- #36
Ayrity
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ya i was thinking that might be easier, but i can't find anywhere online, google or ebay any sort of smallish 4 stroke direct fuel injected engine...
Two-Stroke Engine Tuning: Maximizing Performance
- Thread starter Ayrity
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In summary, the author built a small 2-stroke engine using an internally geared hub and a 20" wheel rim. He plans to tune the engine using adjustments to the expansion chamber design and a valve on the exhaust port to prevent "mileage" and
- #37
Cliff_J
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Ayrity - the job of the fuel management is to try to get the mixture as close to ideal as possible and NateTG did a great job explaining this. Here is one article (of many a google search found) with more detail:
http://www.carcraft.com/techarticles/24549/
Now though, you've completely lost me on your ideas...and purpose, a modern 2-cycle engine can be tuned for incredible performance and it seems you're trying to reinvent a wheel that is round and oval.
You're not going to be able to use a cylinder head off a 4-cycle unless you can match the chamber volumes otherwise you'll affect the compression ratio. Also, the valves would need to be actuated by a camshaft that needs to be driven off the crankshaft, things not found on a 2-cycle. The time in the machine shop just went up dramatically and I don't see the value in it.
We want to injest a mixture, combust it, let it expand to perform work, and then expel it as best possible. The 2-cycle simply let's the events overlap and typically suffers ill effects only at low speed operation where the timing of events isn't very good.
If we have a 10:1 CR and a 100cc engine, we know that if we could fill it with 100cc of mixture then when compressed it would be 10cc and we could figure out the mixture temp (ideal gas law) to make sure we don't have detonation. But the problem is this assumes 100% volumetric efficiency, which is unlikely for a 4-cycle. So if there's only 90cc of mixture in the cylinder, then when compressed to 10cc its only an ECR of 9:1 in that case. However, if properly tuned, a 2-cycle with its intake tuned properly and some mixture in the exhaust temporarily getting pushed back into the combustion chamber, it may be stuffing in 105cc for a volumetric efficiency of 105%!
If you understand harmonics, its a resonance in terms of the behavior and timing. Which means its a frequency dependant event, and will have a different response depending on frequency. And while you can examine it in step-by-step detail, the events all need to be managed and factored at the frequency of operation.
And you won't find much at all on direct injection, its not much more than a very small niche technology on the cutting edge. Its more esoteric than braking assist techologies or some of the other really new and expensive stuff finding its way on production vehicles.
http://www.carcraft.com/techarticles/24549/
Now though, you've completely lost me on your ideas...and purpose, a modern 2-cycle engine can be tuned for incredible performance and it seems you're trying to reinvent a wheel that is round and oval.
You're not going to be able to use a cylinder head off a 4-cycle unless you can match the chamber volumes otherwise you'll affect the compression ratio. Also, the valves would need to be actuated by a camshaft that needs to be driven off the crankshaft, things not found on a 2-cycle. The time in the machine shop just went up dramatically and I don't see the value in it.
We want to injest a mixture, combust it, let it expand to perform work, and then expel it as best possible. The 2-cycle simply let's the events overlap and typically suffers ill effects only at low speed operation where the timing of events isn't very good.
If we have a 10:1 CR and a 100cc engine, we know that if we could fill it with 100cc of mixture then when compressed it would be 10cc and we could figure out the mixture temp (ideal gas law) to make sure we don't have detonation. But the problem is this assumes 100% volumetric efficiency, which is unlikely for a 4-cycle. So if there's only 90cc of mixture in the cylinder, then when compressed to 10cc its only an ECR of 9:1 in that case. However, if properly tuned, a 2-cycle with its intake tuned properly and some mixture in the exhaust temporarily getting pushed back into the combustion chamber, it may be stuffing in 105cc for a volumetric efficiency of 105%!
If you understand harmonics, its a resonance in terms of the behavior and timing. Which means its a frequency dependant event, and will have a different response depending on frequency. And while you can examine it in step-by-step detail, the events all need to be managed and factored at the frequency of operation.
And you won't find much at all on direct injection, its not much more than a very small niche technology on the cutting edge. Its more esoteric than braking assist techologies or some of the other really new and expensive stuff finding its way on production vehicles.
- #38
swedish_lunacy
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Cliff_J said:Lunacy - a few followup points:
1) He's talking about direct injection, it will take a very sturdy injector to survive directly inside the combustion chamber.
2) Most small systems are speed-density, very few use a MAF. There are even quite a few GM V6s that only used speed density!
Oh, and nitro is usually short for "Nitromethane" as in Top Fuel Dragster with oxygen already in the fuel (also used in scale model engines of fractional displacement) and NOT for Nitrous Oxide. Slang might be "the bottle" or "on juice" or NO2 and so on but nitrous is widely understood and a small grammatical omission can lead to big issues. Stoichometric for nitromethane involves a lot less oxygen!
But it isn't the intent of the poster to simply increase his short-term power output for a drag race or something like it, he's trying to improve the efficiency of the 2-stroke motors he has. Little different approach even if a higher specific output is the goal.
