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Steam afterburner

Use steam for forced exhaust
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(+6, -4)
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The 'take a regular car engine and add steam!' trick has been done before and BMW as well as Honda are actively working on their own implementations of this old technology, but I have something different. My idea isn't to increase economy, or power, but rather efficiency.

I'm building a 1991 Subaru Legacy turbo for Rally America competitions. The rule for my setup, which is very common, is that the turbine has a 34mm restrictor on the inlet. This effectively limits the amount of air entering the engine to 360-372 CFM depending on how you do the restrictor. No matter what you do, that's all you got to go fast with. This is of course to limit power and make the sport more competitive and prevent crashes. I plan to ruin all that. Normally Rally Teams design the engine to work around the problem the best they can by using high compression, lean air fuel ratios, adjusting timing and other tricks.

My idea is to use the heat from the exhaust to boil steam and to use the steam to power a turbine and that turbine will suck exhaust. I've been thinking about this for the past week and I can't find a reason why it won't work. Then I did the math and I was shocked to find that if my turbine is even a tiny bit efficient, it should work. I haven't heard of this being done before so it's hard to say what the effects on the engine are, but I can draw a couple conclusions based on some simple principles.

1) The main turbo on my car is driven by pressure from the exhaust. It works off of a pressure difference between the exhaust manifold, and the downpipe behind the turbo. If I reduce the pressure in the downpipe, it takes less exaust pressure to spin the turbo to the same speed.

2) The cylinders push against the pressure in the exhaust manifold. The more pressure there is, the harder the engine has to work and the less power it makes. This is simple exhaust tuning. The more free flowing your exhaust is, the more power you make. I want to take this to an extreme and used forced exhaust to relieve that pressure.

3) As the engine's speed increases, friction increases squared. Because of the limited air my engine will hit a dead end at 5,200-5,500 RPM. Too much friction, not enough air. That's fair BUT my turbo isn't going to spool until 3,000-3,200 RPM. So my power band is extremely limited. Storing steam in a boiler and then releasing it to spool my steam turbine BEFORE I need my main turbo will depressurize my exhaust and start spooling my main turbo before I even need it. Rally Teams do this in a different way with anti-lag systems that work very well but are fairly complex and damaging to a turbo. The steam turbine could improve spool and increase maximum HP.

I want to make the steam afterburner (I know wrong terminology but it sounds cool) from mostly off the shelf OEM parts. It doesn't need to be high pressure, but high flow. The boiler will be an intercooler welded into the exhaust after the downpipe somewhere in the rear of the vehicle. A box for water will be welded around that. As the hot exhaust gasses pass through the intercooler they will transfer heat into the water and eventually create steam. The steam will then rise and be fed into the turbo, which is an off the shelf IHI VF series turbo. The steam will then exit and be vented to atmosphere OR be recondensed into water and pumped back into the boiler. The compressor side of the turbo will be hooked to the exhaust outlet from the boiler and suck the now colder and denser exhaust out of the tailpipe. For times when the water is heating up and not yet producing steam there will be a bypass for the turbo so that the exhaust can flow freely around that restriction. A one way valve will be fitted so that when the turbo depressurizes the exhaust system air is not sucked the wrong way!

What do you guys think? I feel like there is something wrong with this invention but I don't know what yet.

Here are some notes from my math calculations. My math is my weakest subject so I might have the most trouble here!:

970 BTUs /pound of water required to go from 212 degree water to 212 degree steam.

Water = 8.3 pounds/gallon

Gasoline: 125,000 BTUs per gallon

1 KW = 3,413 BTU/hr

1 Btu = 252 calories

1 Btu= .293 watt

1hp = 746 watts

1hp = 42.440 Btu/min

1hp = 33, 479 Btu/hr (boiler HP)

One Gallon of water produces 223 cubic feet of steam.

128 pounds or 15.5 gallons of 212 degree water into steam for every gallon of gasoline (assuming 100% efficiency of course not possible!)

The horsepower output of my engine should be 250-300 HP. We'll use 250 HP because it's a nice round number. Assuming 25% efficiency that means 750 HP is being lost as heat. If 30% of that comes out of the taipipe, then 225 HP of heat is available in the exhaust.

