Fuel consumption: Not filling your tank

[328ii]

The change in frontal area can be calculated from suspension spring rates.

A compression spring has units of force divided by distance, i.e. N/m

F = -k * x

where

x is the displacement vector – the distance and direction in which the spring is deformed
F is the resulting force vector – the magnitude and direction of the restoring force the spring exerts
k is the rate, spring constant or force constant of the spring, a constant that depends on the spring's material and construction.

or rather : x = F / -k

Now all we need is a mathemagician to put it all together in the real world i.e. spreadsheet
Obviously there have to be some assumptions...

Maybe it will be easier to just measure the height of the car with a ruler before and after adding fuel.

Chances are that filling the tank will lower the car at the rear (where the tank is) and not 'really' affect the frontal area.

8?>

 

[moranor@axis]

if your car is heavier is sits lower so you have less drag :fencelook:

 

[killua]

The change in frontal area can be calculated from suspension spring rates.

A compression spring has units of force divided by distance, i.e. N/m

F = -k * x

where

x is the displacement vector – the distance and direction in which the spring is deformed
F is the resulting force vector – the magnitude and direction of the restoring force the spring exerts
k is the rate, spring constant or force constant of the spring, a constant that depends on the spring's material and construction.

x = F / -k

Now all we need is a mathemagician to put it all together in the real world i.e. spreadsheet
Obviously there have to be some assumptions...

8?>

Thanks for putting your google skills to good use.... but these are irrelevant when making silly assumptions. These values are so small, that they offset any assumption you make. The equation you give is something a st 6 will use to calculate displacement in a silly spring. Real springs aren't perfectly perpendicular to the road surface, and some aren't even linear at all...

 

[328ii]

if your car is heavier is sits lower so you have less drag :fencelook:

Chances are that filling the tank will lower the car mostly at the rear (where the tank is) and not 'really' affect the frontal area.

8?>

---

The change in frontal area can be calculated from suspension spring rates.

A compression spring has units of force divided by distance, i.e. N/m

F = -k * x

where

x is the displacement vector – the distance and direction in which the spring is deformed
F is the resulting force vector – the magnitude and direction of the restoring force the spring exerts
k is the rate, spring constant or force constant of the spring, a constant that depends on the spring's material and construction.

x = F / -k

Now all we need is a mathemagician to put it all together in the real world i.e. spreadsheet
Obviously there have to be some assumptions...

8?>

Thanks for putting your google skills to good use.... but these are irrelevant when making silly assumptions. These values are so small, that they offset any assumption you make. The equation you give is something a st 6 will use to calculate displacement in a silly spring. Real springs aren't perfectly perpendicular to the road surface, and some aren't even linear at all...

This thread asked for the math but did not specify the level.
So iI am a supergoogler and therefore can google faster than you..!
Anyways, i think this may become a foundation for further investigation/calculations to whatever level your heart/brain desires.

 

[FaSTandFurious]

imp:
I think a much better way to save on weight is to leave the mother in law at home. :thumb:
I mean, if she really want's to go with then she can fly there on her broom. :mmm:

Bhwahahahahahaha - good one

 

[moranor@axis]

the tank is quite a bit front of the rear wheels so it should not lower the back much more than the front... also the front hangs lower than the back...

so lowering the back slightly will decrease the down force generated under the car so reducing drag...

this is really splitting hairs i think a bug splattered on your side mirror might make more difference than how much fuel you have in a 1200+ kg car

 

[328ii]

the tank is quite a bit front of the rear wheels so it should not lower the back much more than the front... also the front hangs lower than the back...

so lowering the back slightly will decrease the down force generated under the car so reducing drag...

this is really splitting hairs i think a bug splattered on your side mirror might make more difference than how much fuel you have in a 1200+ kg car

and how well polished the car is for better Cd

8?>

 

[LAWLESS BMW]

All righty then...

This thread just got a bit technical. :bangdesk:

 

[328ii]

After doing some more googling, the following consideration(s)

Frontal area of vehicle is determined by :

Frontal area = Width * (Height - Ground Clearance) * Factor

Where factor is due to the car not being an exact boxed shape ~ 95%

So...

The area under the vehicle is NOT considered as part of the drag calculation.

& thus...

The aerodynamic drag will stay the same if the car sits lower to the ground due to more
weight added to the vehicle by, for example adding fuel, passengers - or lowering the car.

