INTAKE and EXHAUST tech long, very long
03-07-2003
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INTAKE and EXHAUST tech long, very long
I’ll start with exhaust since it’s the easiest to explain. Right after the spark plug fires, the pressure in the combustion chamber increases very rapidly as the fuel starts to burn. The pressure causes the piston to move downward on the power stroke but before the piston reaches bottom dead center (BDC), the exhaust valve starts to open. The combustion pressure does most of the work on the piston within the first 1/3 of the piston’s travel. After that, the piston is moving very rapidly downward and so the expanding combustion gasses do less and less work. The period after the exhaust valve opens is called the blowdown period. Since the exhaust gasses are under pressure still and the exhaust piping is somewhere near atmospheric, the pressure of the exhaust gasses help to evacuate the cylinder. Most of the exhaust gasses leave the cylinder at this point but some still remain when the piston reaches BDC. After this point, the piston has to pump the rest of the gasses out. Pumping these gasses out takes some work which is seen as a pumping loss. As you can see, picking a cam that opens the exhaust valve at BDC means that the combustion gasses do a little more work on the piston, but because there is more exhaust to pump out, the pumping looses can be greater than the gain. The secret to cam timing is picking the right point where the gains are higher than the losses. As you can also see, the earlier the valve opening the more of the combustion happens with the exhaust valve open, so generally exhaust noise will get louder as exhaust cam timing is advanced.
So the exhaust gasses leave the chamber and start their way into the header tube. But they are still at a higher pressure than atmospheric. The gasses are really nothing more than a pressure wave traveling down the tube. The wave travels at the speed of sound but the particles of gas themselves travel at a lower speed. The waves however can be used to help move the exhaust particles out better than would be otherwise possible. Some wave theory is due first. The easiest way to explain it I think would be to use a string tied to a metal pole. If you have a long piece of string and you shake one end up and down quickly, then a wave will travel down the length of the string. If the wave is located on top of the string, we’ll call it positive. On the bottom and we’ll call it negative. It’s no different than the waves of an ocean with peaks and troughs. Peaks are positive. Now when a positive wave reaches the end of the string, it hits the pole and starts to make its way back but still located on top. The reason why is because the metal pole is a denser medium than the string. When a wave traveling though a medium hits a more dense medium, than the same wave will be reflected back. The opposite is also true though. If you were to tie the string to a very thin wire and then tie the wire to the pole, then the results would be different. The positive wave reaches the wire and is reflected back on the bottom or as a negative wave. So when a wave hits a less dense medium, its opposite is reflected back. Well sound waves or compression waves are no different. If you shout at a large rock face, you will get an echo which is nothing more than your sound waves reflecting back. If you were standing in a thick fog though and you shouted something, then as the waves reach the end fog the fog, they can be reflected back and you may get an echo. The air at the end of the fog is a less dense medium than the fog. The human ear can’t detect whether sound waves are positive pressure or negative. Ever heard an echo in a fog? Now you know why.
So a positive pressure exhaust wave is traveling down a header tube. When the wave reaches the collector, the cross sectional area increases. To the wave, the medium is less dense than in the tube, so a negative pressure wave is reflected back toward the combustion chamber (CC). A negative pressure wave is nothing more than a little area of vacuum traveling down the pipe. If this wave catches the exhaust valve open, then it will help pull out some remaining exhaust gasses. If both the intake and exhaust valves are open when the wave reaches the CC (valve overlap period), then it will help scavenge exhaust as well as help pull in fresh mixture. If the fuel maps are set correctly to take advantage, then torque will increase around that rpm. We can decide when the negative wave reaches the CC by varying header length. We know that the wave travels at the speed of sound, we also know the length of the header so we can calculate the rpm at which the negative wave will catch an exhaust valve open. If the negative wave hits a closed exhaust valve though, then it will reflect back toward the collector still negative. The whole way down it will impede exhaust gas flow and when it hits the collector, it will reflect back as a positive wave. The positive wave will again impede gas flow all the way back to the CC. If this positive wave then reaches the open exhaust valve, then it will push exhaust gasses back in. If it hits during the valve overlap period, then it will also push some fresh mixture out the intake valve. This process is called reversion. You may have noticed black carbon in your intake port and manifold on engine teardowns. That carbon is from exhaust gasses pushed out the intake by reversion. The rpm at which the negative wave hits the CC and helps make power is usually called the resonant rpm. At that rpm the waves hit resonance and the engine starts making more power (assuming fuel maps are corrected for this phenomenon). Two stroke guys say that the motor is “on the pipe”. The wave action in the intake and exhaust is one reason that the torque curve in an engine is not perfectly flat. As the various waves hit at different rpm, the torque rises and falls.
