Why is rotary exhaust so hot?
12-30-05, 10:04 AM
#51
Piston travel speed is not constant through its stroke, the piston "parks" briefly at TDC and BDC, and it travels at its fastest speed at the middle of the stroke. This provides significant leverage to the ignited mixture to force the piston down for the longest time while the combustion chamber volume expands ever so slowly (at the same speed as the piston is traveling). Racing engine builders take maximum advantage of this phenomenon by using the longest con rods they can fit, as the longer the con rods are the longer the piston will remain “parked” at TDC and BDC, improving the engine’s efficiency. A Wankel rotor, on the other hand, keeps a constant movement speed through its elliptical phase, so it doesn’t count on this phenomenon to improve efficiency.
So a piston engine’s combustion chamber allows for the igniting mixture more time to completely burn, and its expansion is restricted for a longer time so that the pressure force of this expansion coming from the heat of the ignition event is better converted into motive power and thus cooling down the now completely burnt gases. A Wankel engine’s combustion chamber is always expanding at a constant pace, so the igniting mixture doesn’t get burnt as well as on a piston engine’s in this regard. Add to this the fact that a Wankel’s combustion chamber’s shape in itself is far from optimum (the optimum will be a sphere that expands in all directions after combustion, but since this is impossible to make, a piston engine’s cylindrical combustion chamber is the closest to optimum achievable) plus its high area to volume ratio given its “moving chamber” characteristics and you end up with a very inefficient way of converting a flammable air/fuel mixture into motive power. All these inefficiencies combined are good reasons for the high exhaust temps coming out of a Wankel.
Another inherent reason for the hot exhaust temps is the fact that you get a huge, fully open, unrestricted (at least on a peripheral type) and thus very free flowing exhaust port in less “crank angle time” than on a poppet valve piston engine.
So a piston engine’s combustion chamber allows for the igniting mixture more time to completely burn, and its expansion is restricted for a longer time so that the pressure force of this expansion coming from the heat of the ignition event is better converted into motive power and thus cooling down the now completely burnt gases. A Wankel engine’s combustion chamber is always expanding at a constant pace, so the igniting mixture doesn’t get burnt as well as on a piston engine’s in this regard. Add to this the fact that a Wankel’s combustion chamber’s shape in itself is far from optimum (the optimum will be a sphere that expands in all directions after combustion, but since this is impossible to make, a piston engine’s cylindrical combustion chamber is the closest to optimum achievable) plus its high area to volume ratio given its “moving chamber” characteristics and you end up with a very inefficient way of converting a flammable air/fuel mixture into motive power. All these inefficiencies combined are good reasons for the high exhaust temps coming out of a Wankel.
Another inherent reason for the hot exhaust temps is the fact that you get a huge, fully open, unrestricted (at least on a peripheral type) and thus very free flowing exhaust port in less “crank angle time” than on a poppet valve piston engine.
Last edited by EFINI_RX-7_RZ; 12-30-05 at 10:07 AM.
12-30-05, 01:43 PM
#52
Rotary Enthusiast
Before anyone else pontificates with certainty on how a rotary works, they must read and study the writings of Mr Yamamoto.
http://www.rotaryrefs.net/rotaryrefs...-KYamamoto.pdf
From EFINI:
1 "the piston "parks" briefly at TDC and BDC"
Nope1. The piston acceleration is at it's (absolute) maximum at TDC, and near max at BDC. Long arms will make it move more slowly through these positions, but it never parks, with zero acceleration. There is an instant at zero velocity, just as there is an instant at 4K rpm if you go wot with no load and rev the engine to redline. (I do think long arms increase peak piston speeds).
2 "A Wankel rotor, on the other hand, keeps a constant movement speed through its elliptical phase, so it doesn’t count on this phenomenon to improve efficiency."..."A Wankel engine’s combustion chamber is always expanding at a constant pace, so the igniting mixture doesn’t get burnt as well as on a piston engine’s in this regard"
Nope2. Expansion for both engines is described by similar sine/cosine curves. The wankel volume change slows at BDC and TDC, just like a piston engine. In fact, the volume change curve for a wankel chamber will be the same, over the displacement cycle, as a piston of same displacement, with very long rods.
