View Full Version : two stroke engines
hey all, I just completed putting a small 2 stroke on my old bicycle, yes majorly geared down, Im actually rather pleased with it. I used an internally geared 3 speed hub on the rear wheel so i could shift back down to 1st gear after being in 3rd and coasting to a stop (you dont have to have the wheel moving to shift gears, unlike derailers) and basicly welded a 20in wheel rim to the small sprocket off of the hub attached to my 24 in wheel, giving me the reduction i needed.
anyway, having completed the drivetrain, i would like to now move into getting the most out of my 23cc 2 stroke engine. after all my reading, I realized that the biggest problem they have is the fact that they waste some fuel during each cycle out of the exhaust port. Racing dirtbike engines use what they call expansion chambers in the exhaust pipe to reflect sound waves back at the engine, and timed right, they push the unused fuel/air mixture back into the engine. this also helps the compression ratio. take a look at the bottom of this linked page if you want a visual http://www.vf750fd.com/blurbs/stroke.html
I had 2 ideas about how to tune these engines in a better way. the first is an adjustment to the expansion chamber design. on this page http://www-staff.lboro.ac.uk/~elvpc/bikes.html?http://www-staff.lboro.ac.uk/~elvpc/progs/expch.html you can see that all the pipe bits diameters and lengths are all dependant on the RPM that the engine is wanted to run best at (giving a better bottom, middle or top end power band). my idea is to create the different sections of piping of the expansion chamber to be variable in both their lengths and diameters, and have it vary depending on the rpm of the engine, thus giving you the best timed reflection back at the engine. this could be accomplished much in the same way afterburner nozzles change their diameter, and as far as length goes, have the pipes overlap one inside the other and depending on what the equations give you for a particular section of the chamber, have a spring of a certain spring constant attached between the 2 sections (smaller constants for a length that should change more relative to the other lengths, etc).
my second idea may be more of a question, so let me ask it. Why isnt there just a valve on the exhaust port that shuts just before the fresh unburned fuel mixture starts to flow through it? it would prevent the bad gas "mileage" and help control pollution and increase compression ratios as well, and it wouldnt even have to deal with the pressure of the combustion like in a normal 4 stroke engine, so it could be pretty small and light. perhaps a disk valve? anyway please let me get your feedback on these things, thanks.
Why isnt there just a valve on the exhaust port that shuts just before the fresh unburned fuel mixture starts to flow through it?
To save weight is my guess.
the way i see it, it wouldnt be much weight at all, maybe 1/10 of a pound
the way i see it, it wouldnt be much weight at all, maybe 1/10 of a poundThere is a lot of stuff that has to be added to the engine to make valves work, like cams, cam followers, valve return springs, etc, etc and these all drain mechanical power. A properly-designed 2-stroke can out-perform most 4-strokes. The Kawasaki 500 of the 1970's demonstrated this quite nicely, as did the Yamaha RD350 (I had one!), leaving 4-stroke bikes with much larger displacements in the dust.
As for the expansion chambers - when exhaust leaves the cylinder and heads out the pipe, there's lots of complex stuff going on. One of the interesting things is that when the exhaust pops out the end of the pipe, it creates an acoustic "reversion pulse" that can propagate back up the exhaust pipe and restrict the free flow of the next exhaust cycle. There is a whole industry built around designing exhaust systems for motorcycles to deal with this and other isues.
There is a lot of stuff that has to be added to the engine to make valves work, like cams, cam followers, valve return springs, etc, etc and these all drain mechanical power. A properly-designed 2-stroke can out-perform most 4-strokes. The Kawasaki 500 of the 1970's demonstrated this quite nicely, as did the Yamaha RD350 (I had one!), leaving 4-stroke bikes with much larger displacements in the dust.
thats why i thought a disk valve would be good, just a spinning disk with a hole cut into it, have a 90 degree turn of the axle off the disk, and then put a belt from the axle to the drive shaft, then time it correctly tune it using trial and error even.
brewnog
Mar21-06, 01:29 PM
thats why i thought a disk valve would be good, just a spinning disk with a hole cut into it, have a 90 degree turn of the axle off the disk, and then put a belt from the axle to the drive shaft, then time it correctly tune it using trial and error even.
Any idea of the pressures involved, and the amount of pressure a spinning disc is likely to be able to contain? And how exactly would you time it, bearing in mind that one of the key features of a 2 stroke is its simplicity and lack of auxiliary mechanical systems.
