Moparman1

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.

J_D

Will the performance enhacement, on a 2 stroke (if it can be done for these) be worth the cost?

- I think not

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.

SManMTB

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.

Moparman1

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.

87porsche

My Feedback: (1)

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

dfn_doe

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.

guver

My Feedback: (325)

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?

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.

dfn_doe

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....

Moparman1

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?

Moparman1

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.

dfn_doe

The bit about how a tuned piep works was addressed in response to Savagecrazy's post... The rest was just me thinking outloud. As for the exhaust pressurizing the fuel compensating for the additional fuel demands there are a few reasons why your approach won't work, first, as you mentioned the intake will be pressurized meaning any additional fuel pressure from the exhaust will already be counteracted, furthermore flow doesn't scale linearly with pressure which is to say twice the pressure doesn't equal anywhere near twice the flow, thirdly increasing the diameter of the pressure line from the exhaust to the tank will not increase the fuel pressure at the carb because pressure is measured as a force over an area which means that increasing the area will reduce the pressure per area leaving you with the same total force. I.E. 2 PSI over 1 inch becomes 1 PSI over two inches when the area is increased (greatly simplified example)... By all means I wish you the best of luck, but you still haven't addressed the majority of the issues that I've described in my earlier post. As for the port timing issue, I'm far from a 2-stroke guru so maybe I'm visualizing the port changes incorrectly in my head, but suffice to say that changes must be made in order to see the full benefit of forced induction in these applications. And I still assert that with engines of this size that a better cost performance ratio will always exist with buying a larger displacement engine instead of going with forced induction. Even if the two methods (FI/NA) ended up costing the same to implement, the larger displacement NA motor will be a much less complicated system with fewer modes of failure and an overall better power profile since the additional displacement will produce more torque accross the entire power band instead of selectively being available dependant on a number of factors including how quickly the FI system takes to build pressure and how quickly the fuel system can compensate for the additional fuel demand.

Really if you want to experiment with forced induction on an RC your money and time would probably be better spent working with a 4 stroke motor...

inkfreak74x9

Have you had any thoughts on a hi temp fiber resin? I dont know your fin dimentions, but I dont see why a small skellital rod couldnt be inserted into the cast first. This should help keep the small fin ridgid. Will it hold up at 50k RPM? Might be worth a try.

Fuelman

My Feedback: (6)

How do you plan to keep the turbine bearing oiled?

inkfreak74x9

On the topic of exhaust psi to spin the impeller. Have you thought about mounting on the rubber header to mufflur sleeve. You might even add an insert to split the sleeve and cup it to divert the exhaust gasses.

dfn_doe

Any exhaust driven turbine mounted between the exhaust port and the tuned pipe would disrupt the exhaust flow preventing the tuned pipe from working correctly and/or keep the turbo fom being spun up fully due to reversion effects from the tuned pipe causing bidrectional flow over the impeller. Diverting part of the exhaust stream would likely cause wierd issues with variable harmonic resonances in the exhaust system which would also prevent the tuned pipe from working correct and at the same time would provide a greatly reduced flow to the impeller..

edit: reworded for clarity...

inkfreak74x9

Oh well the exhaust split was just a thought. How about the fiber resign idea? Any thoughts there or just not worth it. When you do get these torbos working Ill be first in line. Or at least 10th.

dfn_doe

Material selection and construction method are things you start to consider once all the fundamental questions regarding the whole system's operation are resolved. Worrying about how to machine the turbo parts and what materials to use is really not important until you have determined that there is even a workable way of implementing a turbo... Also, I could be wrong, but I don't think there is any sort of synthetic plastic, polymer, resin, or similar material which could stand up both to the high stresses of rotation, multi directional load and crazy high temperature that an exhaust driven impeller would experience, however if you stick to a crank driven positive displacement super charger then your material selection and design considerations are going to reduced by a not insignifigant factor...

Moparman1

A larger pressure line wouldn't affect pressure in this situation "dfn doe", because it's a closed system. If the line is open , like in a painting application, the larger diameter air line would lower pressure in the line because there's less restriction to air flow. In a closed system pressure becomes equal to the chamber feeding the tank. A larger line would help equalize pressure variations between the chambers better than a smaller line. I know there's still a lot of work to be done on this system, which is why I don't want people to get exited and start asking me for prototypes. I agree that it would be better to focus on designing a turbo for a 4-stroke, which is why I'm trying to make this system universal to all engines. Since the majority of RC'ers in the U.S. use 2-strokes I'm focusing more on them right now. Using a fiber resin would be good, "inkfreak", but I'm pretty sure it can't be done with either pressure die casting or injection molding because the fibers would cause flow issues. Ideally I'd like to use steel in the turbines because this would stand up to any abuse a turbo could throw at it, but it's not possible with pressure die casting. Using a skeletal insert is a good idea. As soon as I get a chance I'm going to try it, but again there might be flow issues. "Fuelman", if you read the second half of post #11 I explain the basics of the oiling system. The design is a lot more involved than oil simply being slung off the turbine blades and onto the bearings, but I don't want to give too much away because it is (in my opinion) a very ingenious system.