3 "So a piston engine’s combustion chamber allows for the igniting mixture more time to completely burn"
Nope3. Already noted that wankel would be similar to piston engine with extreme long rods, regarding slowing down at TDC, based on an engine stroke or cycle. Time is a loaded variable for comparison. At same crank speed , rotary power cycles take 50% more time. Both these facts gives the rotary more time for completing combustion at say 10% of the power stroke (or cycle).
4 "... plus its high area to volume ratio ..... All these inefficiencies combined are good reasons for the high exhaust temps coming out of a Wankel."
Nope4. The high area/vol ratio results in more heat loss to the cooling system during the power and exhaust cycles, lowering exh gas temperatures.
5 "Another inherent reason for the hot exhaust temps is the fact that you get a huge, fully open, unrestricted (at least on a peripheral type) and thus very free flowing exhaust port in less “crank angle time” than on a poppet valve piston engine."
Yup5. Can't comment on crank angle factor now. But can add that a wankel port is shared among 3 chambers, and as such will be flowing exh gas about 75% of the time vs 25% for a boinger exh port. So in any minute, a boinger port will be flowing exh gas for 15 seconds, while a wankel exh port will be flowing for 45 out of the 60 seconds. That would suggest less heat loss during travel through a hotter port.
http://www.rotaryrefs.net/rotaryrefs...-KYamamoto.pdf
From EFINI:
1 "the piston "parks" briefly at TDC and BDC"
Nope1. The piston acceleration is at it's (absolute) maximum at TDC, and near max at BDC. Long arms will make it move more slowly through these positions, but it never parks, with zero acceleration. There is an instant at zero velocity, just as there is an instant at 4K rpm if you go wot with no load and rev the engine to redline. (I do think long arms increase peak piston speeds).
2 "A Wankel rotor, on the other hand, keeps a constant movement speed through its elliptical phase, so it doesn’t count on this phenomenon to improve efficiency."..."A Wankel engine’s combustion chamber is always expanding at a constant pace, so the igniting mixture doesn’t get burnt as well as on a piston engine’s in this regard"
Nope2. Expansion for both engines is described by similar sine/cosine curves. The wankel volume change slows at BDC and TDC, just like a piston engine. In fact, the volume change curve for a wankel chamber will be the same, over the displacement cycle, as a piston of same displacement, with very long rods.
3 "So a piston engine’s combustion chamber allows for the igniting mixture more time to completely burn"
Nope3. Already noted that wankel would be similar to piston engine with extreme long rods, regarding slowing down at TDC, based on an engine stroke or cycle. Time is a loaded variable for comparison. At same crank speed , rotary power cycles take 50% more time. Both these facts gives the rotary more time for completing combustion at say 10% of the power stroke (or cycle).
4 "... plus its high area to volume ratio ..... All these inefficiencies combined are good reasons for the high exhaust temps coming out of a Wankel."
Nope4. The high area/vol ratio results in more heat loss to the cooling system during the power and exhaust cycles, lowering exh gas temperatures.
5 "Another inherent reason for the hot exhaust temps is the fact that you get a huge, fully open, unrestricted (at least on a peripheral type) and thus very free flowing exhaust port in less “crank angle time” than on a poppet valve piston engine."
Yup5. Can't comment on crank angle factor now. But can add that a wankel port is shared among 3 chambers, and as such will be flowing exh gas about 75% of the time vs 25% for a boinger exh port. So in any minute, a boinger port will be flowing exh gas for 15 seconds, while a wankel exh port will be flowing for 45 out of the 60 seconds. That would suggest less heat loss during travel through a hotter port.
12-30-05, 03:39 PM
#53
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I am sure that by "parks" he means the same thing that you stated-- it stops. My car seems to stop when it is parked. It is also true that a longer rod allows the piston to remain at TDC longer or stays within a set parameter of degrees of TDC as measured at the crankshaft.