The strength of the reflected wave is dependent on the geometry of the collector. If a wave hits the closed end of a pipe, it will reflect back at almost the same strength that it hit with. If the wave reaches a partially closed end (like say you drill a small hole in the closed end of the pipe), it will reflect back at much less strength. The same thing is true with an open end of a pipe. If two pipes merge together to form a collector, then the strength of the reflected wave will be less than if four pipes came together to form a collector. This is because the change is cross sectional area of a 4 into 1 is greater than for a 2 into 1. Looking at a 4 into 2 into1 setup though, it can be seen that two negative waves are reflected back toward although at half the strength of a 4 into 1. The two waves also hit at different rpm. So a 4:2:1 header can be used to build power over a larger rpm range. The 4:1 though can build more power but at a narrower rpm range. Some headers have steps down the pipe where the diameter increases slightly. Every time the exhaust wave hits one of these, a tiny negative wave is reflected back. The purpose of these is to broaden the power band slightly by sending back negative waves that will catch the valve open at different rpm. When the main wave reaches the collector, the negative wave reflected back will be somewhat weaker though because all of the little waves that were reflected using the steps. That’s how it goes, a wave has only some much energy in it. You can use that energy all at once, reflecting one strong wave back a many weaker waves. If you can picture a header pipe with all these waves traveling back and forth, then you may also have noticed that the longer the primary pipe, the lower the resonant rpm of the engine. The length of pipe between the head and the collector is called the primary. The pipe between the first collector and the second collector is called the secondary. Obviously 4: and 4:2:1 collectors don't apply to V-6s but it is an important note I think should be mentioned. If you own a four cylinder sportbike, then it applies.
There is one more factor in exhaust design and that is pipe diameter. You may have noticed that large diameter pipes hurt low to midrange power while helping top end. Well, the reason is not because the larger pipe has less backpressure and backpressure helps low end power. Remember this as a cardinal law: Backpressure is always bad. It has to do with exhaust velocity. Forget about waves for a minute and just concentrate on gas flowing through the exhaust pipe. The gas can be modeled as a mass moving though the pipe at high velocity. When the exhaust valve closes behind the mass, a small vacuum is formed. The mass has inertia so it keeps moving. It is the same as if the mass were moving toward an open end and suddenly you close the open end. The mass will compress the air between itself and the now closed end as it slows down. The pressure in front of the mass will increase. The same thing happens when the exhaust valve closes, but because the closed end is behind the mass a vacuum will form. So there is a small vacuum in the exhaust port and next time the valve opens, the vacuum will help to pull exhaust out. Now the smaller the diameter of the pipe, the faster the mass moves though it. The faster the mass moves, the higher the vacuum created when the exhaust valve closes. So to make good power, we want to keep exhaust velocity high. But there is a price to be paid if the pipe is made too small. The smaller the pipe, the higher the backpressure. Backpressure is a resistance to flow. So a small diameter pipe might make good power at lower rpm because velocity is high. As the rpm climb though, there is more and more backpressure. A very large pipe has very little backpressure, so there is little flow restriction at high rpm, but the lower exhaust velocity will hurt low to mid range power. So now you know, choosing the correct pipe diameter is all about exhaust velocity. If redline is increased, or more exhaust gasses are created though better engine breathing, then a larger diameter pipe might be necessary. Exhaust tuning is a combination of wave action and overall gas velocity.
