For any of you out there who may have a very good foundation in acoustics, have you ever considered an exhaust nozzle, up close to the cylinder and dumping out the side of the cowling, that would attempt to match the acoustic impedance rho-V of the exhaust gases to the outside air such that the exhaust would flow un-impeded from the cylinder. C.F.Taylor shows that individual exhaust pipes can increase the volumeetric efficiency 5%-7%. That's like getting 7-11 more HP out of an O-320!
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Acoustic impedance-matched exhaust nozzle
- Thread starter elippse
- Start date
John Clark
Well Known Member
Art and Science
I do not have a background in acoustics, but I do have 50 years experience trying to make horsepower from piston engines. Exhaust design is a balance between science, black art, and practicality. Remember, for instance, that finding a big flow increase on the exhaust side can cause issues in the induction side. The conventional view is that short exhausts may flow better and make a bit more horsepower but tend to lower torque output. Optimizing a piston engine always involves a balance between all the factors. There is some good software available that you might want to consider. here is one that I came across, but have not tried:
http://www.buildandclick.com/html/4t_header_wizard_.html
John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
I do not have a background in acoustics, but I do have 50 years experience trying to make horsepower from piston engines. Exhaust design is a balance between science, black art, and practicality. Remember, for instance, that finding a big flow increase on the exhaust side can cause issues in the induction side. The conventional view is that short exhausts may flow better and make a bit more horsepower but tend to lower torque output. Optimizing a piston engine always involves a balance between all the factors. There is some good software available that you might want to consider. here is one that I came across, but have not tried:
http://www.buildandclick.com/html/4t_header_wizard_.html
John Clark ATP, CFI
FAAST Team Representative
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
Can you hear me now?
"...that would attempt to match the acoustic impedance rho-V of the exhaust gases"
Said "nozzle" for acoustic matching is commonly called a megaphone. Given the proper length and divergence angle, it can be quite effective in preventing reflections back up the pipe, thus maximizing overall flow. Problem is, it takes a lot of megaphone to match up a 360 cube engines exhaust. Where do you put it?
As well, in highly tuned engines the exhaust reflection (and resultant acoustic standing wave) is depended upon to help pull in the fresh charge and scavenge the last of the exhaust during the exhaust - intake valve opening overlap. It only works well for a fraction of the rpm range, but it works. The engine designer is turning lemons into lemonade, and needs his standing wave to do it.
"...that would attempt to match the acoustic impedance rho-V of the exhaust gases"
Said "nozzle" for acoustic matching is commonly called a megaphone. Given the proper length and divergence angle, it can be quite effective in preventing reflections back up the pipe, thus maximizing overall flow. Problem is, it takes a lot of megaphone to match up a 360 cube engines exhaust. Where do you put it?
As well, in highly tuned engines the exhaust reflection (and resultant acoustic standing wave) is depended upon to help pull in the fresh charge and scavenge the last of the exhaust during the exhaust - intake valve opening overlap. It only works well for a fraction of the rpm range, but it works. The engine designer is turning lemons into lemonade, and needs his standing wave to do it.
"...that would attempt to match the acoustic impedance rho-V of the exhaust gases"
Said "nozzle" for acoustic matching is commonly called a megaphone. Given the proper length and divergence angle, it can be quite effective in preventing reflections back up the pipe, thus maximizing overall flow. Problem is, it takes a lot of megaphone to match up a 360 cube engines exhaust. Where do you put it?
As well, in highly tuned engines the exhaust reflection (and resultant acoustic standing wave) is depended upon to help pull in the fresh charge and scavenge the last of the exhaust during the exhaust - intake valve opening overlap. It only works well for a fraction of the rpm range, but it works. The engine designer is turning lemons into lemonade, and needs his standing wave to do it.
Basically what I'm suggesting is a type of megaphone with an outet aperture of about 7"X1.5" and a length of 6"-7".
The resonant system sounds good on paper, but C.F.Taylor dismisses it. It takes a pipe 76" long to provide a low pressure near the end of the exhaust stroke to give the extra filling during the short intake-exhaust overlap at 2700 rpm. I'm not suggesting four or six separate 76" pipes. A short, individual nozzle sort of like a logarithmic speaker dumping the exhaust right out the side of the cowling, through a hole that would also dump the cooling air over board, might just be the trick. You seem to possess knowledge in this area. Why don't you try your hand at coming up with a megaphone with reasonable dimensions. It doesn't have to be perfect; even 10%-20% of perfection would be good! Affter all, the 360ci aren't dumped all at once but over a period of time.
Rocket nozzle
As you rocket scientists know, whenever there is at least a 2:1 pressure ratio across a nozzle the flow through the nozzle is supersonic. Rocket engines use supersonic expansion through an expansion nozzle to lower the pressure and increase the velocity.
Yes, that's right! Supersonic flow in a nozzle with increasing cross-section drops the pressure and increases the velocity. If you've ever passed a car on a hill that had an incredibly noisy exhaust with a lot of hiss, you were experiencing a car whose exhaust pipe was pinched, keeping the exhaust pressure higher on the cylinder-side of the pinch. When you operate your engine at less than half atmospheric pressure, such as at idle at less than 14" MAP at sea level, initially when the intake valve opens there will be a short period when the air in the cylinder, exhausted to atmospheric pressure, will flow supersonically into the intake manifold. Just think of what this effect is when descending power-off when your MAP is a bout 4"-7"! I've seen it on an oscilloscope when testing an ignition system on a plane. 'Bet that is a new one on some of you gear-heads!
On the Atlas Sustainer engine the chamber pressure was about 500 psi and and was expanded to atmospheric pressure at some optimum altitude. The exhaust velocity increased to about 5000 fps.
By the same token, a nozzle could be designed to take the high pressure exhaust gases in the cylinder and expand their velocity and drop their pressure at the nozzle aperture. Since the cylinder pressure is dropping during the exhaust, the expansion ratio would only be optimum at some point in the process, but there might still be energy that can be extracted. How about one or more of you rocket scientists putting your heads to together to see if something like this is practical. Please, let's not waste all of that exhaust energy.
As you rocket scientists know, whenever there is at least a 2:1 pressure ratio across a nozzle the flow through the nozzle is supersonic. Rocket engines use supersonic expansion through an expansion nozzle to lower the pressure and increase the velocity.
Yes, that's right! Supersonic flow in a nozzle with increasing cross-section drops the pressure and increases the velocity. If you've ever passed a car on a hill that had an incredibly noisy exhaust with a lot of hiss, you were experiencing a car whose exhaust pipe was pinched, keeping the exhaust pressure higher on the cylinder-side of the pinch. When you operate your engine at less than half atmospheric pressure, such as at idle at less than 14" MAP at sea level, initially when the intake valve opens there will be a short period when the air in the cylinder, exhausted to atmospheric pressure, will flow supersonically into the intake manifold. Just think of what this effect is when descending power-off when your MAP is a bout 4"-7"! I've seen it on an oscilloscope when testing an ignition system on a plane. 'Bet that is a new one on some of you gear-heads!
On the Atlas Sustainer engine the chamber pressure was about 500 psi and and was expanded to atmospheric pressure at some optimum altitude. The exhaust velocity increased to about 5000 fps.
By the same token, a nozzle could be designed to take the high pressure exhaust gases in the cylinder and expand their velocity and drop their pressure at the nozzle aperture. Since the cylinder pressure is dropping during the exhaust, the expansion ratio would only be optimum at some point in the process, but there might still be energy that can be extracted. How about one or more of you rocket scientists putting your heads to together to see if something like this is practical. Please, let's not waste all of that exhaust energy.
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