Fuel Saving Thread: Motoring Tips & Tricks

[mgb_phys]

The aerodynamics of the back of a car are very important - you are trying to avoid creating a vacuum that pulls the car back. The way to do this is to create turbulence that pulls air from the airstream down into the space and fills the void.

There is an important difference between a spoiler ( the little turned up tip on the edge of the trunk/rear window) and a wing.
The spoiler is to create turbulence and break the airflow away form the rear of the car, the wing, eg. on a F1, car is to create downforce to give more grip. The wing is pretty much pointless on most cars at legal speeds and is generally a bad idea (at these speeds) because the extra drag loses more speed than you gain by the increased grip. One Porsche model has a rear wing that is hidden and deploys at around 80mph.

 

[Topher925]

The aerodynamics of the back of a car are very important - you are trying to avoid creating a vacuum that pulls the car back. The way to do this is to create turbulence that pulls air from the airstream down into the space and fills the void.

In a nutshell, decrease separation as much as possible. Just about all aerodynamic car designs are designed around decreasing the pressure drag. This makes sense since it is the dominant component of drag but I have never seen any design try to reduce the friction drag on a car. I guess the main reason would be cost but there still can be some drag reduction by reducing friction drag on the vehicle.

Most spoilers that are on cars these days are there purely for looks. They offer no down force and only increase the drag on the car. Porsche isn't the only company that puts hidden spoilers on their cars, the crossfire has one as well.

 

[mgb_phys]

Most spoilers that are on cars these days are there purely for looks. They offer no down force and only increase the drag on the car.

Most spoilers are necessary (to detach boundary layers) most WINGS are just for looks.
I don't know if this a UK/US word thing or a general/technical term usage thing.

= yes

= no

Porsche isn't the only company that puts hidden spoilers on their cars, the crossfire has one as well.

They are worthwhile on cars that are fast, rear wheel drive and have stiff enough suspension/chassis that the wheels can be unloaded by aerodynamics. But only when you get to speeds where this matters.

Interestingly - the Porcshe's wing deploy at something like 120Km/h - no problem in unrestricted Germany but above the speed limit in most countries. Since having the wing sticking up was a big badge saying "I'm breaking the law" and since Porsche drivers tended to get pulled for speeding a lot - they changed it so it deployed at a lower than optimum speed limit so the owner at least had some chance of getting away with it!

 

[baywax]

Most spoilers are necessary (to detach boundary layers) most WINGS are just for looks.
I don't know if this a UK/US word thing or a general/technical term usage thing.

= yes

= no
They are worthwhile on cars that are fast, rear wheel drive and have stiff enough suspension/chassis that the wheels can be unloaded by aerodynamics. But only when you get to speeds where this matters.

Interestingly - the Porcshe's wing deploy at something like 120Km/h - no problem in unrestricted Germany but above the speed limit in most countries. Since having the wing sticking up was a big badge saying "I'm breaking the law" and since Porsche drivers tended to get pulled for speeding a lot - they changed it so it deployed at a lower than optimum speed limit so the owner at least you had some chance of getting away with it!

you guys kill me!

America has always led the way in the many innovations surrounding the automobile. The return of the troops from WW2 saw a huge explosion in inventiveness when it came to body builds, hot-rods, engine builds the "whole nine yards". Why haven't some of the more practical and efficient realizations that grew out of those experimental years been modified, bettered and put to use to create "cute", fuel efficient and fast long range American cars?

I suppose the innovations in aerodynamics are transferable to electric cars and require little or no modifications... with the exception of housing a much smaller engine (edit: and potentially larger battery).

 

[mgb_phys]

America has always led the way in the many innovations surrounding the automobile.

Not entirely sure about that - America has led the way in taking the solid axle and ladder chassis technology of a medieval oxcart and adding larger and larger engines.
Citreon probably introduced more of the standard features of a modern car than anyone else - and accidentally invented a lot of the maths behind computer graphics while doing it.

 

[baywax]

Not entirely sure about that - America has led the way in taking the solid axle and ladder chassis technology of a medieval oxcart and adding larger and larger engines.
Citreon probably introduced more of the standard features of a modern car than anyone else - and accidentally invented a lot of the maths behind computer graphics while doing it.

I wondered about that. So there has been more innovation out of Europe concerning autos than out of the US? When you look at some of the funny cars, hot rods and dragsters that came out of the 50s... it makes one think America was at least more imaginative about it.

 

[aerospaceut10]

Not entirely sure about that - America has led the way in taking the solid axle and ladder chassis technology of a medieval oxcart and adding larger and larger engines.
Citreon probably introduced more of the standard features of a modern car than anyone else - and accidentally invented a lot of the maths behind computer graphics while doing it.

Agreed.


Although you have to respect the larger engine + pushrod design + gearing ability of the modern corvette. Impressive gas mileage for such high power output ;)

 

[du nuthin]

I'm talking about this stuff, they use an emulsion "sauce" to make oil and water mix instead of separate and mix water in at like 25% water 75% diesel.

http://inventorspot.com/articles/bre ... _india_13751

http://findarticles.com/p/articles/mi_m0CYH/is_9_6/ai_85591526

I would be interested if anyone could help explain how this all works.

When I read about it 5 years ago, it was supposed to be like 40 year old technology, but that may have been different, not sure if that was diesel like this?