In addition to pressure pulse tuning (and Lotus (I think) are currently experimenting with variable geometry exhaust manifolds) you might want to research scavenging, piston bowl shape and prechamber design. Where most people think of little whizz-bang scooter engines when "2-stroke" is mentioned, all the largest Diesel engines for power generation, pumping, and marine applications are 2 strokes, and achieve very high power densities and efficiencies through careful consideration of these effects.
thats true, werent the old diesel-electric subs 2 stroke? anyway, my idea has evolved a bit since i went through a couple boring classes today haha, i will make a drawing and scan it in if anyone is interested.
the good thing about the disk valve is that it would be an easy instal, run off of the power of the engine, and be able to be easily timed and adjusted, with just a pulley and belt from the crank or drive shaft. also the good part is that it would only have to contain the pressurized gas/air mixture, but not the combusted gasses at the time of combustion like the valves in a 4 stroke do.
anyway, if anyone wants to see i will scan in my drawings after im done with them. i think im going to call it a 2.5 stroke engine, with the 0.5 stroke representing an over-scavenging with just air, but no fuel, therefor not wasting any of the fuel or polluting as much.
thats why i thought a disk valve would be good, just a spinning disk with a hole cut into it, have a 90 degree turn of the axle off the disk, and then put a belt from the axle to the drive shaft, then time it correctly tune it using trial and error even.If you introduce valves of any type, you introduce complexity, weight, and mechanical friction, with is a drag on performance. Typically the best 2-stroke engine performance is acheived when a properly-tuned exhaust system works to scavenge the exhaust and resist the exhaust flow of the about-to-be-burned air/fuel mixture, in effect acting like a little compressor/supercharger. This is done by controlling the geometry of the exhaust to take advantage of the acoustic reversion pulses. If you close the exhaust valve during this part of the cycle, you lose the extra fuel/air compression supplied by the positive-pressure reversion pulse. I used to help my neighbor's son tune his dirt bike for performance (kind of a fill-in father figure) and when the expansion chamber got into a certain resonance with the engine, the torque and power-band were incredible. Did I mention that the bike was really throaty and loud when that happened?
These pulses are a factor in 4-stroke engines, too, but most often (due to the valving) the drive for performance will lead you to go to a stepped (larger and larger diameter) exhaust, with some kind of internal flanging to supress positive reversion pulses, and perhaps wrap the exhaust pipes in heat-resistant tape to keep the pipes very hot, which aids in scavanging. If you know how to cut and weld steel and you are willing to fabricate exhaust systems, you may be able to acheive a lot with a small investment. Inventing a new valving system for 100+ year mature technology might be a bit of an uphill climb. There's nothing wrong with trying if you think you've got a great idea, though. Good luck!
brewnog
Mar21-06, 03:14 PM
How would you time it? How would you achieve dwell? How would you ensure that you would not inhibit trapped mass? Any idea of pressures?
2-stroke diesel is a very different animal in terms of efficiency because diesels don't put fuel into the cylinder until the moment of ignition, so it's only air that's used to scavenge the exhaust.
For small engines (like the one you've got) the goal is typically to get high power density and simplicity which 2-stroke nicely provides.
When IC's get very large (think ship) the power density of two-stroke is also very attractive. AFIAK this 2-stroke is the most efficient production IC in the world: http://www.bath.ac.uk/~ccsshb/12cyl/
Since fuel injection and sophisticated valve control can mitigate many of the problems associated with 2-stroke engines, it does seem a bit odd that we don't see high-power 2-strokes in performance cars.
brewnog
Mar21-06, 04:01 PM
That's fantastic! Makes my 64 litre V16s look tiny!
A lot of engines of that scale aren't designed to be stopped and started, - they're even serviced while they're running (including replacement of bearing shells!). Suppose it makes sense when you have to think about lighting fires inside the bores just to get enough heat in the cylinders to start them.
As for 2-strokes not being used on cars, I think that the fuel economy, emissions and noise issues are still pretty overwhelming. Direct injection can help (and removes the need to mix fuel with oil) but you need so much silencing (particularly when using pressure-pulse tuning) that exhaust lines end up being really quite restrictive, and the injection equipment puts such a high cost onto the engine that they become unviable economically. Reliability and durability are also issues, anyone got any service intervals for the highly strung 2 stroke bike engines? Or should we think back to the old Saabs and Trabants?! Having said all that, Honda and Ford currently have 2 stroke research programmes, so perhaps we might see them reappearing for medium-sized applications.
