a couple of rotary questions
02-06-06, 10:19 PM
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a couple of rotary questions
hi,
I just took the efi 101 course and was left with a few questions as to how some of the things relate to rotaries.
1) How do you calculate the flow rate of the motor? i know on a fourstroke you take the displacement, times the rpms, divided by two, times the V.E. but how many intake strokes does a rotary make per revolution and what is the displacement of one rotor face?
2) how is ignition timing measured on a rotary? IM a little confused in this area since there really is no TDC in a rotary unless your refering to the e-shaft. but if your refernecing it from the e-shaft; isnt the e-shaft in a different position everytime one of the rotor faces needs to be ignited?
3) What kind of an ignition timing is ideal for a rotary? i know on a piston engine depending on what the engine is doing you vary the timing to try and get the flame front to press agianst the piston at around 12-14 degrees ATDC. but im not sure where on a rotary it is ideal to have the flame front meet to maximise power, and more importantly where to avoid the flame front meeting to avoid detonation.
4) When running a standalone on a piston motor i know generally a hall efect sensor is used to measure crank speed with one tooth missing to tell the compy where TDC is, but on a rotary how does the CAS tell the compy where the rotor is in relation to the e-shaft?
I think thats about it for now but im sure ill have more questions. thanks alot for your help!!!
sincerely,
Sean M.
I just took the efi 101 course and was left with a few questions as to how some of the things relate to rotaries.
1) How do you calculate the flow rate of the motor? i know on a fourstroke you take the displacement, times the rpms, divided by two, times the V.E. but how many intake strokes does a rotary make per revolution and what is the displacement of one rotor face?
2) how is ignition timing measured on a rotary? IM a little confused in this area since there really is no TDC in a rotary unless your refering to the e-shaft. but if your refernecing it from the e-shaft; isnt the e-shaft in a different position everytime one of the rotor faces needs to be ignited?
3) What kind of an ignition timing is ideal for a rotary? i know on a piston engine depending on what the engine is doing you vary the timing to try and get the flame front to press agianst the piston at around 12-14 degrees ATDC. but im not sure where on a rotary it is ideal to have the flame front meet to maximise power, and more importantly where to avoid the flame front meeting to avoid detonation.
4) When running a standalone on a piston motor i know generally a hall efect sensor is used to measure crank speed with one tooth missing to tell the compy where TDC is, but on a rotary how does the CAS tell the compy where the rotor is in relation to the e-shaft?
I think thats about it for now but im sure ill have more questions. thanks alot for your help!!!
sincerely,
Sean M.
02-06-06, 11:02 PM
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I'm not an engineer or anything, but I've been messing with RX-7s and the rotary engine since the mid 90s so I'll do my best to explain some of these, but mostly likely in laymens terms.
1.) The cranks shaft makes 1 complete revolution per every power stroke. So with 3 faces of a rotor, each having a power stroke, that means the crank shaft is going around 3 times faster then the rotors. So when the engine is at 3000 RPMs, the rotors are going at 1000Rpms. A good reference for some figures such as volume of the working chamber, etc.. go to www.rotaryrefs.net and click on "misc rotary books & info" on the left side. The only available file will be "The Rotary Engine" and its in PDF format. Check out chapter 2 in there.
2.) Ignition timing is measure via the e-shaft position because the eshaft is in the same position each time the plugs fire. As I mentioned before, the shaft goes around 3 times faster than the rotor. So for each rotor face on a compression stroke, the shaft will come around to the same position for that face as it was for the previous face, having made 1 complete revolution from the previous power stoke.
3.) Ignition timing changes with engine speed, occuring sooner and sooner the faster the engine revs. I'm not sure what you mean by "ideal timing". The factory set timing at the factory set idle speed (750 RPMs) is 5 degrees ATDC for the leading plug, and the trailing plug fires 15 degrees later, or at 20 ATDC. Detonation occurs from the the air/fuel mixture being unstable enough to detonate from the engines compression and heat prior to the ignition from the spark plugs, so having the timing set wrong will not cause detonation as long as the air/fuel mixture is correct. What will happen will be that the spark will either occur late, allowing unburnt fuel into the exhaust manifold, or it will occur early, not maximizing the power potential of the fuel and causing poor performance. If it was mis-adjusted to where the apex seal was between the leading and trailing plugs when they fired, the leading plug would ignite the gases in the chamber on the compression stroke, while the trailing plug would ignite the gases in the follow chamber. This would be too early for this chamber to be ignited, causes extremely poor performance. Some say that igniting this chamber too soon would cause the rotor to try to rotate backwards, but I doubt that would happen because the gases in that chamber are not compressed yet, so theie is less potential energy available. Just a theory of mine. I do agree that the engine would run like a bag of hammers, if it would run at all the the ignition timing so far off.