The exhaust is hot for several reasons--- check post #13
Actually the combustion gases are no hotter in a Wankel than they are in a boinger; assuming they are stoichiometrically correct. Fuel at the correct AFR all burns at the same temp all factors being equal. The heat is derived from the amount and type of fuel burned----- BTU's-----heat. One way it is rated is in BTU's per pound.
The exhaust is hot for several reasons--- check post #13
Actually the combustion gases are no hotter in a Wankel than they are in a boinger; assuming they are stoichiometrically correct. Fuel at the correct AFR all burns at the same temp all factors being equal. The heat is derived from the amount and type of fuel burned----- BTU's-----heat. One way it is rated is in BTU's per pound.
12-30-05, 04:55 PM
#54
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Car parked ... not the same. While you are sitting there, you are not at a high g load. Long rods ... sounds like we agree.
If high heat is just due to the unobstructed port, I would think egt's would be much lower on an RX8, vs a na 13B, at same power?
So you think 3x the mass flow rate through the port (vs boinger) is not related?
If high heat is just due to the unobstructed port, I would think egt's would be much lower on an RX8, vs a na 13B, at same power?
So you think 3x the mass flow rate through the port (vs boinger) is not related?
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12-30-05, 07:04 PM
#55
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Parked---- STOP. What has high G loads have to do with the fact that he was only saying that the piston dwells at TDC longer? A piston which remains at TDC longer allows the pressure to build and results in a greater thrust- force applied to the piston. A greater impulse force if you will. It is analogous to hitting a golf ball. Do golfers simply swing to the point of impact or are they taught to "follow through"? They are taught to follow through same in baseball and other stick and ball sports. The follow through allows the club (force provider--ie combustion) to stay in contact for a fraction of a second longer and transfer more of it's energy to the ball(piston).
I never said that it was the ONLY reason why the exhaust was hot. The new side exhaust rotary engines don't have valves! It is still an unobstructed port compared to a piston engine.
I never said that it was the ONLY reason why the exhaust was hot. The new side exhaust rotary engines don't have valves! It is still an unobstructed port compared to a piston engine.
12-30-05, 07:36 PM
#56
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Golf?
My only point is a parked car analogy is not correct and misleading, as it implies duration with no velocity, which is not true for a 13B or a boinger. Basic kinematics. You can look at varied time near TDC, as you noted at one point, but "at TDC" is an instantaneous event.
Agree the exh valve will have a significant cooling effect.
My only point is a parked car analogy is not correct and misleading, as it implies duration with no velocity, which is not true for a 13B or a boinger. Basic kinematics. You can look at varied time near TDC, as you noted at one point, but "at TDC" is an instantaneous event.
Agree the exh valve will have a significant cooling effect.
12-30-05, 08:06 PM
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Parked --- stopped. What don't you get? Who said that it stayed that way? You said yourself that the piston STOPPED at TDC. The analogy is correct. What do you do when you park your car? You stop. That is all that was meant. If you want to talk about piston speeds, or piston forces we can do that to but it has nothing to do with the Wankel's exhaust temp. It only provides some means of comparison.
There is nothing wrong with Efini's post in reference to the piston at TDC, He only overstated that the rotary engine's combustion expands at a constant pace----Which it does from the standpoint of engine rpm but not from a volumetric stand point. I understand his statements and also understand how he meant them.
What part of the golf example needs more explanation?
Who wants more reasons why it is "hotter"?
There is nothing wrong with Efini's post in reference to the piston at TDC, He only overstated that the rotary engine's combustion expands at a constant pace----Which it does from the standpoint of engine rpm but not from a volumetric stand point. I understand his statements and also understand how he meant them.
What part of the golf example needs more explanation?
Who wants more reasons why it is "hotter"?
