Bimotion. Two Stroke Engine Simulation 2t
Kapil Grunewald
Anybody know if you can download bimotion online somewhere or get a trial version to play with? They want $129 for the full version, and I'm not sure that I am even smart enough to use it yet. I'd like to have a chance to play around with it before I shell out $129 for it. Anybody have it or know where I could download a trial or old version or something?
Yeah, I'm going to build a pipe for my QT60. I was hoping to use bimotion to do it. I was going to buy an exhaust from JEMCO, but they supposedly design their exhaust for the QT50, instead of the QT60 modification that I have done.
I attached a picture of my Java Pipe results. The pipe is a lot longer and skinnier than I expected it to be. Could someone that is familiar with Bimotion run my numbers and see if it comes out the same? I'd sure appreciate it.
Everyone with a QT50 is saying that the JEMCO pipe is the only way to go. To me it looks oversized and overpriced. The expansion chamber is huge, and they cost about $200. If nothing else I want to run my own calculations, and see how accurate their design really is.
yea ive run through the calculations strictly based on jennings' formulae a few times and got some reasonable looking results. of course i dont have that info anymore so i cant compare it to that Java Pipe. id like to see what Java Pipe does differently compared to just the Jennings formulae, or maybe its exactly the same.
im glad this post came up. everything on designing a pipe seems "very cut and dry". now i have use of this applet its even better cause i dont have to do the math out by hand. my only question is this: when asking for the RPM input, what is it looking for? is that the max RPM for the engine you are using, but bone stock? or is that the RPM of the desired powerband?
If you want a good exhaust, I'd advise on using a couple of different formula's and start experimenting. Weld one (or a couple) up, test it, cut off a piece of a cone and try it again etc. See what the changes do. That way you can get a feeling for how different formula's act and how small changes affect your pipe's performance. Ofcourse a dyno would be best to do this, but you can also do it on the road.
Do the results from Java Pipe that I attached above look close at all? This is the first two stroke that I've ever done any performance modifications on, so I'm not at all familiar with what does and doesn't work. To be honest, I was expecting the pipe to be much fatter, and shorter.
Other Available Pipe Design Programs & Why I Think They Are Lacking
MOTA for $175
1) Here's another persons comparison between ECcalc and MOTA: Click here.
2) I know a guy in Australia who races a kart with a KTM SX250 engine in it and he has his own dyno and makes his own pipes. He told me he built an optimized pipe designed by the Mota software and it didn't give him good power.
3) A friend of mine put the Maico AW250 engine details into Mota with theoretical pipe details and gave me the same details to put into ECcalc to compare the two results. The Mota waves didn't change from single to triple cone diffuser. At least the diffuser wave should change but it didn't so Mota doesn't use the detailed wave generation method that I use, otherwise the wave peak would be more to the left with a single cone diffuser.ECcalc says the diffuser wave should start at 1.65ms using 564 degrees C mid header temp (@ 5800RPM), and 1.7ms using 514 degrees (which is what ECcalc calculates it would be close to) but Mota shows 1.83ms. Mota shows a high baffle wave peak but the contrary 2nd generation diffuser wave overlaps it which would lower the peak but Mota doesn't show that lowering. Blairs papers show that the secondary diffuser waves are 1/4 the time duration of the primary diffuser wave but Mota makes them 3/4 the time. My conclusion is that Mota is fancy software but they lack the precision that ECcalc has.
Also a person who has Mota says there is no place to enter the exhaust gas temperature which means it provides only a rough estimate which is OK when using a simulator without the secondary waves but when including the secondary waves (as Mota and ECcalc does) it's best to truly know what the temp is.
Bimotion for $249
It is an Excel spreadsheet program for designing pipes. They admit on their site that they use Blairs formulas in the book "Design & Simulation of two-stroke engines". Unfortunately not even Blair, way back then (>23 yrs) , was very accurate at simulating the return waves. Here is an example of his simulation from one of his research papers, the one on his dyno. Notice how the calculated wave (thin line) lacks the pressure dip before TC that is caused in this example by the negative pressure 2D and 1B1D secondary waves.
2 Stroke Wizard for $27
I had bought this program before starting on making ECcalc and it just wasn't very good at designing a good pipe. And when my hard disk crashed I asked them to send it to me again to replace the lost program and they wouldn't even reply to my email ! This program only uses Blairs formulas which are childs play compared to properly simulating the return waves and calculating the power depending on them.
