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 (https://www.youtube.com/watch?v=mA7l3dpx6t0)
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.
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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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.
Or think of a Detroit 2-stroke as one third of one sixth of one of these: https://www.youtube.com/watch?v=D3bj47TAYiU (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
removed post.
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.
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!)
That U-Tube short video with the Idiot English accented voice over is wrong.
Quote from: HB of CJ on January 12, 2017, 08:18:54 PM
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
Quote from: Scott & Heather on January 12, 2017, 05:45:12 AM
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.
Quote from: Scott & Heather on January 12, 2017, 05:45:12 AM
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
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!
Quote from: dtcerrato on January 13, 2017, 08:51:18 AM
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"!
And Fair play to you Mate!.... ;D
Dave5Cs
Quote from: dtcerrato on January 13, 2017, 08:51:18 AM
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.
A point about the Bypass Valve on turbo Detroits. It is not designed to bypass all the boost from the turbo. Rather it is designed to equalize the pressure on each side of the blower to allow the blower to free wheel-and actually be powered a bit by the turbo boost.
Detroit experimented with a 3 wheel turbo that the 3rd wheel was powered by the engine oil pressure to keep boost up (around 5psi) at idle. But it turned out to be not effective enough.
On the big locomotive 2 stroke Diesels, they have a combination turbo with mechanical drive with an over run clutch. Think of your bicycle. When the turbo reduces in speed, the mechanical gearing takes over to keep boost. When the engine gets up to speed, the turbo speeds up and the mechanical part of the drive is over run. Many different ways. Good Luck, TomC
Quote from: Lee Bradley on January 13, 2017, 10:08:24 AMThe blower is blowing. Air doesn't go through the center but around the around the outside between the lobes and the housing.
Thanks for that Lee. That explains and clarifies a lot -- they should have made the graphic plainer, though.
..."In the diesel engine, air alone is compressed in the cylinder; then, after the air has been compressed, a charge of fuel is sprayed into the cylinder and ignition is caused by the heat of compression.
"In the two-cycle engine, intake and exhaust take place during part of the compression and power strokes, respectively. ...The blower is provided to force air into the cylinders, for expelling the exhaust gasses, and to supply the cylinders with fresh air for combustion. The row of ports on the on the cylinder wall admit the air as soon as the rim of the piston uncovers the ports. The undirectional flow of air toward the exhaust valves produces a scavaging effect, leaving the cylinders full of clean air when the piston again covers the inlet ports. As the piston continues on the upward stroke, the exhaust valves close and the charge of fresh air is subjected to compression...The fuel is sprayed into the combustion chamber and ignited by the heat generated during compression."
Loosely quoted from the Detroit Diesel Allison Service and/or Operators Manual, Page 4. August, 1980
Love the DD Books as much as the MC8 Manuals :)
If memory serves, the exhaust valves, (either 2 or 4) open well before the ports open up. The blower only provides 3-5 pounds of air pressure.
The cylinder pressure is still hundreds of PSI. The remaining air in the cylinder blows out the exhaust valves. It has too. The blower pressure is too low.
THEN ... when timed correctly and when the cylinder pressure has dropped to about nothing, the piston uncovers the sleeve ports. Just an observation.
And when timed and tuned correctly, it runs so sweet! Music...
JC