How a two stroke works!

**clmintie** · 30th June 2009, 21:16

Originally Posted by camchain

Crazy little engines. 42,000 revs is wicked. Would like to see the dainty wee dyno that measured the power output! Tiny carb must be a difficult thing to make and tune.

The HP figure is pretty impressive too............

Motors are very easy to tune, because there is no need to 'throttle' them, it's just a straight through venturi with a spray bar and needle.....

The control line speed glow plug motors run on 80% methanol/20%de-gummed castor (some use synthetic, or syn/castor blend)

Here's a link to 'model' dynos

http://www.rctek.com/engines/dyno/engine_dyno.html

**camchain** · 30th June 2009, 21:20

Originally Posted by Motu

The expansion chamber is using the same sonic impulses as used in 4 stroke tuning - but because the 2 stroke exhaust signal is so weak,it needs to be amplified.Before the loud speaker,people amplified their voices with a megaphone....and at the other of the scale,people hard of hearing amplified sound with a hearing trumpet...the megaphone in reverse. So you have the two main components of an expansion chamber.

Sounds right to me - good description. Weaker 2t sonic pulse due to lower compression ratio?

With simple straight through 4 stroke exhaust, the talk used to be about 'tuned length' for right amount of back pressure.

**camchain** · 30th June 2009, 21:45

Originally Posted by clmintie

The HP figure is pretty impressive too............

Motors are very easy to tune, because there is no need to 'throttle' them, it's just a straight through venturi with a spray bar and needle.....

The control line speed glow plug motors run on 80% methanol/20%de-gummed castor (some use synthetic, or syn/castor blend)

Here's a link to 'model' dynos

http://www.rctek.com/engines/dyno/engine_dyno.html

Interesting. Engine life must be pretty limited, poor little beasties.
(Only thing I ever had to with model planes is when I got interested in Richard Pearse and built a scale (near as I could from book drawings) model of his plane. It didn't glide at all untill I added fabric to wing underside. I bet it would've gone OK with 40k rpm engine dragging it along though!)

**clmintie** · 30th June 2009, 22:00

Slightly off topic...........but.....

Ken Buckley flew this one indoors at the North shore event centre, goes OK......

Back to 2T stuff................

**barty5** · 30th June 2009, 22:15

Talkin bout 2Ts cant rememver where i read it but the reakon it takes about 20min running form when the oil that come through from the carb enters or passes through the carb, Which due the the presure is seperatred out hence you get your lubrication is burnt and passes out the exhaust. interesting little fact i thought.

**noobi** · 30th June 2009, 22:47

Originally Posted by barty5

Talkin bout 2Ts cant rememver where i read it but the reakon it takes about 20min running form when the oil that come through from the carb enters or passes through the carb, Which due the the presure is seperatred out hence you get your lubrication is burnt and passes out the exhaust. interesting little fact i thought.

yea i read somewhere that the fuel/oil mix is seperated out in the crankcases and the fuel moves up the ports and combusts, and the seperated oil in left to lubricate the crank etc. and it takes 20minutes or so before any significant amount of oil is available to lubricate the bottom end?
but this doesnt make sense to me.
given the rate in which oil and fuel bond on a chemical level. it would take a fairly strong catayst to seperate the fuel/oil mix out into the parts again.
i assumed that the atmospheric difference from the piston causes fuel and air to be sucked into the cases. and this lubes the bottom end and the fuel/mix is sucked up the ports to the piston?

thats just my interpretation of it

http://science.howstuffworks.com/two-stroke2.htm

**barty5** · 30th June 2009, 22:59

Not sure on the whole tech side of it but assume the persure formed by the piston stoke is enough. A lot of the 2Ts ive worked on do have a resonable amount of oil in the bottom of the casing when split open. Cause the fuel oil mix isnt going to lube as well the would fuel tend to want to wash the bearings. Try it sometime on a some bit of metal with mixed fuel works but not as well.

**noobi** · 30th June 2009, 23:10

Originally Posted by barty5

Not sure on the whole tech side of it but assume the persure formed by the piston stoke is enough. A lot of the 2Ts ive worked on do have a resonable amount of oil in the bottom of the casing when split open. Cause the fuel oil mix isnt going to lube as well the would fuel tend to want to wash the bearings. Try it sometime on a some bit of metal with mixed fuel works but not as well.

yea and that suggests that fuel/oil seperation is occuring. i still want to know why the fuel and oil would want to seperate. seeing bond together so fast.
why does the fuel vaporise? i understand that it seperates out but WHY???

**barty5** · 30th June 2009, 23:18

Originally Posted by noobi

yea and that suggests that fuel/oil seperation is occuring. i still want to know why the fuel and oil would want to seperate. seeing bond together so fast.
why does the fuel vaporise? i understand that it seperates out but WHY???

only thought on that is cause under presure the oil is heavyer and force it to seperate the fuel vaporises due to the air flow through the carb and the smal amount of fuel being sucked through. Due to the volume of air fuel mix ans air speed it turn to vapour.

**Motu** · 1st July 2009, 00:04

Originally Posted by noobi

i still want to know why the fuel and oil would want to seperate. seeing bond together so fast.
why does the fuel vaporise? i understand that it seperates out but WHY???

It's because of pressure - but negative pressure...what we call vacuum.Lower the pressure and gasoline will vaporise...but the oil won't.

**camchain** · 1st July 2009, 10:31

As I understand it, 2 stroke engine is like a pump. I thought the fuel charge moving through engine was a mix of suck & push. Suck (from carb/intake) when piston moves up - and push (up the transfer ports/into combustion chamber) when it moves down?

I'm wondering how much the charge changes as it passes through a hot engine before the big bang. Being a vapour heat must change it to some extent?

Petrol is much more combustable than oil, so not all the oil in the charge will be getting cleanly burnt during combustion, especially as much (trail) riding not at high revs or at whatever the sweet spot for max engine efficiency is. That's gotta be a good thing for good lubrication. (Bit of crossover info happening on the spark plug thread re different premix ratios depending on intended use)

Barty, how much oil did you see in those crankcases? Spoonful, or half a cup, enough to dip the big end?

**camchain** · 1st July 2009, 10:50

Hey climintie, Ken Buckley any relation to Bill Buckley the genius designer of that Buckley 2 stroke GP bike? (he says in a vain attempt to avoid the issue of being off topic)

That's a nice model (I notice the sneaky rudder at rear of the top aerofoil on), a lot prettier than mine. I hung some weight off it where pilot & engine would sit - it didn't even get close to flying properly.