Rotary engine, how many strokes at the same time?
Thread Starter
Senior Member
Joined: Dec 2001
Posts: 468
Likes: 0
From: Somewhere in California
Rotary engine, how many strokes at the same time?
Hey everybody,
I was having a friendly debate with someone else about which strokes are usually ocurring on a rotary at any one time.
a conventional piston engine has 4 "strokes" right? intake, compression, power, exhaust. say for a 4 cylinder, at any given point, all 4 "strokes" will be occuring at the same time right? so pretty much 4 out of the 4 "strokes" are happening at the same time in different cylinders.
NOW a rotary still has the same 4 "strokes", HOWEVER, since the rotor only has 3 sides, only 3 of the 4 "strokes" will be happening at the same time right? The 4th "stroke" occurs after 1, 2, and 3 move on to their next stage.
I'm the one that agrees with paragraph number 2, but I wanted to get everyones opinion on this. BTW my sources for the 2nd para. is Mike Ancas' RX7 performance handbook.
TIA
I was having a friendly debate with someone else about which strokes are usually ocurring on a rotary at any one time.
a conventional piston engine has 4 "strokes" right? intake, compression, power, exhaust. say for a 4 cylinder, at any given point, all 4 "strokes" will be occuring at the same time right? so pretty much 4 out of the 4 "strokes" are happening at the same time in different cylinders.
NOW a rotary still has the same 4 "strokes", HOWEVER, since the rotor only has 3 sides, only 3 of the 4 "strokes" will be happening at the same time right? The 4th "stroke" occurs after 1, 2, and 3 move on to their next stage.
I'm the one that agrees with paragraph number 2, but I wanted to get everyones opinion on this. BTW my sources for the 2nd para. is Mike Ancas' RX7 performance handbook.
TIA
Old [Sch|F]ool
Joined: May 2001
Posts: 12,855
Likes: 567
From: Cleveland, Ohio, USA
Well, not really.
In a 4-stroke piston engine, only one stroke happens at a time per cylinder. The events happen in one place, spread out over time.
In a rotary, all of the "strokes" are happening at the same time, but they all happen in different places in the engine.
Looking at a single rotor... when the rotor has one apex seal pointing up, one rotor face is roughly 1/2way through the intake cycle, one face is 1/2way through the compression cycle, and one face is between the combustion and exhaust cycles. When one apex seal is pointing down, one face is 1/2way into the combustion cycle, one face is 1/2way into the exhaust cycle, and one face is between the intake and compression cycles.
Note that this is not exact because the port opening and closing means things don't exactly happen at "TDC" and "BDC", just like in a boinger the cycles don't start and end at TDC and BDC. In fact, one cycle will overlap the next one, and in a well tuned engine (piston or rotary) the different cycles will use each other. Like for example, exhaust and intake tuning will cause the leaving exhaust gases to suck intake air into the engine during the overlap period... and the intake will help force the remaining exhaust gases out.
In a 4-stroke piston engine, only one stroke happens at a time per cylinder. The events happen in one place, spread out over time.
In a rotary, all of the "strokes" are happening at the same time, but they all happen in different places in the engine.
Looking at a single rotor... when the rotor has one apex seal pointing up, one rotor face is roughly 1/2way through the intake cycle, one face is 1/2way through the compression cycle, and one face is between the combustion and exhaust cycles. When one apex seal is pointing down, one face is 1/2way into the combustion cycle, one face is 1/2way into the exhaust cycle, and one face is between the intake and compression cycles.
Note that this is not exact because the port opening and closing means things don't exactly happen at "TDC" and "BDC", just like in a boinger the cycles don't start and end at TDC and BDC. In fact, one cycle will overlap the next one, and in a well tuned engine (piston or rotary) the different cycles will use each other. Like for example, exhaust and intake tuning will cause the leaving exhaust gases to suck intake air into the engine during the overlap period... and the intake will help force the remaining exhaust gases out.
boing
piston motors also use the exaust stroke which is nothing more than wasted energy. It wastes a stroke getting the burned gasses out, where as the rotory conviently flushes it out on it's way to the intake stroke. God i love these motors
Thread Starter
Senior Member
Joined: Dec 2001
Posts: 468
Likes: 0
From: Somewhere in California
Originally posted by peejay
Well, not really.