Cliff, you are absolutely correct. Teaches me a lesson for posting after a night of bleary-eyed, hard work . Funny thing is I'm normally the first to point out the difference between Nitro ( Nitromethane ) and Nitrous Oxide ( which is actually N2O ). You're not wrong regards stoich for Nitromethane, 1.7:1 ratio means lots of fuel flowing into those top-fuellers ! Actually real world ratio is even worse due to them running quite richer than stoich.
Agree that N2O is probably not what Ayrity is after. In fact first and foremost I would invest my money/time in an optimised expansion chamber and exhaust combined with an electronically controlled ignition system. Then I would branch into a variable geometry exhaust system ( maybe variable geometry port as well - AKA Yamaha power valves, note valve is a bit of misnomer ).
Direct injection - DI:
ooops didn't notice Ayrity was talking about DI. Not for the faint hearted. It would definitely be too tricky to tackle in a do-it-yourself fashion. Injector durability and fuel pressure required are two limiting factors. The latter requirement probably representing quite a large parasitic lossEFI would still be good for very fine control but I haven't seen many EFI small capacity 2-strokes ( only monstrous 3.5L outboard motors )
Regards MAF vs MAP:
A MAP's output will need to be tuned/calibrated/calculated with respect to both displacement and volumetric efficiency ( the later being a moving target throughout the rev range in a 2-stroke ). MAF just tells you how much air is going in full stop, regardless of VE, displacement, etc. But yeah, chances of finding a MAF small enough is pretty slim
- #39
Ayrity
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well that explains why i can't find anything on the direct injection. one the other comments, youre right, I feel like i am going a bit in circles, but i am only trying to figure out the best way to attack my idea, the one of having direct injection and using the valve on the exhaust. I was getting the feeling that drilling into the head of the 2 stroke engine and setting up a fuel injection system was going to be very hard to do, so i looked for another alternative, like changing a 4 stroke into a 2 stroke, that way i already have the valve on the exhaust and a fuel injection system.
ive been playing around with the idea here, and i think it doesn't have to be direct injection as long as the injector is right next to the intake port, and timed correctly so that it shoots out the gas at the last moment, and make sure the exhaust valve is closed first. so something like, having the fuel injector at the bottom of the downstroke of the piston, that way it could inject seperately from the air intake, so no fuel is wasted on scavenging, and it is low enough so that it is not right next to the spark plug when the combustion happens, in fact when the exhaust touches it, it will be at 1amu, becuase the exhaust valve will always be opened before the injector is exposed in its new "port"
what do you think would be the best way to get this idea to light? (and just a second question, i can't seem to figure out what difference there is between a diesel fuel injector and a gasoline injector, it does seem that a diesel injector can withstand the combustion...)
ive been playing around with the idea here, and i think it doesn't have to be direct injection as long as the injector is right next to the intake port, and timed correctly so that it shoots out the gas at the last moment, and make sure the exhaust valve is closed first. so something like, having the fuel injector at the bottom of the downstroke of the piston, that way it could inject seperately from the air intake, so no fuel is wasted on scavenging, and it is low enough so that it is not right next to the spark plug when the combustion happens, in fact when the exhaust touches it, it will be at 1amu, becuase the exhaust valve will always be opened before the injector is exposed in its new "port"
what do you think would be the best way to get this idea to light? (and just a second question, i can't seem to figure out what difference there is between a diesel fuel injector and a gasoline injector, it does seem that a diesel injector can withstand the combustion...)
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- #40
brewnog
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A Diesel fuel injector will squirt at much, much higher pressures than a petrol injector, - it can not only mechanically withstand and seal under in-cylinder pressure conditions, but it's able to meter and inject fuel at incredibly high pressures (we wouldn't bat an eyelid to run at 200MPa line pressure). Petrol injectors probably max-out at 100MPa, and that's for direct injection. Conventional injection is far less than this.
- #41
Ayrity
- 92
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hmm ok thanks. would it be a bad idea to think about using a diesel injector for this engine idea? do you think it would be difficult to tap a hole for an injector and install it into the head of the engine? what about the changing a 4 stroke engine into this kind of 2 stroke engine? the only reason i bring that up again is because of the dedicated lubrication system. or is there an easier way to make a 2 stroke have a separate oil bath for the crank?
basicly, if you were going to attempt to make this engine, which way would you choose to try and why? thanks
basicly, if you were going to attempt to make this engine, which way would you choose to try and why? thanks
- #42
swedish_lunacy
- 5
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If you want to build an injected two stroke, have a look at the marine industry two strokes. Those outboards run some pretty cool systems. I think they may have a separate oil injector as they run condition-dependant variable fuel/oil ratios. Lubing the crank via any other method will probably requie quite a lot of engineering effort.
I think traditional DI is going to be pretty tricky but I think that an injector just above your intake port might be a good alternative. Diesel injectors tough as nails so will probably be allright. Not sure what the performance of a diesel inj is if run at low pressure, the fuel may not atomise properly.
