The Science of Exhaust Scavenging: Understanding Wave and Inertial Scavenging

When it comes to designing an efficient engine, understanding how to harness the power of exhaust scavenging is crucial. Exhaust scavenging refers to the process of using the pressure wave created by the exhaust gases to pull out remaining exhaust gases from the cylinder, allowing for better combustion and increased power output. In this article, we will delve into the world of exhaust scavenging and explore how it works, including both wave scavenging and inertial scavenging.

Cross-sectional changes in the piping can cause pressure waves to reflect back, creating a positive or negative pressure wave that can affect engine performance. When these pressure waves reach a point where the diameter of the piping changes, they are reflected back, causing any cross-sectional change to have an impact on the system. This is because the amplitude of the reflected wave is proportional to the size of the change in diameter. A larger change in diameter will result in a greater effect, and therefore a greater amplitude.

In a collector, which is responsible for stepping up or stepping down the diameter of the piping, any cross-sectional change can cause pressure waves to reflect back. When these collectors are designed with large diameters, they allow exhaust gases to escape more easily, reducing the pressure inside the cylinder. This reduces the pressure differential between the intake and the cylinder, making it more difficult for air and fuel to enter the combustion chamber.

However, when a collector is designed to be too small, it creates backpressure that prevents exhaust gases from escaping properly. This can cause the engine to work harder to push out the remaining exhaust gases, resulting in decreased power output. Therefore, finding the optimal diameter and design for the collector is critical to achieving efficient exhaust scavenging.

Positive pressure waves are created when there is a change in diameter that causes the pressure wave to become larger. When this happens, the reflected wave is also positive, causing it to push back on the piston. In contrast, negative pressure waves are created when there is a change in diameter that reduces the pressure of the wave. These negative pressure waves can cause the piston to pull down, which can help pull out remaining exhaust gases from the cylinder.

Timing is crucial in the process of exhaust scavenging. The goal is to have the negative pressure wave arrive at the correct time, just before the intake valve opens and the exhaust valve closes. This allows for an empty chamber within the cylinder, making it easier for fresh air and fuel to enter the combustion chamber. With the right amount of scavenging, the engine can achieve better power output and efficiency.

One key concept that affects exhaust scavenging is inertial scavenging. Inertial scavenging refers to the ability of exhaust gases to retain momentum as they travel through the piping system. When exhaust gases are pushed out of the cylinder by the pressure wave, they carry momentum with them, creating a low-pressure area behind the gas. This low-pressure area can be used to pull in fresh air and fuel into the combustion chamber.

The velocity of the exhaust gases plays a critical role in inertial scavenging. If the velocity is too high, the low-pressure area created by the momentum of the gas will be too small, resulting in inadequate scavenging. Conversely, if the velocity is too low, the low-pressure area may not have enough pressure differential to pull in fresh air and fuel effectively.

Inertial scavenging can also be affected by the diameter of the piping system. A wider diameter piping system allows for higher velocities, which can create larger low-pressure areas that are more effective at pulling in fresh air and fuel. On the other hand, a narrower diameter piping system may not allow for sufficient velocity to create an adequate low-pressure area.

The length of the piping system is also crucial in determining the effectiveness of exhaust scavenging. A longer piping system can result in higher velocities, which can create more effective low-pressure areas. However, if the piping system is too long, it can reduce the pressure differential between the intake and the cylinder, making it harder for air and fuel to enter.

In conclusion, exhaust scavenging is a critical component of engine performance that relies on understanding wave and inertial scavenging. By optimizing the design of the collector, piping diameter, and length of the system, engineers can create an efficient exhaust scavenging system that helps pull out remaining exhaust gases from the cylinder, allowing for better combustion and increased power output.