12-30-05, 08:32 PM
#58
Originally Posted by KevinK2
Before anyone else pontificates with certainty on how a rotary works, they must read and study the writings of Mr Yamamoto.
http://www.rotaryrefs.net/rotaryrefs...-KYamamoto.pdf
http://www.rotaryrefs.net/rotaryrefs...-KYamamoto.pdf
BTW, I've been trying to get a hold of Mr. Yamamoto's essay for quite some time, thanks for pointing me to it.My views were based from simple observation on the movement of the rotor through its ellipse: as I saw that its apex traveled through the peritrochoid at a constant speed, I thought that the chamber volumes would also change at a constant speed as well. Thanks for clarifying this for me.
But the other 2 points remain valid: the Wankel’s far from optimum combustion chamber shape and constantly moving characteristic that creates its high area-to-volume ratio are the basics for its inherent inefficiencies.
12-31-05, 12:42 AM
#60
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sccagt3:
Ok, here is a roughly valid "parked" car analogy for you. Slam on the brakes from 60 mph, and just before you "stop", slam it in reverse at wot. You will soon reach a point/instant where forward velocity is zero and you immediatly accelerate in reverse.
"My car seems to stop when it is parked" Hope you can see the diff, otherwise let it go.
I did not bring up TDC activity, I responed to others.
Tiger and Anika have great follow throughs, but I have never heard it linked the to ignition initiation specifics of the power stroke in the tech/sae papers I read. The goal it have the spark advanced sufficiently to result in a pressure peak about 10-15 deg atdc, closer to the point of optimum torqe leverage. A peak at TDC would waste heat (and drop pressure) to all metal boundaries, as it can not create torque there, wankel or recip. The pressure history will determine how effectively torque is created. Mabe that's like a good chip shot from off the green...
I'd like to hear your opinion on the effect of a common exhaust port with high duty cycle, vs boinger, as I asked before. And of course any more reasons the gas is hotter. Righ now I agree with your exhaust valve cooling.
I answered EFINI's post and he recently replied, obviously capable of speaking for himself.
EFINI:
glad the link helped. It also shows that apex seal velocity is a sine curve with a high mean value and low cyclic content, vs piston velocity.
The area thing clearly dumps more usable heat (and thus lowers pressure) to the walls, and yes that would explain high BSFC, or poor thermal efficiency, but not hotter exhaust as you had stated earlier.
Ok, here is a roughly valid "parked" car analogy for you. Slam on the brakes from 60 mph, and just before you "stop", slam it in reverse at wot. You will soon reach a point/instant where forward velocity is zero and you immediatly accelerate in reverse.
"My car seems to stop when it is parked" Hope you can see the diff, otherwise let it go.
I did not bring up TDC activity, I responed to others.
Tiger and Anika have great follow throughs, but I have never heard it linked the to ignition initiation specifics of the power stroke in the tech/sae papers I read. The goal it have the spark advanced sufficiently to result in a pressure peak about 10-15 deg atdc, closer to the point of optimum torqe leverage. A peak at TDC would waste heat (and drop pressure) to all metal boundaries, as it can not create torque there, wankel or recip. The pressure history will determine how effectively torque is created. Mabe that's like a good chip shot from off the green...
I'd like to hear your opinion on the effect of a common exhaust port with high duty cycle, vs boinger, as I asked before. And of course any more reasons the gas is hotter. Righ now I agree with your exhaust valve cooling.
I answered EFINI's post and he recently replied, obviously capable of speaking for himself.
EFINI:
glad the link helped. It also shows that apex seal velocity is a sine curve with a high mean value and low cyclic content, vs piston velocity.
The area thing clearly dumps more usable heat (and thus lowers pressure) to the walls, and yes that would explain high BSFC, or poor thermal efficiency, but not hotter exhaust as you had stated earlier.
12-31-05, 02:59 AM
#61
Yep, from further reading Mr. Yamamoto's I dig the rotor's apex doesn't travel at a constant speed, something that from mere casual watching of a model running can't quite come up as the variation is ever so slight compared to its piston analogy, in which a piston reaches a speed of 0 at TDC and BDC. "Park" was my choice of wording, it is not very much unlike "stop" in a figurative meaning, so as long as everybody understands what I meant it's just fine then, I guess. In the meantime, I've got Mr. Yamamoto's fine essay to read and digest, so I'll be busy doing this for the time being. Until then, I'll just stop and park...