Also need to look at the parasitic losses of having to pressurise the fuel. It may not be efficient enough on such a small scale.
PS: Have a look at the following, http://www.dolmar.com/755.php twenty four chainsaw motors in a motorcycle chassis
I think traditional DI is going to be pretty tricky but I think that an injector just above your intake port might be a good alternative. Diesel injectors tough as nails so will probably be allright. Not sure what the performance of a diesel inj is if run at low pressure, the fuel may not atomise properly.
Also need to look at the parasitic losses of having to pressurise the fuel. It may not be efficient enough on such a small scale.
PS: Have a look at the following, http://www.dolmar.com/755.php twenty four chainsaw motors in a motorcycle chassis
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- #43
brewnog
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Parasitic losses: On the 4 stroke direct injection Diesels I work with, each unit injector takes ~2 tonnes of force to actuate, when running balls out. Bearing in mind that it's doing this 15 times per second, and there's one per cylinder, you can see that they're pretty tough things.
- #44
Ayrity
- 92
- 0
they do have small, like 4 and 5 hp four stroke diesel engines though. maybe i could lift off the injection system off of one of those.
by the way, how come they don't make smaller 2 stroke diesels? is it the cost of the turbo/supercharger needed would be as much as the engine? why can't diesels use the downstroke of the piston to push their air into the cylinder (my guess is its not enough compression, but i don't know).
a benifit of maybe modifying a diesel engine is i could use biodiesel to run my bike :) i have experimented with biodiesel and had some good results, so maybe that's a better plan...modify a regular smallish 4 stroke diesel into a 2 stroke? what do you guys think of that?
again, I am worried about the lubrication system... this is why I am leaning to 4 stroke to modify
by the way, how come they don't make smaller 2 stroke diesels? is it the cost of the turbo/supercharger needed would be as much as the engine? why can't diesels use the downstroke of the piston to push their air into the cylinder (my guess is its not enough compression, but i don't know).
a benifit of maybe modifying a diesel engine is i could use biodiesel to run my bike :) i have experimented with biodiesel and had some good results, so maybe that's a better plan...modify a regular smallish 4 stroke diesel into a 2 stroke? what do you guys think of that?
again, I am worried about the lubrication system... this is why I am leaning to 4 stroke to modify
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- #45
Averagesupernova
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Ok I've been reading over this for the last couple of days and my memory isn't perfect so I may repeat something that has already been said.
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Ayrity, let me get this straight. You basically want to stay with a 2 stroke design that fully scavenges the exhaust out of the combustion chamber yet doesn't waste fuel out the exhaust port right? A 2 stroke engine more or less RELIES on the incoming air-fuel mixture to help push the exhaust out. So, if you want to scavenge the combustion chamber without wasting fuel you need a blower. Here is what I would do:
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Conventional porting arrangement with the exhaust slightly higher than the intake port. BUT, the intake port is only going to be porting fresh air into the engine and under pressure by the blower. The intake ports ONLY function is to scavenge. The exhaust port would probably be a very small distance above the intake port. Only enough to avoid the exhaust blowing back into the intake port. This distance could probably be less than the average conventional two-stroke design.
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Next step, getting fuel into the combustion chamber:
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Ok, here's an idea that isn't really anything new, but maybe just a little bit. Put a valve in the piston. Not a reed or poppet valve, but a slide valve with its own port that feeds out the bottom of the piston. Stay with a carburetor and reed valve to feed the air-fuel charge into the crankcase.
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Here is how the valve in the piston would work: It could be improved on or simplified, but I'll explain it in such a way to best illustrate it. Instead of having one connecting rod for the piston, have 2. Yep, you read it correctly. They are spaced slightly to allow for linkage to fit between them. The linkage goes from the crankshaft journal up to the piston. The journal on the crank is the same for the outside 2 rods, but different in between. It is offset ground. So as the crank turns, the piston goes up and down and the linkage goes up and down relative to the piston. The linkage is attached to it's own miniature piston within the regular piston. This second piston is the one that covers and uncovers the port within the main piston which forms a valve. This way when the crank is at a certain position the linkage is pulled away from the piston which opens the in-piston valve. The offset journal on the crank is positioned in such a way as to open the in-piston valve just as the exhaust port is covered up when the piston is on the way up. The one problem I can see with this idea is that the combustion chamber is sealed up when the in-piston valve opens. Also, since the trapped fresh air in the combustion chamber WILL be part of the combustion, it seems that the charge coming up through the in-piston valve will need to be a fairly rich mixture. It seems like a properly designed exhaust system should draw enough air out of the combustion chamber to leave it in a slight vacuum in order to help get the fuel up through the in-piston valve. One other thing that could be done is to use a 2-stage blower. I'm not sure of which specific one, but older planes, bombers I think, used this. Basically the crankshaft turned a turbine on the exhaust pipes to help draw out the exhaust as well as turning a turbine on the intake for obvious reasons.