225 HP of steam means 9,844 pounds/min of 212 degree water into steam or 1,186 gallons of 212 degree water into steam every minute.

Formula for exhaust volume:

Exhaust Temp. (°F) + 460 / 540 x Intake Airflow CFM = Exhaust Flow

Using my engines numbers (360 CFM intake air) I get: 1,373 CFM of exhaust at 1,600 degrees F and 1,106 CFM of exhaust at 1,200 degrees F.

GodSquadMandrake, Mar 17 2009

BMW blurb http://www.autoblog...-gets-hot-and-goes/
EGR [eight_nine_tortoise, Mar 17 2009]

And a bit or research work on the same lines http://www1.eere.en...2005_deer_crane.pdf
more EGR [eight_nine_tortoise, Mar 17 2009]

LOX for Boost LOX_20for_20boost
[21 Quest, Mar 17 2009]

Steam Anti Turbo-Lag System Steam_20Anti_20Turbo-Lag_20System
[phoenix, Mar 18 2009]

Ricardo http://www.ricardo....agenda_22-10-08.pdf
What we don't know about IC engines is'nt worth knowing. [eight_nine_tortoise, Mar 19 2009]

Exhaust flow calculation discussion http://www.eng-tips...m?qid=104735&page=1
Not conclusive [knowtion, Mar 19 2009]

Steam turbo Another_20Version_20of_20a_20SteamTurbo
[bungston, Jul 07 2010]

[link]






       The idea to use the waste energy from the exhaust is being baked [see BMW links]. I think the trouble with your version is that you are will be trying to add a small bit of suck to the supersonic blowdown of the exhaust valves opening, minus whatever energy is lost to the turbine. You can only ever suck down to some fraction above atmos, otherwise your gas will not have the energy to leave the system. Plus you have to trade off exhaust system flow losses against what you use to recover the energy. So no matter how efficient your vacuum pumping is to empty the exhaust it only ever adds a small fraction of momentum to the exhaust gas. Also if you recover too much energy to heat the new system you could just end up with a vehicle that does not have the energy to expel its exhaust.   

       Like the idea of keeping the turbo spinning, you can do this with compressed air injection (make your roll cage into an air cylinder). I am sure someone was banned for using that in the past (apart from the old trick of filling the cage with Nitrous Oxide). Any extra plumbing is sure to fall fowl of the scrutineers.   

       I think you just need a water-jacketed exhaust post turbo to heat water, and use the steam to spin a second turbine on the turbo spool. Watch the head gaskets fly!
eight_nine_tortoise, Mar 17 2009
  

       //I plan to ruin all that.//   

       [Marked-for-Tagline]   

       Welcome to the 'Bakery, GSquad! I don't know jack shit about car engines or how to improve them, so I don't really know if this would work. I had an idea once, I'll post a link to it, that you may be interested in, however. See if you can make mine work. If you do, I'll split the profits with you. Whaddayasay?
21 Quest, Mar 17 2009
  

       21_quest - Your idea is a bit more complicated than mine actually because you are changing the way the oxygen is introduced into the engine. I'll take a look at it. I'll be honest I had the same idea years ago but it got shot down. There are some new technologies out now though.   

       eight_nine_tortoise - Thanks for the links! It looks like their research turned out similar results to mine. I am very surprised at how small their heat exchanger in the exhaust is. I was thinking of a large box, there's is merely the size of an exhaust pipe. I can probably learn a lot from that research.
GodSquadMandrake, Mar 17 2009
  

       Look, this idea is hopelessly naive. If this isn't your first RAC attempt I would be SHOCKED. First of all, if you think that you can defeat the restrictor by spinning your current compressor faster then you need to change your A/R, clip your compressor, or change your hot side wheel. If it was as easy as that F1 would have figured it out long ago. Also if increasing the EGP and thus the speed of the turbo using the heat of the exhaust was your goal then simply spraying the liquid water into the manifold would work best (still basically no). Placing a restriction in the exhaust and expecting that to reduce back pressure is paradoxical (heat shmeet, it's still CFM).   

