• Tuned Pipe Designer

     
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    © Martin Hepperle

    This small Java applet will help to size a tuned pipe for a 2 stroke piston engine. The calculations are assuming a speed of sound in the exhaust gas, which is not known exactly; also, it depends on the temperature and thus on fuel/air mixture and external conditions. In this applet, the gas conditions for methyl alcohol (methanol) and gasoline have been implemented. The results of the calculations can be used as a good starting point for experiments and fine tuning, deviations of +/-10% are surely possible.

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    Usage

    • Measure the timing of your engines exhaust and transfer ports in degrees (see below).
    • Measure the dimensions and calculate the area of the exhaust port opening in the sleeve.
    • Assume the velocity of rotation of your engine in rpm (dreaming doesn't help much though).
    • Either:
      • use the sliders to adjust the values according to your engine, or
      • enter the corresponding values into the text fields and press [enter] to update the graph.
    • Printing and Exporting only works, after you have adjusted the appropriate Java security settings of your Internet Explorer browser. If you use a different browser, you can run JavaPipe as a standalone application which has access to files and printers (see below).
      You could create a hardcopy of the program window, though, using your systems suitable tools.
    • You can manually plot the cones to build a paper model or to transfer the shape onto sheet metal by using the values of R and phi noted below each cone. Plot two concentric circles (having the same center) using the two radii R. The flattened cone spans a segment with an opening angle of phi degrees between the two circles.
    • The Print... command will print a copy of the general arrangement and a scaled view of the cones on a single sheet as shown below:

    • The Export... command will create three AutoCad compatible DXF files for the two cones plus a side view of the complete tuned pipe. You specify a base file name and JavaPipe appends an extension as shown in the picture to the left.
      Later, you can import these into your favorite CAD program for printing or further processing.

     

    Determining the Timing

    		 The animation shows, how to measure the timing angles of exhaust and transfer port. These angles define how long the ports are open, expressed in crankshaft angle. You can attach a disk with divisions in degrees to the crankcase and a pointer to the crankshaft (or vice versa).
    Then you adjust disk and pointer so that the it indicates 180° when the piston is at its bottom dead center. Now you turn the crankshaft until the port of interest closes (when the piston crown arrives at the upper edge of the port). It is helpful to point a flashlight or a halogen lamp through the plug hole or to remove the cylinder head.
    Reading the angles in both directions must be symmetrical to the bottom dead center, e.g. a port opening at at 100° must be closing again at 360°-100° = 260°, as shown in the animation. The port timing angle (shown in blue) is then 260° - 100° = 160°. This means, that port is open while the the crankshaft turns 160°.
    Usually the transfer port will have a shorter timing than the exhaust port.

    Background

    (metric units)

    The opening angles of the cones should be 4° to 10° for the opening cone and approximately twice this value for the closing cone.


    Cut view of a tuned pipe with silencer.
    Enlarged view of the muffler section.

    © Martin Hepperle