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I know it sounds like a strange animal and I wouldn't expect you to make it work, but this IS the place to discuss it. So everyone feel free to shoot this down or modify it or whatever. BUT, before you get too carried away with the bashing, valves in the piston HAS been done before. The radial engine where the block spins and the crank was stationary used a valve in the piston. The fuel was piped in through the crankshaft. I'm not sure about fresh air. If I recall correctly, it had ONE speed of wide open.
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Ayrity, let me get this straight. You basically want to stay with a 2 stroke design that fully scavenges the exhaust out of the combustion chamber yet doesn't waste fuel out the exhaust port right? A 2 stroke engine more or less RELIES on the incoming air-fuel mixture to help push the exhaust out. So, if you want to scavenge the combustion chamber without wasting fuel you need a blower. Here is what I would do:
-
Conventional porting arrangement with the exhaust slightly higher than the intake port. BUT, the intake port is only going to be porting fresh air into the engine and under pressure by the blower. The intake ports ONLY function is to scavenge. The exhaust port would probably be a very small distance above the intake port. Only enough to avoid the exhaust blowing back into the intake port. This distance could probably be less than the average conventional two-stroke design.
-
Next step, getting fuel into the combustion chamber:
-
Ok, here's an idea that isn't really anything new, but maybe just a little bit. Put a valve in the piston. Not a reed or poppet valve, but a slide valve with its own port that feeds out the bottom of the piston. Stay with a carburetor and reed valve to feed the air-fuel charge into the crankcase.
-
Here is how the valve in the piston would work: It could be improved on or simplified, but I'll explain it in such a way to best illustrate it. Instead of having one connecting rod for the piston, have 2. Yep, you read it correctly. They are spaced slightly to allow for linkage to fit between them. The linkage goes from the crankshaft journal up to the piston. The journal on the crank is the same for the outside 2 rods, but different in between. It is offset ground. So as the crank turns, the piston goes up and down and the linkage goes up and down relative to the piston. The linkage is attached to it's own miniature piston within the regular piston. This second piston is the one that covers and uncovers the port within the main piston which forms a valve. This way when the crank is at a certain position the linkage is pulled away from the piston which opens the in-piston valve. The offset journal on the crank is positioned in such a way as to open the in-piston valve just as the exhaust port is covered up when the piston is on the way up. The one problem I can see with this idea is that the combustion chamber is sealed up when the in-piston valve opens. Also, since the trapped fresh air in the combustion chamber WILL be part of the combustion, it seems that the charge coming up through the in-piston valve will need to be a fairly rich mixture. It seems like a properly designed exhaust system should draw enough air out of the combustion chamber to leave it in a slight vacuum in order to help get the fuel up through the in-piston valve. One other thing that could be done is to use a 2-stage blower. I'm not sure of which specific one, but older planes, bombers I think, used this. Basically the crankshaft turned a turbine on the exhaust pipes to help draw out the exhaust as well as turning a turbine on the intake for obvious reasons.
-
I know it sounds like a strange animal and I wouldn't expect you to make it work, but this IS the place to discuss it. So everyone feel free to shoot this down or modify it or whatever. BUT, before you get too carried away with the bashing, valves in the piston HAS been done before. The radial engine where the block spins and the crank was stationary used a valve in the piston. The fuel was piped in through the crankshaft. I'm not sure about fresh air. If I recall correctly, it had ONE speed of wide open.
- #46
Cliff_J
Science Advisor
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Averagesupernova - little complex, but like the idea since it moves away from DI for now, that technology is just a little bleeding edge right now. Oh, and those old stationary crank airplane motors had another advantage - there's no major reciprocating parts, the pistons travel in a circle. And on the TV program I saw it on, the German version simply cut a number of ignition pulses to idle and then let all the pulses through for normal operation - idle or wide open, like an on/off switch! I guess the plane could turn one direction on a dime, but like a freight train in the other.
Ayrity - turbos and superchargers cost a lot of money because of the precision work needed. A 2-cycle involves casting 3 main engine parts, a crank/rod/piston with a couple bearings and its operational with 10 machined parts that require maybe two dozen machine operations. Simple and super cheap with surprisingly good power output for the displacement and weight and cost.
But Ayrity, you are certainly fascinated with 2-strokes even if to a extreme.
The 2-stroke effectively gives up some of its power stroke to begin the exhaust stroke as they overlap, and the intake stroke also begins while the exhaust stroke is still occurring and so on. You're talking about trying to tweak those a little more by adding a ton of technology.
Now the biggest question is how to make that technology pay for itself. A military vessel needs high-power and lightweight so they use turbines. A transport truck needs to be economical on long trips but have power to navigate steep grades and varying loads, so a turbo is able to make it more economical. For a dirtbike or motorcycle, pretty tough economics, especially with the new emission standards even in off-road parks. Just the silencer/muffler on a 2-stroke can steal 20% or more of the power to comply with noise requirements.
Ayrity - turbos and superchargers cost a lot of money because of the precision work needed. A 2-cycle involves casting 3 main engine parts, a crank/rod/piston with a couple bearings and its operational with 10 machined parts that require maybe two dozen machine operations. Simple and super cheap with surprisingly good power output for the displacement and weight and cost.
