Why do two strokes have pea shooter exhausts?

[gareth_r]

There is a fascinating story in the attempts to make 2-strokes work. The problems led to
deflector pistons
split singles
disc valves
reed valves
stepped pistons

and, no doubt, loads more.

I might even overcome my prejudice against 2-strokes if I could own a DKW piston-supercharged split single as used in the pre-war GP racer I posted above

[Wedg1e]

Diesel powered 2-stroke aero engine? 105bhp but 270 lb.ft at 1200rpm...

http://en.wikipedia.org/wiki/Commer_TS3

Supercharged 2-stroke diesel with more ingenuity than you can shake a stick at...

http://en.wikipedia.org/wiki/Napier_Deltic

Not an expansion pipe in sight

[crofty1984]

upsidedownmark said:

Two strokes are very different animals to four strokes - they don't even need to have valves (although many do), in the most simple form relying instead on ports in the cylinder that are opened/closed by the cylinder moving past. Obviously they also get through a whole combustion cycle 'suck-squeeze-bang-blow' in one revolution rather than two. Thus they struggle rather more than a 4 stroke to both fill the combustion chamber and scavenge the exhaust gas - largely because you're trying to move the mixture in a much shorter time.

The exhaust design has a pretty major effect on efficiency - tuned pipes are quite common; decent article here explains the concept better than I can:
http://en.wikipedia.org/wiki/Expansion_chamber

Note particularly that you're 'tuning' the pressure wave to arrive back at the exhaust port in time with when you want to stop the outflow. Obviously these are effective over a narrow range of RPM.. two strokes tend to be peaky anyway - largely because there is much less flexibility available to control gas flow through the engine than there is with a 4 stroke, so the design will be 'optimised' around a particular powerband, outside of which things aren't really working very efficiently.

Edit - found the expansion chamber.. it's hiding in the belly pan - they are what I'd call a tuned pipe..

As for having separate pipes, pulse tuning is a lot harder (and probably less effective) when you start with N->1 manifolds etc. Probably far more effective to each cylinder as a separate unit.

[edit to add link, and correct my lack of observation]

Edited by upsidedownmark on Tuesday 28th April 11:29

Really interesting link, thanks.

[fergus]

Tim85 said:

Even worse than not knowing about the exhaust is that I literally have zero knowledge about anything 2stroke. I've never ridden one, never really seen one. I've seen the odd one on the road.
I've only been riding bikes for 4 years and no one in my family is into bikes so I've not known any older history really.
If it was made before 2000 I'm completely lost which seems like a shame.

See if you can ride a 4T and 2T motocross bike back to back, although I'd probably do the 2T first to avoid launching yourself through a corner (they have next to no engine braking compared to the 4T).

[srob]

gareth_r said:

There is a fascinating story in the attempts to make 2-strokes work. The problems led to
deflector pistons
split singles
disc valves
reed valves
stepped pistons

and, no doubt, loads more.

I might even overcome my prejudice against 2-strokes if I could own a DKW piston-supercharged split single as used in the pre-war GP racer I posted above

Whilst not quite as exotic as the DKW, we have a 1928 Scott 2-speeder. It's a very advanced bike for its day, with water cooling and upside down forks. Ours was recently found to be bored out to a 600cc engine, and actually wheelies, which isn't bad for an 87 year old bike

[MrOrange]

Wedg1e said:

Supercharged 2-stroke diesel with more ingenuity than you can shake a stick at...

http://en.wikipedia.org/wiki/Napier_Deltic

Not an expansion pipe in sight

And the sound. The glorious sound.

[TooMany2cvs]

bass gt3 said:

Everyone should have a little 2T in the garage...

<nods>