2 stroke supercharger...?

[naderson]

A four strokecycle engine may or may not have a pressure boost (supercharger/turbocharger) in the intake ystem. why must a two-stroke engine always have an intake pressure boost???

[psychopathicdude]

Let's begin at the top (when the piston is all the way up). On a Four-Stroke, an intake valve opens, and the piston begins to move downward. This lowers cylinder pressure below that of the intake manifold; be it pressurized or not. Air is drawn into the cylinder by this vaccuum (Intake Stroke) until the intake valve is closed, and the piston begins to travel back up, this is the Compression Stroke. At a point determined by ignition timing, (degrees before Top Dead Center), the spark plug is fired. Combustion occurs, there is a rapid increase in cylinder pressure (and temperature) which pushes the piston down, your Power Stroke. Roughly at the bottom of the pistons travel, the exhaust valve opens, and the piston begins up again, pushing the exhaust gases out, the Exhaust Stroke. 1.) Suck [down] 2.) Squish [up] 3.) Bang [down] 4.) Blow [up]. and repeat!

To fit the whole "suck, squish, bang, blow" into only two piston movements, you have to make some compromises. Generally, (though designs may vary) you are asking the intake air to enter the cylinder without the vacuum effect of downward motion. The piston may actually be on its way back up the cylinder, while intake air is still squeezed in (until the valve is closed). Then at the top of travel, the spark plug fires, and the power stroke occurs. But before the piston reaches the bottom, the exhaust valve opens (or is uncovered) and the still expanding gases now have an easier path, and exit. The inrush of pressurized intake air is actually used to help "push" the remaining exhaust out. Then the exhaust valve is closed, the intake valve is closed and the whole cycle begins again. 1) "Suck"/Squish [up] 2.) Bang/Blow [down]

more or less....many details left out, because I get easily confused and seem to relay that gift to people with whom I converse. If I left out something important, or gave horribly innaccurate information (gasp!) feel free to chime in.

[curtis73]

2 strokes don't always have to have an intake pressure boost... only in certain situations.

4 strokes actively draw in their own intake charge so they don't need boost. Most 2-strokes don't either because they use the compression stroke to draw intake charge into the crank case. The intake charge lubes the bearings, then when the piston drops again on the power stroke it becomes pressurized. As the intake and exhaust ports become uncovered, the pressurized crankcase intake charge is pushed into the cylinder by the falling piston.

In the case of SOME 2-strokes, like for instance an old Detroit diesel, the crankcase is separate and filled with oil like a normal 4-stroke. When the piston drops and uncovers the intake and exhaust ports, there is no pressure variant. No air rushes in on its own. Forced induction is required in order to get things going.

Many 2-stroke designs with the separate crankcase will operate (very inefficiently) on their own without forced induction after they're started. The high-pressure combusted gasses will exit the bore fast enough to scavenge new air in, but for starting, forced induction is necessary to get that first charge in there.

[UncleBob]

the only supercharged/turbo'd 2 stroke I can think of is the old detroit diesels. Never seen it on a bike/atv stock.

When bikes/atv's have a turbo, it has a habit of going through reed valves/rotary valves at a high rate. They don't like the added pressure against whatever valving they use. But they can make some pretty sick power. 130hp out of a 350cc engine type levels. Quite sick when you consider that stock 671 detroits weren't terribly far from that level of HP (426ci, or 7.0L engines)

[The Dude]

Curtis is right not all 2 strokes need boost to fill the cylinders. But some do. Like your detroit diesels. I am a detroit diesel mechanic, i dont work on alot of two strokes but the two strokes have what you call liners (cylinders). These liners have holes on the bottom of them. When the piston uncovers these holes in the "AIRBOX" at the bottom of the stroke, booost is needed to force the charge into the cylinder because there is no difference of pressure that far down on the detroit. That is where your supercharger comes in.
A detroit 2 stroke has no intake valves these holes in the liners are the only way a charge can enter the cylinder. If you want more power you add a turbo. Detroit 2 strokes are only good for Constant High RPM, Like something you would see in a genset, but they put those darn things in firetrucks and buses. Alot of 6v71 and 92's are still out there in city buses and firetrucks.