But Ayrity, you are certainly fascinated with 2-strokes even if to a extreme.
The 2-stroke effectively gives up some of its power stroke to begin the exhaust stroke as they overlap, and the intake stroke also begins while the exhaust stroke is still occurring and so on. You're talking about trying to tweak those a little more by adding a ton of technology.
Now the biggest question is how to make that technology pay for itself. A military vessel needs high-power and lightweight so they use turbines. A transport truck needs to be economical on long trips but have power to navigate steep grades and varying loads, so a turbo is able to make it more economical. For a dirtbike or motorcycle, pretty tough economics, especially with the new emission standards even in off-road parks. Just the silencer/muffler on a 2-stroke can steal 20% or more of the power to comply with noise requirements.
- #47
Ayrity
- 92
- 0
supernova, great thinking, i really do apprecicate some new ideas like that. i do see what you mean with the valve in the piston. i was thinking about doing the same kind of thing actually, only i was going to do it like this... allow just air to come into the crank and have it push out all the exhaust, but at the last moment of the downstroke of the piston, have a valve switch that puts the air coming from the crank through a different tube that has the carb on it, and also leads to the intake port, again, like in Novas idea it would probably have to be a pretty rich mixture. if any of that was unclear to anyone i would be happy to draw it out and scan it, I am probabbly going to do it for my own benifit anyway haha.
Cliff, its not that I am obsessed with 2 strokes, its just, when i put my mind on something, i work with it until it gets where i want it, or until i run out of money, or until i don't have the tools to do it yet. in which case i put them away for later working :) thanks for humoring me though haha, i love this place
Cliff, its not that I am obsessed with 2 strokes, its just, when i put my mind on something, i work with it until it gets where i want it, or until i run out of money, or until i don't have the tools to do it yet. in which case i put them away for later working :) thanks for humoring me though haha, i love this place
- #48
Averagesupernova
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Ayrity, I've given a bit more thought to my idea. I figured why have a slide valve in the piston? Why not just make it so it slides in the block along side of the main bore? It is still run off of the crank. The slide valve would be closer to a spool valve though since it would control a scavenge port as well as a port that let's fuel in. This would give us a chance to go to indirect fuel injection right ahead of the spool valve which would probably guarantee that enough fuel-air charge gets into the combustion chamber. Basic cycle timing would be the same as my previous example. Also, you could have a conventional pressure oil system in the bottom end so there is NO FUEL MIX. So in this idea contrasted to my previous you have a trade off of going to a more conventional piston/rod assembly and eliminating the reed valves and a carburetor while going to a fuel injection system and a slightly more complicated valve. My previous example had a valve, so adding a bit of complexity to it isn't that big of a deal. Of course the simplest is no valve, but to achieve what you want I don't think it is possible. Lubrication for the valve is now a consideration though since there won't be oil going around it at all. Just fresh air and fuel. You may want to consider an oil metering pump for it the same way the Wankel rotary meters oil for lubrication. I think you would also be guaranteed to be going away from the need for the 2 stage blower.
- #49
Ayrity
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- 0
ya, that's basicly what I was thinking of doing, but just your idea uses an injector, and mine a carb i think.
*************************EDIT: SUPERNOVA- would you mind drawing your idea its a bit unclear to me... sorry thanks
*************************EDIT: SUPERNOVA- would you mind drawing your idea its a bit unclear to me... sorry thanks
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- #50
Averagesupernova
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That's SUPERnova to you buster! I'll give it a whirl. I've never attached anything before. So we'll see. About the spool valve lube, I suppose with a conventional oil system oil could be pumped to the valves and since they don't pass exhaust I think a seal could be made to stand up just fine.
I'll give it a whirl. I've never attached anything before. So we'll see. About the spool valve lube, I suppose with a conventional oil system oil could be pumped to the valves and since they don't pass exhaust I think a seal could be made to stand up just fine.- #51
Averagesupernova
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Ok. It's a crude drawing but I think it will get the point across. I've drawn it as you would view it from the end of the crank. The blue is the piston and rod, the white represents the bore and block, the green is the crankshaft. The red is the valve. It isn't really a 'spool valve'. I decided that there is no need to run the scavenging air through the valve. Do it as I described in my first post. The little circle on the left represents the exhaust port. The lower circle on the right represents the fresh air only port for scavenging. The upper circle on the right is the air-fuel charge port. It is connected to the slide valve. The diagonal lines that are red represent a spring that pushes down on the end of the valve. The little horizontal lines are sealing rings. The valve is basically a straight rod with sealing ring grooves cut into it. The bottom end rides on a cam on the crank shaft. There is a LOW spot on the cam that causes the valve to drop when the crank gets to the right spot in its rotation. The upper sealing rings drop below the port in the cylinder bore opening up the port to the pressurized air. This would be about the moment that the injector fires. The injector is the pointy thing on the right side of the valve bore. The diagonal line at the top of the valve bore is a throttle valve. The main valve which is drawn in red and air-fuel port are actually drawn 90 degrees off of where they would actually be on the engine. The valve naturally needs to be arranged in-line with the crank.
