Diffuser

dzx · Post by **dzx** » Sun Sep 12, 2004 9:13 am

Just curious if anyone has ever thought of making their exhaust into a diffuser. I was talking with my uncle and roughly calculated we figured a 3 inch exhaust pipe the length of the car ending in a 6 in diffuser, 100 lbs of pressure would equal about 2800 lbs of boost. Rather than using a custom turbine setup, you could just use a pressurized bottle since you would only need a 5 or 6 second burn. The most difficult part would be keeping the car straight if the exhaust was mounted on one side. Probably would be best to mount it in the middle of the rear bumper tho it'd look kinda strange. Might do some testing in the future just for fun... Theoretically a 5 or 6 second quarter mile would be easily attained. Tho the drag strip would certainly throw you out after your first pass.

georryan · Post by **georryan** » Sun Sep 12, 2004 9:25 am

What's a diffuser and how does it work?

dzx · Post by **dzx** » Sun Sep 12, 2004 10:09 am

I found this the easiest way to explain it. Got it off of howstuffworks.com
A modern turbine engine is extremely efficient, and there is still a lot of oxygen available in the exhaust stream. The idea behind an afterburner is to inject fuel directly into the exhaust stream and burn it using this remaining oxygen. This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.

The big advantage of an afterburner is that you can significantly increase the thrust of the engine without adding much weight or complexity to the engine. An afterburner is nothing but a set of fuel injectors, a tube and flame holder that the fuel burns in, and an adjustable nozzle. A jet engine with an afterburner needs an adjustable nozzle so that it can work both with the afterburners on and off

Theoretically you could burn just about any kind of fuel, with each one having diff results. Alcohol would give an invisible flame during the day. Hydrogen would probably give the most power and have the advantage of being invisible also. It's not a really difficult concept, just not one that is well known, if i do try to develope one for an exhaust pipe, i'll do pics and a writeup just for anyone's curiousity.

The only drawback i can really think of is that your car would be well over its max speed long before you finished the run. I'm not exactly sure how the gearing would work. Also you'd have to shift gears extremely fast to keep up with it. The good thing is you wouldn't have to worry about breaking transmissions

Legacytuner · Post by **Legacytuner** » Sun Sep 12, 2004 3:40 pm

COOL I WANT ONE, LET ME KNOW IF YOU DO ANY TESTING !

Post by **Legacy777** » Sun Sep 12, 2004 4:36 pm

Turbine engines and car engines are quite different.

I really doubt you'll get this to work.....

vrg3 · Post by **vrg3** » Sun Sep 12, 2004 4:38 pm

Uh, I'm confused... is this a joke?

The stuff you pasted in from HowStuffWorks is describing an afterburner on a jet engine.

georryan · Post by **georryan** » Sun Sep 12, 2004 6:55 pm

Yeah I understand the idea of an afterburner from a jet engine, but i never really thought it could be applied to a car engine.

Also, i didn't think jet engines just burn exaust gases, I thought the dumped more fuel post turbine as well.

BAC5.2 · Post by **BAC5.2** » Sun Sep 12, 2004 7:30 pm

Ryan - That's exactly what they do. They dump more fuel and then ignite it. The exhaust gas from the turbine keeps the newly formed fireball from going back up through the turbine and forces it out. Since the fuel ignites and rappidly expands, as long as the turbine is spinning it is blowing the huge burst of energy out the rear of the engine, and producing a significant amount of boost. Think of it like firing a gun out the rear of the engine. Opposing forces and all that jazz. Send something one way (gas) and push something else the opposite direction (the plane).

I suppose this COULD work in a car, but it's unlikely.

What is MORE likely, is that the newly formed fireball would either shoot UP through the turbine, (on shifts). OR, it would stop the turbine dead thanks to our friend Backpressure, which would stall the engine or shoot the wastegate clean off the turbo. OR... it would simply blow the exhaust right off the car.

It'd be cool to try, but I'm not gonna test-dummy it. I'll stick with test-dummying Vikash's ingenious creations. At least I KNOW those will work.

evolutionmovement · Post by **evolutionmovement** » Mon Sep 13, 2004 12:50 am

Might as well strap a JATO rocket to it.

Steve

Ruby the Sooby · Post by **Ruby the Sooby** » Mon Sep 13, 2004 1:01 am

If you didn't blow up the engine by having the flame back up into your exhaust system, the thing would rip your exhuast system off of your car anyway, you would have to brace your pipes to be able to transfer all of that thrust to your chassis, which would be interesting...

And by the way, the amount of oxygen required is probably only found in JET engine exhaust streams, a car's won't have as much oxygen as this thing would require.

So I have reached this conclusion...

You my friend are full of shit

petridish38 · Post by **petridish38** » Mon Sep 13, 2004 3:14 am

I think a difuser on a car engine would be very cool if you could ever get it to work. I don't know too much about them, and im no physicist or engineer, but it doesn't seem like that kind of a system would be feasible on a car engine. But if you have the time, money, and expertise, then by all means go for it!