12-31-05, 06:34 AM
#62
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Does anyone have the math skills to plot the increase in area from ignition event to when the leading apex seal reaches the exhaust port, over time (in degrees) vs that of a piston engine? This would be some serious calculus though heh.
Also, bare in mind the speed of the piston does not ever reach "zero", it asymtopically slows to zero and then goes in the other way, just like a graph of y=x².
Regardless, even with the heat loss, the data tell us that its either not completely using the energy of the charge to do work to move the cylinder, or its still burning when the exhaust port is open.
Ive been up to long to remember this but I know there are certain things you can do to the timing of a piston engine to increase EGTs... could the same effective thing be happening here?
Also, bare in mind the speed of the piston does not ever reach "zero", it asymtopically slows to zero and then goes in the other way, just like a graph of y=x².
Regardless, even with the heat loss, the data tell us that its either not completely using the energy of the charge to do work to move the cylinder, or its still burning when the exhaust port is open.
Ive been up to long to remember this but I know there are certain things you can do to the timing of a piston engine to increase EGTs... could the same effective thing be happening here?
12-31-05, 09:35 AM
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Originally Posted by Nihilanthic
Does anyone have the math skills to plot the increase in area from ignition event to when the leading apex seal reaches the exhaust port, over time (in degrees) vs that of a piston engine? This would be some serious calculus though heh.
Also, bare in mind the speed of the piston does not ever reach "zero", it asymtopically slows to zero and then goes in the other way, just like a graph of y=x².
Regardless, even with the heat loss, the data tell us that its either not completely using the energy of the charge to do work to move the cylinder, or its still burning when the exhaust port is open.
Ive been up to long to remember this but I know there are certain things you can do to the timing of a piston engine to increase EGTs... could the same effective thing be happening here?
Also, bare in mind the speed of the piston does not ever reach "zero", it asymtopically slows to zero and then goes in the other way, just like a graph of y=x².
Regardless, even with the heat loss, the data tell us that its either not completely using the energy of the charge to do work to move the cylinder, or its still burning when the exhaust port is open.
Ive been up to long to remember this but I know there are certain things you can do to the timing of a piston engine to increase EGTs... could the same effective thing be happening here?
12-31-05, 09:57 AM
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Originally Posted by KevinK2
sccagt3:
Ok, here is a roughly valid "parked" car analogy for you. Slam on the brakes from 60 mph, and just before you "stop", slam it in reverse at wot. You will soon reach a point/instant where forward velocity is zero and you immediatly accelerate in reverse.
"My car seems to stop when it is parked" Hope you can see the diff, otherwise let it go.
I did not bring up TDC activity, I responed to others.
Tiger and Anika have great follow throughs, but I have never heard it linked the to ignition initiation specifics of the power stroke in the tech/sae papers I read. The goal it have the spark advanced sufficiently to result in a pressure peak about 10-15 deg atdc, closer to the point of optimum torqe leverage. A peak at TDC would waste heat (and drop pressure) to all metal boundaries, as it can not create torque there, wankel or recip. The pressure history will determine how effectively torque is created. Mabe that's like a good chip shot from off the green...
I'd like to hear your opinion on the effect of a common exhaust port with high duty cycle, vs boinger, as I asked before. And of course any more reasons the gas is hotter. Righ now I agree with your exhaust valve cooling.
I answered EFINI's post and he recently replied, obviously capable of speaking for himself.
EFINI:
glad the link helped. It also shows that apex seal velocity is a sine curve with a high mean value and low cyclic content, vs piston velocity.
The area thing clearly dumps more usable heat (and thus lowers pressure) to the walls, and yes that would explain high BSFC, or poor thermal efficiency, but not hotter exhaust as you had stated earlier.
Ok, here is a roughly valid "parked" car analogy for you. Slam on the brakes from 60 mph, and just before you "stop", slam it in reverse at wot. You will soon reach a point/instant where forward velocity is zero and you immediatly accelerate in reverse.