In an ICE (Internal Combustion Engine) it is the total volume of combustion gases that provide the motive energy (expansion pressure of combustion gases) to drive the pistons.

The water component of emulsion fuel is instantly transformed into superheated steam, upon entry into the engine's combustion chamber, by both the heat of compression (compression stroke) and combustion heat. 1 litre of water will generate >1600 litres of steam.

 

[mgb_phys]

The water component of emulsion fuel is instantly transformed into superheated steam, upon entry into the engine's combustion chamber, by both the heat of compression (compression stroke) and combustion heat. 1 litre of water will generate >1600 litres of steam.

And the energy to do this comes from ?

 

[du nuthin]

In a diesel engine, fuel is ignited by the heat generated during the engine's compression stroke. The combustion of the fuel component of the emulsion fuel also releases heat. The excess heat of combustion is a waste heat that is released into the atmosphere via the engine's cooling system.

 

[AWOC]

The problem with cars today is aerodynamics. Yes they are way better than they used to be be not nearly enough where they need to be. One area is the under neath of cars today. Unless you are riding 1/2" off the ground the drag created by the undercarriage of your car is a huge problem. The weird floor shapes and exhaust pipes create a lot of drag in this higher pressure area. The drag created by the wheel wells are also an aerodynamic mess. More needs to be done in these areas to improve fuel mileage.

Here's what Honda did for the undercarriage. Seems cheap and with little assembly line work.
http://world.honda.com/NSX/technology/t3.html

Undercarriage and wheel well.
http://www.insightcentral.net/encyclopedia/enaero.html

Manufacturers need to do the same for all passenger cars!

 

[baywax]

The problem with cars today is aerodynamics. Yes they are way better than they used to be be not nearly enough where they need to be. One area is the under neath of cars today. Unless you are riding 1/2" off the ground the drag created by the undercarriage of your car is a huge problem. The weird floor shapes and exhaust pipes create a lot of drag in this higher pressure area. The drag created by the wheel wells are also an aerodynamic mess. More needs to be done in these areas to improve fuel mileage.

Here's what Honda did for the undercarriage. Seems cheap and with little assembly line work.
http://world.honda.com/NSX/technology/t3.html

Undercarriage and wheel well.
http://www.insightcentral.net/encyclopedia/enaero.html

Manufacturers need to do the same for all passenger cars!

I totally agree. So far it has been style dictating the design instead of efficiency. Whereas a design based on efficiency will be more pleasing in more ways than simple aesthetics. I don't get it.

 

[mgb_phys]

A front air dam will stop a lot of the air going under the car.

But making the underside smooth will create downforce which while improving cornering and acceleration (the reason it's done in F1 cars) will overall increase drag - since it effectively makes the car heavier.

 

[mheslep]

A front air dam will stop a lot of the air going under the car.

But making the underside smooth will create downforce which while improving cornering and acceleration (the reason it's done in F1 cars) will overall increase drag - since it effectively makes the car heavier.

s/drag/rolling resistance/

 

[mgb_phys]

Yes - I meant drag in the sense of retarding force.

 

[AWOC]

A front air dam will stop a lot of the air going under the car.

But making the underside smooth will create downforce which while improving cornering and acceleration (the reason it's done in F1 cars) will overall increase drag - since it effectively makes the car heavier.

How's making the bottom of your car smoother going to create down force, and increase drag?? I still don't understand how a few light sheets of plastic are going to me the car heavier. If anything it will create lift and less drag!
Formula cars are not a good example because they way they channel the air out is a science and different because we're not going to be traveling at 200 mph anytime soon. The way the air is channeled out is to keep the car steady at those kind of high speeds.

 

[mheslep]

How's making the bottom of your car smoother going to create down force, and increase drag?? I still don't understand how a few light sheets of plastic are going to me the car heavier. If anything it will create lift and less drag!
Formula cars are not a good example because they way they channel the air out is a science and different because we're not going to be traveling at 200 mph anytime soon. The way the air is channeled out is to keep the car steady at those kind of high speeds.

Bernouli's principle. The idea would be that the air will flow faster past this new undercarriage, and all else being the same, faster moving air exerts less pressure. Thus the pressure of the faster moving air under the vehicle now exerts less pressure on the bottom of the vehicle. Air flow pressure on top has not changed; the net result is more downward force on the vehicle.
http://en.wikipedia.org/wiki/Bernoulli's_principle#Compressible_flow_in_fluid_dynamics

 

[mgb_phys]

Thank you mheslep - the funny part is that in the link AWOC provided Honda claim more downforce as the main advantage!

 

[AWOC]

Thank you mheslep - the funny part is that in the link AWOC provided Honda claim more downforce as the main advantage!

Yeah sorry guys. I took Fluid Dynamics last semester and very aware of Bernoulli's Law, but I got confused because if you think of it as an airplane wing where the faster moving air is underneath and the slower up top it would create lift. I forgot that flow goes towards the lower pressure, but that's why you guys are here to remind me and threads like this are great little refreshers.
So anyways.. by putting a lip that allows no air underneath the car is the best thing to do, but does anyone know of any real proven gains from this?

 

[du nuthin]

And the energy to do this comes from ?

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species.

Water-Diesel-Emulsion fuels can capture some of that heat of combustion which is essentially a waste heat that need to be removed.

A mixture of one part water and two parts diesel can deliver the same power as pure diesel.