Naked Science Forum

General Science => General Science => Topic started by: satrah on 16/03/2015 04:26:29

Title: By how much does carrying extra weight affect car fuel economy?
Post by: satrah on 16/03/2015 04:26:29

Hello all

I am doing a bit of traveling in my small 1.4 litre car (Hyundai excel). I have not weighed the content inside, but it may be around 150 Kg in extra weight. How much will this affect fuel economy, and is this weight burning out my car quicke? I do about 30 kg per day around town.

Do I look into physics for these answers ect.   (momentum)  Is there any math formulas to be used as a guide for vehicle per weight?

Thanks in advance.

Title: Re: By how much does carrying extra weight affect car fuel economy?
Post by: evan_au on 16/03/2015 10:49:35

Assuming you are driving well below 80km/h in stop/start city traffic, you can ignore wind resistance.
Assuming your tires are properly inflated, you can ignore rolling resistance.

Most of the mechanical energy produced by the petrol (or diesel) engine is spent accelerating you and your car (and it's load) up to (say) 60km/h. You then apply the brakes, which turns this kinetic energy into heat at the next traffic lights, when you do it all again!

An equation which describes this is E =½mv²
Where:

• E is the kinetic energy of the car (which is less than 10% of the chemical energy put into the engine)
  
• m is the mass of the car and contents
  
• and v is the velocity of the car before you hit the brakes (think of it as the speed) 

So if you increase the mass of the car by 10% (for example by refilling the tank several times per day, or carrying unused items around in the trunk), it will take 10% more energy to reach 60km/h, and your brakes will dissipate 10% more energy when you stop.

Note that this logic does not apply to electric cars, which often have quite heavy battery packs. This is because they use regenerative braking (http://en.wikipedia.org/wiki/Regenerative_brake), so most of the kinetic energy is put back into the battery when you stop, and then used to power the car when you accelerate. So if you want to carry around a lot of unecessary weight, and it bothers your green conscience, get an electric car.