"My car seems to stop when it is parked" Hope you can see the diff, otherwise let it go.
I did not bring up TDC activity, I responed to others.
Tiger and Anika have great follow throughs, but I have never heard it linked the to ignition initiation specifics of the power stroke in the tech/sae papers I read. The goal it have the spark advanced sufficiently to result in a pressure peak about 10-15 deg atdc, closer to the point of optimum torqe leverage. A peak at TDC would waste heat (and drop pressure) to all metal boundaries, as it can not create torque there, wankel or recip. The pressure history will determine how effectively torque is created. Mabe that's like a good chip shot from off the green...
I'd like to hear your opinion on the effect of a common exhaust port with high duty cycle, vs boinger, as I asked before. And of course any more reasons the gas is hotter. Righ now I agree with your exhaust valve cooling.
I answered EFINI's post and he recently replied, obviously capable of speaking for himself.
EFINI:
glad the link helped. It also shows that apex seal velocity is a sine curve with a high mean value and low cyclic content, vs piston velocity.
The area thing clearly dumps more usable heat (and thus lowers pressure) to the walls, and yes that would explain high BSFC, or poor thermal efficiency, but not hotter exhaust as you had stated earlier.
I used the golf analogy to show why it was advantageous to use a longer rod in a piston engine; because the piston is parked at TDC longer. The longer rod allows the piston to dwell within TDC longer which in turn reduces the need to advance the timing and initiate the burning. This brings us to the golf analogy, the longer rod allows the combustion to pack more punch on the piston----just like the impulse force applied to the golf ball. The longer the club head stays in contact with the golf ball the greater the transfer of force.
Efini can speak for himself and has after your post-- It seems his meaning was understood by some myself included.
12-31-05, 10:06 AM
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You guys need to think about the intake/exhaust overlap. The time that both the intake and the exhaust are open at the same time. This was eliminated on the renesis. Think about how this might affect efficiency.
12-31-05, 10:10 AM
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In a post to why someone would want a rotary engine i just finished watching a show in which they explained how the engine was light weight and compact which is ideal for for faster times. they raced an fd tuned with about $6000 with 350hp and a BNR34 same tuning but 600hp and the fd came out on top. this engine with less hp is lighter and in turn you can have better cornering, lower you car more because its compact design and ultimately compete against the bigger hp cars with a much much less weighted car
12-31-05, 01:28 PM
#67
Rotary Enthusiast
area comparison
Originally Posted by Nihilanthic
Does anyone have the math skills to plot the increase in area from ignition event to when the leading apex seal reaches the exhaust port, over time (in degrees) vs that of a piston engine? This would be some serious calculus though heh.
Also, bare in mind the speed of the piston does not ever reach "zero", it asymtopically slows to zero and then goes in the other way, just like a graph of y=x².
....?
Also, bare in mind the speed of the piston does not ever reach "zero", it asymtopically slows to zero and then goes in the other way, just like a graph of y=x².
....?
https://www.rx7club.com/forum/showth...t=thermal+area
Velocity is the slope of the displacement vs time curve. So at tdc and bdc, slope is zero, as is velocity, at that instant.
12-31-05, 01:49 PM
#68
Rotary Enthusiast
"Do golfers simply swing to the point of impact or are they taught to "follow through"? They are taught to follow through same in baseball and other stick and ball sports. The follow through allows the club (force provider--ie combustion) to stay in contact for a fraction of a second longer and transfer more of it's energy to the ball(piston)."
Follow through allows maximum club head speed to be achieved, which is what distance is all about. Longer contact with the ball is a secondary effect.
Gas pressure is always in contact with the piston, from ignition to exhaust opening. If you can get higher peak pressures in the combustion event (ATDC) this will carry through the power stroke for more torque generated, but duration of the contact in not a variable.
But, in simple terms, a bigger bang in the chamber will get more power, as a harder (proper) hit of a golf ball will get more distance. I can buy that.
Follow through allows maximum club head speed to be achieved, which is what distance is all about. Longer contact with the ball is a secondary effect.