So the exhaust gasses leave the chamber and start their way into the header tube. But they are still at a higher pressure than atmospheric. The gasses are really nothing more than a pressure wave traveling down the tube. The wave travels at the speed of sound but the particles of gas themselves travel at a lower speed. The waves however can be used to help move the exhaust particles out better than would be otherwise possible. Some wave theory is due first. The easiest way to explain it I think would be to use a string tied to a metal pole. If you have a long piece of string and you shake one end up and down quickly, then a wave will travel down the length of the string. If the wave is located on top of the string, we’ll call it positive. On the bottom and we’ll call it negative. It’s no different than the waves of an ocean with peaks and troughs. Peaks are positive. Now when a positive wave reaches the end of the string, it hits the pole and starts to make its way back but still located on top. The reason why is because the metal pole is a denser medium than the string. When a wave traveling though a medium hits a more dense medium, than the same wave will be reflected back. The opposite is also true though. If you were to tie the string to a very thin wire and then tie the wire to the pole, then the results would be different. The positive wave reaches the wire and is reflected back on the bottom or as a negative wave. So when a wave hits a less dense medium, its opposite is reflected back. Well sound waves or compression waves are no different. If you shout at a large rock face, you will get an echo which is nothing more than your sound waves reflecting back. If you were standing in a thick fog though and you shouted something, then as the waves reach the end fog the fog, they can be reflected back and you may get an echo. The air at the end of the fog is a less dense medium than the fog. The human ear can’t detect whether sound waves are positive pressure or negative. Ever heard an echo in a fog? Now you know why.
So a positive pressure exhaust wave is traveling down a header tube. When the wave reaches the collector, the cross sectional area increases. To the wave, the medium is less dense than in the tube, so a negative pressure wave is reflected back toward the combustion chamber (CC). A negative pressure wave is nothing more than a little area of vacuum traveling down the pipe. If this wave catches the exhaust valve open, then it will help pull out some remaining exhaust gasses. If both the intake and exhaust valves are open when the wave reaches the CC (valve overlap period), then it will help scavenge exhaust as well as help pull in fresh mixture. If the fuel maps are set correctly to take advantage, then torque will increase around that rpm. We can decide when the negative wave reaches the CC by varying header length. We know that the wave travels at the speed of sound, we also know the length of the header so we can calculate the rpm at which the negative wave will catch an exhaust valve open. If the negative wave hits a closed exhaust valve though, then it will reflect back toward the collector still negative. The whole way down it will impede exhaust gas flow and when it hits the collector, it will reflect back as a positive wave. The positive wave will again impede gas flow all the way back to the CC. If this positive wave then reaches the open exhaust valve, then it will push exhaust gasses back in. If it hits during the valve overlap period, then it will also push some fresh mixture out the intake valve. This process is called reversion. You may have noticed black carbon in your intake port and manifold on engine teardowns. That carbon is from exhaust gasses pushed out the intake by reversion. The rpm at which the negative wave hits the CC and helps make power is usually called the resonant rpm. At that rpm the waves hit resonance and the engine starts making more power (assuming fuel maps are corrected for this phenomenon). Two stroke guys say that the motor is “on the pipe”. The wave action in the intake and exhaust is one reason that the torque curve in an engine is not perfectly flat. As the various waves hit at different rpm, the torque rises and falls.
The strength of the reflected wave is dependent on the geometry of the collector. If a wave hits the closed end of a pipe, it will reflect back at almost the same strength that it hit with. If the wave reaches a partially closed end (like say you drill a small hole in the closed end of the pipe), it will reflect back at much less strength. The same thing is true with an open end of a pipe. If two pipes merge together to form a collector, then the strength of the reflected wave will be less than if four pipes came together to form a collector. This is because the change is cross sectional area of a 4 into 1 is greater than for a 2 into 1. Looking at a 4 into 2 into1 setup though, it can be seen that two negative waves are reflected back toward although at half the strength of a 4 into 1. The two waves also hit at different rpm. So a 4:2:1 header can be used to build power over a larger rpm range. The 4:1 though can build more power but at a narrower rpm range. Some headers have steps down the pipe where the diameter increases slightly. Every time the exhaust wave hits one of these, a tiny negative wave is reflected back. The purpose of these is to broaden the power band slightly by sending back negative waves that will catch the valve open at different rpm. When the main wave reaches the collector, the negative wave reflected back will be somewhat weaker though because all of the little waves that were reflected using the steps. That’s how it goes, a wave has only some much energy in it. You can use that energy all at once, reflecting one strong wave back a many weaker waves. If you can picture a header pipe with all these waves traveling back and forth, then you may also have noticed that the longer the primary pipe, the lower the resonant rpm of the engine. The length of pipe between the head and the collector is called the primary. The pipe between the first collector and the second collector is called the secondary. Obviously 4: and 4:2:1 collectors don't apply to V-6s but it is an important note I think should be mentioned. If you own a four cylinder sportbike, then it applies.