there is my hand drawn design. as you can see it has a fuel injector, and that means that the scavenging can be done by just air, wasting no fuel, however, the disk valve aids in keeping both higher compression ratio and a longer power stroke. it works like this. starting with how the picture shows it, the spark ignites, sending the piston down, right after the top of the piston reaches the bottom of the exhaust port, and just before reaching the top of the air intake port (seen on the left of the engine) the disk valve allows the exhaust out (this is the longer power stroke as compared to the average 2 stroke), letting it equal normal atmospheric pressure, then as the piston reaches the bottom of the intake air port, the disk valve shuts and allows pressure to be built up again in the cylinder. you should note that this means there is a bit of overlap when both valves are open, this time will be used to scavenge the exhaust completely out of the engine and the disk valve will be timed to shut again exactly when the unburned air begins to come out of the exhaust pipe.
then the piston will begin traveling back up, sucking in fresh air in past the reed valve, and injecting the fuel (the gas can be injected way before the ignition happens unlike diesel because of the spark plug, this allows the gas time to atomize with the air). the disk valve is closed this entime stroke, allowing for a slightly higher compression ratio than the average 2 stroke which would have had to wait for the piston to seal it. the air fuel mixture is pressurized, spark is ignited and it begins again.
i realize that a valve on the engine may add a bit of complexity, but it could be very very lightweight and does not have to suffer high heat because it only seals the cylinder during the weakest compression time, and during combustion, the piston also helps to seal the exhaust outlet, protecting the valve from the combustion. therefor the valve would add almost no extra drag or friction.
what do you guys think? any comments?
what do you guys think? any comments?The mechanisms for timing the valve would add weight, the fuel injection system would add weight and rob power, and most importantly, your valve would probably have to be lightweight and would be subject to erosion from hot exhaust gases. Maybe if you could devise a lightweight adjustable-timing drive train to run a lightweight ceramic disk valve with fair sealing capacity, you'd have a chance. Intuitiviely, light metal disk valves would fail pretty quickly from overheating unless there is a way to conduct the heat away from them faster than they accumulate it. With a hot 2-stroke at full chat, this may be hard to achieve.
i understand that the fuel injection and valve would add some weight, but i dont need a carberator anymore so that removes some weight, and also, this would be way lighter than any 4 stroke with the same power output, and far cleaner than any average 2 stroke, that said, i think the addition and slight power robbing of turning a valve and fuel injection is an easy price to pay for that kind of engine. it could go faster than a normal 4 stroke, and further (better mpg) than a normal 2 stroke.
i do see what you mean about the valve material though, perhaps ceramic is the way to go
brewnog
Mar21-06, 04:56 PM
Again, have you thought about dealing with the in-cylinder pressure? The moment the piston moves past the outlet port, you'll have a LOT of pressure trying to blow that flimsy, poorly sealed valve right out of your exhaust system. You don't just have an instant pressure pulse at the moment of ignition, the pressure continues to rise at an incredible amount. You only need to look at the design of conventional valvegear on a 4-stroke (or the pressure pulse effect exploited on a small 2 stroke) to appreciate that. Also, why does it not have to suffer high heat? It's dealing with exhaust gases!
Am I missing something with this? You've obviously thought about it, but that was my concern from the out.
brewnog, i was thinking that as the top of the piston moves down past the bottom of the exhaust port, the hole in the disk valve would begin to open, and it would be all the way open by the time the air inlet port was all the way uncovered. this is the overlap of both ports being open, helping to scavenge. i dont know if there is something that could hold in the pressure at that point, but it would only have to hold it in at almost the smallest point of pressure, because the piston has moved almost all the way down, almost at the end of its power stroke.
i guess it does have to deal with high heat of exhaust gasses, what i meant to say was that it doesnt have to deal with the initial explosion of combustion.
and as far as heat goes, if its found that the material cant take that much heat, it could always be flipped so the axle is on the bottom (below the exhaust pipe) and have it dip one side into oil or something.
Gigabyte111
Mar21-06, 05:35 PM
if its found that the material cant take that much heat, it could always be flipped so the axle is on the bottom (below the exhaust pipe) and have it dip one side into oil or something.
ever seen a bucket of oil burn? i would sugest a fin system simmilar to that used to cool engines with out raidiators. your 2-stroke probably has this on the cylender head. or if you do want to use the above idea, use a nonflamable substance such as water. one more thing. the people who built this engine most likely knew what they were doing. dont go trying to reinvent the wheel. try it like it is. anything you add will probably just have a neagative effect. if you really want good preformance, use the gasoline with the highest octane rating you can find.
Well, let's start with:
You're not lubing the crank-case anymore.
(You're not lubing the crank-case anymore.)
It's important enough to write twice. Your 2-stroke motor has lubricant mixed in with the fuel so that, as it goes through the crank case, it lubricates. Unless you set up an alternative lubrication system, you'll be siezing up.
