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14109Re: [Small4-strokeEngines] single carb

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  • Bart Ferguson
    Mar 3, 2014
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      Thank you Gary for reassuring my suspicions about the use of a scavenging collector on a 90* v-twin as well as the sharing of a common manifold. In both cases, one or the other cylinder is favored.

      Bart


      From: "garywolf@..." <garywolf@...>
      To: Small4-strokeEngines@yahoogroups.com
      Sent: Monday, March 3, 2014 5:07 AM
      Subject: Re: [Small4-strokeEngines] single carb

       
      Darrell,
         Noise level must always be a consideration for an aircraft, and a resonant system is going to produce an increased noise level at the design rpm. When I started building Rotax and Geo 1300 systems I first did traditional 4-1 and later 4-2-1 systems to produce a wide powerband that did not have a mixture hump that would result in a bad spec fuel. What I was finding was that the sound level and sound quality  that were acceptable for race vehicles were not acceptable for aircraft. Also at ~5K rpm these systems were heavy and so long that the silencer was extending to the rear of the cabin. Power was good but it was becoming an exhaust system with an aircraft attached to it. The tail was wagging the dog.
        For the Rotax I ended up with a hollow transverse canister, usually 5" diameter and a foot long, positioned under the centre of the engine. The Rotax is 20-25 hp per cylinder so 1-1/4" OD tubes will flow enough air, and they are just slightly larger, so a good match for the ports. The lengths of tube are not critical because they are usually in the range of 20", a length that would resonate at well over 10,000 rpms. The outlet diameter for the can is not critical as long as it is large enough to flow the mass. I use 1-3/4" O.
        In the case of a V-twin tis is impossible to produce a resonating system if the two pipes are joined into a traditional small-funnel collector. To produce a resonance the pressure pulses must arrive at the collector in a regular manner, evenly distributed over time, and a 90 degree V twin cannot do this. A flat twin can, and so can a parallel twin with a 360 degree crank, each sending a pulse every 360 degrees of crank rotation. A 90 degree V twin will have uneven pulsing because of the 90 degree cylinder separation.
         Joining the tubes together does have a silencing benefit though. A silencer's volume must be large enough that its fundamentall will be below the lowest frequency you wish to attenuate. Joining two tubes doubles the frequency, so a smaller flask can be used. Use a flask of several quarts size and run both tubes to it without their ends pointing towards each other. You can even do this if you decide to carry the weight of tubes that will resonate at your rpm, something in the range of 4 feet. I wouldn't recommend this length for an aircraft because of the weight.
        If you want a quick way to experiment with tube lengths you can use the telescope method. 18 gauge mild steel tube has a wall of .050 and will slide nicely over the next smaller size tubing, at least until everything rusts up. As long as you have six inches of overlap the leakage will be minimal. Set your engine up with its prop, and wind it up to max prop-limited rpms. Start sliding the overlapped tube up and down (no silencer) and you will eventually find a length that produces a cleaner, louder exhaust sound, and if your carb has rich enough jetting you will find that the rpms increase. I emphasize the rich jetting because going to a resonant system will inevitably produce a hump in the mixture curve. A 2 stroke will go lean very quickly and can seize in this condition. A 4 stroke is more forgiving but it too will require more fuel when more air is being processed.
         One carb per cylinder is best for a resonant system. A single carb on a 90 degree v twin can end up favouring one of the cylinders, to the detriment of the other. A shared intake likes even pulsing too, same as the exhaust. A V twin is nice for packaging but lousy for intake and exhaust design.
          If you do get to the point where you have the tubes resonating and producing more power, you will then have to deal with how to silence the increased noise level.
       
      Gary Wolf
       
       
       

       


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