Gas pressure is always in contact with the piston, from ignition to exhaust opening. If you can get higher peak pressures in the combustion event (ATDC) this will carry through the power stroke for more torque generated, but duration of the contact in not a variable.
But, in simple terms, a bigger bang in the chamber will get more power, as a harder (proper) hit of a golf ball will get more distance. I can buy that.
12-31-05, 02:06 PM
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Originally Posted by KevinK2
"Do golfers simply swing to the point of impact or are they taught to "follow through"? They are taught to follow through same in baseball and other stick and ball sports. The follow through allows the club (force provider--ie combustion) to stay in contact for a fraction of a second longer and transfer more of it's energy to the ball(piston)."
Follow through allows maximum club head speed to be achieved, which is what distance is all about. Longer contact with the ball is a secondary effect.
Gas pressure is always in contact with the piston, from ignition to exhaust opening. If you can get higher peak pressures in the combustion event (ATDC) this will carry through the power stroke for more torque generated, but duration of the contact in not a variable.
But, in simple terms, a bigger bang in the chamber will get more power, as a harder (proper) hit of a golf ball will get more distance. I can buy that.
Follow through allows maximum club head speed to be achieved, which is what distance is all about. Longer contact with the ball is a secondary effect.
Gas pressure is always in contact with the piston, from ignition to exhaust opening. If you can get higher peak pressures in the combustion event (ATDC) this will carry through the power stroke for more torque generated, but duration of the contact in not a variable.
But, in simple terms, a bigger bang in the chamber will get more power, as a harder (proper) hit of a golf ball will get more distance. I can buy that.
Longer contact is not a secondary effect. What if contact time were reduced to zero. One analogy might be the surviveability to a high g load event. People can survive high g loads if the time they are exposed is limited- very limited, because there is less total transfer of energy to the person. Why is pine tar limited on major league bats? Because it allows the bat to stay in contact with the baseball for a fraction of a second longer---more transfer of energy from the bat to the ball. Not to mention it is a might messy. The first thing I do is "rough up" my aluminum bats.
12-31-05, 02:38 PM
#70
Rotary Enthusiast
"Total duration is variable. Timing of the ignition sooner or later varies the time that combustion pressure is applied."
It does not count before TDC, as this is creates negative torque. The time pressure is applied in a productive way is constant, for a given rpm, defined by mechanical events, as I said. I'll agree to disagree here.
It does not count before TDC, as this is creates negative torque. The time pressure is applied in a productive way is constant, for a given rpm, defined by mechanical events, as I said. I'll agree to disagree here.
12-31-05, 02:56 PM
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It is variable, engines don't ignite the air fuel after TDC they do it before. A small block Chevy can run 28 to 36 degrees of advance on the ignition. This VARIES the time that the combustion gases have time to do their work. It Is variable however there is one "sweet" spot that provides the greatest combustion pressure at the right time to send the piston on its way which is dependent on many factors and changes with RPM. As an engine speeds up the combustion gases have less time to be at the optimum pressure for the cylinder, this is why the ignition is advanced--ignited sooner as engine speed increases.
12-31-05, 05:49 PM
#72
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Join Date: May 2005
Location: Jacksonville, Florida
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So I guess its settled then that its a matter of the rotary cant as compeltely burn all of the fuel air charge as a piston engine before it has to exhaust it?
sccagt3 - I havent taken calculus yet, but it looked to me like it never in reality actually came to a stop unless the engine wasn't spinning. If you want to correct someone you dont have to flame them in a TECH forum.
sccagt3 - I havent taken calculus yet, but it looked to me like it never in reality actually came to a stop unless the engine wasn't spinning. If you want to correct someone you dont have to flame them in a TECH forum.
01-03-06, 09:05 PM
#75
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Join Date: Aug 2004
Location: New Zealand
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By definition a piston has to stop at top and bottom dead centres. The piston velocity is also not nessesarily sinusoidal either (offset gudgeon pins). The fact that the piston is stopped or has a low velocity at the point of highest cylinder pressure is why a piston ring and liner wear much more in this area.