As a benefit, you no longer need to use 2-stroke fuel, so you can actually get something that won't wreck your injector right away. Of course, getting the right injector, and getting it to work right is going to be an adventure in its own right.
With the geometry you've drawn, closing the exhaust valve on the power strok will make the exhaust gasses pressurize the crank case so you'll have trouble getting air in, and closing it on the compression stroke will make the piston push air/fuel back into the crank case. So I don't see a whole lot of benefit there unless you're also willing to stick a valve on the intake port.
first of all, i would lube the crank case, of course. probably use a thicker oil in the bottom of the crank, that way it wouldnt tend to blow up into the cylinder with the air.
youre right, i know nothing of fuel injectors, but they work on alot of engines, and i bet i could get it to work on this one with some experimenting.
maybe my draving doesnt make it clear, but i have said it a few times now, the valve would be opened just before the piston uncoveres the intake air port, so just before the intake port is uncovered, the pressure in the cylinder is at 1 atmosphere of pressure, so NO exhaust would be forced into the crank case where the air comes from.
Cliff_J
Mar21-06, 09:56 PM
The hole in your disk would need to look more like a slot. But why bother? If all you're trying to do is create a one-way flow, you could use another reed valve. If you have a situation where the incoming mixture is only air, you can afford to shovel it out the exhaust port in droves without hurting fuel consumption, but you will have less in the combustion chamber. You may want to copy most 2-cycle diesel applications that use a turbocharger (or supercharger or both) to pressurize the incoming air charge. Yes, more complex but definitely higher potential for power/weight.
You will note there are very very few examples of direct injection gasoline applications. Meaning parts would be expensive and difficult to come by.
You could easily replace the bearings on the crank and rod with sealed units and eliminate the need for constant lubrication.
thanks for the tips on lubrication. i was thinking of trying to modify an existing 2 cycle i have by just drilling a hole in the top next to the spark plug and threading it and putting in a small 4 cycle fuel injector, like from a lawn mower. the reason for the disk valve and not a reed valve is that i want the valve to be closed as soon as the piston moves to block the intake port, this way it can increase the compression ratio without the use of a turbo or supercharger. like in a regular 2 cycle, as the gas and air mixture is injected, it is pushed upwards by the piston, but since the exhaust port is higher than the intake port, it doesnt start compressing the mixture until it passes by the exhaust port, sealing it. with a disk valve, you can seal it early on the compression stroke, and open it late on the power stroke, letting it last longer and extracting more power out of each stroke. Is this thinking wrong? i understand it would be hard to keep everything cool, but in principle is my thinking correct on these ideas?
Don't get me wrong, I'd definitely like to see a more developed 2-stroke gasoline motor. It's just that I'm pretty sure you'd be better off starting with a 4-stroke or, even better, starting with something like:
http://rbowes1.11net.com/dbowes/
(especially if you add a super/turbo charger)
interesting idea. any clue why this hasnt been wide spread on automobiles yet? i know i should start with something like that, but i have 2 or 3 2 strokes just sitting around and i want to try to use them, and make them better, so i was hoping for some help/constructive crit on my idea specifically thanks
interesting idea. any clue why this hasnt been wide spread on automobiles yet? i know i should start with something like that, but i have 2 or 3 2 strokes just sitting around and i want to try to use them, and make them better, so i was hoping for some help/constructive crit on my idea specifically thanks
Electronic valves on production cars are a year or two away.
2-strokes are notorious for being hard to break. I don't think you'll get a big performance increase, but it won't hurt to try.
kleinjahr
Mar22-06, 01:21 PM
Interesting idea. I do wonder how you will seal the valve seat so there is no blowpast. Perhaps rather than a disk some type of ball valve? Rather than a straight through hole, use one that is cut to give proportional opening. I have seen hand valves, in plastic, of this type but not in metal.
actually since nate showed me that link with the electronic valves, i was thinking of using one of those... maybe use it in conjunction with the magneto on the flywheel to have it activate?
on another note, does anyone know how i could rig up a fuel injection system? i know how they work, and how the ecu controls them, do you think it would be possible to use a system lifted from a car? use the 02 sensor and stuff like that?
Cliff_J
Mar22-06, 04:24 PM
Note that the electronic valves are moving like 2mm.
An easier place to get an EFI system more closely matched would be a motorcycle, snowmobile, or ATV. They are closer in displacement and number of cylinders so the injector size would be in the ballpark so the pulse width would still be large enough for good atomization. Also they would be likely speed density so with a MAP and IAT you'd be set once you adjusted the tables to compensate for your displacement. How long they'd last inside the combustion chamber when they are not designed for it might be measured in 2-cycles too though.