       Ok, so what do I suggest? Well this is a rally event so reliability and robustness trump everything else, forget increasing the engine power, 160hp is going to be plenty once you get going, focus on getting the car on a dyno and making sure that your cooling and braking systems are adequate. If you have gobs of extra time to do development focus on suspension tuning (nothing else will gain you more time than finding good settings for different conditions and driving wet/dry rough/clean). If you have money to burn buy tire sets, struts and spares.   

       Something about this smells of armchair racing.
WcW, Mar 17 2009
  

       Get the restrictor, install it, and get behind the wheel, if you can't get practice because you spend your time trying to make a few extra HP, you will never set good times. Get friendly with your navigator, get familiar with hearing and following notes. Go in the ditch. Try different settings. Go in the ditch. Break stuff. Drive in the foulest weather. Learn to follow notes. Go in the ditch. FORGET THE ENGINE, FORGET THE HP, FOCUS ON YOURSELF. FOCUS ON THE LINES. FOCUS ON READING THE ROAD. CONTROL FRUSTRATION. LEARN TO HEAR THE NOTES WITHOUT THINKING.
WcW, Mar 17 2009
  

       Ditto on [WcW], but I'm pretty sure you'd be better off injecting the steam into the exhaust manifold, as in the link. The alternative would be 'turbocompunding' through a similar method to the BMW. I like the concept of using the intercooler as a heat exchange membrane though.
Skrewloose, Mar 18 2009
  

       Once again, I'm amazed at just how smart/experienced the ½bakers are: [WcW], this time. (Never surprised, though: everybody seems to have there own area(s) of expertise.)   

       Not knowing much about I.C.E./racing, etc, a bun to [GodSquadMandrake] [+]   

       (Anybody who can use "Mandrake" PLUS "GodSquad" in their name deserves at least ONE bun... Do I smell an Ox and an Moron here?)
Wily Peyote, Mar 18 2009
  

       WcW - // First of all, if you think that you can defeat the restrictor by spinning your current compressor faster then you need to change your A/R, clip your compressor, or change your hot side wheel. If it was as easy as that F1 would have figured it out long ago. //   

       I'm not trying to spin my compressor faster. I already pointed out that the restrictor can only flow 360 CFM. After that point the air going through the restrictor will be moving at sonic speeds and a shock wave will form in front of the inlet preventing any more air from entering regardless of the pressure difference. You could have infinite vacuum on one side, and infinite pressure on the other and no more air would go through. So spinning the compressor faster will only make more heat. Also I know I can work to make the turbo and the engine more efficient. Instead of wasting time and money on that, I want to focus on reducing pumping losses which is a greater loss. I know this wouldn't work on a normal car for economy, or a normal car just for power. Even in F1 this wouldn't work because if it isn't in the Formula, you can't do it! Their rules are way more strict than Rally America so they would never have the freedom to engineer this way. In Rally America the rules are very lenient about exhaust: //The exhaust system is unrestricted, except that the stock exhaust manifold(s) must be retained, the pipe(s) must exit behind the driver and external to the body. A functioning (meets the Federal emissions standards) catalytic converter must be retained or installed.// So yeah I can run this on my car in Rally America.   

       //Also if increasing the EGP and thus the speed of the turbo using the heat of the exhaust was your goal then simply spraying the liquid water into the manifold would work best (still basically no).//   

       I'm actually DECREASING the exhaust gas pressure (if that's what EGP is). The second turbine sucks on the exhaust side of the first turbine. Normally this would be innefficient, but the second turbine is using free energy so it increases efficiency. If I just sprayed water into the manifold I would be increasing pressure in the exhaust manifold and I would get more pumping losses although I might have an increase in spool. Plus the water droplets in steam (unless it's 1,500-3.000 degree F superheated steam) will have a very corrossive effect on the turbine blades. My secondary turbine won't have this problem because there will be a lot less pressure on it and the steam won't be as hot.   