There is one more factor in exhaust design and that is pipe diameter. You may have noticed that large diameter pipes hurt low to midrange power while helping top end. Well, the reason is not because the larger pipe has less backpressure and backpressure helps low end power. Remember this as a cardinal law: Backpressure is always bad. It has to do with exhaust velocity. Forget about waves for a minute and just concentrate on gas flowing through the exhaust pipe. The gas can be modeled as a mass moving though the pipe at high velocity. When the exhaust valve closes behind the mass, a small vacuum is formed. The mass has inertia so it keeps moving. It is the same as if the mass were moving toward an open end and suddenly you close the open end. The mass will compress the air between itself and the now closed end as it slows down. The pressure in front of the mass will increase. The same thing happens when the exhaust valve closes, but because the closed end is behind the mass a vacuum will form. So there is a small vacuum in the exhaust port and next time the valve opens, the vacuum will help to pull exhaust out. Now the smaller the diameter of the pipe, the faster the mass moves though it. The faster the mass moves, the higher the vacuum created when the exhaust valve closes. So to make good power, we want to keep exhaust velocity high. But there is a price to be paid if the pipe is made too small. The smaller the pipe, the higher the backpressure. Backpressure is a resistance to flow. So a small diameter pipe might make good power at lower rpm because velocity is high. As the rpm climb though, there is more and more backpressure. A very large pipe has very little backpressure, so there is little flow restriction at high rpm, but the lower exhaust velocity will hurt low to mid range power. So now you know, choosing the correct pipe diameter is all about exhaust velocity. If redline is increased, or more exhaust gasses are created though better engine breathing, then a larger diameter pipe might be necessary. Exhaust tuning is a combination of wave action and overall gas velocity.
03-07-2003
#2
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The same principles apply to intake tuning. By intake I mean the manifold and CAI together. Intake velocity plays a large part in making power. As rpm rise, intake velocity increases. This should be obvious, as the same amount of air needs to fill the same amount of space in a shorter time. So velocity must increase in order to deliver the same amount of air. Again the air can be modeled as a mass traveling down a pipe. The mass starts entering the combustion chamber as the piston moves downward. But a smart engine designer will know to keep the intake valve open even as the piston passes BDC and starts coming up. The designer knows that the intake charge has a lot of velocity (inertia) and he knows that the mass isn’t going to stop moving just because the piston starts coming up. At very high rpm, the velocity is extremely high and so we can keep the intake valve open well after the piston crosses BDC. The charge will continue filling the cylinder, possible even slightly supercharging the cylinder. This phenomenon is called inertial supercharging. At low rpm though, intake velocity is low. Because the valve is open as the piston is coming back up, then some of the mixture will be blown back out of that intake valve. In the bad old days of carburetors, this caused big problems. As the mixture was initially drawn past the carb, fuel was added in the correct ratio. But at low rpm, the mixture could be blown out and past the carb again, where more fuel was added, then the mixture got sucked past the carb one more time and even more fuel was added. In those days, big cams caused terrible idle and drivability problems because of over rich mixtures. If the carb was leaned out though, then it would be too lean at high rpm. Fortunately, fuel injection solved that problem and now you can have an engine with big cams and decent drivability.