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Now for the shortcomings of this. This particular engine is not going to be able to be a real high reving engine because of the spring. The valve will float if it revs too high. This valve pops up and down TWICE as often as a valve in a 4 stroke engine. In order to prevent this it will have to be run with a connecting rod, wrist pin, and extra crank journal. That could be made to work ok I guess, but then the valve moves constantly throughout the cycle which means if the valve is to act quickly at the right moment it needs to move a fair distance. The port always needs to have sealing rings below it to prevent combustion pressure from blowing by down between the valve and the bore. The injector also needs to have sealing rings below it to prevent combustion pressure from reaching it. Sooooooo, if the valve were to be run off of a crank journal instead of a cam like I have it drawn, the injector will have to be higher up in the valve bore to guarantee that it always has sealing rings below it. I don't like it as high as it is in the bore and I certainly wouldn't want it any higher. I would prefer from a fuel delivery standpoint that it is directly across from the port.
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Now for the shortcomings of this. This particular engine is not going to be able to be a real high reving engine because of the spring. The valve will float if it revs too high. This valve pops up and down TWICE as often as a valve in a 4 stroke engine. In order to prevent this it will have to be run with a connecting rod, wrist pin, and extra crank journal. That could be made to work ok I guess, but then the valve moves constantly throughout the cycle which means if the valve is to act quickly at the right moment it needs to move a fair distance. The port always needs to have sealing rings below it to prevent combustion pressure from blowing by down between the valve and the bore. The injector also needs to have sealing rings below it to prevent combustion pressure from reaching it. Sooooooo, if the valve were to be run off of a crank journal instead of a cam like I have it drawn, the injector will have to be higher up in the valve bore to guarantee that it always has sealing rings below it. I don't like it as high as it is in the bore and I certainly wouldn't want it any higher. I would prefer from a fuel delivery standpoint that it is directly across from the port.
Attachments
- #52
Ayrity
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very cool SUPERnova haha. I think it might be easier to just use an electronic valve than messing with all that stuff, at least for me rigging an engine up, and keeping weight down. I planned on adding a battery for an electric start anyway...
again i think we both have the same basic idea, ill draw out mine tomorrow sometime and show you.
again i think we both have the same basic idea, ill draw out mine tomorrow sometime and show you.
- #53
Ayrity
- 92
- 0
Sorry I havnt drawn out my idea yet, busy school schedule... I am getting to it.
just another quick idea btw, how about making a mini super charger for the engine by moding a smog pump off of a car?
just another quick idea btw, how about making a mini super charger for the engine by moding a smog pump off of a car?
- #54
Ayrity
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ok so I've acquired a small B+S 4 stroke engine (3hp) and I now think I am going to try to convert it into a 2 stroke and try to almost double my power. My plan so far is to get a smog (AIR) pump from a car at the local junkyard, and hook that up to a pulley on the crank somplace so as to pump air into the intake, like a mini supercharger. I will then calculate the amount of air needed to fully scavenge the exhaust out (a bit more volume than the displacement i would guess) and get the right ratio for the pulley wheel so the air pump will provide the correct volume of air to the engine.
Step 2 will be to create an intake tube with 2 paths that then lead back into one path, there would be 1 line that branched into 2 lines with a y splitter, than back together with another y splitter.
there will be a valve in the first Y split that will be electronically controlled. in the top path the carb will be split in. this will allow the air path to either go PUMP-->CARB-->ENGINE or PUMP-->ENGINE. this will let me use just air to scavenge the exhaust out rather than risk losing fuel. (im pretty sure the carb would have to put out a rather rich ratio)
the spark plug will have to fire twice as often, the second one being exactly 180 degrees off from when it fires now. I am pretty sure it uses a magnet on the flywheel so if this is right, it is probably sparking at that time anyway (during the exhaust stroke of the 4 stroke engine design). so i think that's covered.
the hardest problem to solve will be control of the intake and exhaust valves. they run on a cam shaft with lobes and everything like in a car engine. I have to be able to modify these valves to open and close twice as often... and maybe even modify when they do so. so far I have 2 ideas: the first is to take off the cam shaft timeing gear and replace it with another smaller gear, half as big=twice as often and run a chain to bridge the gap, or maybe find new gears for both the crank and cam shaft that makes the right gear ratio and mesh.
so far that is the idea, any help especially on the technical side of things would be GREATLY appreciated thank you everybody.