You're more likley to be able to use a mechanical injector for direct injection with better success, they have been used in diesel applications for years with success and are designed for this type of operation. But how to retrofit one of those is another massive problem to itself, but an old gas fuel injection system (GM had mechanical gasoline injection on the infamous 57 Chevy 283HP 283CID engine and on Corvettes and so on) might lend itself better. The all metal construction seems a lot more robust than plastic...
hmm i like the idea of the mech injection, any details or sites that could explain better how that works? and also could i find a mechanical system in the general size i would need for my small engine? i have tried looking for 2 stroke diesels on like ebay, and there are none..... and one more question, is gasoline injection different than diesel injection? how so?
GENIERE
Mar22-06, 06:06 PM
hmm i like the idea of the mech injection, any details or sites that could explain better how that works? and also could i find a mechanical system in the general size i would need for my small engine? i have tried looking for 2 stroke diesels on like ebay, and there are none..... and one more question, is gasoline injection different than diesel injection? how so?I believe Orbital (Australian Company) uses some of the 2-stroke injection ideas you discuss in their engines.
Two stoke diesels have been around for a while, at least since WW2. To fit the diesel engine into an aircraft wing, the Germans used pairs of pistons. They compressed the fuel/air mixture between the pistons (no head) with a crankshaft at both ends of the cylinder yielding a very flat engine. Later a triangular setup using multiple banks of 6 pistons (3 cylinders) was used in high-speed boats and locomotives.
Cliff_J
Mar22-06, 09:01 PM
http://www.hilborninjection.com/product.asp?Id=42&CatId=35
Welcome to Fuel Injection Engineering Co., Inc. We are the original designer and sole manufacturer of the HILBORN fuel injector. We provide a complete product line of HILBORN fuel injectors, fuel pumps, and related parts to fit a variety of engine types and racing applications. We also offer technical information, service and flow testing of all HILBORN injection systems.
HILBORN fuel injectors have been successfully used in all types of racing including oval track, drag, dry lakes, super modified, off-road, motorcycle, tractor pulling, hydroplanes, and Indy Lite series.
Google works well
http://www.auto-solve.com/mech_inj.htm
Wiki is always good, even math to figure out pulse width
http://en.wikipedia.org/wiki/Fuel_injection
http://en.wikipedia.org/wiki/Gasoline_Direct_Injection
Oddly, the wiki article isn't using SI, and you'll find a couple things about injectors different (some in CC instead of lbs but the conversion is nearly 10 so you can just move the decimal) and also the pressure affects the fuel delivery. The mechanical gets a little tricky to tune since its not a computer value, and without working on diesels its a pretty rare sight to even see a mechanical injected vehicle (even at a car show) so that valuable tuning information might be tricky to come by for mechanical injection.
swedish_lunacy
Mar23-06, 04:09 AM
Ayrity,
Good on you for thinking outside the square. It's great to see people like you rolling up their sleeves and getting into it. Two strokes can produce tremendous power under the right conditions.
1) If you are thinking about fuel injection then use electric rather than mechanical. You WILL need the added control and granularity that it affords you over mechanical systems. EFI is very reliable. You will need the following: MAF sensor, fuel rail, fuel pressure regulator, injector, computer ( try and get one that will handle fuel AND spark ). Can be quite cheap these days. Remember the power is in the tune, tweak, tweak and more tweaking
2) Use a MAF rather than a MAP air flow sensor. Might be hard to get a small enough MAF but it is useful in that it requires much less aggro to setup as it is not so concerned about plenum geometry etc.
3) Don't bother with an O2 sensor, it is only required in "closed-loop" mode. Your EFI will run on a "fixed" map, ie "this much air needs this much fuel". The EFI will tell you how much air is coming into the motor ( mass in kg's or lb's ) and your map just adds fuel to suit. This way if your mods allow more air to come through, more fuel will be added automatically.
4) Have a look at Yamaha's EXUP system which is basically a RPM-variable exhaust geometry setup.
