This is from the DYNOmite Forum. They make engine transducer and data recording kits.
I suspect that they know more than just a little bit about these measurements!
The coloured and resized text is my work.
Link here: http://forums.land-and-sea.com/showthread.php?t=64
2-stroke crankcase pressures
________________________________________
How should I set up to measure pressures in my snowmobile’s crankcase? I already have the -12 to 22 PSI manifold pressure sensors but it seems to read rather erratically.
#2
07-05-2005, 08:49 AM
Tech
Land & Sea Engineering Join Date: Mar 2004
Location: Concord, NH
Posts: 210
Re: 2-stroke crankcase pressures
________________________________________
1) If you are trying to map the cyclical crankcase pressure changes that occur during each revolution of the crankshaft, you need more than just your sensor. This requires the same type of equipment and software Option Pack as combustion chamber pressuring mapping.
Our DYNOmite-Pro boards collect data at either 200 Hertz or 1,000 Hz (in USB mode). This is ideal for traditional dynomometer testing - where you are collecting data on Hp, Torque, RPM, fuel flow, etc. But, to map what goes on during a single revolution of the engine, you'll need individual data readings taken consecutively at each degree of crankshaft rotation. At high (two-cycle range) RPM's we are talking about board sets that can handle over 100,000 Hertz - like the 500,000 Hz board found in our Combustion Pressuring Mapping Kit.
You also need an encoder that provides an index for each pressure reading (to the exact degree position of the crankshaft when that data is captured). Again, the encoder in the Combustion Pressuring Mapping Kit is appropriate.
Note: It's mounting (which is designed for an automotive crankshaft dampener) would have to be custom adapted to your magneto housing.
Lastly, you will want to custom order the kit with a much lower range pressure transducer. Instead of the normal 2,000 PSI range, you will want one that is appropriate for the -14 to +30 PSI readings of a normally aspirated two-stroke's crankcase.
2) If instead all you want is to monitor the average crankcase pressure (a relatively low positive number), then your existing sensor can do that. However, you can not just screw the sensor into the crankcase. Those erratic readings are caused by the arbitrary synchronization aliasing) between your DYNOmite's 200 Hz sampling rate and the position of your engine's crankshaft. You're never taking the pressure readings at the same index twice.
Instead, plumb it to your engine's crankcase using a restricted dampening orifice. A hose with about a 0.030" fuel jet stuck in it works well as the orifice. Also, put a small dampening chamber (plenum) between the orifice and your pressure sensor. A small in-line fuel filter makes a convenient chamber. Once your have the restrictor and plenum in place I think you will find your erratic readings cured.
Note: We recommend that the pressure sensor be oriented so any residual fuel droplets will gravity drain back into the plenum and crankcase between tests. That greatly extends the sensor's life.
__________________
Land & Sea Engineering
Pity old mate didn't stick a transducer in the transfer duct and the cylinder, with a 360 tooth wheel and an encoder to graph crank degrees all at the same time. That would have showed the pressure in the transfer duct highest
AFTER the transfer port opens. Which is still higher than the crankcase.
Any-one who would like to read what Emeritus Professor Gordon P. Blair's thoughts on how 2-Stroke engines work, might like to download this:-
http://www.dragonfly75.com/motorbike/2StrokeDesign.pdf
Cheers, DJ
That book isn't about how a engine works it's more about mathmatical models and making test rigs to prove the model. I can post plenty of screen grabs from simulators (loosely) based on Blairs work.
Lozza , YOUR THEORY says its impossible for the engine to start without the pipe.
Because you state that " ALL the work is done by the pipe at BDC "
if you had said "SOME of the work is done by pipe at BDC" or even "MOST of the work is done by the pipe at BDC" then it would still be possible for the engine to run without the pipe.
But if the pipe is doing "ALL the work" then surely the engine cant run without the exhaust fitted,
because you also state that " there is never going to be high enough pressure in the crankcase to push intake out of the transfers"
without the pipe fitted how then does any air/fuel get into the cylinder if its not from crankcase compression ?
Without the pipe you have a greater cylinder purity ie no exhaust gas to mix with the fresh charge, the piston going up and down, the rod and crank shaft spinning together with the crankcase getting filled with dense inake charge each time the piston goes up will create enough turbulence to get fresh charge into the cylinder.
If you look at say a DT 175 cylinder how does all that black carbon get into the transfer ducts? On rgv engines it is possible to see carbon shadow of the C or boost port duct on the reed stops.
This isn't
my theory at all first person to highlight this to me was Wayne "Wobbly" Wright from NZ, as confirmed by Jan Thiel, Frits Overmars and the late Calvin Pollet of aftermarket CPI Banshee cylinder fame. Again plain to see on even the most basic engine simulator.
Twin, triples and inline 4 cylinder engines have labryinth seals on the crankshaft between the webs that bleed off and allow pressure to pass either way (decreasing as it passes) they seem to run just fine
The window is to allow
cool gas out from under the piston