"WEBVTTKind: captionsLanguage: enhello everyone and welcome in this video we're going to be talking about why exhaust back pressure is bad we're also going to be talking about exhaust velocity and exhaust scavenging which are both good and so first we're going to start pretty basic and then we're going to get in some more complicated aspects of how exhausts work so starting with the very basics we have our engine here it's got an intake it's got an exhaust the power stroke has just happened so our piston is at bottom dead center and we're about to press out all that exhaust those spent combustion gases once this exhaust valve opens so within this chamber pressure is about six to seven times atmospheric it could be more or less depending on the engine design of course outside pressure is atmospheric so about six to seven times as high inside the cylinder and of course one atmosphere outside coming in your exhaust now once this exhaust valve opens of course that high pressure is going to want to leave and go towards the lower pressure and so we're going to define back pressure as pressure with the opposite direction of flow so of course as that valve opens the high pressure in here wants to escape because there's lower pressure out here so it moves out the exhaust now back pressure would be resisting that so we have atmospheric pressure outside of the exhaust so one way to think about this why back pressure is bad take for example if you were to lower atmospheric pressure uh extremely low so now you have a vacuum out here well now the pressure differential between outside and within your engine is even higher so the exhaust is going to want to go out even faster now the whole purpose of an exhaust is to evacuate those spent gases as quickly and as efficiently as possible now if you were to take pressure that was way above atmospheric let's say it was 10 times atmospheric pressure well now that's going to resist that exhaust coming out so once that exhaust opens if ambient pressure was 10 times what's inside here you're actually going to have air flowing reverse it's going to go the opposite direction so you can see that as you increase back pressure it does the opposite of what you want your exhaust to do which is to evacuate and get rid of those exhaust fumes as quickly and as efficiently as possible so back pressure is bad it's not a good thing so why do people say well you need some back pressure well it's kind of misleading and it has to do with exhaust velocity so restrictions create back pressure if you have a super restrictive exhaust that's a really narrow diameter exhaust that's restrictive that's going to have a lot of back pressure it's also going to have a very high velocity because you're forcing all those fumes out to a very tiny channel so they're going to move very quickly if you have a super large exhaust you're not going to have much restriction but consequently you're not going to have much velocity of your exhaust flow so your exhaust is going to flow too slowly to escape out into the atmosphere ideal is somewhere in between so a restriction causes a negative effect which is back pressure but it also causes a positive effect which is high velocity so you want some balance of you know velocity and minimal back pressure somewhere in the middle is that sweet spot where you have good exhaust flow good speed to it you have good scavenging characteristics but however you don't have too much back pressure resisting that flow resisting your exhaust escaping meaning you would be making less power okay so now let's get a bit more complicated and talk about header design and exhaust tube length design so you have your four-cylinder engine right here and here you have the header of course exaggerated with these diameter widths here but regardless the four cylinders they all merge at one point as they come out into these exhaust pipes into a collector where all four then float together through the rest of the exhaust well there's a process here that happens that is ideal for exhaust scavenging so we're going to talk about how this process works so the first thing that happens is you have of course your combustion phase now your pistons at bottom dead center and you need to release those exhaust gases so the exhaust valve opens and immediately a positive pressure wave is created that travels outward at the speed of sound of course behind that following that are the exhaust gases coming out because of that pressure differential so that pressure wave continues to move down your exhaust pipe through your exhaust and then of course your exhaust gas is following behind and once your piston gets about halfway up now you don't have quite as big of a pressure differential between inside the cylinder and outside the cylinder and so the problem with this is is it means your exhaust gases aren't going to want to travel the remaining exhaust gases that are still within that cylinder aren't going to want to travel out very quickly so that's a problem we of course don't want those exhaust gases to remain there because that means our next combustion cycle won't be as effective okay so this pressure wave is traveling along and anytime a pressure wave changes reaches a point in the piping that changes diameter it's going to reflect back a pressure wave so any cross-sectional change will cause this so a collector for example if the piping were to step up or step down and of course as it exits the exhaust at the very end of your tailpipe so anytime there's a cross-sectional change and the amplitude of that reflected wave that's coming back uh is you know proportional to how big or small that step is that change in diameter so if the change in diameter is larger the wave that's going back is going to have a greater effect the amplitude will be increased now a positive pressure wave which is what comes out initially is reflected back if you were to step down so if this collector were to get smaller which you of course wouldn't want to do it would send back a positive pressure wave because it's getting larger it sends back a negative pressure wave and so that negative pressure wave starts traveling back and what you want to happen so that piston all the while is of course still moving upwards what you want to happen is for that negative pressure wave to come back at the exact right time and there's going to be a bit of a window so it doesn't have to be you know one specific rpm but generally this will be a good effect for specific rpm range rather than across the entire rpm range of course based on the tubing length and you know based on diameter things like that which affect how this pressure wave comes back so it's coming