2 Stroke cycle explained

[Jim Eh.]

Not sure if this has been posted but for those who have not seen it, this is how it works ...

https://www.youtube.com/watch?v=mA7l3dpx6t0

[CrabbyMilton]

I never saw that one but there is a WWII film that GM produced pitching all of their engines in military equipment.
For those of you much younger people out there, GM later became DETROIT DIESEL and has not been affiliated with GM for about 30 years now.

[Scott & Heather]

Ok so explain a couple of things to a green behind the ears youngin:

1. What are the two vertical rectangular air chambers for on either side of the piston and do our engines have them?

2. It would seem to me that if your exhaust valve is opening just before the piston reaches BDC it wouldn't be pushing exhaust out, but they say that the exhaust is "scavenged" out by the fresh air coming in? Is that because the blower is creating positive pressure and pushing the exhaust gas out? Is there any pressure created on the piston side of the blower since the piston is traveling up?


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[CrabbyMilton]

Well, you highlighted the principle very well but perhaps, a 2 stroke expert can address the air pressure on the side of the piston.
It should also be noted that turbo versions had a blower bypass since the turbo was pulling in most of the air at higher RPM's anyway.

[Iceni John]

Or think of a Detroit 2-stroke as one third of one sixth of one of these:  https://www.youtube.com/watch?v=D3bj47TAYiU    How about 18 cylinders, 36 pistons, 3 crankshafts and scavenge blowers, and not a single valve or head.   Then double that for the legendary Deltic locomotives  -  once heard, never forgotten.   I guess that's where I developed my interest in 2-stroke diesels, along with Commer TS3 engines (3 cylinders, 6 pistons and only one crankshaft  -  work that one out!) during my formative years.

John

[Lin]

removed post.

[eagle19952]

proof that pistons do not suck...

atmospheric pressure fills voids

further the same motor without adjustment or difference runs better below sea level (Death Valley, Bad Water Basin) than at sea level, than at altitude.

airplanes have the same problem...compounded.

[TomC]

Same principle as the giant engines in overseas container ships. But instead of a 4.25" bore x 5.00" stroke (71 Series), they have a 39" bore by 135" stroke (yes that's over 11ft!)

[HB of CJ]

That U-Tube short video with the Idiot English accented voice over is wrong.

[Iceni John]

That U-Tube short video with the Idiot English accented voice over is wrong.

Sounds more like bonzer Strine to me.   Fair dinkum, cobber?

John

[Oonrahnjay]

Ok so explain a couple of things to a green behind the ears youngin:

1. What are the two vertical rectangular air chambers for on either side of the piston and do our engines have them?

       Those are "plenums" (or, perhaps more appropriately "pleni").  You could think of their function as a reservoir for air.  It takes a fair volume of air to purge and refill the cylinders and that plenum capacity accepts the air being pumped when the ports aren't open and holds it until the ports open again.  Without them, you'd have to have pressure release valves because the blower would be blowing against a closed port and when the port opened, you'd just get a little puff of air.

2. It would seem to me that if your exhaust valve is opening just before the piston reaches BDC it wouldn't be pushing exhaust out, but they say that the exhaust is "scavenged" out by the fresh air coming in? Is that because the blower is creating positive pressure and pushing the exhaust gas out? Is there any pressure created on the piston side of the blower since the piston is traveling up?

      It's just like valve timing and "overlap" of regular gasoline car engines.  Remember the "loping" idle of the big muscle cars of the 70s?  (No, you don't but most of us do.)  Their valve timings were so wild that inlet valves were open when there was compression and exhaust valves were open before the expansion cycle was completed.
      It basically because gas has mass (think of weight).  Like everything with mass, it wants to stay still when it's still and keep moving when it's moving -- thank you, Sir Isaac.  If you have a cylinder full of gas that you want to get out of there and you open the valve just when the piston reaches the bottom, that gas is going to just sit there for a microsecond before it starts to move and that makes it too late for all of it to go out.  If you crack the valve a little early, the pressure makes the gas begin to move and since it's moving when you want it to be moving, it's working for you when the scavenge gas comes in through the ports under pressure from the blower (through the plenum).  That increases the efficiency of the air flow through the engine.  And all engines - 2 and 4 strokes - are just big air pumps with things going on during their cycle of pumping; if you pump air better, they work better.

       HTH,  BH

[dtcerrato]

One thing I noticed in both the videos is that the blower is sucking air through - unlike the GM blower blowing or pushing air through the air box.
That would create positive pressure in the air box to help expel exhaust air/gas out exhaust valves as piston comes to bottom of its stroke (scavenging ports open)... I think I got that right!

[Oonrahnjay]

One thing I noticed in both the videos is that the blower is sucking air through - unlike the GM blower blowing or pushing air through the air box.
That would create positive pressure in the air box to help expel exhaust air/gas out exhaust valves as piston comes to bottom of its stroke (scavenging ports open)... I think I got that right! 

   I think you did.  And I think that the video was done sloppily; the way it's shown just isn't right for anything.  I think that the animation should show the rotors turning the other way - the little blue wisps that imply airflow (and doing it badly in my opinion) seem to indicate pressure and not suction from the blower.  After all, they call it the "blower" and not the "sucker"!

[Dave5Cs]

And Fair play to you Mate!....
Dave5Cs

[Lee Bradley]

One thing I noticed in both the videos is that the blower is sucking air through - unlike the GM blower blowing or pushing air through the air box.
That would create positive pressure in the air box to help expel exhaust air/gas out exhaust valves as piston comes to bottom of its stroke (scavenging ports open)... I think I got that right!

The blower is blowing.  Air doesn't go through the center but around the around the outside between the lobes and the housing.