       //Placing a restriction in the exhaust and expecting that to reduce back pressure is paradoxical (heat shmeet, it's still CFM).//   

       The only restriction on the exhaust is the intercooler but that is a very small restriction to trade off the potential gain in flow. Also adding the intercooler allows me to remove the muffler because the intercooler will break up the exhaust pulses and deaden sound. So I gain some there and I lose some. You're still thinking old school here. Less restriction in the exhaust and more CFM = power right? Well that's how you tuned an induction system before turbochargers right? Well now things are different, we have forced induction. Pretty soon we will have forced exhaust too. I'm sure a turbocharger seemed like a huge restriction to old school tuners when it first came out too.   

       //Ok, so what do I suggest? Well this is a rally event so reliability and robustness trump everything else, forget increasing the engine power, 160hp is going to be plenty once you get going, focus on getting the car on a dyno and making sure that your cooling and braking systems are adequate. If you have gobs of extra time to do development focus on suspension tuning (nothing else will gain you more time than finding good settings for different conditions and driving wet/dry rough/clean). If you have money to burn buy tire sets, struts and spares. //   

       Suggestions taken. I'm not relying on my engine to win the race, but I would be kicking myself if I never tried to fully cook this half baked idea!
GodSquadMandrake, Mar 18 2009
  

       I'm not a thermodynamic expert by any means. But I do know that getting the air out of the engine means more can come in, means more combustion. This is why people put 3"+ diameter exhaust piping if they are looking to make some serious horse power. But I'm struggling with the concept of being able to "suck" exhaust flow. I *think* exhaust flow is roughly twice the intake flow (2 x 350 = 700 CFM)(someone please correct me if I'm wrong). Your heat exchanger will restrict flow as well (making your vacuum work harder still). It really comes down to the numbers in the end. Would you be so kind as to add a space between your equation lines so that it's a bit more readable. You need   

       to press   

       enter   

       twice for it to work.   

       cheers
knowtion, Mar 18 2009
  

       Exhaust gas flow - Exhaust Gas Recirculation (If you have it fitted) = Intake gas flow + Fuel combustion products (Gas) + Unburned Fuel/Air Mixture (Liquid and Gas) - Piston Ring Blowby - Exhaust Gas Recirculation (If you have it fitted)   

       I don't think it would be 2:1 in volume but have never measured it to be honest.   

       [link] might help or read Internal Combustion Engine Fundamentals by John B. Heywood. Knowledge of chemistry and thermodynamics needed to understand it though.
eight_nine_tortoise, Mar 19 2009
  

       //make the sport mire competitive //
For very muddy stages?
coprocephalous, Mar 19 2009
  

       The mass of exhaust will be equal to the mass of intake + fuel. That is *mass* not *volume*. The volume is effected by temperature and chemical composition. A added a link to a discussion. One guy says a ratio of 1.6 to 2.2 cfm (per horsepower) is a good rule of thumb. I don't suggest this to be gospel truth necessarily. Still waiting for a halfbakery expert to chime in.   

       [Godsquad] Thanks for updating your equations. I'll look closer at them when I get a chance.
knowtion, Mar 19 2009
  

       knowtion - Good point about the exhaust flow calculations. I hadn't researched that yet, I just assumed that it would be something like the intake flow. I looked up a formula and I found:   

       Exhaust Temp. (°F) + 460 / 540 x Intake Airflow CFM = Exhaust Flow   

       So if I plug in 1,600 degrees F, which should be the max I'd let my engine hit, I get 1.373 CFM. At 1,200 degrees F it's 1,106 CFM. It's hard to guess what the temperature of the gasses will be after they pass through the heat exchanger but 1,000-1,200 degrees F at balls to the wall is a good estimate being I only want this when going balls to the wall.   

       1,100 CFM is about 4 times what I wanted, that makes this whole thing harder. I honestly can't think of an off the shelf turbine that can flow that. That brings my water demand up to 5 gallons per minute! I still have enough heat to boil that, sure but it means this whole contraption is going to have to be about 4 times bigger than what I imagined. This project may have come to a grinding halt unless I can figure out a solution to this. Any ideas?
GodSquadMandrake, Mar 19 2009
  

       Ok this math is really kicking my butt!   

       My conversion chart says that: 1hp = 42,440 Btu/min   

       I questioned that so I decided to see how much total gas I can put into the engine.   

