Cylinder Offset From Crank 1

[thundair]

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 don't know anything but the people that do.

 

[FoMoCoMoFo]

Not sure any amount of offset will get you max. pressure and max. leverage at the same time. unless you have a very high rod ratio diminishing returns can begin almost immediately. I've read arguments for offsetting both ways. years ago i used a single cylinder engine and made some graphs plotting torque,crank angle,offset. gotta go

 

[FoMoCoMoFo]

do any OEMs offset piston pins (the other way for wear/noise) anymore ? I seem to recall reading years ago of some European manufacturer who offset cylinder/crank center the direction your talking about no ? anyway in your typical auto engine I would expect the gains to be small and hard to find .

 

[GregLocock]

Yes, or at least 10 years ago, it was a recognised technique for dealing with cold engine piston slap. More recent F1 engines also tend to have a slight offset, but this may be for packaging rather than any other benefit.

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 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

If you want to big offsets, check out the Volkswagen range of VR engines.

 

[ivymike]

piston pin offset is still done to reduce slap - but typically in the 0.25-1.0mm range.

 

[thundair]

My reasoning was different in that the maximum force generated by the burn happens at one angle and the crank throw is not at an optimum angle.
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.

 

[ivymike]

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.

if we turn off inertia, yes...

 

[thundair]

I was reminded of the old Husky that use to start back wards.
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.

 

[FoMoCoMoFo]

The negative rod angle comment causes me to wonder how your defining TDC. that "negative angle" pre TDC was present before you offset the piston. but if you take an engine with an accurate degree wheel on it and then switch to an offset piston you will find that TDC has moved as has BDC and they are not even 180* apart.so the degree wheel is no longer accurate. TDC being defined as piston at the top of it's travel.

 

[thundair]

Exactly..
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.

 

[Kier14]

If the piston is at TDC and the crank angle still has 10deg to go then you have actually made it worse as the conrod is now making more of an angle to the cylinder axis thus increasing friction. The new Toyota Aygo/Citroen/Peugeot 107 has an offset crank, the idea is to minimize friction by making the conrod closer to parallel on the power stroke.

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.

 

[thundair]

Aha
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.

 

[thundair]

Is there an edit? I really screwed up this time.
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.

 

[FoMoCoMoFo]

one of the "funny" effects of this sort of offset is that piston stroke is greater than crank stroke. but at 1 or 2 mm and a 6" rod the difference would be very small.

 

[anyoldname]

Some OEMs (notably Mercedes) have made V engines without a piston pin offset, so that they don't need handed pistons for each bank. Unfortunately I don't have any data to quantify the NVH implications of this.

 

[thundair]

FoMoCoMoFo
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.

 

[FoMoCoMoFo]

.160" is much to big. thats larger than your offset. typo, measuring some portion twice ? for getting leverage sooner offset pin (the same direction) does the same thing but the piston is going to like it less

 

[thundair]

Quote..From previous post..
"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.

 

[FoMoCoMoFo]

I guess I'm confused.