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RC turbo design problems... - 10/1/2005 7:27:33 AM   
Moparman1


 

Posts: 68
Joined: 9/16/2005
From: Denver, CO, USA
Status: offline 		Before anyone starts arguing about why forced induction does or doesn't work on 2 stroke RC engines please read post #54 of the "Anyone with actual supercharger, intercooler, or nitrous experience." thread where I explain why forced induction can work on these engines. Here's my probem: I'm designing a turbo system for RC's that requires verry small components, some of which (specifically the turbine) are so small that they can't be conventionally machined, so they must be pressure die cast. This process is significantly cheaper after the inital molds are made than CNC machining the parts as an added bonus, but the materials that can be used are limited (pure aluminum, zinc and silver) and the castings tend to be brittle. At 50k+ RPM's the turbine has the potential to shatter. Right now I'm using LM24 aluminum for the turbine and plating it with hard chrome, but the chroming process leaves marks from the contact points of the wire used to suspend it in the plating tank because there's really no good place to mount it. Like I said before it's just a prototype and is a long way from being marketable, so don't get exited. If anyone who has experience with this casting process has suggestions on a better material to use or a way to strengthen the castings without increasing costs much please help me out. Thanks.
       Post #: 1

RE: RC turbo design problems... - 10/1/2005 8:00:24 AM   
J_D



Posts: 2810
Joined: 8/18/2005
From: Brisbane, AUSTRALIA
Status: offline 		Will the performance enhacement, on a 2 stroke (if it can be done for these) be worth the cost?

- I think not

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(in reply to Moparman1)
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RE: RC turbo design problems... - 10/2/2005 12:01:13 AM   
Moparman1


 

Posts: 68
Joined: 9/16/2005
From: Denver, CO, USA
Status: offline 		If you read more of the forum you would have seen that I've managed to get the price of the actual turbo unit down to about $67 per unit, but like I said this doesn't include the additional equipment needed to make it a full kit. Eventually I hope to get the price down to $120 for the full kit, which is considerably less than what RB innovations is charging for their "blower", and is still cheaper than a nitrous kit. Look at it this way; let's say you buy a cheap engine for $150 that puts out 1.5 hp and equip a turbo that increases hp by about 30-40% for $120. That engine would now put out 1.9 to 2.1 hp for just $270, which is still less than the cost of a high performance engine that puts out equivalent hp. Equipping a high performance engine with a turbo like this would produce even better results because the performance modifications that are designed to smooth airflow would accentuate gains from a turbo. If you ask me, the prices charged for "performance" modifications like RB innovation's blower are bloated. I'm just trying to "trimm the fat" on my design and leave myself a profit I feel happy with. I'm not a company, so profits aren't a huge concern for me.

(in reply to J_D)
       Post #: 3

RE: RC turbo design problems... - 10/2/2005 12:15:13 AM   
SManMTB



Posts: 4793
Joined: 4/28/2005
From: Charlottesville, VA, USA
Status: online
quote:

ORIGINAL: Moparman1

If you read more of the forum you would have seen that I've managed to get the price of the actual turbo unit down to about $67 per unit, but like I said this doesn't include the additional equipment needed to make it a full kit. Eventually I hope to get the price down to $120 for the full kit, which is considerably less than what RB innovations is charging for their "blower", and is still cheaper than a nitrous kit. Look at it this way; let's say you buy a cheap engine for $150 that puts out 1.5 hp and equip a turbo that increases hp by about 30-40% for $120. That engine would now put out 1.9 to 2.1 hp for just $270, which is still less than the cost of a high performance engine that puts out equivalent hp. Equipping a high performance engine with a turbo like this would produce even better results because the performance modifications that are designed to smooth airflow would accentuate gains from a turbo. If you ask me, the prices charged for "performance" modifications like RB innovation's blower are bloated. I'm just trying to "trimm the fat" on my design and leave myself a profit I feel happy with. I'm not a company, so profits aren't a huge concern for me.


Hmm the OS RG is $110 and rated for 1.9 hp.

