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Two stroke engines

  1. Mar 20, 2006 #1
    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.
     
  2. jcsd
  3. Mar 20, 2006 #2
    To save weight is my guess.
     
  4. Mar 20, 2006 #3
    the way i see it, it wouldnt be much weight at all, maybe 1/10 of a pound
     
  5. Mar 20, 2006 #4

    turbo

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    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.

    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.
     
  6. Mar 20, 2006 #5
    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.
     
  7. Mar 21, 2006 #6

    brewnog

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    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.
     
  8. Mar 21, 2006 #7
    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.
     
  9. Mar 21, 2006 #8

    turbo

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    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!
     
  10. Mar 21, 2006 #9

    brewnog

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    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?
     
  11. Mar 21, 2006 #10

    NateTG

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    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.
     
  12. Mar 21, 2006 #11

    brewnog

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    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.
     
  13. Mar 21, 2006 #12
    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?
     

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  14. Mar 21, 2006 #13

    turbo

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    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.
     
  15. Mar 21, 2006 #14
    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
     
  16. Mar 21, 2006 #15

    brewnog

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    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.
     
  17. Mar 21, 2006 #16
    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.
     
  18. Mar 21, 2006 #17
    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.
     
  19. Mar 21, 2006 #18

    NateTG

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    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.
     
  20. Mar 21, 2006 #19
    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.
     
  21. Mar 21, 2006 #20

    Cliff_J

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    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.
     
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