I need someone to explain the science and how it works in this topic "Should cars be more efficient".I have to have a paragraph explaining this. I know why it is important but unsure of the science...

I need someone to explain the science and how it works in this topic "Should cars be more efficient".

I have to have a paragraph explaining this. I know why it is important but unsure of the science and how it all works.

Asked on by gsstom

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unkyd's profile pic

unkyd | High School Teacher | (Level 2) Adjunct Educator

Posted on

Efficiency is defined as the ratio of Work out/ Work in.  You can also look at this as Energy out / Energy in.  When comparing efficiency of two machines you would typically look at how much work you get by placing a fixed amount of work or energy into the machine.

A car typically gets its energy from the fuel (gas) it uses.  Usually, cars will have a fuel efficiency rating of miles per gallon.  The miles are being used as a measure of work out while the gallon of gas is used as a measure of the work in.  Neither are appropriate values for work or energy but they can be for handy for consumers who ultimately look at the gallons of gas as their hard earned dollar (work in) will get them further down the road (work out).

To be more correct with efficiency you need to know more about work and energy.  In its simplest form, work is the product of force applied to an object times the distance the object moves because of that force.  Energy is the ability to do work.  If we modify the efficiency equation  we get:

   Efficiency (Eff) = Work Out / Energy In

                         =  (Force * distance) / Energy In

The energy in is the gas used in the car.  Let's assume that is the same for all cars we want to compare.  If so, efficiency is all about the Work out or the product of Force the car applies times the distance it travels.  If you also assume that all cars take the same force to move down the road at 55 mph (bad assumption - but let's pretend) then really Miles (distance) per  (/) gallon (Energy In) would be a good measure of effeciency.  

Personal experience tells me it is a lot easier to push a little economoy class car compared to a heavy duty SUV.  So maybe (I stress MAYBE) the big SUV travels less miles but requires greater force so the physical efficiency of the SUV is a lot closer to the economy car.

So the "physics" of the efficiency may be better than the miles per gallon show, but consumers are still concerned with getting where they want to go so the miles per gallon will be the best we have for combution engines.

Now with the advent of new fuel sources for cars (electric, fossil fuel, Hydrogen) finally we are looking at the bottom half of that equation (the Energy In).  Unfortunately, there is no quick and easy way to compare these especially with Hybrids thrown in the mix as well.  So as consumers we may have to throw out the physics (did I say that?) and look at the dollar signs along with environmental impact (for those of us that are socially aware that this will add cost down the road - so to speak) to determine if the 'Energy In' part of the equation is worth the 'Work Out'

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