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one large exhaust vs. smaller exhaust plus auxiliary exhaust ports
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Posted by: rasputin
hello,
even though i don't race moto-x, i've been enjoying reading the drn forum for quite some years now because of the impressive two-stroke knowledge of the members. and even though the subject of my topic is not directly related to moto-x engines, i'm hoping for your advice, because my question relates to a general two-stroke issue.
the picture shows the 3d port map of an aftermarket cylinder for vespa ( 
) engines that i intend to use for dragracing (1/4 mile) purposes.
it seems like i'm not allowed to post links... here's the link to the picture. you still need to add "w_w_w" before the address:
250kb.de/u/070107/g/c6e072a5.gif
bore: 56 mm
stroke: 54 mm
conrod length: 105 mm
transfer timing: 124°
desired engine output: 155 psi @ 10.000 rpm
necessary effective compression ratio: 7.2 / 1
in order to achieve the necessary time-areas (calculated with tsr software) i have designed two possible solutions for the exhaust port. the time-areas of these two solutions are identical.
1. the first solution is one large exhaust port (shown in red) that is 73% of bore wide and has 190° duration (the top and bottom of the port will be generously chamfered, of course). the necessary compression ratio is 14.4 / 1
2. the second solution is one smaller exhaust port with two auxiliary axhaust ports. the duration is 187°, the necessary compression ratio 14.1 / 1
my question is: which solution is preferable, and why?
my two cents:
- the 73% width of the first solution is slightly on the "critical" side, and compared to the second solution it gives 0.7 mm less effective stroke. however, this can be compensated with a slightly higher compression ratio (and a higher octane fuel).
- the second solution will be superior in piston and piston ring longevity, but i think it will not flow as well as the first solution. so even when the time-areas are identical it will probably result in less power.
thank you in advance.
Posted by: blackduc98
My totally uneducated thought, and therefore to be taken as a suggestion for investigation, rather than actual design guidance, is that the situation of 3 ports vs 1 port is more complex than simply having an equivalent time area. The 3 ports will have significantly larger surface area (i.e. port wall area) and therefore much more drag and thermal transfer, so they will flow very differently, compared to a single port with the same area.
Also note that sub-ports will have a different length paths compared to the main port, which makes for very complex wavefront propagation and reflection. And of course the expansion chamber will further compound this complexity. If you don't pay close attention to this issue, you might end up with destructive (as opposed to constructive) wavefront interference between main and sub ports, and thus terrible flow characteristics. If you believe that a single port is impractical, then what about 2 perfectly symmetrical ports? They will have less combined wall area than 3 ports, as well as identical paths so the wavefront complexity is more on par with a single-port configuration.
And now some complete wild-ass speculation on my part: I made the preceding statements assuming that you'll have a single expansion chamber. But if you are willing to have 2 independent expansion chambers, then perhaps you could turn the multi-port complexity to your advantage. I wonder if it's possible to tune the 2 independent exhaust tracts (i.e. port timings and expansion chamber shapes) towards slightly different peak RPM targets to obtain a slightly broader powerband, which I presume would be beneficial for drag racing?
Last but not least, a study of 2-stroke design literature is in order, as I'm sure these subjects have been investigated ad nauseum by now. www.sae.org is a good place to start.
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