Still, sounds like an interesting project. I see another problem with the turbo. It will make a tuned exhaust unusable since the turbo is in-line with the gas flow and therefore ruins the gas flow needed and used by a tuned exhaust.


(in reply to Moparman1)
       Post #: 4

RE: RC turbo design problems... - 10/2/2005 6:16:16 PM   
Moparman1


 

Posts: 68
Joined: 9/16/2005
From: Denver, CO, USA
Status: offline 		O.k., I'm not too familiar with current engine prices, but you get the idea. I've seen some high performance engines going for as much as $400 and even one for $500. My turbo isn't inline with the exhaust either, it's small enough that a line can be tapped off of almoast any area of the exhaust that experiences a large surge of pressure when the exhaust port opens (the header and the foreward part of the tuned pipe usually see the most pressure). What's great about these engines is that they produce such a large amount of pressure for their size. I'm experimenting with sevral ways to tapp the exhaust system, like an insert that's placed between the manifold and the block, a screw-in system that requires you to drill a hole in the manifold or tuned pipe (which is my least favorite, but seems to be the most effective), and a system that is driven directly off the tuned pipe. I would preffer to use the last system because it wouldn't effect the pressure waves that a tuned pipe is designed to use. Although a turbo provides relatively little resistance to the flow of exhaust gases exiting the system, you are correct in saying that a turbo system mounted in front of the tuned pipe would interrupt and ultimately eliminate the pressure waves produced by the engine. To work effectively a tuned pipe must have a smooth transition from the exhaust port through the header, and placing a turbo in this path would create a solid barrier (specifically the blades of the turbine) that would absord the energy of pressure waves traveling through it.

(in reply to SManMTB)
       Post #: 5

RE: RC turbo design problems... - 10/5/2005 5:49:59 AM   
87porsche


 

Posts: 87
Joined: 1/21/2005
From: East Quogue, NY, USA
Status: offline 		Hello,
I was just thinking a little about how i can increse proformance of a gas engine with something very simple. I know that the was a tuned pipe works is by sending already burned fuel back into the engine wich turbocharges it, but they are expensive or very hard to make. I was wondering if you send a tube ( small like a fuel hose tube) from the exhast header to the intake manifold so at the exhast stroke it will sent the burned fuel through the tube and into the intake, then in the intake stroke the engine sucks the burned air and fuel back in. ( or would this only work if the engine has reeds because i dont think a homie or something does).

Also if any one knows what a boost bottle is i was wondering if it would be like a expansion chamber if you put it on the exhast.
If yor think this idea is completly stupid or just wont work just tell me because in 13 and my dad thinks i have some crazy ideas.


James

(in reply to Moparman1)
       Post #: 6

RE: RC turbo design problems... - 10/5/2005 3:52:26 PM   
dfn_doe


 

Posts: 58
Joined: 9/19/2005
From: , CA, USA
Status: offline
quote:

ORIGINAL: savagecrazy

Hello,
I was just thinking a little about how i can increse proformance of a gas engine with something very simple. I know that the was a tuned pipe works is by sending already burned fuel back into the engine wich turbocharges it, but they are expensive or very hard to make. I was wondering if you send a tube ( small like a fuel hose tube) from the exhast header to the intake manifold so at the exhast stroke it will sent the burned fuel through the tube and into the intake, then in the intake stroke the engine sucks the burned air and fuel back in. ( or would this only work if the engine has reeds because i dont think a homie or something does).


I think you are misunderstanding how a tuned pipe works. During the overlap phase where both the intake and exhaust ports are uncovered some *unburned* fuel/air mixture is blown out the exhaust port during the "high pressure wave" this high pressure wave is moving at near the speed of sound and is then reflected back so that the unburned fuel/air in the exhaust is pushed against the "low pressure wave" which follows it. When the frequency of the waves corresponds with the length/volume of the tuned pipe the reflected highpressure wave overcomes the low pressure wave causing a reversion flow back into the exhaust port. Saying it "turbocharges" the engine is really a misnomer because a positive pressure can only be achieved for a narrow rpm band and even then only when everything is working correctly together. However this does provide a signifigant increase in efficiency over using an open exhaust wherein the highpressure wave just blows out the exhaust pulling the fuel/air mixture along with it and leaving the cyilinder with a much lower volumetric efficiency.