Wave action plays a large role in the intake as well. As the high velocity charge reaches the cylinder, it basically slams into a wall the same as an exhaust waves hits the closed end of a pipe. A positive pressure wave gets reflected back toward the plenum. The plenum being a lower density medium, will cause a pressure wave to now be reflected back toward the intake valve. If this wave hits while the valve is open, then it will help push more charge into the cylinder. If the wave hits a closed valve, then the positive wave will reflect back toward the plenum. When it hits the plenum, a negative wave will be reflected toward the valve. If this wave arrives while the valve is open, then it will impede flow into the cylinder. Since we know that the waves travel at the speed of sound, and we know the length of the intake runner, we can calculate at which rpm the positive pressure wave will reach an open intake valve. By now you should see that wave tuning is not free. You can gain at a certain rpm but you will lose at another. Intake and exhaust tuning can be used together depending on what you want. You can use the correct lengths so that both intake and exhaust hit resonance at the same rpm. You will make nice power but it will be peaky. Or you can use the exhaust to make power at a certain rpm and use the intake to cover the torque dip from the exhaust at another rpm. One will give you a broad power band, while the other will give you a more powerful but narrower power band. Nothing is free. There is always a price to be paid for making power. Usually that price comes from having to give up power at a particular rpm to gain power at another.
quoted
The same principles apply to intake tuning. By intake I mean the manifold and CAI together. Intake velocity plays a large part in making power. As rpm rise, intake velocity increases. This should be obvious, as the same amount of air needs to fill the same amount of space in a shorter time. So velocity must increase in order to deliver the same amount of air. Again the air can be modeled as a mass traveling down a pipe. The mass starts entering the combustion chamber as the piston moves downward. But a smart engine designer will know to keep the intake valve open even as the piston passes BDC and starts coming up. The designer knows that the intake charge has a lot of velocity (inertia) and he knows that the mass isn’t going to stop moving just because the piston starts coming up. At very high rpm, the velocity is extremely high and so we can keep the intake valve open well after the piston crosses BDC. The charge will continue filling the cylinder, possible even slightly supercharging the cylinder. This phenomenon is called inertial supercharging. At low rpm though, intake velocity is low. Because the valve is open as the piston is coming back up, then some of the mixture will be blown back out of that intake valve. In the bad old days of carburetors, this caused big problems. As the mixture was initially drawn past the carb, fuel was added in the correct ratio. But at low rpm, the mixture could be blown out and past the carb again, where more fuel was added, then the mixture got sucked past the carb one more time and even more fuel was added. In those days, big cams caused terrible idle and drivability problems because of over rich mixtures. If the carb was leaned out though, then it would be too lean at high rpm. Fortunately, fuel injection solved that problem and now you can have an engine with big cams and decent drivability.
Wave action plays a large role in the intake as well. As the high velocity charge reaches the cylinder, it basically slams into a wall the same as an exhaust waves hits the closed end of a pipe. A positive pressure wave gets reflected back toward the plenum. The plenum being a lower density medium, will cause a pressure wave to now be reflected back toward the intake valve. If this wave hits while the valve is open, then it will help push more charge into the cylinder. If the wave hits a closed valve, then the positive wave will reflect back toward the plenum. When it hits the plenum, a negative wave will be reflected toward the valve. If this wave arrives while the valve is open, then it will impede flow into the cylinder. Since we know that the waves travel at the speed of sound, and we know the length of the intake runner, we can calculate at which rpm the positive pressure wave will reach an open intake valve. By now you should see that wave tuning is not free. You can gain at a certain rpm but you will lose at another. Intake and exhaust tuning can be used together depending on what you want. You can use the correct lengths so that both intake and exhaust hit resonance at the same rpm. You will make nice power but it will be peaky. Or you can use the exhaust to make power at a certain rpm and use the intake to cover the torque dip from the exhaust at another rpm. One will give you a broad power band, while the other will give you a more powerful but narrower power band. Nothing is free. There is always a price to be paid for making power. Usually that price comes from having to give up power at a particular rpm to gain power at another.
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03-07-2003
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wow,,very nice post,,,answered a few questions ive had for a long time,,,by the way,,my friend had a 93 probe till he wrecked it,,but its theonly 11 second car ive been in
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