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14099Re: [Small4-strokeEngines] single carb

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  • garywolf@rogers.com
    Mar 2, 2014
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      Mike,

        I would like to chime in here. I have been designing and manufacturing tuned exhausts for 2 and 4 strokes since 1970, and I see printed in a recent (not this one) email a few of the common misconceptions about what makes them work.
      People seem to think that it is the slug of gas going down the pipe that makes it all work. The slug of gas moving down the pipe is the result - the cause is the pressure wave released by the opening of the exhaust valve. Depending on the temperature of the exhaust ithe wave usually travels between 1700 fpm and 2200 fpm. You can use 2000 fpm and a useful number for most engines. The slug of mass will travel about 600 fpm depending on the exhaust port diameter.

         If you have the simplest system, a straight pipe on a single cylinder engine, the exhaust pressure is initially above atmospheric. It travels down the pipe and when it hits the open end it pops and reverses in sign to become a below atmospheric pressure. I am going to use suction to describe this although the concept does not really exist.

        The suction wave travels back up the pipe, and when it reaches the still-open exhaust valve it helps to clear the burnt and unburnt gases from the combustion chamber, hopefully leaving it empty for when the intake opens. It also reduces some of the pumping losses so more work can be done by the crank to turn the propellor.

        if the length has been chosen to match the wave speed to the desired rpm this phenomenon will build a resonance, which is a repeated reinforcement of the wave, and you will hear the resonance. This is the same as when you blow into a length of tubing, tightening or loosening your lips like a trumpet player to produce a buzz at the frequency that results in resonaance of the tube. The effect is possible over only a narrow bandwidth and if you tighten your lips more you might hit the first overtone, one octave above which is double the original frequency. Trombone players are adjusting both the tightness of their lips and the length of the tubing to resonate all frequencies. it would be nice if a shifting-length exhaust system were built, and I have done this for test purposes. Unfortunately for real world use it is not practical.

         When an engine's cam has overlap about tdc, the resonance can be exploited for another gain. An intake valve that opens before tdc is seen by the engine as just anothe rplace to push the burnt exhaust gases out. This frequently happens in engines with long cam timings, running at low rpms. The cure is to choose an exhaust length that returns the suction wave just before the intake valve opens. This suction will then draw the burnt gases into the exhaust port, and also provide a pressure differential between the intake and exhaust ports, encouraging fresh mixture to come into the combustion chamber even before the piston has crested tdc. This improves cylinder filling, and therefore provides more power.

         There have been stationary engines that had the exhaust cut to a length that match the desired rpm. The effect produced by a parallel wall pipe is over such a narrow band that it actually acts as an acoustic governor.

        You are asking about the effect of bends in a system. The pressure waves will not care one bit about the bends, but the mass of exhaust gases will. A system that uses mandrel bent tubes means that the tubes can be bent over only so tight a radius before the outside wall thins too much. A 1-1/4" OD tube will generally not be bent tighter than a 2.5" centreline radius, a ratio that tube benders call a 2D bend. Pumping losses though such a mandrel bent tube will be minimal. However when the fabricator cheats and welds together something with sharp outside corners and abrupt transitions on the inside of the bend there can be appreciable pumping loss. I see this all the time on VW aero engines where the prop is in danger of clipping one of the front exhaust bends. Same happens of the Rotax 912. Fabricators cut a wedge and produce an abrupt change in direction, at just about the worst spot possible.

         Is a long tuned exhaust worth the trouble and weight for a small engine with minimal overlap? Not in my experience. I have built tuned systems for just about every motorcycle and formula car out there. These rev much higher so the lengths are shorter, and because they have lots of overlap the resonance can be well exploited. For an engine under 5000 rpms with little overlap you are just wasting time and carrying around extra weight. Just use tubes that are no smaller than the exhaust port ID and put them into a flask of several quarts volume, with an outlet pipe of 1.5" to the atmosphere. That is the simplest lightest system that will not attenuate power or produce unreasonable sound levels. The length hardly matters because you will always be too short to resonate.

      Gary Wolf

       

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