I'm doing some research for work on crank offsets. I was wondering if there are any experts out there who might be able to help me out. I'm aware of many benefits of crank offsets, but I don't know any disavantages. And what the benefit of crank offset is vs. pin offset... Here's some information I've gathered so far (you might be able to learn something anyway). My gathered info is below.
My questions are:
- Difference and advantages crank vs. pin offset
- Piston design constrains crank offset
- Can you do crank AND pin offset?

Effects of Crank Offset


- Assuming offset direction is to the thrust side of the part, an offset will decrease the amount of side forces on the piston by decreasing the deflection angle of the connecting rod during the power stroke. Thus it will also reduce skirt/ring wear as a result.

- An offset will increase the maximum deflection angle of the conrod. With a tapered pin boss, this would increase pin boss spacing due to clearance, and therefore increase pin boss contact pressure.

- An offset requires a slightly longer conrod for the same stroke. Offset and conrod length are directly correlated.

- Reduction of friction:

o “The advantages claimed for this construction are: that it eliminates the dead center of the crank shaft and thereby increases the efficiency of the motor by reason of the greater leverage and more direct thrust of the piston; that friction is very much reduced by reason of the fact that there is a more direct thrust on the drive stroke. The same direct thrust reduces vibration and increases the steady operation of the motor.”
§ (http://www.car-nection.com/yann/dbas_txt/Igniter.htm)

- Longer powerstroke/exhaust stroke
o “When the crankshaft is rotated in a clockwise direction, the distance the piston travels from the top of the stroke (piston at maximum travel) to the bottom of the stroke (piston at the bottom of its travel) is greater than the diameter of the crankshaft rotation. The angle through which the crankshaft moves during the downstroke is greater than 180.degree.. The engine therefore has a longer time power stroke than exhaust stroke. The intake cycle is longer in time than the exhaust cycle which improves aspiration of the engine. This concept can be applied to Otto cycle engines, Diesel engines, two stroke engines, and may be applied to compressors. When used in compressors, the intake stroke is extended which improves aspiration.”
§ http://www.freepatentsonline.com/5146884.html
o A large offset is required for this to be noticeable
§ For example, using a 192 rod and 124mm stroke,
· an offset of 20mm gives 0.759 mm of extra power stroke
· an offset of 40mm gives 3.138 mm of extra power stroke

- Engine is less likely to be stalled
o http://books.google.com/books?id=BtY...8#PRA3-PA81,M1

- Reduction of friction loss is greatest at low speeds, and
- Greater offset increases friction at higher speeds
o “Analytical results show that the crankshaft offset has some influence mainly on the side force upon the piston and effects variation in the piston sliding speed. The crankshaft offset can reduce significantly friction loss of the piston skirt, whereas friction loss in other parts is negligible. The optimum offset to minimize skirt friction loss depends on the operating conditions. Upon calculation and measurement it is determined that reduction in friction loss occurs mainly at low engine speed and low engine load. When the speed and load increase, benefit is confined to the lowest offsets, and at higher offsets the friction increases. Analytical and experimental results indicate that crank offset is effective in reducing engine friction and improving fuel economy in the low and medium engine speed region.”

- http://cat.inist.fr/?aModele=afficheN&cpsidt=15208906:




In summary,
Less side-force on piston skirt
Less wear on skirt and rings
Increased power stroke per rotation of crank
Increased inertia forces due to increased piston velocity
Increased connecting rod deflection angle[/ul]

Any ideas?

 

Can you restate the question in english?

 

Basically, if off-setting a crank gives you less side-forceand wear on the piston skirt and rings,plus an increased power stroke over therotation of the crank, thenwhy isn't every inline engineoffset? What's the disadvantage?
Also, has anyone ever seen a pin and crank offset in an engine?
I understand its a pretty technical question, but I just thought I'd throw it out there cuz you never know...

 

Some of your answers are IN your answer:

Increased pin-boss pressure means increased pin-boss side loading, true? The increased loading would require either a larger pin-boss, or a HARDER pin-boss, to compensate. A larger pin-boss means you'd need more mass to counter-balance the extra ricprocating mass. More mass means slower rotation, for a given amount of power (the bang-blow part of S-S-B-B). You'd need higher compression to compensate. Highr compression means beefier rings and pistions to handle the extra compression. beefier pistions means bigger/stronger conrods to handle the extra mass of teh piston. Exponentionally, everything keeps growing, each in turn. Somewhere in and amongst all that metal is a trade-off point. It is determined by the law of diminishing returns.

A HARDER pin-boss would cost extra money to manufacture, and, not inconsequentially, be mmore brittle. Making sure the MTBF is long enough to make the manufacturing process WORTH the cost of actually manufacturing the part takes more money still. Money spent somewhere, on a part, is money unavailable to be spent somewhere else. Expensive pin-bossessess's means you may have to put up with cheap, prone-to-failuremain bearing-bolts, or cheaper, thinner head-gaskets. Again, the law of diminishing returns. Trade-offs!

Life if FULL of them. Some are obvious, like, longer lines to get on a plane, for the sake og giving the *illusion* of security, so "da-Man" can *feel* safe, without actually BEING any safer from all those millions of travellers who would otherwise pack their Keds with fertilizer (think about it); others are more subtle, like when you make the bed, in the morning, to shut your wife up, even though you'll most likely be messin it up, ALL OVER AGAIN, in half-a-day, or so.

I'll bet you ONLY pick your nose when you think no one is looking, doncha? Then, you understand! It's a trade-off.

Same basic answer, different startin point. Again, the law of diminishng returns will define the point at which, for any given engine design, offset is determined.

...I could go on, for each of your points, but, all roads point to the fact that, if you change ONE parameter, others MUST be changed to compensate, which will in turn define other, downstream changes, again, in compensation. And so on, and so on, and so on. Those engineers don't spend hundreds of thousands of dollars on their edumuhcashuns for nuthin. To say nothing of all that loot spent on all that expensive software, mainframes, and workstations the companys pay through the nose for.

In short: It's a trade off. You give a little here, to get a little somewhere else.

 

While I'm glad (and impressed) that you took the time to read and contemplate, I don't think I explained myself well enough. The side forces on the piston will be decreased from an offset. Same reason for a pin boss offset (which is quite common), it decreases the amount of side forces on the piston.

When combustion occurs, it puts a force down on the piston (obviously). The conrod will resist this force at an angle (throughout the rotation of the crank). Thus the resisting force will be a vector towards one side of the piston (the thrust side). If one can decrease the angle of this vector (for example, with an offset) then more of the force goes down the conrod and less on the skirt of the piston.

So, using Jazzian logic of diminishing returns, the engine should actually be cheaper. The conrod length would be longer, so in that sense increase reciprocating mass and decrease efficiency, and this would have be wieghed against other options...

Besides a small amount of mass (I'm talking a mm (max) longer conrod) I don't see a disadvantage of an offset. Unless there is an engine infrastructure problem that I don't know about...

I believe the answer lies in the middle east somewhere - somewhere between Iraq and Ahardplace. (I watched hot shots deux last night).