Exhaust only Turbo mod idea

[bs0u0155]

I've had an idea, I might be insane but here goes anyway.

If we assume that in normal operation turbochargers recover energy from the exhaust and use it to force more air into the engine creating the opportunity to generate lots more power. If we also assume that our nice efficient high-compression ratio engines, tuned for FE, are not really looking for more power and in fact are really poorly set up for forced induction (advanced ignition/running hot/high compression). However, there is still energy in the exhaust to recover.

So, if we take a turbocharger and mount it to the exhaust of an engine with all of the oil feeds etc, we have a "spare" efficient centrifugal compressor. These flow huge amounts of air in some cases. For a Mitsubushi lancer evo, 300cfm was mentioned and 600cfm for some RX7s. Interestingly they can flow these large amounts even against 15psi+ back pressure.

So why can't this air be taken from a high pressure area at the front of the car and routed out into a low pressure area like behind the car... therefore mitigating the wake.

Now I'm not suggesting you can bolt up a turbo capable of blowing a 600bhp engine to a 1.3l 4 cyl engine. However because there's actually a negative pressure gradient, you may get much larger cfm figures out of a turbo or be able to create a hybrid turbo with a small turbine large compressor to take advantage. Could this be used to mitigate somewhat the hole in the air behind the car? Or directed in some way to improve existing over-car flow?

Given the simplicity of this it could be worth a try, no engine management issues just a turbo/manifold/exhaust/some tubing.

[Arragonis]

Apologies, couldn't resist.

However I have wondered about a slightly different approach in using the turbo to do other things such as battery charging for Hybrids as this is, as you say, wasted 'free' energy. You would need to gear down the turbo speed to drive a generating device but as the turbo may not have all that much torque in it...?

My original thought came about from the issues that early turbos had with lag, for example pressurising a cylinder of some kind with normal air from outside captured from the air filter box, and having that air released to provide additional torque - a little like the Honda IMA idea - or using it to overcome the inertia of the turbo until the exhaust pressure builds up enough to take over. This would be via a 3rd turbine so the exhaust system isn't being pressurised.

For the former those air-hogs compressed air motor things are what triggered the thought. Obviously it would need to be larger, not plastic and not mounted to a model airplane that immediate demolishes itself against a tree - or maybe that was just my experience

One issue - does anyone know the pumping loss from the engine of having a turbo being spun on the exhaulst but not providing boost input to it ?

[bs0u0155]

also, how much air in cfm does a car displace and would 5-15cubic feet /second make a dent in that?

[Zerohour]

Well There may be benefits if you duct the air to places like mirrors, or have a rear mounted turbo system (believe these are "STS" systems, i might have been the company), and you could try to use place the pressured air at the outlines of the rear bumper to build a aero stream that works similar to a kammback but without the physical kammback.

[vtec-e]

Thats starting to sound like the "blown flaps" the tsr-2 had. They made the airflow stick to the flaps during landing and take off and provide massive lift. In our case, it would make the airflow stick to the body and with a suitable body shape, reduce the wake.
If you are thinking of using it to fill the wake from the rear view mirrors, first remove the mirrors and see what mpg gains you get. If you get something then go ahead with the wake filling idea, otherwise don't bother!
I believe renault are doing some research on "synthetic jets" but are using piezo type jobbies in slots around the tail of the car to make the airflow stick to the body.
Heres a link to an article on it:Green Car Congress: Renault Altica: 44MPG Diesel Concept with Active Airflow Management
Renault Altica : News & Reports : Motoring : Web Wombat
http://www.jafmonline.net/modules/ht...-022069-A4.pdf
That last link appears to mention reducing drag by creating your own wake.......familiar?

ollie

[bs0u0155]

That was pretty much what I was thinking. That stuff was very analytical but the premise seems sound. I've just been to the Garrett website to see if I could get some flow/pressure information. This was to see if I could work out how much an easily available standard turbo would flow with a -1/2psi pressure gradient rather than a +15/18psi. They seem to have it but not in a way I could decode!!

i was also thinking that the intake could be mounted behind the radiator. The radiator would act as a rudimentary air filter and the airflow would be greatly enhanced across it meaning that more of the rest could be blocked off.

[SlideWRX]

Originally Posted by bs0u0155

also, how much air in cfm does a car displace and would 5-15cubic feet /second make a dent in that?

a 2.0 liter 4 cycle engine (at 100% volumetric efficiency) takes in 1 liter per revolution. At, say 2000 rpm that's 2000 lpm. That's 70.6 cfm, or about 1.2 cubic feet /second. That's air *intake*. Anyone want to figure out what combustion turns that into?

[bs0u0155]

Slide, That's not quite what I meant, I was thinking about how much the car itself displaces which I can kind of work out, I think. 2.4sq m for my car travelling at 100kph (27.8 m/s) 27.8*2.4=66.7 metres cubed/second = 2344.9 cubic feet/second. So 10% is possible, but the cool thing is, that you're REMOVING 10% from in front of the car then adding it behind so actually is it closer to 20%?

[DonR]

Some where on here is a topic about injecting air into the wake of a semi truck. They talk about seeing decent results with 1/4 psi (compared to what, I don't know). They used compressed air to evaluate results & were working on how to produce the air efficently afterwards. So the idea has some validity.

Something to consider is that a turbo charger will quiet the engine down to a degree. You may be able to reduce restriction in the muffler & maintain the exhaust backpressure that helps the engine run at low RPM. Like Zerohour suggested, ducting the air to a shape near the outer perimiter of the back of the car makes sense to me as well. What makes sense doesn't always work though.

Don

[bs0u0155]

Don
I was thinking exactly that this morning.
I was also wondering about the best location for the catalytic converter (if fitted). One assumes that turbocharger turbine stages work best with the best pressure/temperature gradient across them. So the quieting effect of the turbo allowing the removal/simplification of the muffler setup is very handy. In addition, you may be able to loose some of the weight gained in the turbo.

Perhaps adding a catalytic converter ahead of the turbo would place it in a high temperature/high pressure environment created by the turbocharger's back pressure therefore speeding up warm-up time and increasing efficiency with regard to the pressure. Also loosing the cat from behind the turbo may give some gains.

Perhaps the biggest problem to overcome is the ducting. Getting a large duct from the engine bay to a situation where it can deliver air in a large car-rear-sized square isn't going to be particularly easy. In my car there is certainly space to run it under the dashboard, around the gear lever and around the handbrake. Then it would have to go up over the fuel tank through the back seat and across the boot floor to the hatch back. Then some form of sealed duct built into the hatch which would interface with the boot floor duct. All quite difficult and a potentially tortuous air path, and to be usefully non-draggy, the duct has to be quite big.... 3" diameter?

An alternative would be to simply vent the turbo into the car interior, then create "exit" holes around the hatch perimeter. Simple, but how would the interior react to having a lot of air vented into it which may be hot from compression or cold from a cold exterior.

Another alternative, which is quite simple would be to only address the bottom of the car, simply run a duct paralell to the exhaust and have (both?) feed a lower trailing edge extension. Sort of a "blown diffuser" if there are any F1 fans in.

A simple experiment could be to build the rear perimeter ducts and have them open to the interior then simply give the heater fan everything and look for small changes. An electrical load control would be needed of course, but probably easily achievable with combinations of non-fan interior devices.

I wonder if the many interior channels of coroplast make it the ideal material to go around the perimeter? :-)

With regard to the "STS" remote turbos, they gain points for not having any specific manifold/header modifications but loose them for efficiency and they retain the ducting problem, just on the suction side.