       Fuel injectors = 565 cc/min (3785 cc in 1 gallon) x 4 fuel injectors.   

       So I get .597 gallons of gas/min into engine MAX although 300 HP should only require .33 gallons/min at .7 BSFC (found on fuelinjectors.com calculator).   

       So 125,000 BTU's in a gallon of gas x .33 = 41,250 BTU/min from my engine. Now only about 25% of that should come out the tailpipe so we're left with 10,312 BTU/min to make steam. If I still need 5 gallons of steam per minute I'll need 40,255 BTU/min which means I am 29,943 BTU short assuming 100% efficiency out of my heat exchanger which would be impossible.   

       Guys, I think this one might be busted. Apparently there isn't even enough heat unless you get more heat out of this engine than from just the gasoline. I don't know where that 1hp = 42,440 Btu/min came from but that one threw me off and gave me a false hope. Unless there is something I am missing here I don't have enough heat.   

       Ok I checked my conversion chart and apparently a comma was supposed to be a period!!! DOH!   

       1hp = 42.440 Btu/min So I get 10,610 btu/min using this formula. My longer method gave me 10,312 btu/min. So they are close enough together and sound more reasonable. I'm willing to accept that is probably the case.   

       My dreams went out the tailpipe due to a decimal point!
GodSquadMandrake, Mar 19 2009
  

       //Any ideas?//
Guttering and a rainwater butt.
coprocephalous, Mar 19 2009
  

       you seemed to have some contradictory goals, I hope you the best of luck and that you enjoy the events that you enter. Don't forget to crash a few times before your first event to get it out of your system.
WcW, Mar 19 2009
  

       Whew! You guys are taking this pretty seriously; I'm gonna have to sit down for this.
daseva, Mar 19 2009
  

       I was thinking of this concept after watching Junkyard Wars. An afterburner is usually a thing on a jet engine, I learned. Usually more fuel is shot in, capitalizing on the heat present in the rocket exhaust and whatever leftover oxygen might be there to make an additional explosion of thrust.   

       I like the idea of doing that with water. Not so keen on powering anything with exhaust pressure, or powering an engine. But a plain steam explosion sounds like a good way to harnest waste heat captured from the exhaust.   

       I envision the Steam Afterburner to have angled interior metal bits that capture exhaust heat and get very hot. On injecting steam the steam explodes out the exhaust pipe, which must be augmented to be strong enough to propel the car. The interior metal bits are pushed together when the explosion happens, so steam is not driven back up into the engine. The interior bits are on levers that open some more proximal escape holes so that exhaust from the engine does not back up in the manner of the proverbial potato in the tailpipe.   

       This would work only in a burst as steam output falls right off once metal is cooled. But when metal is hot again, good to go with another burst. An advantage is that unlike ether or additional gasoline, both of which could be used in exactly the same way, but which would probably burst into flame on exiting the exhaust pipe, the steam explosion is unlikely to set anything else on fire.   

       Maybe dry ice would work better. I am not sure which would produce more volume on expansion, water or dry ice. It is a knowable thing, though.
bungston, Jul 06 2010
  

       Aftersteamer.
ShawnBob, Jul 06 2010
  

       There will be a lot of exhaust backpressure in the heat exchanger which will greatly impact the power production of the IC engine will it not?
jhomrighaus, Jul 07 2010
  

       I have to admit I'not read all the way through your idea, but it seems to me that the premise is flawed.   

       There is a limited quantity of energy in the exhaust as it leaves the engine. This can be used in a variety of ways, including resonating pipes (like 2-strokes) and turbochargers. Whatever means you use, you can only gain by increasing the effeciency with which energy is extracted from the exhaust.   

       A second heat engine in the system may offer some additional energy recovery, but this has to be offset against it's weight and durability. There will also be significant delay between an increase in power at the engine and the corresponding increase in energy recovery at the exhaust, giving the effect of massive turbo lag.   

       I'll read right through when I'm not at work.
Twizz, Jul 07 2010
  

       Maybe I should liberate my steam afterburner concept so it is not associated with the idea of powering anything with exhaust pressure.
bungston, Jul 07 2010
  
      
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