculations assume the drive train (manual/auto trans) is 95% efficient so if anyone has a better approximation of efficiency of transmissions i would love to put these numbers in.
I agree to an extent with your comment on engine efficiency being different for different power outputs. However i disagree with the comment that when looking at a very powerful engine like a v6 or v8 that the fact there is excessive power means that weight reduction will have no effect. As far as i see it the more powerful engine has a different percentage of efficiency for converting the chemical energy in petrol to energy for movement. So the V8 still has to put out less energy to accelerate less mass and assuming the car is still driven the same way with the same throttle position the V8 will use less energy with less weight.

as a side comment it is my impression that auto makers reduce weight not just for the gains in not accelerating the extra weight but also the gains had by reducing the size of the engine and associated systems. The gains in reducing the engine and associated systems probably out weigh the effect from accelerating the extra weight. It is very hard for ecomodders to reduce the engine and other system sizes so we will always get less improvement than the car manufacturers will when reducing weight.

[RobertSmalls]

Kinetic energy, which you've termed "theoretical weight energy", and is every bit as theoretical as gravity, is conservative. That is to say, when you add kinetic energy to a car, that energy is not consumed or dissipated, but 100% of it is stored in the car. The same is true of work done against gravity: it is stored as gravitational potential energy, and 100% of it will be released when you bring the car back to the velocity or elevation where you started.

The fact that kinetic and gravitational energy are conservative mean that their impact on fuel economy is ZERO, provided that you never have to brake.

You can calculate kinetic energy to estimate the fuel consumption impact of braking. When you brake to a stop, you lose 1/2*m*V², where V is your initial velocity and m is your mass. As cr45 mentioned, weight has no place in a discussion of kinetic energy.

Weight does play a part in rolling resistance, though...

If you'd like a wiki article on how to do some math to estimate fuel consumption, I'd be happy to tap out something on the road load equation. Aero and rolling resistance are far more important to fuel economy than anything else, and should be given a proper treatment in the wiki.

[cr45]

The title "energy of momentum" within your wiki should be changed to "kinetic energy". Momentum has the units kG.m/s or N.s (and the equation is p = m * v). Momentum is thus not a form of energy as the units are not equivalent.

[NeilBlanchard]

Aero loss = Total loss.

Weight loss = partially recoverable by coasting and/or engine braking.

I think aero improvements are more helpful. The other factor is that aero drag completely swamps all other losses above 35-40mph; because it goes up by the square of the speed.

[saand]

robertsmalls yes id be very happy if you were able to make up something for the wiki that gives a indication of the theoretical impact of rolling resistance and aero.
I have also made up one for aero impact specifically for aero impact of a antenna. the link is in the initial post in this thread, it is very simplistic though so i welcome any further input into this.

for people that are suggesting in their posts that weight isn't important because it only gets wasted when you break, that is the point of the calculations and that is what they prove. It will show people that are considering removing weight to improve efficiency that weight wont give you much of an improvement unless you break a lot in stop go traffic.

regarding kinetic / momentum discussion after rechecking the equations and physics books i haven't opened for a while I assume we are all just talking about the wording rather than the numbers (Please correct me if this is wrong). I personally am not worried about the wording / definitions i am only worried in the numbers that come out, i think its my engineering attitude compared to a physicists attitude. I have attempted to fix up the wording but please let me know if its still wrong and ill modify again or anyone is welcome to modify the wiki page themselves

[PaleMelanesian]

Why reinvent the wheel? We already have a tool that lets you quickly and easily calculate the impact of aero and weight (and many other) improvements.

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

[saand]

the aim of putting these calculations in the wiki is so that the formulas are viewable by users and users can therefore understand (if they want to) the impact of various assumptions.

The main reason for putting these in the wiki initially was because i have seen many posts saying "if i reduce weight in my car what will my efficiency gain be". This is not easy to pick out of the current calculator, actually im fairly sure its not in the calculator.

[cr45]

It would be interesting to look at the numbers for the energy consumed when operating in a pulse glide situation, using real aero numbers. You could compare what happens if the mass of one vehicle is say 10% less than another, with both vehicles accelerating at the same constant rate. The next step would then be to add the rolling resistance.