In a 4-stroke piston engine, only one stroke happens at a time per cylinder. The events happen in one place, spread out over time.
Well, not really.
In a 4-stroke piston engine, only one stroke happens at a time per cylinder. The events happen in one place, spread out over time.
I'm not really an expert on this but from my interpretation of a 4 stroke on lets say a 4 cylinder. each of the 4 cylinders will at a different stage of the "cycle" at any one time.
So while cylinder # 1 is on the intake stroke, cylinder #2 is on the compression stroke, cylinder # 3 is on power stroke and cylinder # 4 is on the exhaust stroke. I thought that that's how it worked...that's what I was getting on for 4-strokes on pistons.
Originally posted by peejay
In a rotary, all of the "strokes" are happening at the same time, but they all happen in different places in the engine.
Looking at a single rotor... when the rotor has one apex seal pointing up, one rotor face is roughly 1/2way through the intake cycle, one face is 1/2way through the compression cycle, and one face is between the combustion and exhaust cycles. When one apex seal is pointing down, one face is 1/2way into the combustion cycle, one face is 1/2way into the exhaust cycle, and one face is between the intake and compression cycles.
In a rotary, all of the "strokes" are happening at the same time, but they all happen in different places in the engine.
Looking at a single rotor... when the rotor has one apex seal pointing up, one rotor face is roughly 1/2way through the intake cycle, one face is 1/2way through the compression cycle, and one face is between the combustion and exhaust cycles. When one apex seal is pointing down, one face is 1/2way into the combustion cycle, one face is 1/2way into the exhaust cycle, and one face is between the intake and compression cycles.
Do you have a copy of Mike Ancas' book RX7 performance? I could show you what I was reading and also the picture I was looking at. I'm not saying you're wrong pee-jay, I just wanted to show you my references.
Last edited by ludeowner; Mar 20, 2002 at 03:38 AM.
Old [Sch|F]ool
Joined: May 2001
Posts: 12,855
Likes: 567
From: Cleveland, Ohio, USA
All strokes overlap. The end of every stroke assists in getting the next one started. (This is true for piston engines too... and yes the exhaust stroke on a boinger is VERY important, and is not "wasted energy")
I've never read his book, unfortunately. Next time I go down there I'll buy a copy from him
I've never read his book, unfortunately. Next time I go down there I'll buy a copy from him
Lives on the Forum
Joined: Feb 2001
Posts: 26,664
Likes: 22
From: n
You need to define what a "stroke" is...
For me a 4-stroke motor is intake, compression, ignition, and exhaust.
A piston motor needs TWO revolutions of the crankshaft to complete this on all it's pistons.
The equivalent would mean two revolutions on a rotary engine.  But the rotor spins at 1/3rd the speed of the eccentric shaft, so this means only 2/3rd's of a full rotation for each rotor.  If we pick a starting point for the front rotor, in 2/3rd's of a rotation you have two rotor faces spitting out it's exhaust.  At the same time, the rear rotor is starting out in the middle of it's exhaust position on one face (1/2 an exhaust cycle?) and then one full rotor face then another half before we stop - how do you count 1/2-full-1/2?  You really can't, unless you count that as 2 exhaust faces.  Now technically, the rotor hasn't gone through a FULL cycle, where one rotor face goes through intake/compression/ignition/exhaust, but it spits out 2 exhaust pulses from each front and rear rotor.  Technically, it spit out twice as much exhaust pulses as there are rotors equivalent to each piston spitting out an exhaust pulse...
Purely speaking on the definition of a piston 4-cycle motor spitting out each piston exhaust in two crankshaft rotations, the rotory spat out two in the same two rotations of the eccentric shaft.  Therein lies the 2x multiplier on single rotor face displacement (x2 for front and rear rotor), and you end up with 2.6l piston engine equivalent for a 13B...
-Ted
For me a 4-stroke motor is intake, compression, ignition, and exhaust.
A piston motor needs TWO revolutions of the crankshaft to complete this on all it's pistons.
