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Turbofan car
Mitigate some of a car's drag with turbofan technology | |
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.
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Annotation:
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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? |
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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. |
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hmm... that almost sounds doable dunnit. |
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// Didn't Jay Leno build one of these?// |
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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. |
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Jay leno's is a jet powered car. If I'm reading this idea right, it's completely different. |
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Anyway +1 because you could direct the exhaust *upwards to gain downforce. |
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Where are you going to get all the extra power required to turn the fan? it has to come from someplace. |
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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? |
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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? |
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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. |
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Wouldn't it just be more energy efficient to design vehicles that are more aerodynamic? |
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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. |
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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. |
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To get a given x-section are through the air at a given speed, the only way to reduce drag is to reduce Cd. |
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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. |
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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. |
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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. |
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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. |
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There are passive ways to disturb the slipstream. |
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There is a reasonable quantity of energy in the exhaust of an IC engine, but most of it is in the form of heat. |
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The energy available from the gas pressure created by combustion can be extracted at the piston and by a turbo. |
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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. |
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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. |
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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. |
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