In a few posts a bunch of anti-forced induction naysayers are going to pop in here and crap up this thread, so let me point this out now. Forced induction on a valvless 2-stroke is a dicey proposition and one that can only be truly exploited with modified intake/exhaust timing events. This means that in order for any signifigant gains to be made the overlap period must be reduced by moving the exhaust port higher on the cylinder wall and also the turbo/super charger must maintain a positive pressure on the intake side of things. Ideally one would use a positive displacement style supercharger driven from the crank and also a well designed tuned pipe to maximize the exhaust pulses' aid in building and maintaining an ideal dynamic cylinder pressure. Alternatly if an exhaust driven turbo charger is used it would ideally be placed after the tuned pipe such to maximize the exhaust volume spinning the turbine while still using the tuned pipe as described above to help contain dynamic cylinder pressure.

That said, I'm doubtful that any great gain will be seen in forced induction for RC applications until someone with the capabilities and knowledge to manufacture a matched engine, pipe, turbo/super charger setup, which ultimately would be far more costly than just building a well tuned naturally aspirated motor and will create a motor which will prove to be much harder to tune and will require a great deal more maintainence. Additionally there are other practical design considerations, for example when the intake air charge is a greater density than atmospheric how would one maintain an appropriate relative fuel pressure at the carburetor. In the 1:1 world blow through carburetion on forced induction systems involves matched rate fuel pressure regulators and mechanical or electrical pumps. Also since the carburetor utilizes venturi effect past the needle/seat to meter air your metering will be thrown off as the intake air density changes since the fuel provided will not increase to match this. If one switches to a drawthrough induction setup to avoid these issue then you have the further problems of fuel being inside the compressor housing of your super/turbo charger which will require additional design consideration as to seal material and what not and that isn't even getting into the effects caused by having increased intake length and the assosciated problems of loss of fuel suspension, puddling, intake resonance effects etc...

wow, that turned out alot longer than I intended... Good luck Moparman, but I think that perhaps you have quite a bit more work ahead of you than you realize.

(in reply to 87porsche)
       Post #: 7

RE: RC turbo design problems... - 10/5/2005 6:43:15 PM   
guver


 

Posts: 8800
Joined: 3/7/2003
From: LaGrange, OH, USA
Status: online 		Here's a possible solution for the carb. Split the carb so that the mixing is done upstream and the butterfly/throttle is downstream. You would still have the fuel mix in the turbo, but this way gets rid of the vacuum inside the compressor. I have had great succes with doing this on carbs that can be split this way. RC carb, well I don't know about them, maybe they cannot be done like that, maybe a dummy butterfly can be had?

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RE: RC turbo design problems... - 10/6/2005 12:28:33 PM   
popadel


 

Posts: 79
Joined: 9/21/2005
From: Johannesburg, SOUTH AFRICA
Status: offline 		It wont work because 2 stroke expell oil in the exhaust and this will prevent the turbine from reaching the desired speed to presurise the intake. I know plenty about reall turbo application on cars, and i wish you good luck, but your gains will be minimal due to engine desing.

I sugest geting a turbop app on a wankle motor since it has a closed chaber and less issues with straight through feeding.

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RE: RC turbo design problems... - 10/6/2005 2:41:57 PM   
dfn_doe


 

Posts: 58
Joined: 9/19/2005
From: , CA, USA
Status: offline
quote:

ORIGINAL: popadel

It wont work because 2 stroke expell oil in the exhaust and this will prevent the turbine from reaching the desired speed to presurise the intake. I know plenty about reall turbo application on cars, and i wish you good luck, but your gains will be minimal due to engine desing.

I sugest geting a turbop app on a wankle motor since it has a closed chaber and less issues with straight through feeding.