I digress.
4.) The rotary engine uses a crank angle sensor that it geared directly off of the e-shaft. To index the shaft, you rotate it until the yellow mark on the main pulley (which can only be installed 1 way) is lined up with the pointer on the front cover. There is an index mark on the crank angle sensor. Like it up and keep it lined up when installing it. I'm not sure exactly how the sensor works but its a pretty simple design, nothing overly complicated. There are two pointers inside (180 degrees apart from each other, just like the rotor lobes on the e-shaft) that rotate when the e-shaft turns it and I think it uses a magnet to trigger the signal to the ECU. The ECU then uses engine speed to determind how much longer before it delivers the spark. It just has to do it for the leading plug and trailing plug, so after the leading coil is fired, it waits 15 degrees before firing the trailing.
Hope that helps a little. Like I said, I didnt' invent the rotary, just been working on them and learning about them for a while.
1.) The cranks shaft makes 1 complete revolution per every power stroke. So with 3 faces of a rotor, each having a power stroke, that means the crank shaft is going around 3 times faster then the rotors. So when the engine is at 3000 RPMs, the rotors are going at 1000Rpms. A good reference for some figures such as volume of the working chamber, etc.. go to www.rotaryrefs.net and click on "misc rotary books & info" on the left side. The only available file will be "The Rotary Engine" and its in PDF format. Check out chapter 2 in there.
2.) Ignition timing is measure via the e-shaft position because the eshaft is in the same position each time the plugs fire. As I mentioned before, the shaft goes around 3 times faster than the rotor. So for each rotor face on a compression stroke, the shaft will come around to the same position for that face as it was for the previous face, having made 1 complete revolution from the previous power stoke.
3.) Ignition timing changes with engine speed, occuring sooner and sooner the faster the engine revs. I'm not sure what you mean by "ideal timing". The factory set timing at the factory set idle speed (750 RPMs) is 5 degrees ATDC for the leading plug, and the trailing plug fires 15 degrees later, or at 20 ATDC. Detonation occurs from the the air/fuel mixture being unstable enough to detonate from the engines compression and heat prior to the ignition from the spark plugs, so having the timing set wrong will not cause detonation as long as the air/fuel mixture is correct. What will happen will be that the spark will either occur late, allowing unburnt fuel into the exhaust manifold, or it will occur early, not maximizing the power potential of the fuel and causing poor performance. If it was mis-adjusted to where the apex seal was between the leading and trailing plugs when they fired, the leading plug would ignite the gases in the chamber on the compression stroke, while the trailing plug would ignite the gases in the follow chamber. This would be too early for this chamber to be ignited, causes extremely poor performance. Some say that igniting this chamber too soon would cause the rotor to try to rotate backwards, but I doubt that would happen because the gases in that chamber are not compressed yet, so theie is less potential energy available. Just a theory of mine. I do agree that the engine would run like a bag of hammers, if it would run at all the the ignition timing so far off.
I digress.
4.) The rotary engine uses a crank angle sensor that it geared directly off of the e-shaft. To index the shaft, you rotate it until the yellow mark on the main pulley (which can only be installed 1 way) is lined up with the pointer on the front cover. There is an index mark on the crank angle sensor. Like it up and keep it lined up when installing it. I'm not sure exactly how the sensor works but its a pretty simple design, nothing overly complicated. There are two pointers inside (180 degrees apart from each other, just like the rotor lobes on the e-shaft) that rotate when the e-shaft turns it and I think it uses a magnet to trigger the signal to the ECU. The ECU then uses engine speed to determind how much longer before it delivers the spark. It just has to do it for the leading plug and trailing plug, so after the leading coil is fired, it waits 15 degrees before firing the trailing.
Hope that helps a little. Like I said, I didnt' invent the rotary, just been working on them and learning about them for a while.
02-07-06, 07:22 AM
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thanks for your answers they are very helpful...what i meant by ideal ignition timing is this.
In a piston motor i know that to get the most torque an engine can produce you want the peak cylinder pressure to occur at about 12-14 degrees ATDC. to accomplish this you vary the ignition timing based on engine load to time the flame propagation to meet the piston at 12-14 ATDC. Im wondering at about what degree ATDC do you want the flame to meet the rotor to create peak torque?
