2 Stroke Tech.

[Montynut]

Erm not really. When the exhaust port opens to the pipe you have a hot high pressure wave, following the high pressure outbound wave is a trough lower than atmospheric pressure. That trough/depression stays there until BDC when that is the lowest pressure in the engine and when most bulk flow happens.After this is when the reflection from the baffle cone arrives to push most of the intake charge back into the cylinder. There never is any 'pushing' through the transfers by the decending piston, there is never any negative pressure in the combustion chamber. You still want the pipe for these reasons.

Correct panch and outboards also use water to vary the diameter of the stinger outlet.

Surely Lozza the "trough lower than atmospheric pressure" is a negative pressure although that is on the scale of gauge pressure not absolute pressure of course. A fuel/air mixture or just air can only flow from an area of higher pressure to an area or lower relative pressure. The difficult thing for people to understand is the idea of relative pressure as opposed to absolute or gauge (atmospheric) pressure. The pressure may always be more positive than the atmosphere but be negative or positive relative to the pressure in the cylinder or crankcase area. I think that is the concept that most people struggle with.

It is a bit the same as how we breathe by expanding the chest volume creating a relative low pressure in the chest cavity causing the lungs to take in air until equilibrium is reached. The low pressure is only relative. That is why the air supply pressure in scuba gear increases to the diver as their depth increases to maintain that relative pressure relationship.

Or am I just crapping on

[maico police]

Just showing off (again).......

[Mike52]

Erm not really. When the exhaust port opens to the pipe you have a hot high pressure wave, following the high pressure outbound wave is a trough lower than atmospheric pressure. That trough/depression stays there until BDC when that is the lowest pressure in the engine and when most bulk flow happens.After this is when the reflection from the baffle cone arrives to push most of the intake charge back into the cylinder. There never is any 'pushing' through the transfers by the decending piston, there is never any negative pressure in the combustion chamber. You still want the pipe for these reasons.

Correct panch and outboards also use water to vary the diameter of the stinger outlet.

Surely Lozza the "trough lower than atmospheric pressure" is a negative pressure although that is on the scale of gauge pressure not absolute pressure of course. A fuel/air mixture or just air can only flow from an area of higher pressure to an area or lower relative pressure. The difficult thing for people to understand is the idea of relative pressure as opposed to absolute or gauge (atmospheric) pressure. The pressure may always be more positive than the atmosphere but be negative or positive relative to the pressure in the cylinder or crankcase area. I think that is the concept that most people struggle with.

I

Are you guys saying that the pipe would help increase volumetric efficiency ?

[pancho]

to my mind that is the guts of it.

[Lozza]

Surely Lozza the "trough lower than atmospheric pressure" is a negative pressure although that is on the scale of gauge pressure not absolute pressure of course. A fuel/air mixture or just air can only flow from an area of higher pressure to an area or lower relative pressure. The difficult thing for people to understand is the idea of relative pressure as opposed to absolute or gauge (atmospheric) pressure. The pressure may always be more positive than the atmosphere but be negative or positive relative to the pressure in the cylinder or crankcase area. I think that is the concept that most people struggle with.

It is a bit the same as how we breathe by expanding the chest volume creating a relative low pressure in the chest cavity causing the lungs to take in air until equilibrium is reached. The low pressure is only relative. That is why the air supply pressure in scuba gear increases to the diver as their depth increases to maintain that relative pressure relationship.

Or am I just crapping on

Pipe pressure goes lower than atmospheric, same as your lungs, as I'm pretty confident when I breathe I am breathing air at 1.00 atmospheres.Same as I'm confident the air a engine intakes is at atmospheric(disregarding any ram air boxes).If the pressure in the header pipe was not less than atmospheric nothing would flow toward it at BDC. Prof Robert Fleck at QUB has a excellent SAE paper in this.

[Montynut]

Surely Lozza the "trough lower than atmospheric pressure" is a negative pressure although that is on the scale of gauge pressure not absolute pressure of course. A fuel/air mixture or just air can only flow from an area of higher pressure to an area or lower relative pressure. The difficult thing for people to understand is the idea of relative pressure as opposed to absolute or gauge (atmospheric) pressure. The pressure may always be more positive than the atmosphere but be negative or positive relative to the pressure in the cylinder or crankcase area. I think that is the concept that most people struggle with.

It is a bit the same as how we breathe by expanding the chest volume creating a relative low pressure in the chest cavity causing the lungs to take in air until equilibrium is reached. The low pressure is only relative. That is why the air supply pressure in scuba gear increases to the diver as their depth increases to maintain that relative pressure relationship.

Or am I just crapping on

Pipe pressure goes lower than atmospheric, same as your lungs, as I'm pretty confident when I breathe I am breathing air at 1.00 atmospheres.Same as I'm confident the air a engine intakes is at atmospheric(disregarding any ram air boxes).If the pressure in the header pipe was not less than atmospheric nothing would flow toward it at BDC. Prof Robert Fleck at QUB has a excellent SAE paper in this.

I'm not an engine expert but pressure and measurement of pressure is an area I'm familiar with. Atmospheric pressure is a very variable value as you know from tuning. It is also a variable that is taken out of measurements of closed cycles such as steam condensers by using absolute values. Typical pressures in this instance would be 6Kpa absolute (0Kpa Absolute being a perfect vacuum) or nominally -100Kpa gauge.

If you are breathing at the top of a 2000m mountain you are not breathing at one atmosphere you are breathe at some value less than 1bar absolute (nominally 106Kpa absolute). Even though this is the case the pressure differential across your lungs is the same as that at sea level (1 atmosphere). That is why you suffer oxygen deprivation at high altitudes due to the lower atmospheric pressure but your lungs take in the same volume of air which is now lower in pressure and hence less dense therefore less oxygen leading to, well, eventually death.

What causes air to flow into or out of an engine or any other vessel or system is a pressure differential the only time these laws do not apply is when some form of mechanical pumping is applied (ram air effect or mechanical/turbo supercharging). People commonly discussing pressure it is taken as gauge pressure where atmospheric pressure is considered to be zero.

I think we are saying the same thing but the atmospheric pressure versus pressure differential thing is confusing the issue.

[Lozza]

I think so too.