I don't think that oily exhaust is really going to be a problem with getting the turbo spinning, especially since turbocharged two-strokes which run on premix are not unheard of. The primary issues are just design considerations such as those I've outlined above. If one were to take the time to engineer a solution to all my above problems then you could very possibly have a workable system, but the cost value of designing and implementing it would be rather low especially if produced in low volumes. Compare several hundred dollars worth of custom parts and lots and lots of R&D to the cost of simply buying a bigger displacement production engine and you will see that the primary concern when discussing forced induction RC cars is simply that there isn't enough performance available through this technique to offset the cost to achieve it....

(in reply to popadel)
       Post #: 10

RE: RC turbo design problems... - 10/7/2005 4:08:05 AM   
Moparman1


 

Posts: 68
Joined: 9/16/2005
From: Denver, CO, USA
Status: offline 		Looks like this forum has been busy in the time I was gone... "Savagecrazy", routing a line from the exhaust to the intake is the last thing you want to do. The only way the system you are describing could work is if there were a way to filter the unburned oil/fuel out of the exhaust and then recirculate it into the engine, but realistically there's no way to do this. I suppose you could cool the exhaust to the point that the oil/fuel would condense into droplets, but again the problem would be extracting the oil/fuel from the exhaust system. "Dfn doe", there's one thing you are forgetting about RC's that counteracts a pressurized intake; the pressure line that runs from the tuned pipe to the fuel tank. Although I'm not sure if all 2-stroke powered RC's have this system, I know that most cars and boats do. Obviously an increase in air/fuel being combusted would lead to an increase in exhaust pressure, and inturn increased pressure in the fuel tank. This alone will not compensate for the additional fuel needed, especially when trying to counteract a pressurized carb, but I will include a section of larger diameter fuel tubing in the kits that will help increase pressure in the tank. Unfortunately this would mean that the fuel tank would need a latch or something to hold it closed under the additional pressure. I'd like to understand your thinking behind your idea to move the exhaust port up in the cylinder, because doing this would make the exhaust open earlier in the cycle, reducing power output. If your idea was to increase the amount of time it would be open to aid in scavenging, a simpler solution would be to widen the exhaust, which would allow more exhaus gas out without affecting timing. I understand that valve overlap must be reduced on a 4-stroke to minimize pressure losses through the exhaust when super/turbocharging, but a 2-stroke is an entirely different machine. No matter how it's designed there will always be port timing overlap. Honestly I don't see the need to change the exhaust timing on these engines, especially since they consume fuel as quickly as they do. Finally, "popadel", as I said before my turbo's bearings are oiled by the unburned oil in the exhaust, so oil entering the turbo (whether from the exhaust or intake) is nessisary. A spinning turbine acts just like a centrifuge, it flings anything in contact with it outward. When oil comes in contact with the turbine it is flung against the walls adjacent to the travel of the turbine, and on my design, onto the bearings. This provides the bearings with a constant supply of oil. I'm quite aware of the dynamics of a running engine and the effects of the sonic waves in a tuned pipe contacting a solid object, which is to bounce back. "Dfn doe" I don't know if you were trying to explain how a tuned pipe works to me or to "Savagecrazy" (or both), but what I was saying on the other forum was that the explosion that's created in the combustion chamber traveling into the exhaust port and a portion of header is what causes this pressure wave. All waves traveling through a gas are technically sound waves, what we refer to as the speed of sound is actually the lowest audiable sound we hear. I'm not trying to be rude, I'm just showing that I am prepared for the many hurdles involved with designing a fully functional turbo. By the way, does ANYONE have suggestions on how to strengthen my turbine castings?

(in reply to dfn_doe)
       Post #: 11

RE: RC turbo design problems... - 10/7/2005 4:34:52 AM   
Moparman1


 

Posts: 68
Joined: 9/16/2005
From: Denver, CO, USA
Status: offline 		Oh, P.S., a "boost bottle" is just a hollow piece of crap that holds a little fuel so your engine doesn't cut out when your car flips over. It doesn't have any performance value at all. As for it being used as an expansion chamber it would have little effect, if any on power. Splitting the carb's not a bad idea "guver", I might try that some time. Thanks.

(in reply to Moparman1)
       Post #: 12

RE: RC turbo design problems... - 10/7/2005 6:51:20 PM