5) Have a look at Yamaha's "power-valves". http://www.dansmc.com/powervalve.htm
http://en.wikipedia.org/wiki/2-stroke_power_valve_system
These were an eccentric lobe that changed the exhaust port geometry based on RPM. They changed the timing by adjusting the height of the top or bottom ( can't remember which ) of the exhaust port cutaway on the side of the cylinder wall. The lobe would slowly occlude or reveal more of the port as the rev's changed. NOTORIOUSLY unreliable due to the fact that rotating parts need to be gasketed and are subject to high particulate content and extreme heat. FYI, lobe was rotated by cables run from a servo elsewhere on the bike
Look at nitro for more power, it is non-parasitic and it will afford the single biggest gain. Your motor needs much more fuel when it is running a shot of NOS so get a system that injects NOS and more fuel simultaneously
Cliff_J
Mar23-06, 12:39 PM
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. :smile:
thanks for all the ideas guys, cliff is right about my goal, so i think NO2 is out, but i loved the rest of your ideas lunacy. I appreciate all of your inputs on the fuel injection, but this is the area where i get a bit lost, i dont know much about this kind of stuff, like the computer control unit etc. like cliff, what does "speed density" mean? also i was thinking, is it at all possible to just swap the head off of a fuel injected engine, put it on a 2 stroke block (so it has the air injection set up in the crank), or otherwise convert backwards from a 4 stroke to a 2 stroke? it would already have a seperate oil set up, and it would have the valve on the exhaust there and ready to adjust, obviously it would take more than this, but basicly, just readjust the valves, figure a way to have the power stroke pump fresh air to scavenge the exhaust, and then just double the injection and spark? is this worth thinking about?
thanks for all the ideas guys, cliff is right about my goal, so i think NO2 is out, but i loved the rest of your ideas lunacy. I appreciate all of your inputs on the fuel injection, but this is the area where i get a bit lost, i dont know much about this kind of stuff, like the computer control unit etc. like cliff, what does "speed density" mean? also i was thinking, is it at all possible to just swap the head off of a fuel injected engine, put it on a 2 stroke block (so it has the air injection set up in the crank), or otherwise convert backwards from a 4 stroke to a 2 stroke? it would already have a seperate oil set up, and it would have the valve on the exhaust there and ready to adjust, obviously it would take more than this, but basicly, just readjust the valves, figure a way to have the power stroke pump fresh air to scavenge the exhaust, and then just double the injection and spark? is this worth thinking about?
The speed density stuff talk is because EFI systems can be more efficient if they know the mass of air in the cylinder so there are sensors for measuring the air flow into the cylinders. There are a variety of methods for measuring - speed density refers to one of them.
Modern 4-stroke engines generally have all of the valves at the top of the cylinder, and don't use the crank case for aspiration or as a fuel pump. Some do have superchargers which are effectively air pumps used to improve the compression - but you can use them to scavenge in addition to aspirating.
Realistically, you've got to replace or retool the air, fuel, and timing systems but it's likely to be easier than retrofiting valves onto a 2-stroke.
Apparently orbital does offer modern 2-stroke and direct injection tech...
http://www.orbeng.com.au/orbital/nonflash_index.htm
ya i was thinking that might be easier, but i cant find anywhere online, google or ebay any sort of smallish 4 stroke direct fuel injected engine....
Cliff_J
Mar23-06, 06:36 PM
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 lets 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.
swedish_lunacy
Mar23-06, 09:18 PM
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. :smile:
Cliff, you are absolutely correct. Teaches me a lesson for posting after a night of bleary-eyed, hard work :yuck: . 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:
:uhh: 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 loss
EFI 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
well that explains why i cant 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 doesnt 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 cant 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....)
brewnog
Mar24-06, 12:45 PM
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.
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 seperate 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
swedish_lunacy
Mar25-06, 03:23 AM
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
brewnog
Mar25-06, 09:15 AM
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.
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 dont make smaller 2 stroke diesels? is it the cost of the turbo/supercharger needed would be as much as the engine? why cant diesels use the downstroke of the piston to push their air into the cylinder (my guess is its not enough compression, but i dont 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 thats a better plan....modify a regular smallish 4 stroke diesel into a 2 stroke? what do you guys think of that?
again, im worried about the lubrication system....... this is why im leaning to 4 stroke to modify
Averagesupernova
Mar25-06, 08:51 PM
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.
Cliff_J
Mar26-06, 12:35 PM
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. :smile:
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 occuring 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.
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, im probabbly going to do it for my own benifit anyway haha.
Cliff, its not that im obsessed with 2 strokes, its just, when i put my mind on something, i work with it untill it gets where i want it, or untill i run out of money, or untill i dont 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
Averagesupernova
Mar26-06, 05:23 PM
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 lets 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.
ya, thats 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
Averagesupernova
Mar26-06, 09:37 PM
That's SUPERnova to you buster! :wink: 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.
Averagesupernova
Mar27-06, 12:01 AM
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.
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.
Sorry I havnt drawn out my idea yet, busy school schedule.... im 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?
ok so ive aquired a small B+S 4 stroke engine (3hp) and I now think im 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. Im 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 thats 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.
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,
i do have access to a lathe here at my mechE lab, although i dont 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?