Ruby the Sooby wrote:You my friend are full of shit

How is he full of shit? I too don't think it would work... not without MAJOR modification, but he said he was just curious if anyone ever thought of doing it. He wasn't claiming anything. All of his numbers were theoretical values.

Andrew

scottzg · Post by **scottzg** » Mon Sep 13, 2004 3:56 am

Ruby the Sooby wrote: You my friend are full of shit

nice third post, buhqueet.

skid542 · Post by **skid542** » Mon Sep 13, 2004 4:33 am

As an aerospace engineering student who has been working with rockets and planes and such since I was little don't see this a viable idea. An afterburner has a tremendous amount of oxygen still available to it compared to what is coming out the back of our cars. The fuel setup to the combusion chamber (which I'm assuming is basically what you are calling a diffuser?) would require a lot of plumbing and valving. Not to mention the need for a seperate control unit with it's owning wiring.

Then there is the issue having enough mass flow rate through the combustion chamber. An afterburner has both a high mass flow rate but also a high intitial velocity. In order to get something useful for a car you need to compress the exhaust before the combustion chamber which would require a set of high tolerance turbines and more additional weight and bracing. To give you an idea, in RC airplanes we have small jet engines (same simple design as the V-1) that don't utilize turbines and they don't do anything close to efficient until they hit about 60 mph and even then only produce about at most 10 lbs. At about 100mph I would say those little jet engines have about the same mass flow rate going through them as our exhausts (don't feel like doing the math). So if you take that into consideration unless you are drawing in more air throw out the back you need to be really compressing the air in exhaust sytem to reach high exhaust velocities, remember F = ma. An actual jet engine run high compressions, usually at least 5 compressor turbines, in order to achieve the required velocities.

Next comes the issue of fuel. Hydrogen is not an option due to the extreme nature of it and the usual need for an additional oxidator. It is rarely used for anything but in rocket motor applications, different than an afterburner. Jet fuel would be the best fuel due to it's relative saftey and ease of working with (corrosion to plumbing, valving, etc.). So you would now need another tank and more additional weight. Along with fuel is how much actual oxygen is in the exhuast. I would say if you wanted to produce much more than 5-10lbs of thrust you would also need a seperate intake or be supplying a seperate oxidizer to the combustion chamber (oxygen, nitrous, etc.). Again more weight and bracing.

Now in theory you could definately get metals strong enough to withstand the pressures and you could pay 10's of thousands for the machining and plumbing and control valving but in the end you'd just have another easy 100-200lbs of weight on your car for maybe 20lbs of thrust, I'll be libral and say 40, still a negative increase in performance though.

Please understand I am by no means trying to say your idea was bad, I like the idea but I just don't see it as being anything that would be benificial and not result in the loss of a lot of money.

Lee

georryan · Post by **georryan** » Mon Sep 13, 2004 4:34 am

Don't knock him for thinking outside of the box. At least he has been creative.

-Ryan

dzx · Post by **dzx** » Mon Sep 13, 2004 5:11 am

Ok, maybe you missed the point of having a pressurized bottle of oxygen which is fuel or maybe i forgot to post that part. Let me restate and emphasize this part. Also go back and read the explanation. A turbine is not required to run a diffuser.
Pressurized oxygen is liguid. Pressurized Oxygen when unpressurized turns back into a gas. O2 is a fuel. In fact, Oxygen is the main ingredient in Nitrous Oxide. The nitrogen is only there to absorb some of the heat. Why dont they just use oxygen you might ask. Using pure oxygen would cause engine temperatures to get out of hand way too fast. The amount of oxygen needed for a 5 or 6 second burn would be fairly small.
I said nothing about mounting a turbine in the exhaust pipe or anywhere in the car for that matter. There are also things called check valves, same thing keeps the flame on your propane torch from going back into the bottle otherwise you would have quite a mess of your hand and whatever was hit by the shrapnel caused by the resulting explosion. You could rig it to shut off between shifts and as far as bracing the exhaust i dont think that would be too much of a problem.
The idea was actually brought up by my uncle who nasa brought over in the mid 60's from ireland to work on rockets, I myself am a chemistry major and can only draw my ideas on what is feasible from my physics classes and my understanding of elements. If anybody knows about Ireland from that time, their main export was engineers. (edit). Compared to school here, school there was/ could possibly still be murder.
As for just strapping a rocket to your car, that wouldnt be too advisable. During the 70's in washington, when he did contract work for lockheed martin, two engineers strapped a B-52 booster rocket to the top of their volkswagen and managed to take out telephone poles for quite some distance. Evidently they had slight miscalculation when planning it. Obviously they didnt walk away from it, more like were scraped away. As for practical uses of a diffuser in the exhaust, its not very practical, its not something you would use on your way to go buy groceries. It is something that would be entertaining to light up at a drag strip before they threw you out.
And i do think its funny about how some of you spend more time thinking about why it wont work rather than how it could work. Really, I'm impressed. Especially when you try to draw your conclusions from what someone else wrote without understanding it completely.
Thanks georryan and some of you others who realize its only a concept.
Am i full of shit: Generally only after eating lots of steak and potatos or junk food, but thanks for your concern.

skid542 · Post by **skid542** » Mon Sep 13, 2004 5:59 am

Allow me reitterate " Please understand I am by no means trying to say your idea was bad, I like the idea but I just don't see it as being anything that would be benificial and not result in the loss of a lot of money. " I gave my opinion on my knowledge and I didn't put it down, prove me wrong, show me the math and build it. As I said, I like the idea. Make it happen and keep us posted.

