compression in cylinder

[ccs]

I was wanting to know why the compression is different depending on whcih way you rotate the crankshaft.Most 2 cycles I have checked have about 10 pounds more compression turning them backwards versus the way they were designed to run.It looks like the manufacture would design them to run the way with the most compression.I know that most 2 cycles that are used to run both ways ussually have more power in reverse.It must have something to do with the air flow in the block as to which way the crankshaft is turning.The port timing would be the same in either direction.I have check engines with reed valves and piston ported and get about 10 pounds differance with both.

 

[Crofter]

CCS; I dont think you can correctly say that most two strokes have more power running the opposite to design direction. Some will barely run backward. Timing is BTDC one direction and after in the other. Crank rotation is often quite an influence in moving the charge and doesnt help at all in the other direction. Some things that may cause the directional difference in compression is that the ring gap (s) are usually to the non thrust face of the piston and there is more of the ring gaps exposed when rotated forward. This would be more noticeable on a worn engine.

 

[lucky]

If they ran the other way, you'd have to put the chain on backwards Or file the back of the tooth......

 

[stihltech]

reverse

You mean there is no reverse on the saw?

 

[DanMan1]

CCS; Some things that may cause the directional difference in compression is that the ring gap (s) are usually to the non thrust face of the piston and there is more of the ring gaps exposed when rotated forward. This would be more noticeable on a worn engine.

Are you saying the rings in your engines don't rotate like they do in mine?

 

[sedanman]

The thrust side of the piston changes with reverse rotation. Where the rod may be angled toward to exhaust port in normal reoation on the power stroke it will be toward the intake in reverse. This will load the piston differently in the bore, it may seal better I don't know.

 

[spacemule]

I was wanting to know why the compression is different depending on whcih way you rotate the crankshaft.Most 2 cycles I have checked have about 10 pounds more compression turning them backwards versus the way they were designed to run.It looks like the manufacture would design them to run the way with the most compression.I know that most 2 cycles that are used to run both ways ussually have more power in reverse.It must have something to do with the air flow in the block as to which way the crankshaft is turning.The port timing would be the same in either direction.I have check engines with reed valves and piston ported and get about 10 pounds differance with both.

When you turn th engine backward by hand, you're putting pressure on the internal parts in the direction they are loaded with the engine running. The difference in compression is at least partially due to the engine being broken in and the parts forming a more effective seal in the running direction.

 

[Al Smith]

Good question

Which brings up another one.I may be mistaken,but if I remember correctly, the westbend 580's and 820 Kart engines, could be reversed.If I recall correctly,a cam was used on the points,that could ,in fact reverse the timing for this purpose.I will add,in my defense,it's been 40 some years since I've been around one.

 

[glens]

If the clutch screws on it would only be feasible to run it in the one direction.  Were the output shafts grooved for keys and circlips?

 

[ccs]

You're right Al the cam lobe is designed to close the points at the same degree ATDC that way it fires in reverse when the points open.That is the way it is on golf carts the starter just turn the motor over which ever way you want to go.Harley Davison said they do that to give the carts more power in reverse and also to make sure the engine is assembled right.With 10 pounds difference that's about as good as you are going to get.

 

[dozerdan]

Al
You are correct about the old westbend engines. Year ago a friend of mine had a cart with two of the 820s on it. Each engine drove one rear wheel and the one engine ran in the opposite direction of the other.
Later
Dan

 

[bwalker]

Oh boy. I dont even know if I should answer this one, but a two cycle doesnt care which way it turns and will not make more power in one direction or another. At least ones with cdi ignitions. Points where before my time so I cant say how the would function in reverse. My snowmobile actually has push button reverse in which the computer shuts slows the motor and restarts it in reverse with out shutting the machine off.

 

[Crofter]

DanMan1;

The rings definitely do not rotate in my two strokes. They are pinned in the gaps to keep ring ends from falling out into the ports.

Ben;

That a two stroke does not care which way it runs is a blanket statement that has some flaws I think.

I am not sure whether it is common now to offset either the piston pin or the center line of the bore to crank center slightly, but this gives a very slight difference in port timing and effective compression ratio dependant on direction of rotation. The most difference in compression whether cranking forward or back has to do with the relationship of the thrustface and the position of the ring gaps.

 

[DanMan1]

Frank,

Goes to show how well I know 2-strokes.

 

[bwalker]

am not sure whether it is common now to offset either the piston pin or the center line of the bore to crank center slightly,

I have never seen a two stroke like this or even heard of one. Does such a engine exist? Besides, I fail to see wh this would change port timing.

The most difference in compression whether cranking forward or back has to do with the relationship of the thrustface and the position of the ring gaps.

I'm not buying this statement either. Just for ????s and grins I went out and put a comp gauge on my snowmobile with the engine running on one cylinder and took a reading of 150lbs. I then with much effort reversed the motor using the elctronic reverse I mentioned above and took a reading of 150. Hows that for practical data,lol.

 

[Crofter]

I have never seen a two stroke like this or even heard of one. Does such a engine exist? Besides, I fail to see wh this would change port timing.



Ben, If you cannot see why an offset in the piston pin location or the bore to crank centerline could affect timing, you are not looking too hard.

It was not I who said there will always be a difference in compression depending on which way you rotate. I only said why it could exist. There is a big difference in cranking speed compression and running a two cylinder while checking the compression on the other cylinder. The difference in leakage depending on which side of the piston and ring gaps is pressed to the thrust face is not going to show. Do you still say a two stroke doesnt care at all which direction it turns. Be careful here Ben!

I have not seen mechanical drawings on the newer two strokes, but pin or cylinder offset used to be very common. It actually gives improved efficiency,and eases the job of stabilizing the piston. I can't remember all the reasons why, but I haven't studied it for a year or two.

 

[bwalker]

Ben, If you cannot see why an offset in the piston pin location or the bore to crank centerline could affect timing, you are not looking too hard.

Frank, Its quit easy to see how it would effect port timing, it however is as clear as mud as to how the timing would change when the motor switched directions.

The difference in leakage depending on which side of the piston and ring gaps is pressed to the thrust face is not going to show. Do you still say a two stroke doesnt care at all which direction it turns. Be careful here Ben!

Yes, I do. As long as the ignition is compatible the engine is just as happy in reverse as it is going forward. Keep in mind that most ring designs today are of the low tention design. Compression during the power stroke would tend to press the rings down and out which is what does the sealing, not the piston acting on the rings. The only reason you dont see the gaps on the exhuast side of a two stroke is because the exhaust port prohibits it.
FWIW I have never seen a two cycle motor with a off set wrist pin. I am not sure they even exist when one considers the possible pitfalls of such a design. The constant rocking motion that such a design would empart on the piston would deffinalty cause some odd wear paterns.

 

[sedanman]

I can see how an offset piston pin could affect port timing. THe piston is not truly square in the bore under load. there is friction of the rings against the bore and the force of combustion acting on the piston. An offset pin MIGHT let the piston rock more in one direction than the other, the effect would be miniscule but none the less present. This is moot for me because my saws only run forward.

 

[bwalker]

the effect would be miniscule but none the less present

BINGO! Of course Frank is the splitting hairs king!

 

[spacemule]

BINGO! Of course Frank is the splitting hairs king!

Hey Ben, is bingo your favorite word? For ????s and gigles, do a search of your posts and see how many times that word comes up.