Moparman1

Just to clarify the last post the fuel tank is a closed system, not the exhaust system. Since exhaust gas doesn't exit through the fuel tank gases entering it will equalize with pressure in the tuned pipe.

primedynasty

I am a huge fan of turbos on 1:1 cars. I drive a 1.8 liter Turbocharged Jetta and love it. I have also been thinking about how you could get a turbo to work on a Nitro car.

My first question/statement I guess is this: On a 4 stroke engine when it comes to exhaust and pressure there are two different applications as far as exhaust gas exiting the engine. If you are running a NA car then you don't want to have this crazy open exhaust because you will lose the critical back pressure these cars need. However, on a turbocharged car this isn't necessary because the turbo itself provides plenty of back pressure so you want to run the most wide open exhaust that you can to make sure that after the turbo there is no pressure what so ever. What I am not understanding is how this wouldn't apply to the 2-stroke engine? The turbo is going to provide a level of back pressure, or at least I would imagine it would. So in theory, IN THEORY, wouldn't it be possible to use the turbo to generate back pressure and eliminate the concept of a tuned pipe all together? You would still want to be able to pressurise the fuel tank which would be possible by using a exhaust tailored to that purpose.

Next, I don't see why a gas oil mixture would be bad hitting the compression fan. If it can flow around the crank then it should be alright in the compression housing. The oil would provide at a minimum some, if very little, lubrication which is never bad in a turbo.

I guess I am not very familar with the 2 stroke exhaust system and how much effect the back pressure actually possess. If you could stear me toward some data or information that explains the system I would really appreciate it.

I would also love to see what you are working on as far as a design. What kind of measurements are you using for the housings as far as diameter and length, etc. Are you using a liquid bearing design or a ball bearing design? Are you at all worried about spool times or have you figured the size of the turbo in order to deal with the appropriate amount of exhaust coming from the engine? I am glad to see someone is actively pursuing this, I don't really care about how practical it is, I just think it is a great project.

guver

My Feedback: (325)

I can speak to the back pressure issue. The 2 stroke pipe is a "tuned" chamber with set dimensions that is designed to reflect back the pressure pulse and mixture that came out the exhaust port. At the proper time it will "supercharge" the cylinder from the exhaust side and accounts for maybe up to 50 % of the horsepower. It would be good to put the turbo after that tuned pipe and then just as you say to eliminat any backpressure after the turbo with large short exit.

this other stuff with fuel pressure and tank pressure are minor issues that can be overcome later on.

Moparman1

Like "dfn doe" explained in post 7 the purpose of a tuned pipe is to increase the efficiency of an engine at a specific rpm range through resonance, not backpressure. When the frequency of pulses from the exhaust corresponds to the frequency of the specific volume of the tuned pipe the result is the higher frequency waves produced by the tuned pipe overcoming the lower frequency waves coming out of the exhaust port and causes the reversion of gases and unburned fuel/oil into the engine. A simple way to think of it is to imagine a soda bottle, when you blow across the top of it at just the right angle and intensity it will humm, the humming is actually the resonating air waves coming out of the bottle overcoming the air outside of it. Backpressure is (simply put) resistance to flow. Although a turbo does create some resistance it's very small. Increasing the backpressure in a 2 stroke engine could be benificial if applied properly, but slowing the flow of exhaust gas would be detrimental to the scavenging (how efficiently the exhaust gases exit) ability of the engine as the extra resistance would also slow the entrance of fresh fuel/air into the cylinder. It is possible to run these engines without tuned pipes, but you should never try to because, 1:it would be deafeningly loud; 2: it would run much hotter due to the lack of backpressure and the resonation (tuned) effect; and 3: it would be nearly impossible to tune. If you really want to get into the details there are sevral RC magazine companies that produce DVDs devoted entirely to the functions of model engines and the secrets of tuning them, but a little web searching could go a long way if you stay analytical. As far as the dimensions of my turbo go, turbo output is adjusted to provide roughly an additional 5 psi in the crankcase dependent on the application. Initially I based turbo output on cylinder size, but this created unpredictable results because engine manufacturers will often base larger displacement engines off of earlier small block designs. The amount of boost that provided an additional 5 psi in the cylinder of a .15 based on a .12 crankcase turned out to be enough pressure to stop the engine as pressure was multiplied in the crankcase. A good example is the now discontinued O.S. .15 CV-R engine family which is based off the crankcase of the .12 CV-R. The bearings are tungsten carbide ball bearings which have extreme heat resistance and high wear resistance, but I'm considering using steel to reduce costs. Spool time is a VERY difficult thing to asses nomatter what the application, but on an engine this size it isn't really a problem because the turbine weighs next to nothing and the engine produces a huge amount of exhaust compared to a 4-stroke of equivalent size so I'm not really concerned with spool time. Man that's a lot to type! I'll try to keep awnsers short from now on, but please try to help me out with strengthening the castings instead of getting into the details of engine function.

Moparman1

I talked to the guys at my local machine shop today and they said it would be pretty hard to put an insert into the turbine casting and get it to come out right, but the main issue would be the cost. Not only would I pay for the casting, but I'd have to pay for the machining of the insert too. Even if I put simple rods in the casting it would upp the price to what it would cost to machine it on a mill, which is why I switched to pressure die casting in the first place. It was a good idea, though.