Step 2 will be to create an intake tube with 2 paths that then lead back into one path, there would be 1 line that branched into 2 lines with a y splitter, than back together with another y splitter.
there will be a valve in the first Y split that will be electronically controlled. in the top path the carb will be split in. this will allow the air path to either go PUMP-->CARB-->ENGINE or PUMP-->ENGINE. this will let me use just air to scavenge the exhaust out rather than risk losing fuel. (im pretty sure the carb would have to put out a rather rich ratio)
the spark plug will have to fire twice as often, the second one being exactly 180 degrees off from when it fires now. I am pretty sure it uses a magnet on the flywheel so if this is right, it is probably sparking at that time anyway (during the exhaust stroke of the 4 stroke engine design). so i think that's covered.
the hardest problem to solve will be control of the intake and exhaust valves. they run on a cam shaft with lobes and everything like in a car engine. I have to be able to modify these valves to open and close twice as often... and maybe even modify when they do so. so far I have 2 ideas: the first is to take off the cam shaft timeing gear and replace it with another smaller gear, half as big=twice as often and run a chain to bridge the gap, or maybe find new gears for both the crank and cam shaft that makes the right gear ratio and mesh.
so far that is the idea, any help especially on the technical side of things would be GREATLY appreciated thank you everybody.
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- #55
NateTG
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If I were trying to do this my approach would probably be:
Convert to electronic ignition.
Convert to electronic valves.
Add driven air system (supercharger)
Add driven air scavenging system.
Use reprogramming of ignition/vales to switch between 2 and 4 stroke.
You could, very reasonably, get away with doubling the timing on the sparks, but the valve behavior is likely to be something that you're going to have to explore, and, quite frankly, you'll have an easier time of that with electronic valve control.
If you're good with a lathe, you could replace the camshaft.
Of course,
Convert to electronic ignition.
Convert to electronic valves.
Add driven air system (supercharger)
Add driven air scavenging system.
Use reprogramming of ignition/vales to switch between 2 and 4 stroke.
You could, very reasonably, get away with doubling the timing on the sparks, but the valve behavior is likely to be something that you're going to have to explore, and, quite frankly, you'll have an easier time of that with electronic valve control.
If you're good with a lathe, you could replace the camshaft.
Of course,
- #56
Ayrity
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i do have access to a lathe here at my mechE lab, although i don't see how you can make lobes with a lathe... they arent circular all the way around. and also what would I use as an electronic valve? a solenoid or something?
- #57
Cliff_J
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Remember the electronic valves in the link earlier were moving 2mm which is about 10x too short and the valve weight is substantially lower as well. You will be fighting a bad economies of scale problem on the wrong side of exponential problems.
With a B&S you do have a very inexpensive and plentiful platform to work with. But if you only wanted to double the HP, you could easily fit it with an OHV setup and seal off the flathead portion of the chamber, increase the CR, and fit it with adequate carberation for that increased power level. Basically, leapfrog it from 1940 to 1990.
With a B&S you do have a very inexpensive and plentiful platform to work with. But if you only wanted to double the HP, you could easily fit it with an OHV setup and seal off the flathead portion of the chamber, increase the CR, and fit it with adequate carberation for that increased power level. Basically, leapfrog it from 1940 to 1990.
- #58
Ayrity
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would an overhead valve design really double the power output? where would i find that sort of a retro fit head? this could be useful info or a useful thing to do no matter what else i decide to do to the engine, thanks! and i also assume by increased compression ratio you mean a supercharger or something of the sort. on a side note, do you guys think that idea for using a smog pump is worth looking into as a mini supercharger?
- #59
NateTG
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Ayrity said:
I'm not sure it's an ideal cam profile, but one way is to use a 4 jawed chuck, (or some other excentric chuck) and split the lobes of the cam into circular sections, and then a similar set up on the cylindrical grinder.
If it's available, and you're familiar with it, CNC milling is also a reasonable option for creating a camshaft.
The EVIC guy was using solenoid controlled poppet valves. One of his friends was apparently using solenoid controlled rotary valves. Cliff's comment that the characteristics of the valves are non-ideal is certainly appropos, but if you're doing some sort of MechE experiment, that's less of an issue.
Similarly, you could bootstrap (or even run) with an external source of compressed air using regulators and whatnot to see what kind of flow you're going to want.
- #60
Ayrity
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ya i think that's a good idea, the use of the compressed air to test with. I don't know why everyone is always saying that we don't have the ability to make electric valves work. the way i see it, i have 3 options: 1) use solenoid controlled poppet valves 2) solenoid controlled slide valves just attached in line to the port holes. 3) use an electric motor with a big thread or ramp wrapped around it and use that like a cam lobe, have the neck of the valve ride on it and as the electric motor spins (at a selected speed) it would make the valve open and close at different speeds.
- #61
Ayrity
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ok so i can't find ANY sort of solenoid poppet valve... hmm maybe that will limit my choices
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- #62
NateTG
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Ayrity said:
The EVIC guy rolled his own.
Hmm, how many cylinders does your motor have?
- #63
Ayrity
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just the one. I've been doing searches and i really need to figure out the psi and temperature that this engine will be putting out. anyone have an easy way of figuring that out? it is a 127cc engine and I am planning to have a little bit of blowing from that supercharger so i figure the pressure can't be more than 700psi at its greatest. am I way off here? thanks for all the help
- #64
sid_galt
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Isn't a solenoid valve implementation going to be tough?
