Question about Crank Stuffers Case Volume and Primary Compression

[DavidP71801]

Am I correct in thinking that the volume in the cases and transfers when the intake closes must be greater than the displacement of the cylinder?

 

[Mastermind]

Am I correct in thinking that the volume in the cases and transfers when the intake closes must be greater than the displacement of the cylinder?

I don't believe that would be an accurate statement.

 

[DavidP71801]

I don't believe that would be an accurate statement.

Why? I’m trying to get a better understanding of how a 2 stroke works and how modifications affect the performance my thinking was if I have say a 100cc cylinder but the total available volume in the cases and transfers when the intake closes is only 80cc only that 80cc is available to fill the cylinder when the transfer opens since 80cc is less than the 100cc of cylinder volume wouldn’t that mean that the cylinder is not being filled to capacity with fresh charge? Or am I missing something?

 

[Mastermind]

Why? I’m trying to get a better understanding of how a 2 stroke works and how modifications affect the performance my thinking was if I have say a 100cc cylinder but the total available volume in the cases and transfers when the intake closes is only 80cc only that 80cc is available to fill the cylinder when the transfer opens since 80cc is less than the 100cc of cylinder volume wouldn’t that mean that the cylinder is not being filled to capacity with fresh charge? Or am I missing something?

You said when the intake port closes....

That happens when the piston is about 80° of crankshaft rotation away from TDC. Think about that. Especially if stuffers are in place.

 

[DavidP71801]

You said when the intake port closes....

That happens when the piston is about 80° of crankshaft rotation away from TDC. Think about that. Especially if stuffers are in place.

Ok but doesn’t primary compression start when the intake closes? I’m really trying to learn and most of what I’ve read here and in books makes sense but how stuffers can improve performance just doesn’t if it makes the volume available to be compressed is less than the volume you are trying to fill

 

[Mastermind]

I'm really glad to see someone interested in this stuff. I'll do all I can to help. Time is always short for me though.

Think about it not in such stark terms if you can. Keep in mind that air stretches and compresses. It is also influenced by inertia.

Look at this from the point of the intake opening rather than when it closes. The intake opens....low pressure is created as the piston travels up in the bore. Even after it passes TDC, inertia in the intake tract continues in fill the crankcase.

Is the fill equal to displacement? Not sure.....I can't really answer that.

 

[Mastermind]

Why (or even if) stuffers increase performance is about the intake and transfer action being more responsive to the movement of the piston. Stuffers take up case volume, so there is less room for the air/fuel mixture to be stretched or compressed.

 

[EFSM]

An engine could get by with less crankcase volume if the intake air had more velocity/inertia and did more momentum compression after TDC. Does this mean an engine's performance could be increased with a longer intake tract? or maybe that's the whole reasoning behind velocity stacks. I've never messed with them.

 

[Mastermind]

An engine could get by with less crankcase volume if the intake air had more velocity/inertia and did more momentum compression after TDC. Does this mean an engine's performance could be increased with a longer intake tract? or maybe that's the whole reasoning behind velocity stacks. I've never messed with them.

Velocity stacks do a couple of things. They "organize" air flow entering the carb for better flow.....and they help to trap spit back and get it back in the airstream before it gets lost.

 

[DavidP71801]

Why (or even if) stuffers increase performance is about the intake and transfer action being more responsive to the movement of the piston. Stuffers take up case volume, so there is less room for the air/fuel mixture to be stretched or compressed.

That’s what I’m thinking about if the amount of room for air/fuel mix to be compressed is less than the amount of room it’s trying to fill in the cylinder how could it be beneficial? The way I’m thinking about it is when the intake closes that defines the amount of room for compression to take place and that amount of room needs to be greater than the amount of room in the cylinder. Or is my thinking flawed?

 

[DavidP71801]

I'm really glad to see someone interested in this stuff. I'll do all I can to help. Time is always short for me though.

Think about it not in such stark terms if you can. Keep in mind that air stretches and compresses. It is also influenced by inertia.

Look at this from the point of the intake opening rather than when it closes. The intake opens....low pressure is created as the piston travels up in the bore. Even after it passes TDC, inertia in the intake tract continues in fill the crankcase.

Is the fill equal to displacement? Not sure.....I can't really answer that.

I know your time is valuable I really appreciate you taking the time to help me wrap my mind around all this. I’ve done a ton of reading including lots of stuff you posted here and 99% makes sense and I have a decent grasp on the how and why but crank stuffers and what the right amount of case volume is has me bum fuzzed.

 

[Mastermind]

I know your time is valuable I really appreciate you taking the time to help me wrap my mind around all this. I’ve done a ton of reading including lots of stuff you posted here and 99% makes sense and I have a decent grasp on the how and why but crank stuffers and what the right amount of case volume is has me bum fuzzed.

I believe you are thinking about it as if it were a four stroke engine. It isn't like that at all.

The timing events in a two stroke piston ported engine are defined in crankshaft rotation....and some are very short. For instance....from the time the intake closes.....until the transfer ports open is only about 40 degrees of crankshaft rotation. That period is where primary compression is developed. Nothing going in.....nothing going out of the case. Then the uppers transfer ports open, and the mixture charge is sent into the combustion chamber. Those ports are open for about 120 degrees. Part of that is on the down stroke....and part of it is on the up stroke. So you might expect to have some charge go back down those ports. All this is to just get you to thinking about all the variables that can be occurring, and to realize that this stuff isn't just black and white. The movement of the piston is what drives the charge into the case.....and upward through the transfer ports. So all that can be displaced is the amount that piston moves. The tighter the crankcase....via stuffers, or other means, the more responsive that movement will be.