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. :smile:
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?
i do have access to a lathe here at my mechE lab, although i dont 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?
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.
ya i think thats a good idea, the use of the compressed air to test with. I dont know why everyone is always saying that we dont 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.
ok so i cant find ANY sort of solenoid poppet valve.... hmm maybe that will limit my choices
ok so i cant find ANY sort of solenoid poppet valve.... hmm maybe that will limit my choices
The EVIC guy rolled his own.
Hmm, how many cylinders does your motor have?
just the one. ive 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 im planning to have a little bit of blowing from that supercharger so i figure the pressure cant be more than 700psi at its greatest. am I way off here? thanks for all the help
sid_galt
Apr4-06, 04:23 AM
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.
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. im open to ideas though!
sid_galt
Apr5-06, 09:27 AM
You mean you want variable valve timing in your engine?
well, yes, because with such a different engine set up, Im pretty sure im 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.
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.
just the one. ive 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 im planning to have a little bit of blowing from that supercharger so i figure the pressure cant be more than 700psi at its greatest. am I way off here? thanks for all the help
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.
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, lets say 100g, a certain distance and in a certain time. Lets 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.
sid_galt
Apr5-06, 11:47 AM
In that case, these thesis about electronic valve actuation may help you.
innovexpo.itee.uq.edu.au/2001/projects/s366921/thesis.PDF
legend.me.uiuc.edu/~astubbs/acc_2001.pdf
a few papers
www.seas.ucla.edu/~ttsao/Publications/ACC02_ChunTai.pdf
http://ghost.engin.umich.edu/SAE2002_01_1106.pdf
http://www.math.rutgers.edu/~sontag/FTP_DIR/03cdc-papers-refs-eds/WeA08.1.pdf
Regarding Cliff's post, you should also consider reliability. As soon as anything goes wrong with your solenoid or controlling system, or anything in between, there's a good risk of killing your engine when the valves hit the piston.
i have class soon, and will respond in full more later, but just to touch on your post brewnog, luckily the B+S engine I have is pretty low compression, and if i mod the engine to be OHV i can choose the height of my head and valves over the piston, and with a big enough blower, that wont be a problem.
Hola, Ayrity
Joined PF so I could get in on your discussion regarding 2-cycles and injection.
I own an ancient 1973 Yamaha scooter called a U7e. It looks like a moped <em>sans</em> pedals. It's a single-cylinder, 80cc, and for reasons I can't figure out, I'm quite fond of it. Maybe "character" is of more value than little traits like "reliability"?
The pollution issue is really getting up my nose, as it were. There are tweaks--using different oils, keeping the machine in tune, etc. But the best available solution seems to be adding fuel injection, ideally direct injection. According to the miles of what I've read out here on the weerd wibe wob, properly tuned injection can bring a 2-cycle down to California ULEV status (probably not THIS one, but any improvement is an improvement).
Since it seems no-one's doing commercial conversion, I am left with experimentation on my own. It'll be slow going, but I intend to try and create a bolt-on system similar to the U of Colorado mini-taxi conversion gadget pictured here:http://www.envirofit.org/technology/kit.php
I don't think it's actually as complex as it looks. I hope it ain't.
Anyway, since you seemed to have questions along the same line, I thought I'd quit lurking and chime in on the chorus.
Recent thought: Your bike engine might be small enough to make use of scavenged parts from a scooter salvage yard. Many late models from Honda, Aprilla, et al, come with injection systems.
Re exhaust valves: howabout a simple valve held closed with a spring and opened by chamber pressure ... Or an adaptation of the leaf valves many 2-strokes use for intake control?
fredric21
Mar13-11, 06:32 AM
Your idea for a valve in the exhaust in the form of a fenestrated disc is a poor one I'm afraid.
You would have major problems with both sealing, friction and lubrication. You would also have to deal with uneven heat distribution within the disc itself, not to mention the problem of carbon build-up (always a problem in conventional two-strokes).
Ad to this the increased weight of your drive mechanism, which would require some form of dwell period and things don't look too good.
