Cylinder Offset From Crank
Cylinder Offset From Crank
(OP)
Is there a diminishing return on offsetting the bore over the crank?
I have moved it 1 and 2mm toward rotation and did not get the results I expected, although it is easier then stock to push the piston down by hand . This is a parallel twin but I guess applies to all in line engines.
Is there any studies on moving the cylinder to have the highest force on the piston at the largest moment on the crank
Thanks
I have moved it 1 and 2mm toward rotation and did not get the results I expected, although it is easier then stock to push the piston down by hand . This is a parallel twin but I guess applies to all in line engines.
Is there any studies on moving the cylinder to have the highest force on the piston at the largest moment on the crank
Thanks
I don't know anything but the people that do.





RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
In any case we are talking about 1 or 2 mm, which given the geometry involved is going to make about one poofteenth of a horsepower difference.
check out manolis's balance.exe program if you want some hard numbers.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
I did some rotational studies and placed the bore directly over the rod throw at 90* and on a 3.5" stroke w/6" rod. The piston is down the bore 45%. It should be closer to 30% down but you get the idea.
The scary part is the negative rod angle to rotation when coming up to TDC, any advance timing is going to send it back-wards.
So I was hoping that I could find an optimum angle to take advantage of the longer moment on the crank at the highest force of the burn...
Thanks for your replies
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
if we turn off inertia, yes...
RE: Cylinder Offset From Crank
Surprised a lot of people....
I am back to the multiple plugs that will allow the timing to be set closer to TDC.
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
When the piston is at TDC the crank still has ~10* to go...
You are right about the existing condition though. I get locked in to a micro analysis sometimes, but this angle seems extreme. Anyways the movement on a 3.5 stroke with the crank at 90* is 1.599 up to piston TDC and down 2.072 to BDC, I know that adds up to more than 3.5 and I have a feeling it is the hypotenuse of the angle.
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
Maximum cylinder pressure on most gasoline engines is around 15deg ATDC. If the your crankshaft still has 10deg to go before it gets to TDC then in theory max cylinder pressure is at 5deg ATDC which will produce less torque. The best time to produce maximum torque is when the crankshaft is at 90deg ATDC, ideally you would want max cylinder pressure to correspond to this but combustion related issues prevent you from doing so, the higher the cylinder pressure later in the stroke then the more torque will be produced.
Regards,
Kieran.
RE: Cylinder Offset From Crank
At 15* my piston is .086 down the hole. I changed the cylinder to crank alignment by 1.353 the crank throw now is 90* and my rod is almost vertical. Running out of time I will check in later.
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
Anyways the last post is hosed. I got in a hurry I guess.
So I took the .086 down the hole and was able to get another 10* on the crank over the initial 15*. I don't have time to run any numbers on length of moment arm and % of increase in torque
Exit stage left....
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
Yes..I was surprised to find ~.160 more piston travel than the stroke.
anyoldname
Is the pin offset the same as changing the cylinder alignment to the crank? Although I have heard of the offset pin I am not that familiar with the advantages.
A side note the % increase in torque from force at 15* (.086 down the hole) and 25* (.086 down the hole) is 63%
seems like a lot. But isn't it the difference in the sine of 15* and 25*?
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
"Anyways the movement on a 3.5 stroke with the crank at 90* is 1.599 up to piston TDC and down 2.072 to BDC, I know that adds up to more than 3.5 and I have a feeling it is the hypotenuse of the angle."
I guess it is 2.072+1.599=3.671-3.5(stroke)=.171
this is an offset of 3.5..
The next offset I looked at with a 63% increase in torque is the sine of 10* against 3.5 stroke..
I did a search on offset pins and the offset toward the rotation is healthy for pistons and an offset against the rotation is good for torque but hell on pistons.
I don't know anything but the people that do.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
One mathematical model by some chaps at Hyundai showed a reduction in friction at low engine speeds, but increased friction at higher engine speeds (cross-over point depends on offset). There have been some SAE papers published on the topic.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
It is packed away from my last move but I'm sure you can search 4- bar linkages and come up with something.
Maybe check martindales reference desk.
Sorry I don't have a better answer. HTH
RE: Cylinder Offset From Crank
Offsetting the piston pin away from the centerline of the piston but keeping the crank centerline on the centerline of the cylinder, has a different effect from having the piston pin centered on the piston but the crankshaft shifted off the centerline of the cylinder. The former approach requires oddball pistons. The latter approach requires oddball crankcase machining. I suspect the V8's are using a millimeter or two of the former approach. The above-referenced Toyota Aygo (and Echo and Prius, and Honda diesel) use the latter approach.
I suspect that aside from piston-slap control, any overall effect on the engine's power and efficiency will be very small.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
Mike Halloran
Pembroke Pines, FL, USA
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Cylinder Offset From Crank
RE: Cylinder Offset From Crank
If you want to have a play have a look at
ht
A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com
RE: Cylinder Offset From Crank
I simplified my statement by showing 15* ATDC = .085 down the hole with a 3.5 stroke and a 6" rod. That being arguably the highest pressure. Then using that .085 down the hole at 25* should give you a larger moment arm and more torque.
I don't know anything but the people that do.
RE: Cylinder Offset From Crank