Here is why i was wondering about the danger of detonation occuring if the timing is too advanced.
i know in a piston motor if you have the timing too advanced then two things occur. one the cylinder pressure rises too quicky slowing the motor down more then it speeds it up, and two; since you lit the flame before the charge is full compressed you've effectivly heated the whole mixture. what happens is beacaue the charge is still compressing it has far more heat then it should due to early ignition, then as the cylinder compresses farther that extra heat has caused the charge to reach its flash point in certain areas of the charge causing detonation. So i was wondering at about what degree this becomes an issue in a rotary at WOT?
on a side note the difference between pre-ignition and detonation is pre-ignition is caused before the sparkplug is fired due to a hot spot. and detonation occurs beacause the plug has started a flame kernal in one part of the combustion chamber which has heated up the rest of the combustion chamber causeing the charge to ignite in different areas before the flame propogates to that point.
one more question...the 1.3 litre displacement of the rotary engine, is that measured by the displacement of one revolution thereby hinting that that is the displacement per 2 rotor faces (since there are two rotors)...or is that the displacement of all six rotor faces?
Thanks again
sincerely,
Sean M.
In a piston motor i know that to get the most torque an engine can produce you want the peak cylinder pressure to occur at about 12-14 degrees ATDC. to accomplish this you vary the ignition timing based on engine load to time the flame propagation to meet the piston at 12-14 ATDC. Im wondering at about what degree ATDC do you want the flame to meet the rotor to create peak torque?
Here is why i was wondering about the danger of detonation occuring if the timing is too advanced.
i know in a piston motor if you have the timing too advanced then two things occur. one the cylinder pressure rises too quicky slowing the motor down more then it speeds it up, and two; since you lit the flame before the charge is full compressed you've effectivly heated the whole mixture. what happens is beacaue the charge is still compressing it has far more heat then it should due to early ignition, then as the cylinder compresses farther that extra heat has caused the charge to reach its flash point in certain areas of the charge causing detonation. So i was wondering at about what degree this becomes an issue in a rotary at WOT?
on a side note the difference between pre-ignition and detonation is pre-ignition is caused before the sparkplug is fired due to a hot spot. and detonation occurs beacause the plug has started a flame kernal in one part of the combustion chamber which has heated up the rest of the combustion chamber causeing the charge to ignite in different areas before the flame propogates to that point.
one more question...the 1.3 litre displacement of the rotary engine, is that measured by the displacement of one revolution thereby hinting that that is the displacement per 2 rotor faces (since there are two rotors)...or is that the displacement of all six rotor faces?
Thanks again
sincerely,
Sean M.
Last edited by incubus_boader; 02-07-06 at 07:49 AM.
02-07-06, 06:22 PM
#5
Originally Posted by incubus_boader
1) How do you calculate the flow rate of the motor? i know on a fourstroke you take the displacement, times the rpms, divided by two, times the V.E. but how many intake strokes does a rotary make per revolution and what is the displacement of one rotor face?
A 2-rotor has two intake strokes per eccentric shaft revolution -- one for each rotor.
One rotor face is 654cc on a 13B. There are 6 654cc combustion chambers in a 2-rotor 13B, but it takes 3 full rotations of the eccentric shaft to fire them all.
-Max
02-07-06, 10:31 PM
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thanks for all the info guys!
just to make sure i understand correctly, the 13b flows 1.3 litres of air per e-shaft revolution at 100% V.E.
So that just leaves one question...
In a piston motor i know that to get the most torque an engine can produce you want the peak cylinder pressure to occur at about 12-14 degrees ATDC. to accomplish this you vary the ignition timing based on engine load to time the flame propagation to meet the piston at 12-14 ATDC. Im wondering at about what degree ATDC do you want the flame to meet the rotor to create peak torque?
just to make sure i understand correctly, the 13b flows 1.3 litres of air per e-shaft revolution at 100% V.E.
So that just leaves one question...
In a piston motor i know that to get the most torque an engine can produce you want the peak cylinder pressure to occur at about 12-14 degrees ATDC. to accomplish this you vary the ignition timing based on engine load to time the flame propagation to meet the piston at 12-14 ATDC. Im wondering at about what degree ATDC do you want the flame to meet the rotor to create peak torque?
02-08-06, 12:28 AM
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All of that has to do with the crank/con rod angle being ideal for the flame front hitting the piston head to be the most efficient.
For a Rotary its... not so simple. And I have no idea, but I was just letting you know the theory behind it. Also, remember there are TWO combustiom chambers and the second plug fires twice to get that last pocket of air/fuel at the corner before its swept away, so you have more than one flame front to deal with.
For a Rotary its... not so simple. And I have no idea, but I was just letting you know the theory behind it. Also, remember there are TWO combustiom chambers and the second plug fires twice to get that last pocket of air/fuel at the corner before its swept away, so you have more than one flame front to deal with.
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