I mean it brings along tons of issues with it like the high power required and valve seating and things like that. Besides it will likely end up being costlier than normal valve procedures.
I mean it brings along tons of issues with it like the high power required and valve seating and things like that. Besides it will likely end up being costlier than normal valve procedures.
- #65
Ayrity
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well to allow for an easier valve timing this is an option to go in, and it would also take the stress off of the engine somewhat. I am open to ideas though!
- #66
sid_galt
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You mean you want variable valve timing in your engine?
- #67
Ayrity
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well, yes, because with such a different engine set up, I am pretty sure I am going to have to mess with the valve timing a bit. and plus with an electronic valve set up, I can use a pretty simple circuit to do a derivation and give me the optimum timing and have it do it on its own, granted i give it the right sensors which i would need to adjust it myself anyway.
- #68
NateTG
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Well, solenoids are an obvious choice (as used by the EVIC guy). I haven't seen the engine, but it should be feasible to place a solenoid so that it replaces the cam lift on the rocker arm. Leaving the return spring in place can work, or you could use a double-acting solenoid.
Another option is to hook servo (or stepper) motors to a cam of some kind.
Conventional cams should work, but the "skew disk circular cam" at http://www.ducati.com/bikes/techcafe.jhtml?artID=5&detail=article&part=technical also looks interesting.
Edit: Having missed your earlier post -- if it's a single cylinder engine with a cam shaft (rather than push rods) consider taking the timing belt/chain off of the crank shaft and hooking it to a servo motor.
Another option is to hook servo (or stepper) motors to a cam of some kind.
Conventional cams should work, but the "skew disk circular cam" at http://www.ducati.com/bikes/techcafe.jhtml?artID=5&detail=article&part=technical also looks interesting.
Edit: Having missed your earlier post -- if it's a single cylinder engine with a cam shaft (rather than push rods) consider taking the timing belt/chain off of the crank shaft and hooking it to a servo motor.
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- #69
NateTG
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Ayrity said:
Heat is going to be an issue for you when you're running 2-stroke.
If you know the typical operating temperature for the thing, you could use something like a tempilstick, or an infrared thermometer to monitor temperature during test runs. If you don't know the operating temperature, you can get a pretty good idea of what it's going to be by running the motor for a while.
Regarding figuring out pressure:
You can use a compression tester to figure out what you're running at currently. From there the natural gas law will give you a decent prediction on what sort of pressure you'll be getting from supercharging.
Noteably: You may be seeing lower rather than higher pressures when you're running as a 2-stroke - especially at high RPM - because you've got to scavenge and charge the cylinder in a single stroke, you're likely to see lower compression. Of course, this does depend on how much supercharging you're getting.
- #70
Cliff_J
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Ok, an OHV from a small Honda or something like it has been done before, a roommate in college had done so at his high school when they competed in the high mileage contest.
There is a simple reason you don't see solenoid activated valves, and its not because everyone is some Luddite who thinks cams are cool because they're old school.
You need to accelerate a mass, let's say 100g, a certain distance and in a certain time. Let's say we have 6000 RPM, that's 100 revs per sec. For a 4-stroke engine, that's 50 times per second or 20msec period. Now it needs to open and close, so just the opening needs to happen in 10msec. We don't want a trianglular wave vavle action though, we want a sharp ramp rate and lots of vavle hang-time to get good airflow, a regular camshaft is similar to a sine wave but with the electronic valve we could get a near square wave right? Sure, if the motor operated at 60RPM.
A typical solenoid has the property of a varying magnetic field strength where it will be strongest in the activated position and weakest when first activated, which could be of benefit since the valve spring will increase its tension as its compressed, but our ramp rate is now compromised.
Please, do the math and figure out what force is needed to accelerate 100g a distance of 12mm in 10msec or less. Now the spring will need the same force, so double it to get the total force. Then locate a solenoid that can pull that force and complete its cycle in 10msec or less.
There is a simple reason you don't see solenoid activated valves, and its not because everyone is some Luddite who thinks cams are cool because they're old school.
You need to accelerate a mass, let's say 100g, a certain distance and in a certain time. Let's say we have 6000 RPM, that's 100 revs per sec. For a 4-stroke engine, that's 50 times per second or 20msec period. Now it needs to open and close, so just the opening needs to happen in 10msec. We don't want a trianglular wave vavle action though, we want a sharp ramp rate and lots of vavle hang-time to get good airflow, a regular camshaft is similar to a sine wave but with the electronic valve we could get a near square wave right? Sure, if the motor operated at 60RPM.
A typical solenoid has the property of a varying magnetic field strength where it will be strongest in the activated position and weakest when first activated, which could be of benefit since the valve spring will increase its tension as its compressed, but our ramp rate is now compromised.
Please, do the math and figure out what force is needed to accelerate 100g a distance of 12mm in 10msec or less. Now the spring will need the same force, so double it to get the total force. Then locate a solenoid that can pull that force and complete its cycle in 10msec or less.