Lee
Virginia Polytechnic Institute

dzx · Post by **dzx** » Mon Sep 13, 2004 6:25 am

I wasn't singleing you out, more Ruby the scooby. It is something that would be a challenge but so is anything that hasn't been done before. There is a chance that it has been done before actually. A very long time ago, there was a little porsche 914 at a drag strip with a 4 cylinder engine that was killing all the muscle cars it went up against. It had one thing that struck as particular as it had a large cone shaped exhaust in the middle of the car. That of course was quite some time ago. Your equation force equals mass multiplied by acceleration kinda threw me tho since thats a formula for determining force required to make a solid object accelerate at a given speed. Were you referring to amount of thrust necessary to move a 1,361kg (3000lb assuming 9.8m/s2 G) car since weight is different than mass? I believe this would require more work than making a diffuser in the exhaust. http://www.science.tamu.edu/story.asp?storyID=37[/url]

"His idea has been dubbed a ‘quantum afterburner’ because of its parallel with the devices that extract useful energy from the exhaust of a jet engine"
Does it matter what type of engine the device would be extracting left over useful energy from? And if efficiency isn't your goal, it stands that you could create more power.

skid542 · Post by **skid542** » Mon Sep 13, 2004 6:41 am

That porsche sounds wicked. Can you give us more details? When I said F = ma I was refering to the amount of thrust you could produce by expelling a gas out the back. In this case since we have fluids it would be a combination of your fuel flow rate into the stream in conjuction with the pressure increase and acceleration of the mixture (which is a low density unless you can pump a lot of fuel). Newton's Second Law applies to anything that produces a force, you have to have a mass of some form experience an accleration. That's why I question how much thrust you could produce but I am definately curious, that porsche has me intrigued.

Lee

dzx · Post by **dzx** » Mon Sep 13, 2004 6:44 am

It would be interesting... Unfortunately i don't have any more details about the porsche although a 914 weighs almost nothing. The guy didn't talk to anyone and was gone as soon as the races ended. Kinda strange for someone who wins. Such are some engineers. They will develop something then decide they dont want to share how they did it with anyone else or even tell what it is. lol
About 100 lbs of pressure would flow a lot of fuel in a short time depending on how big the injector and line was.

skid542 · Post by **skid542** » Mon Sep 13, 2004 6:50 am

Perfectly put, it would be interesting...

Hehe, yeah there are some true odd engineers out there. But my props to guy if he was wasting the muscle cars with a 4 banger

.

Second edit (it's late)
100lbs would allow a lot of flow but think about this, in the morning I will get actual numbers if you want but for name sake. When you turn on the water hose you are propelling a liquid 10 times more dense than a fuel and air mixture, even with a lot of fuel, and propelling it at a considerable velocity. It pushes back on your hand say with 2lbs of force hardly anything to tie to the back of a car. Now say you have your fuel air mixture and you can propel it 200 times faster than the water hoze, you now have a force of 40lbs. You will gain some more from the pressure difference but I do not think it would be that much, again I'll check in the morning, I'm curious.

dzx · Post by **dzx** » Mon Sep 13, 2004 7:10 am

I'll see if i can get a diagram or better explanation out of my uncle so i can explain it better. He's pretty busy and i only see him about once a week when i go to his house to work on my car.

If your not cheating, your not trying

J-MoNeY · Post by **J-MoNeY** » Tue Sep 14, 2004 6:41 am

O2 isn't a flammable gas. It's a ingiton inhancer. It helps combustable things burn more easily.

dzx · Post by **dzx** » Tue Sep 14, 2004 8:51 am

your right

skid542 · Post by **skid542** » Tue Sep 14, 2004 4:24 pm

After running through the equations, I still don't think this is a very plausible idea. I may be wrong and would still like to see your uncle's notes but I don't know how you'd get very much appreciable additional thrust, or least be able to break even with the weight you'd gain.

Lee

G-reg · Post by **G-reg** » Tue Sep 14, 2004 5:38 pm

I’m not an engineer, but I am a certifiable Prop-head and an F-16 mechanic. So I thought I’d throw my 1.5 cents into the ring. The first thing that came to my mind is the weight of all of this. You’ll need one Hoss of a fuel pump, some kinda huge exhaust including the flame holder/injector/igniter apparatus, and a lot of fuel. You might get some positive thrust, but you’ll be lucky if it is enough to push the extra weight. That and a jet engine has just a touch more airflow. And just for shits, some fun F-16 AB facts: in Full AB they burn 2 GALLONS PER SECOND, that’s 25 TONS of fuel per hour folks. And the 16 is a single engine fighter, double that figure for an F-14/15/18.

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