Crappy free programs
Some programs are even based on the formulas proposed by Gordon Jennings from way back in the early 70's. Man they were just beginning to investigate back then. Basically they knew next to nothing.
Jennings main shortcomings;
0. He assumes the same temperature and sound wave speed inside all pipes.
1. Because of his temperature error his calculations for the tuned length were off by 3.5" compared to a Maico 440 pipe, and 1.75" compared to a Maico 250 pipe. (both pipe specs along with full engine data, horsepower data, and EGT were given to me for analysis.)
2. He uses formulas to calculate everything but belly length, letting it take up the remaining distance. This is totally wrong because the belly length should be set to allow a slight overlap of the diffuser and baffle waves. That set length is optimally almost always below 4" (100mm).
3. He had no idea whatsoever about the return waves the exhaust pulse causes, their return time or their time duration. His ideology was not based on the reality of what was happening inside the pipe but just on scant trial and error data.
4. He calculates the header length as a multiple of its diameter. It's correct length is that which locates the secondary waves in correct relation to exhaust closing time at top RPM.
5. He calculates the belly diameter as a multiple of the header diameter although the header can be too narrow or too wide. Proper sizing depends on the exhaust pulse pressure.
6. He calculates the stinger diameter as a fraction of the header diameter whereas it should be calculated by the exhaust pulse pressure.
7. He calculates the stinger length as a multiple of its diameter whereas in reality the same back pressure can be had by a fat long stinger as by a skinny short stinger.
8. He recommends not going over a combined baffle angle of 30 degrees (15 from centerline) although many successful pipes use much larger angles. According to physics there is no limit to the angle. It's just that the shorter the baffle, the shorter its return wave. The return wave length should match the RPM, the timing between transfers closing and exhaust closing, and the desired RPM spread of the power band. And its length is dependent on the length of the exhaust pulse.
9. He was calculating for a single coned diffuser which gives weak results compared to a multi-coned diffuser of gradually increasing angles.
10. He calculated for a 2 to 1 ratio of angles between the baffle and diffuser which is too limiting. Any way when you go to a multi-coned diffuser there is no single angle of overwhelming importance to compare the baffle angle to. So in modern pipes there is no ratio formula.
Well he was a pioneer 46 years ago but anyone who uses his formulas now is way behind the times. We have progressed so much farther than his formulas and now have computers to simplify the complex process. (But thank you Gordon for helping along pipe research in the early days. RIP)
At a certain online pipe calculatorI entered specs for an engine reving to 10,000 RPM and only changed the area for the exhaust port to get four different pipes. You can seehow the timing is different for each one as evidenced by the differenttotal lengths till the baffle cone. That is a disaster because the needed wave timing to match the ports timing would remain almost the same but they changed it drastically on each pipe, evidenced by changing their lengths. Here's howthe program works: It makes the header diameter according to theexhaust port size, then the belly diameter according to the headerdiameter, then the baffle and diffuser angles (at a 2:1 ratio)according to the desired exhaust temperature which remained the samefor all four pipes. Its emphasis was on areas, not timings. All 4 pipes have different tuned lengths (to the start of the baffle wave) and 3 of them didn't even have a belly which is needed to separate the diffuser and baffle waves so that your're not wasting energy by letting then cancel each other out where they cross (since they are of opposite pressure). And it designed only single cone diffusers which are ineffective compared to 3 cone diffusers.
Additions to the Expansion Chamber Construction Utilities
The Expansion Chamber Construction utilities have been extended considerably and are now accessed under a separate item on the Main MOTA Menu. A sub-menu offers the two selections "Constructing the Development Pattern of a Cone" and "Printing the Development Pattern of a Cone". It is the options provided under the second selection which have been added to MOTA. You can now print the development pattern of a cone and this may extend over several A4 pages. Of particular note, you can produce the pattern of a cone having either end or both ends angled to the cone axis. A set of explanatory diagrams with text can be displayed. You may also define a single straight cone and print the patterns of each of the pieces which, when welded together, will provide an equivalent bend section. The number of pieces and the overall bend angle are entered through the keyboard. A MOTA engine data file may also be accessed and the pattern of each section of the expansion chamber printed. Alternatively, any one section may be selected and patterns suitable for the construction of an equivalent bend section printed This new feature allowing creation and printing of Expansion Chamber sections