8?>

 

[Carbon M3]

I think affect in drag is not the major issue here :fencelook:

There are other considerations also eg. will you going more uphill were added weight will be a disadvantage, engine power is also a huge consideration.

A more powerful engine will be strained less by having to pull around a further 50kg's

Just my 1c.

:fencelook:

 

[Doomsdaya]

A more powerful engine will be strained less by having to pull around a further 50kg's

+ 1

my 328i got so much torque, I hardly notice a difference in consumption whether laden or not.

Unfortunately same cannot be said for my 1600 vw

 

[Carbon M3]

A more powerful engine will be strained less by having to pull around a further 50kg's

+ 1

my 328i got so much torque, I hardly notice a difference in consumption whether laden or not.

Unfortunately same cannot be said for my 1600 vw

KW gets you to speed, Torque keeps you there :thumbs:

 

[killua]

After doing some more googling, the following consideration(s)

Frontal area of vehicle is determined by :

Frontal area = Width * (Height - Ground Clearance) * Factor

Where factor is due to the car not being an exact boxed shape ~ 95%

So...

The area under the vehicle is NOT considered as part of the drag calculation.

& thus...

The aerodynamic drag will stay the same if the car sits lower to the ground due to more
weight added to the vehicle by, for example adding fuel, passengers - or lowering the car.

8?>

Your formula is way too simple to take everything into account. The formula is assuming the ground has no effect on the drag, which is a uber lie. When the car is lower, the effect of drag caused by the ground is less, probably due to less turbulance behind as well. It is a common phenomenan I have seen that a lower car has a higher top speed.... But all the formulas are just estimations, otherwise they wouldn't ever have to test anything, they would just use formulas on google :joy:

 

[naushad]

I know a few stingy people but u my friend just take the cake!:roflol:

 

[Yuvan]

Ass-u-me we consider 1/2 tank and full tank to be 30 and 60 liters.
The question then is how will ~<30 kg affect frontal area and tire drag
If the car weight distribution is 50/50 that is ~7.5 kg per wheel

A basic equation for the force it takes to push something through air :

Aerodynamic drag = 1/2 D x A x V^2

Where...

D is the density of the air
A is the frontal area of the vehicle
V is its velocity relative to the air

For real body shapes & air at standard conditions...

Drag = K x Cd x A x V^2

Where Cd is the drag coefficient

Above shows that velocity is more of a factor than area as it is squared.

Nice physics there...LOL

---

I have noticed a change in consumption for the worse after installing sound in my 320i and thats just 1X12" sub in box and a amp, i have also noticed that uphills are more of a battle.

I think that any amount of weight change can influence the fuel consumption on your car and power although some cars are so powerful that the influence that the weight has is hardly noticeable but there will be a change.

I guess i notice the weight difference more because the compression rings are already messed up on my 320i....need to sort that out....

 

[ETTIENNE]

imp:

:rollsmile:

HAHAHA!

 

[whattingh]

while we are talk about minnor stuff.

Sound system can effect things too. The louder you play it the more power the amp takes the more power the alternator needs to output, more engine power is eaten, more fuel.

"Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed."

 

[Yuvan]

while we are talk about minnor stuff.

Sound system can effect things too. The louder you play it the more power the amp takes the more power the alternator needs to output, more engine power is eaten, more fuel.

"Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed."

Very True, Nice point:thumbs: i have seen this difference even with the sound off though,i will experiment a trip with the sound thumping and the same trip in complete harmony, completely no sound or unneccesary electronics to see the difference......there could be a mental change that will influence the consumption...LOL, with the sound on, im sure you unconciously become more heavy on the accelerator pedal and aggressive and if the environment is calm, you state of mind is calm...hence more fuel efficient...LOL, might sound nerdy but i love these technical stuff..:rollsmile:

 

[Carbon M3]

while we are talk about minnor stuff.

Sound system can effect things too. The louder you play it the more power the amp takes the more power the alternator needs to output, more engine power is eaten, more fuel.

"Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed."

A huge sound system doesn't help the weight issue either
:=):

 

[Yuvan]

while we are talk about minnor stuff.

Sound system can effect things too. The louder you play it the more power the amp takes the more power the alternator needs to output, more engine power is eaten, more fuel.

"Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed."

A huge sound system doesn't help the weight issue either
:=):

Its a small system bru, just 1X12" sub.....LOL