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Turbofan car

Mitigate some of a car's drag with turbofan technology
  (+4, -2)
(+4, -2)
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A car fitted with a turbocharger is effectively a zero- bypass turbojet with the combustion chamber being inside the engine block. All of the air going in the front end compressor passes through the combustion chamber before all of it exits via the turbine. A turbofan has a bypass, some of the air taken in the front never passes through the turbine, instead it simply goes directly out of the back. In a car, I envisage a larger compressor, possibly axial, with some air being bled off to provide forced induction for the engine. The rest is directed down a large diameter tube, to the back of the car. I don't see this as producing any serious thrust, however it can be used, if combined with clever bodywork, to mitigate the enormous hole in the air behind the car, which produces a good portion of the drag. Of course putting an enormous compressor on the front of an engine will make turbo-lag that much worse. This is why I propose this be used in constant load applications. Trucks and trains perhaps, or some form of gas/diesel-electric hybrid. Here a small engine with a relatively large turbo running at a specific RPM would make a very efficient way of getting about.

Additional benefits could be as follows.. in normal cars ~33% of the drag comes from the "cooling package" the radiator, A/C oil coolers etc. With a source of constant air flow, these could be designed into the bypass system and made much smaller and more efficient. Thereby making the front of the car less draggy, and possibly gaining a small boost from warming up the air and causing it to expand on it's way out.

The biggest problem with this is the packaging of a big tube through the car, but I reckon something the size of a transmission tunnel for a RWD car should do.

bs0u0155, Jul 06 2010

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       So to reduce the drag of the air moving around the car you propose that you pump the air through the car? Why would this have less drag?
MisterQED, Jul 06 2010
  

       at 100 feet per second and a 12 square foot rear end, that's 1200 cubic feet per second to overcome. A 300ci engine @ 2,000 rpm is 350 cubic feet of intake.   

       hmm... that almost sounds doable dunnit.   

       [+]
FlyingToaster, Jul 06 2010
  

       To the Batmobile!!!
MaxwellBuchanan, Jul 06 2010
  

       // Didn't Jay Leno build one of these?//   

       No but he owns one, there is at least one company that builds them and there have been numerous DIY versions. Also there were a number of concept cars from days past that used turbines.
jhomrighaus, Jul 07 2010
  

       Jay leno's is a jet powered car. If I'm reading this idea right, it's completely different.   

       Anyway +1 because you could direct the exhaust *upwards to gain downforce.
DIYMatt, Jul 07 2010
  

       Where are you going to get all the extra power required to turn the fan? it has to come from someplace.
jhomrighaus, Jul 07 2010
  

       I think the word "turbo" fogs my mind. I think it is turbo because it is an addon to something which is turbo, not because the thing itself has any turboness. Turbosity?   

       Seriously: does this need to be turbo to work as you hope, [bs0]? Could one achieve the same desired benefits with a big fan on the car (axial? does that mean the top?) car blowing air through a tube to the back?
bungston, Jul 07 2010
  

       the air is coming through the normal intake area (ie: the portion of the car commonly known as "the front bit") and, after annoying all the components under the hood, is driven out the back, along with the exhaust, powered by a turbocharger turbofan, for the purpose of improved aerodynamics.
FlyingToaster, Jul 07 2010
  

       Wouldn't it just be more energy efficient to design vehicles that are more aerodynamic?
Cedar Park, Jul 07 2010
  

       If the car has space for a huge duct to allow bypassed air to be passed from front to rear, just start with a smaller car and forget the duct & bypass fan. A small duct requires compression of the airflow and that requires a lot of power.   

       Some of the drag is not simply displacement of air around the body, but the friction at the surface. The duct and fan add significant surface, so the whole design would actually have more drag than a conventional body.   

       To get a given x-section are through the air at a given speed, the only way to reduce drag is to reduce Cd.
Twizz, Jul 07 2010
  

       I'm aware that simply designing a car with a nice smooth shape and a lower frontal area will improve the aerodynamics, but this is both baked and not, depending upon the circumstances, and various factors affecting that. I am also not proposing a gas turbine power plant for a car. It's been tried, GM made and released some, they have their own benefits like size and power capabilities, and their downsides, like spool-up and gearing.   

       What I am proposing is that there is more energy to be extracted from ICE exhausts than is currently implemented. Lots more. Extracting it needs big turbos, and then there's the question of what to do with it. Normally you'd use it to cram more air into the engine, add more fuel for a bigger bang which gives more BHP. This is valid. However, when you have a finite need for BHP and you want to increase efficiency WITHOUT using more fuel what do you do? Well you can extract energy from the exhaust stage, and use it to help the aerodynamics. A well-designed duct could utilize grill area, compress the air and shoot it out at a strategic point to mitigate the wake. Along the way radiators etc could be made to work and the normal cooling fan ditched.   

       While I admit that it may increase surface area, that doesn't matter if there is extra free energy available from the exhaust to overcome this and more.
bs0u0155, Jul 07 2010
  

       I still think that purposely channeling and pumping air through the car and out the back to mitigate the lost efficiency is a net power loss. The majority of drag on a vehicle is pushing the air in front of the car and loss through the tires and drivetrain. The small amount of suction in the slipstream is negligible compared to the other losses.   

       There are passive ways to disturb the slipstream.
Cedar Park, Jul 08 2010
  

       There is a reasonable quantity of energy in the exhaust of an IC engine, but most of it is in the form of heat.   

       The energy available from the gas pressure created by combustion can be extracted at the piston and by a turbo.   

       It is most effecient to extract this energy when the gas temperature is highest, i.e. immediately after combustion. This is the job of the piston. The turbo can extract some residual energy by operating at a lower pressure delta, but this is a less effecient extraction of energy than the piston.   

       To extract more energy from exhaust without impacting the effeciency of the engine proper, you need to look at a low delta heat engine. In real terms, this means something like a stirling engine, which has a low power density and is not worth carrying around in a motor vehicle.
Twizz, Jul 08 2010
  

       @Twizz   

       That's what I'm trying to do. An aerodynamic shape, e.g. a teardrop allows air flow to remain attached and energised all the way to the back where the flows may rejoin behind the vehicle leaving minimal turbulent wake. The downside with a teardrop is that it's very long for a given cross section, the length comes from the fact that air at a given speed can only remain attached to a surface of a moving object if the angle of that object is shallow. However, blowing air out of the trailing edge of a slightly-to-steep an angle object can encourage attached flow (see F1 blown diffusers), so you can make the rear of the car smaller, faster, reducing the wake.
bs0u0155, Mar 15 2011
  
      
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