The equivalent would mean two revolutions on a rotary engine.  But the rotor spins at 1/3rd the speed of the eccentric shaft, so this means only 2/3rd's of a full rotation for each rotor.  If we pick a starting point for the front rotor, in 2/3rd's of a rotation you have two rotor faces spitting out it's exhaust.  At the same time, the rear rotor is starting out in the middle of it's exhaust position on one face (1/2 an exhaust cycle?) and then one full rotor face then another half before we stop - how do you count 1/2-full-1/2?  You really can't, unless you count that as 2 exhaust faces.  Now technically, the rotor hasn't gone through a FULL cycle, where one rotor face goes through intake/compression/ignition/exhaust, but it spits out 2 exhaust pulses from each front and rear rotor.  Technically, it spit out twice as much exhaust pulses as there are rotors equivalent to each piston spitting out an exhaust pulse...
Purely speaking on the definition of a piston 4-cycle motor spitting out each piston exhaust in two crankshaft rotations, the rotory spat out two in the same two rotations of the eccentric shaft.  Therein lies the 2x multiplier on single rotor face displacement (x2 for front and rear rotor), and you end up with 2.6l piston engine equivalent for a 13B...
-Ted
Thread Starter
Senior Member
Joined: Dec 2001
Posts: 468
Likes: 0
From: Somewhere in California
Originally posted by peejay
I've never read his book, unfortunately. Next time I go down there I'll buy a copy from him
I've never read his book, unfortunately. Next time I go down there I'll buy a copy from him
Trending Topics
Full Member
Joined: Dec 2001
Posts: 140
Likes: 0
From: Antioch,CA
There are many ways to describe the combustion cycle of a rotary vs a pistion engine. Some of the simpler ways I like to look at it are as follows:
*a rotor has 3 sides. each side of the rotor can be equated to 1 piston in terms of doing all 4 phases of the combustion cycle( each rotor side will intake fuel/air, compression, combustion, exhaust )
*One revolution of a rotor yields 3 complete combustion cycles(intake,compression,combustion,exhaust) - this is is comparable to a 3 cylinder pistion engine.
*Since the most common rotary engine has 2 rotors each with 3 faces, this is ;very much like a 6 cylinder piston engine in terms of #of combustion cycles completed per revolution of the crankshaft.
*The "eccentric" shaft turns 3 times for each revolution of the rotor. Recall from above: rotor has 3 faces, rotor completes three combustion cycles per rotor revolution.
Therefore: #combustion cycles per revolution of the eccentric shaft = 1
For 2 rotor engine that is 2 combustion cycle per crankshaft revolution.
*Go look at this animation it really helps visualization ( only thing is it might mislead you about the # of crankshaft rotations vs "rotor" rotation( it's not one to one )).
http://www.howstuffworks.com/animation15.htm
http://www.rotaryengineillustrated.c...-thecycle.html
*
*a rotor has 3 sides. each side of the rotor can be equated to 1 piston in terms of doing all 4 phases of the combustion cycle( each rotor side will intake fuel/air, compression, combustion, exhaust )
*One revolution of a rotor yields 3 complete combustion cycles(intake,compression,combustion,exhaust) - this is is comparable to a 3 cylinder pistion engine.
*Since the most common rotary engine has 2 rotors each with 3 faces, this is ;very much like a 6 cylinder piston engine in terms of #of combustion cycles completed per revolution of the crankshaft.
*The "eccentric" shaft turns 3 times for each revolution of the rotor. Recall from above: rotor has 3 faces, rotor completes three combustion cycles per rotor revolution.
Therefore: #combustion cycles per revolution of the eccentric shaft = 1
For 2 rotor engine that is 2 combustion cycle per crankshaft revolution.
*Go look at this animation it really helps visualization ( only thing is it might mislead you about the # of crankshaft rotations vs "rotor" rotation( it's not one to one )).
http://www.howstuffworks.com/animation15.htm
http://www.rotaryengineillustrated.c...-thecycle.html
*
Thread
Thread Starter
Forum
Replies
Last Post
combustion, combution, crankshaft, cycle, cycles, cylinder, cylinders, engine, events, motor, revolution, rotary, rotation, rotor, strokes