 

[DavidP71801]

I believe you are thinking about it as if it were a four stroke engine. It isn't like that at all.

The timing events in a two stroke piston ported engine are defined in crankshaft rotation....and some are very short. For instance....from the time the intake closes.....until the transfer ports open is only about 40 degrees of crankshaft rotation. That period is where primary compression is developed. Nothing going in.....nothing going out of the case. Then the uppers transfer ports open, and the mixture charge is sent into the combustion chamber. Those ports are open for about 120 degrees. Part of that is on the down stroke....and part of it is on the up stroke. So you might expect to have some charge go back down those ports. All this is to just get you to thinking about all the variables that can be occurring, and to realize that this stuff isn't just black and white. The movement of the piston is what drives the charge into the case.....and upward through the transfer ports. So all that can be displaced is the amount that piston moves. The tighter the crankcase....via stuffers, or other means, the more responsive that movement will be.

OK when you explain it like that It definitely has me thinking in a different way, but it It does raise a new question when you mentioned the transfer ports being open both up and down stroke, and some of the charge being pulled back into the cases could it potentially be beneficial to put a reed valve in the transfer? Just a thought that popped in my head it may have been tried. It may have been proven a bad idea. Maybe one of these days we’ll meet somewhere and we can have a conversation about how all this works. I bet I could learn a lot from you. I find it So fascinating how something seems so simple on the surface, but is so complex.

 

[Mastermind]

OK when you explain it like that It definitely has me thinking in a different way, but it It does raise a new question when you mentioned the transfer ports being open both up and down stroke, and some of the charge being pulled back into the cases could it potentially be beneficial to put a reed valve in the transfer? Just a thought that popped in my head it may have been tried. It may have been proven a bad idea. Maybe one of these days we’ll meet somewhere and we can have a conversation about how all this works. I bet I could learn a lot from you. I find it So fascinating how something seems so simple on the surface, but is so complex.

All the ports in a piston ported engine flow in both directions at times. Just the nature of the beast.

 

[drf256]

I believe the optimal case to displacement ratio is 60-70% IIRC.

One also has to remember that displacement in a 2T motor is defined by bore x stroke. Unlike a 4T, the true displacement that’s being compressed for ignition is much less. It’s really the cylinder volume from the exhaust roof to the squish band that’s being compressed for ignition.

AKA “swept volume”. This is without an exhaust pipe.

 

[DavidP71801]

All the ports in a piston ported engine flow in both directions at times. Just the nature of the beast.

All this thinking about it makes me think how nice it would be if we could see inside while it’s running and know exactly what is happening which has given me an idea my brother works for a company that manufactures stuff out of acrylics like aquariums and bulletproof glass I asked him one time about making a cylinder out of acrylic and he said that it wouldn’t work because there is heat and pressure they have types of acrylic to deal with one or the other but not both. So now I’m wondering if they could make a set of cases out of acrylic for a saw that uses bolt on transfer caps and make the caps and intake out of acrylic. Then we would have an engine with see through cases, transfer caps and intake so we could see exactly what is happening. If it’s possible to do would you be interested in helping run test? I don’t know enough to know what to test or really recognize or understand what I’m seeing. If it’s possible and there is knowledge to be gained and someone can help with the testing I’ll provide the acrylic parts.

 

[stihl #1]

You would need a slo mo camera since 10,000 RPM is 166 revs per second. Even at idle speed of 3000 RPM that is 50 revs per second. All you would see is a blur.
Maybe these pics can help with visualization:

 

[huskihl]

I look at primary case compression like this… Grab 2 16 ounce solo cups and a 24 ounce solo cup. Put 16 ounces of water into a 24 ounce cup and a 16 ounce cup. Take the other 16 ounce cup and push it down into each one flush with the top. Which example pushes out more water?
Higher case compression pushes more charge quicker out through the transfers.
Chainsaws need to be lightweight and don’t have the stroke length to take advantage of bigger cases like in a motorcycle engine

 

[el33t]

All this thinking about it makes me think how nice it would be if we could see inside while it’s running and know exactly what is happening which has given me an idea my brother works for a company that manufactures stuff out of acrylics like aquariums and bulletproof glass I asked him one time about making a cylinder out of acrylic and he said that it wouldn’t work because there is heat and pressure they have types of acrylic to deal with one or the other but not both.

Chylo Racing

Teile deine Videos mit Freunden, Verwandten oder der ganzen Welt

www.youtube.com

 

[DavidP71801]

I look at primary case compression like this… Grab 2 16 ounce solo cups and a 24 ounce solo cup. Put 16 ounces of water into a 24 ounce cup and a 16 ounce cup. Take the other 16 ounce cup and push it down into each one flush with the top. Which example pushes out more water?
Higher case compression pushes more charge quicker out through the transfers.
Chainsaws need to be lightweight and don’t have the stroke length to take advantage of bigger cases like in a motorcycle engine

I guess where I get confused or have trouble understanding is if the 24oz cup is the cylinder and the 16oz cup is the cases how can the cylinder ever get all the way filled? I understand the idea that stuffers increase the compression ratio which increases pressure and therefore velocity.