12 replies to this topic

[me madjoe 90]

Basicly i was wondering about the four-stroke cycle and the two-stroke cycle. And the effect knowen as scavanging on the exhaust stroke on two strokes.

my question is dose this same principle happen on a four stroke engine when the valves come on overlap.
Speaking in generaly as obviously it depends on cams and valve timeing.

For people who dont know what scavanging on a two stroke engine is its when - The piston is on its power/ exhaust stroke it moves down first uncoving the exhaust port the gasses begin to escape from the cylinder. As it moves down and fully uncovers the exhaust port the transfer port is also uncoverd and the remaining exhaust gasses are forced out by incoming fuel and air mixture underpressure from the crank case. This is knowen as scavanging, in a piston ported two stroke engine.

So am i right in thinking in a four stroke engine when the inlet valves and exhaust valves over lap (Its on the exhaust stroke). And the same scavinging efect ocures.

[AndyMiniMad.]

Yes I think you are correct, This would only happen if your cam is allowing both the valves to be slightly open at the same time. I think it says in the yellow bible that this scavanging effect would be more prevelent at the top of the rev range.

[me madjoe 90]

Thanks for that Andy, and the other bit is some good information to about it working better at higher revs.

I shall now get back to c/w now thats stopped anoying me . Diddent want to not metion about scavanging on overlap in my fourstroke cycle questions. If it was applyed to some fourstrokes.

[bmcecosse]

The LCB exhaust is supposed to help with this - but it's all a bit doubtful because of the shared centre exhaust port...........

[me madjoe 90]

The LCB exhaust is supposed to help with this - but it's all a bit doubtful because of the shared centre exhaust port...........

Yes i would have though it would only realy work on engines with indevidual ports per cylinders.

It was just a question i dident realy relate it to minis but cheers Bmcecosse for making it relavent to minis

Dont know how an LCB would make much of a differance tho, would have though port shapes and sizes would. But then it might do as the exhaust on two strokes has alot of influance on the scavanging as the expantion chamber stops to much fuel and air mix escaping.

[Dan]

Properly designed exhausts and especially manifolds are tuned to suit the engine, in that the length of each section up to each joint is very important. The distances between the joints where each of the pipes blend into each other affect the time it takes the pulses of exhaust gas to meet each other, if it's all setup properly the pulses will flow into each other at the joints more or less seamlessly which keeps the gas speed up which is what causes scavenging. If the pulses colide with each other badly out of sync, they have to slow.

[me madjoe 90]

Properly designed exhausts and especially manifolds are tuned to suit the engine, in that the length of each section up to each joint is very important. The distances between the joints where each of the pipes blend into each other affect the time it takes the pulses of exhaust gas to meet each other, if it's all setup properly the pulses will flow into each other at the joints more or less seamlessly which keeps the gas speed up which is what causes scavenging. If the pulses colide with each other badly out of sync, they have to slow.

I think we are talking about to slightly different things hear altho it is still relavent.

[Dan]

You asked how the LCB would make a difference over the standard manifold, that's how it does it. Poor manifolds cause the pulses to collide which sends a pressure wave back up the exhaust into the cylinder and stalls the scavenging effect.

[R1minimagic]

Unless it's a 2 stroke diesel engine in which case it is completely different!

[me madjoe 90]

You asked how the LCB would make a difference over the standard manifold, that's how it does it. Poor manifolds cause the pulses to collide which sends a pressure wave back up the exhaust into the cylinder and stalls the scavenging effect.

Ahh sorry fair enougth!

[me madjoe 90]

Unless it's a 2 stroke diesel engine in which case it is completely different!

But my 2stroke methanol compresstion model engines work the same and there basicly two stroke diesels.

[Dan]

The way scavenging works, in terms of how the exhaust interacts with the cylinder and in terms of what it actually does, is different in 2 stroke and N/A 4 stroke engines. Remember the charge is already compressed in a 2 stroke. Cylinder scavenging in a 2 stroke uses the pressure of the charge to clear the cylinder, but to avoid loosing charge and so compression pressure straight out of the exhaust the system has to be a specific design. I don't know a lot about 2 strokes but I believe that the pressure wave is actually desirable here because it acts to effectively close the exhaust port earlier than it otherwise would be, which is why they employ tuned reverberation chambers. N/A 4 stroke engines are slightly different in that they have only atmospheric pressure driving the charge in. So to get the most charge possible to flow in, they use the momentum of the exhaust leaving the cylinder to slightly depressurise the cylinder below atmospheric which is why it's important to keep the flow up.

I don't know if I've explained that very well.

[me madjoe 90]

The way scavenging works, in terms of how the exhaust interacts with the cylinder and in terms of what it actually does, is different in 2 stroke and N/A 4 stroke engines. Remember the charge is already compressed in a 2 stroke. Cylinder scavenging in a 2 stroke uses the pressure of the charge to clear the cylinder, but to avoid loosing charge and so compression pressure straight out of the exhaust the system has to be a specific design. I don't know a lot about 2 strokes but I believe that the pressure wave is actually desirable here because it acts to effectively close the exhaust port earlier than it otherwise would be, which is why they employ tuned reverberation chambers. N/A 4 stroke engines are slightly different in that they have only atmospheric pressure driving the charge in. So to get the most charge possible to flow in, they use the momentum of the exhaust leaving the cylinder to slightly depressurise the cylinder below atmospheric which is why it's important to keep the flow up.

I don't know if I've explained that very well.

Yes its a good explanation. I kind of got it i was just making sure that four strokes do have a scavanging effect when on overlap.

Thanks for taking the time to explain it. Hope others my find it useful or intesting.