back you want to time it so that just before that exhaust valve closes and your intake valve opens that negative pressure wave arrives and helps lower the pressure in there pulling out the remaining exhaust gases so you have basically an entire empty chamber then with that low pressure once your intake valve opens it helps pull in fresh air because you've got a bigger temperature or a bigger pressure differential between the intake and within the cylinder so if you get that pressure really low within the cylinder right before that intake valve opens it helps pull in additional gas and of course that means you have better scavenging better scavenging and then as a result you have more air and fuel that gets pulled in and you can make more power now again this is all dependent on that pipe length of course because it has to do with how fast does that wave travel it reaches the collector creates that reflected wave once it hits the collector and then travels the way back through those exhaust gases so during that process uh you want to time it so that it arrives at the right time and these can help out different cylinders it has doesn't have to necessarily be the exact same cylinder but you get what's going on here where you want that negative pressure wave to arrive help pull out those remaining exhaust gases and then help pull in that fresh intake air now on top of this there's something called inertial scavenging and so if you think about like throwing a ball or a car driving of course behind that car is a low pressure area because it's pushing the air out of the way and the same thing happens with these exhaust gases so when they open up and it sends out that high pressure exhaust pulse that exhaust that air has inertia to it because it's traveling at a very high speed and so as it travels through the pipe it creates a low pressure area behind it so as it's traveling it can start filling up this area with low pressure and that's of course dependent on its velocity and its velocity is also of course dependent on the piping diameter so if it's too wide it'll have a low low velocity and it won't have that very low pressure area behind it if it's too restrictive you have too much back pressure it's not allowing those exhaust gases to escape and that's causing your engine to have to work harder to push out those exhaust gases so you know there's a sweet spot where you have the perfect amount of scavenging from both wave scavenging and from inertial scavenging where this piping diameter and the piping length are both very critical in the performance of your engine for a specific rpm range of course if you were to tune this in correctly if you were to have the wrong length you can hurt the performance of it if you have this wave arrive at the incorrect time it can actually be detrimental and cause you to have even less power than if you didn't do this at all so timing is very important tubing length tubing diameter all very critical in how this system works and how you have that exhaust scavenging so hopefully you know this is a decent uh basic overview of how this all works gives you an idea of why you don't want back pressure but why you do want scavenging and you do want exhaust velocity to help out with that scavenging so thank you all for watching and if you have any questions or comments feel free to leave them belowhello everyone and welcome in this video we're going to be talking about why exhaust back pressure is bad we're also going to be talking about exhaust velocity and exhaust scavenging which are both good and so first we're going to start pretty basic and then we're going to get in some more complicated aspects of how exhausts work so starting with the very basics we have our engine here it's got an intake it's got an exhaust the power stroke has just happened so our piston is at bottom dead center and we're about to press out all that exhaust those spent combustion gases once this exhaust valve opens so within this chamber pressure is about six to seven times atmospheric it could be more or less depending on the engine design of course outside pressure is atmospheric so about six to seven times as high inside the cylinder and of course one atmosphere outside coming in your exhaust now once this exhaust valve opens of course that high pressure is going to want to leave and go towards the lower pressure and so we're going to define back pressure as pressure with the opposite direction of flow so of course as that valve opens the high pressure in here wants to escape because there's lower pressure out here so it moves out the exhaust now back pressure would be resisting that so we have atmospheric pressure outside of the exhaust so one way to think about this why back pressure is bad take for example if you were to lower atmospheric pressure uh extremely low so now you have a vacuum out here well now the pressure differential between outside and within your engine is even higher so the exhaust is going to want to go out even faster now the whole purpose of an exhaust is to evacuate those spent gases as quickly and as efficiently as possible now if you were to take pressure that was way above atmospheric let's say it was 10 times atmospheric pressure well now that's going to resist that exhaust coming out so once that exhaust opens if ambient pressure was 10 times what's inside here you're actually going to have air flowing reverse it's going to go the opposite direction so you can see that as you increase back pressure it does the opposite of what you want your exhaust to do which is to evacuate and get rid of those exhaust fumes as quickly and as efficiently as possible so back pressure is bad it's not a good thing so why do people say well you need some back pressure well it's kind of misleading and it has to do with exhaust velocity so restrictions create back pressure if you have a super restrictive exhaust that's a really narrow diameter exhaust that's restrictive that's going to have a lot of back pressure it's also going to have a very high velocity because you're forcing all those fumes out to a very tiny channel so they're going to move very quickly if you have a super large exhaust you're not going to have much restriction but consequently you're not going to have much velocity of your exhaust flow so your exhaust is going to flow too slowly to escape out into the atmosphere ideal is somewhere in between so a restriction causes a negative effect which is back pressure but it also causes a positive effect which is high velocity so you want some balance of you know velocity and minimal back pressure somewhere in the middle is that sweet spot where you have good exhaust flow good speed