Why not have a look at the Yamaha power valve system for ideas, instead!
gear head 460
Aug4-11, 09:47 PM
i've been reading on this string. why are you worried about the waist of gasses out the exhaust. if the gasses doesn't suspend fuel. direct injection!!! much like a 2 stroke detroit diesel. supercharged and with direct injection, there is no waist of fuel on the return trip of the piston. the spray of fuel starts as soon as the piston rings pass by the exhaust port. and the inrush of air is pushed in by forced induction system. supercharger, and that only, a supercharger can start the engine with cranking rotation. a turbo can't start the combustion process. unless you pre-spool the shaft before firing the combustion process. breathing the cylinder with air. this would make the engine the best a 2 stroke could be. the less parts the better. the more simple the parts the better. the fewer reciprocal parts the better. kiss, keep it simple stupid. there would be no wasted oil mix in the fuel. sealed crankcase, much like a fore stroke. this is the best of the best in 2 stroke ideas. and it can be multi-fuel operated. given a varied emission output. just a thought.
fredric21
Aug5-11, 02:34 PM
Yes, direct fuel injection would indeed appear to be the answer. The problem is, however, that at high speeds, there is very little time available after exhaust closing to allow sufficient fuel to be injected before the point of ignition, (typically less than 2msec.) in a conventional, piston ported, two-stroke engine.
Earlier injection would only result in short-circuiting of the charge, hence fuel lost to the exhaust.
gear head 460
Aug8-11, 10:20 PM
ok, then multiple high flow injectors in the head, should cover the fuel needs and a balanced air fuel mix across the piston, with a centrally located spark gap plug. may cover the needs.
gear head 460
Aug8-11, 10:28 PM
i never saw a bore size. so this may be a problem. with size limiting factors for extra injectors. so how many can be mounted in a given space without detrimental head strength loss.
fredric21
Aug9-11, 12:15 AM
It's the actual time available for injection that's the problem. I have discussed this with Siemens technical staff.
HowlerMonkey
Aug9-11, 10:33 PM
The problem with 2 strokes is scavenging and overlap.
Direct injection does away with the issue of overlap allowing unburnt fuel/air being passed through without being combusted by only adding fuel after the ports are closed.
Direct injection also helps scavenging because it allows manufacturers to achieve much higher percentage of scavenging because they are no longer worried about passing fuel along with the air through the engine unburnt.
This allows for port or valve timing schemes that are more favorable for power production.
For gasoline engines, Goliath cars used direct injection as far back as 1952 to clean up their two-strokes.
OMC made outboards with direct injected two strokes in 1996 but you have to wonder why so many outboard makers have an ever increasing percentage of 4-stroke engines in thier lineup every year.
fredric21
Aug10-11, 12:29 AM
If it were only so simple, then we would have Saab two-strokes, Trabants, Warburgs, and NSU's dominating the Automotive market, but we don't, because direct injection is not the magic wand for the genre.
We don't have any two-stroke motorcycles being manufactured currently, not even for racing.
gear head 460
Aug10-11, 03:26 AM
if its a time issue, wouldn't the addition of more injectors put the proper fuel mix into the cylinder. or is this because it wouldn't have time to spread the fuel about the area fast enough to mix properly. even using fast spraying high flow injectors. but it that's still the problem. so how about the use of exhaust valves like in the detroit diesels. like the v6 71 run in semi-tractors. but with less stroke. for faster running engines. and less compression for gas or run a higher compression for propane or other high no knock fuels. you would defeat the issue of time to have a good spread of fuel across the cyl. by using a valve train. but going back to having a rpm limit and issues with a 4 stroke. but power would still be in abundance of power. having a power strokes on every return trip to tdc. and with the forced induction system. you can still get a clean cyl. every time too. just calculate the time to clean out the cyl, and pressure flow factors of the forced induction system, then there you go. because a drtroit diesel has the fastest running diesel out there to date. and as you know diesels don't turn up that high in rpm's. but detroits do. over 3800 rpm's, more than a cummings or cat ever thought about.
gear head 460
Aug10-11, 03:36 AM
more 4 stokes, is because of epa reg's that all, but put a valve train on the exhaust side and a forced induction system. this will clean the old burnt fuel, for the new air charge. and there you go. with the time to have direct injection.
fredric21
Aug11-11, 12:26 AM
If only life was that simple for the two-stroke! The problem with poppet exhaust valves is two-fold. a) the tremendous amount of heat that needs to be dealt with at high bmep's, due to the extra power strokes, and b) inertia loading on the valve train, limiting rpm.
gear head 460
Aug16-11, 03:36 AM
ok, did i say poppet valves. i said valves. just like those used in a 4 stroke engine. research the detroit engine. from and inline 2 to a v24 they used mushroom valves just as those in all cars & trucks and alike. using a cam, rockers, springs and push rods. only on the exhaust side, every thing else is normal two stroke, but a sealed crankcase. no carter influence involved on the induction system. being a forced induction. you can use any fuel type on it being a direct injection system. like i stated. look up the detroit diesel engine. then get back to me on this and say it wont work. because it has already worked before. http://www.expertdiesel.com/71series.htm this is just one lead to what i'm stating.
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