Why do smaller cars get better gas mileage?

Question

I think it is because they weigh less therefor they require less energy, but my husband says that bigger cars have bigger motors that should make up the difference. I still say it doesnt matter what the size of the motor is it doesnt change the fact that it takes more eneregy to move a bigger car. Please help us end this debate

Answer 1

You are right.

more weight requires more energy to move, bigger motors might be more effient pound for pound but more engery is still required to move the rest of the car.

Answer 2

Yes, it takes more energy to move a heavier car, that is why they get poorer mileage!

heavier car means more friction, more wind resistance. More friction and more resistance means you need a larger engine and more power to move it, which add to the weight and the friction.

More power means more gasoline and lower MPG.

.

Answer 3

not all smaller cars get better mileage, almost all sports cars are small...e.h....

the size of engine is what that matters....

larger the engine size and the number of cylinders they have, will give a fair chance of reducing the mileage.

Answer 4

EDIT

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You are right.  He's wrong.  I'm not sure why the Ford Mustang website didn't convince him.  The Mustang GT with the bigger engine (V8) gets less mpg than the Mustang V6 which has a smaller engine in essentially the same car.

Here's some more data from Honda showing the same exact car (Honda Accord) with two different engines (V4 vs V6) with the same result - bigger engine gets lower gas mileage.  It's a 10% increase in weight for the V6 as opposed to the in-line 4.  All the data is on this page:

http://automobiles.honda.com/accord-seda...

And as for the energy used to lift a 50 pound weight.  The energy required depends on its mass - not the person lifting it.  It's the mass times g (the "acceleration due to gravity" which is 9.8 m/s^2 or 32 ft/s^2) times the height the mass was lifted.  The same energy is required by a 10 year old or a 30 year old or a 50 year old.

Oh, and bigger engines require more gas because they have more volume.  There's more air to mix with more gas and get more kick per piston stroke.

ORIGINAL POST

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You are right.  The larger and more powerful an engine, the more fuel it uses.  Larger, more powerful cars generally have lower gas mileage

As a control, you need to check the numbers for the same exact car with different engines in it.  Loke up the Ford Mustang on their website and compare the Mustang V6 and Mustang GT (virtually the exact same car):

http://www.fordvehicles.com/cars/mustang...

Mustang V6 (4.0L) gets 17 MPG city / 26 MPG HWY

Mustang GT (4.6L) gets 15 MPG city / 23 MPG HWY

"There's no Replacement for Displacement!  It's an old saying, but it works. An engine's displacement is a measure of how much of that air/fuel mixture it moves in one cycle, or to put it simply, how much work gets done. Push more air through the engine, make more power, pretty simple. A larger engine will generally make more power, even if it doesn't show up in the car's horsepower rating. Many larger engines push out more torque (A measure of twisting force) instead of horsepower.  There is a cost to this, however. More than just air is needed to power an engine; gasoline is also required. A bigger engine pushes more fuel through it as well as air, raising fuel consumption. This is why larger, more powerful cars generally have lower gas mileage."

Also, check out this reference:

http://seattlecentral.edu/qelp/sets/036/...

Mileage goes down as engine size goes up.

Answer 5

You will need auto-maker data to answer this conclusively I think.

THe smaller engine does weigh less for sure, BUT, the smaller engines are usually of the 4-cylinder type. Some 2 or even 4 cylinders LESS than the 6 or 8 cylinder (larger) engines. So, aside from weight there is simply a smaller number of cylinders  burning fuel, which is what I tend to think is the major factor.

However...what you have to consider is how much energy per cylinder are you getting per volume of whatever cylinder you're considering.

This energy will be a function of OVERALL weight, that includes the weight of the engine itself.

The smaller car gets better mileage because its cylinders are smaller, therefore it necessarily has to use less gasoline. However it has a smaller gas tank....smaller torque available to turn the wheels per unit weight. The larger car has greater torque, but a lot of it has to be spent making the extra mass move.

It's like asking the question:

Is it better to sell a lot of something that costs less, or better to sell less of something that costs more?

There's a line there.....you need parameters to help define where this line is, which is what I am saying here....you need more data to really know for sure.

The relationship between the larger car and mileage and the smaller cars and mileage is NOT linear.

In other words...you can't take a 4 cyl. engine in a smaller car and say it will get TWICE the mileage of an 8 cyl. engine (larger) car. Even if the weight were exactly twice that of the smaller car I would not expect the mileage to follow suit.

You cannot make the blanket statement that your husband is making and have it hold.

With bigger engines there is more friction, more heat, more oil needed for lubrication, which may not give the same 'slipperiness' as in the smaller cyl. engine. The bigger engine needs more efficient cooling, maybe a bigger pump (proportionally) than the smaller engine to keep it within a reasonable operating temperature.

So, in my opinion, you're both right, and both wrong. There's much more to it than simply accounting for weight and # cylinders. You're also dealing with the fact that humans want things to be linear in relation to each other. Nonlinear things cause troubles.

For example....with linear momentum. If you double the speed you double the momentum....if you triple it you triple the momentum, and so on.....it's linear.

BUT

With kinetic energy (ke) say...if you double the speed the 'ke' goes up FOUR times! Very non-linear!

And unless you knows this, you can't sit around and carry on a cavalier 'party' conversation about such things and make much sense...especially to someone that knows better. When you get more than one non-linear thing coming into play, then you can't really carry on a discussion unless you start breaking out pencil and paper, unless everyone is 'in the know'.

Which is where I think this topic is at now....you need data to split the difference and find where the line actually lies.

Interesting question for sure....can't say I really helped much, but thanks for the interesting thought-gymnastics.

Answer 6

You're right, he's wrong, (don't rub it in too hard).

Smaller car = less weight , less air resistance,

less rolling resistance, (tire heat & friction loss).

The bigger engine uses more gas to make more energy,

(also more gas just idling).