sub piston induction. what does it do?

that makes sense

oh yes, not every engine has it. the ones with an exhaust throttle or muffler do not as they would breathe in their
own exhaust gases

aw yes, you bring up a good point, im just sayn that my engines dont have spi

It is not so much in order to get "extra" oxygen as it is a way to increase the crankcase pressure at high revs, as I see it.

At high revs, and especially with a reduce intake area or a reed valve, there is not enough time for the pressure in the crankcase to equalize before the piston turns and starts to go down. If that happens the reed will not close and there is also less compression in the crankcase and hence a lower gas flow trough the transfer channels, and a poor replacement of the gas charge above the piston will occur.

ok, thank you

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carbuerator venturi.

TDC and the downstroke: As the piston goes through top dead center, the skirt clears the bottom of the exhaust port. The crankcase vacuum created by the rising piston causes fresh air to rush into the crankcase and causes additional pressure as the now descending piston forces the freshly inducted air to combine with the existing fuel/air mix.* This gives the crankcase a belly full of extra pressurized f/a mix ready for the next charge. The increasing pressure of the descending piston forces the mix to scoot up through the bypass ports until sufficient pressure is relieved. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM.

*Obviously, surprising the premix from the venturi with bonus air pressure leans out the mixture. The clueless sucker with his fingers on the needle valve has to open it more to let in more fuel to balance the mixture.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the A/F ratio, now you have a higher charge in the crankcase than if there had been no sub induction.

Thanks Rusty. By far, the best explanation of sub-port induction I've ever read. +rep

Bob

I'll second that so so clear.
Surely though all two strokes need a transfer port to get the charge from
crankcase to above piston, so do they all have spi or is the special bit you
explained so clearly the piston skirt clearing the exhaust port when nearing
tdc allowing extra air in to crankcase, allowing you by opening needle to
increase the charge the type of porting we call spi.
Would love to hear more about porting as there is lots of stuff I still don't
understand, what kind of porting you can use with mufflers/cox throttle
and which you can't for example.
Great explanation! Thanks

Sorry Beez answered that last bit about engines with mufflers not having
spi, but I,m sure there's lots more about porting on cox engine that
I don't understand.
John

Many use SPI pistons/cylinders with muffler and throttle systems to gain lower RPM.

proctor wrote:... Surely though all two strokes need a transfer port to get the charge from
crankcase to above piston, so do they all have spi or is the special bit you
explained so clearly the piston skirt clearing the exhaust port when nearing
tdc allowing extra air in to crankcase, allowing you by opening needle to
increase the charge the type of porting we call spi.

If you are asking if other engines besides Cox use SPI, the answer is yes, it's common. Other model 2 strokes as well as full sized motorcycle engines and all kinds of others use various methods of boosting crankcase pressure while the piston is near TDC.

Hey guys!
I am new to the forum and I do some engine works and rebuils.Mostly diesel engines.And ofcourse the diesel bee was also tested by me.They are lovely engines,thats for sure!
I also wonder many times about SPI. I think ,maeby its also moore in it,with that story.Read my side of it..I got also a theorie about that.
If you imagine the fuel are like air with tenissbals,sucked in a tube...(the carb)...As fuel is moore heavy than air, its need moore time to start moving,right?.And the flow of a running engine at high rpm looks maeby constant,but its not ,...its a pulsing motion,with a very high frequence.But to go to the start of my story of motion....I think,that in the very first stage,when the engine start to suck a fuel mix "puls",...the very first tiny part of it, is only air,because as I say,the moore heavy massa of fuel needs moore time to start mooving,as the light air.I am not sure about my theorie,but I think the SPI do the trick,to just blow out the "empty" first part of "fuel mix",what is actually empty.But with SPI is the mixed air is in the crankcase filled to a maximum.
And as a second thought,I think,the airpressure from the prop dont create a overpressure,Its just blow through,right?... so also out... maeby its creates a bit of vacuum,because its a open cilinder?.So it blows through the 2 windows and sucks the "empty" mix with it out,to filling the crankcase even better till the edge with the fully mixed gas.If you want a overpressure from a blowing prop,it shoud be a closed window on the other(back) side of the cilinder,so pressure can be catched and build up.But maeby I am wrong..I just think about what are happend to moving air and gases.
I have no proof about my theories,but I think its a interresting but controversing possibility.
But it does indeed give a higher performance,thats for sure guys.But I read, its very sensitiv for the right gap.A lot testing is needed to get a better performace for a engine.(other than cox) I have a lot old diesels,and some good runners have SPI too.
Regards (and sorry for my poor english)
Peter,Netherlands

Welcome Peter!

Yes...welcome to the bunch !!!!

Fore those that don't know Peter has a fantastic collection of diesels posted on YouTube!

Welcome, Peter. We'll get back on the SPI topic soon. I need to think over some of your post rather than offer a quick response.

Hey Guys!
Yip ,I wil post soon my intro.I hope its Oke..
As soon I have moore time I wil do that.. promised guys!
Like Arni say in Terminator:
....I BE BACK!

yes! we have arnie joke, people!! WE HAVE ARNIE JOKE!!! HAHA!!!
oh yeah, welkom op het forum, Peter!!

welcome peter

Thanks al of you guys for warm welcome! moore souls moore fun,right?
Its nice that there are still so much peopel,that loves the cox engines.Lot of modelers fly electric ore the huge airplanes with a " grass cutter engine" with 40 cc..
Cox is actualy the company,that make the largest numbers of modelengines in the modeling engine history.And ofcourse thats great,still a lot parts left,even new from Xena look.So its a lot plessure for a lot peopel ...at low costs.
Every body a cool weekend and a great time with making (flying) plans for the spring!!

Welcome Peter!

We are in the same country!

Yes Bastiaan I showe you him in YouTube remember?
Indra

Yes, i remember.

But he did not know me yet!