to it you have good scavenging characteristics but however you don't have too much back pressure resisting that flow resisting your exhaust escaping meaning you would be making less power okay so now let's get a bit more complicated and talk about header design and exhaust tube length design so you have your four-cylinder engine right here and here you have the header of course exaggerated with these diameter widths here but regardless the four cylinders they all merge at one point as they come out into these exhaust pipes into a collector where all four then float together through the rest of the exhaust well there's a process here that happens that is ideal for exhaust scavenging so we're going to talk about how this process works so the first thing that happens is you have of course your combustion phase now your pistons at bottom dead center and you need to release those exhaust gases so the exhaust valve opens and immediately a positive pressure wave is created that travels outward at the speed of sound of course behind that following that are the exhaust gases coming out because of that pressure differential so that pressure wave continues to move down your exhaust pipe through your exhaust and then of course your exhaust gas is following behind and once your piston gets about halfway up now you don't have quite as big of a pressure differential between inside the cylinder and outside the cylinder and so the problem with this is is it means your exhaust gases aren't going to want to travel the remaining exhaust gases that are still within that cylinder aren't going to want to travel out very quickly so that's a problem we of course don't want those exhaust gases to remain there because that means our next combustion cycle won't be as effective okay so this pressure wave is traveling along and anytime a pressure wave changes reaches a point in the piping that changes diameter it's going to reflect back a pressure wave so any cross-sectional change will cause this so a collector for example if the piping were to step up or step down and of course as it exits the exhaust at the very end of your tailpipe so anytime there's a cross-sectional change and the amplitude of that reflected wave that's coming back uh is you know proportional to how big or small that step is that change in diameter so if the change in diameter is larger the wave that's going back is going to have a greater effect the amplitude will be increased now a positive pressure wave which is what comes out initially is reflected back if you were to step down so if this collector were to get smaller which you of course wouldn't want to do it would send back a positive pressure wave because it's getting larger it sends back a negative pressure wave and so that negative pressure wave starts traveling back and what you want to happen so that piston all the while is of course still moving upwards what you want to happen is for that negative pressure wave to come back at the exact right time and there's going to be a bit of a window so it doesn't have to be you know one specific rpm but generally this will be a good effect for specific rpm range rather than across the entire rpm range of course based on the tubing length and you know based on diameter things like that which affect how this pressure wave comes back so it's coming back you want to time it so that just before that exhaust valve closes and your intake valve opens that negative pressure wave arrives and helps lower the pressure in there pulling out the remaining exhaust gases so you have basically an entire empty chamber then with that low pressure once your intake valve opens it helps pull in fresh air because you've got a bigger temperature or a bigger pressure differential between the intake and within the cylinder so if you get that pressure really low within the cylinder right before that intake valve opens it helps pull in additional gas and of course that means you have better scavenging better scavenging and then as a result you have more air and fuel that gets pulled in and you can make more power now again this is all dependent on that pipe length of course because it has to do with how fast does that wave travel it reaches the collector creates that reflected wave once it hits the collector and then travels the way back through those exhaust gases so during that process uh you want to time it so that it arrives at the right time and these can help out different cylinders it has doesn't have to necessarily be the exact same cylinder but you get what's going on here where you want that negative pressure wave to arrive help pull out those remaining exhaust gases and then help pull in that fresh intake air now on top of this there's something called inertial scavenging and so if you think about like throwing a ball or a car driving of course behind that car is a low pressure area because it's pushing the air out of the way and the same thing happens with these exhaust gases so when they open up and it sends out that high pressure exhaust pulse that exhaust that air has inertia to it because it's traveling at a very high speed and so as it travels through the pipe it creates a low pressure area behind it so as it's traveling it can start filling up this area with low pressure and that's of course dependent on its velocity and its velocity is also of course dependent on the piping diameter so if it's too wide it'll have a low low velocity and it won't have that very low pressure area behind it if it's too restrictive you have too much back pressure it's not allowing those exhaust gases to escape and that's causing your engine to have to work harder to push out those exhaust gases so you know there's a sweet spot where you have the perfect amount of scavenging from both wave scavenging and from inertial scavenging where this piping diameter and the piping length are both very critical in the performance of your engine for a specific rpm range of course if you were to tune this in correctly if you were to have the wrong length you can hurt the performance of it if you have this wave arrive at the incorrect time it can actually be detrimental and cause you to have even less power than if you didn't do this at all so timing is very important tubing length tubing diameter all very critical in how this system works and how you have that exhaust scavenging so hopefully you know this is a decent uh basic overview of how this all works gives you an idea of why you don't want back pressure but why you do want scavenging and you do want exhaust velocity to help out with that scavenging so thank you all for watching and if you have any questions or comments feel free to leave them below\n"