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Old 02-26-2013, 11:15 PM   #1 (permalink)
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Piston and bearing friction ARE affected by compression because your analogy fails. There isn't more gravity pushing down on the car when you're going up hill. The gravity goes in the same direction. Instead, increase the weight of your car and tell me that the resistance of the driveline doesn't increase.

Higher compression means a higher cylinder pressure that builds exponentially as the piston rises. A bigger fill [speaking to air density] obviously correlates to higher dynamic compression [DIFFERENT from static compression].

Typically, a higher STATIC compression ratio will increase power output for the same fuel/air charge, provided the increased combustion force is sufficient to defeat the losses of compressing the charge tighter into a smaller space.

Dynamic compression refers not to the relation between cylinder volume at TDC and BDC, but to the relation between actual air charge versus capacity at TDC. Another term is 'volumetric efficiency' and although the two sets of numbers can be used differently, they are similar.

IF your intake valve opens late, for instance, you lower your VE and at the same time, your dynamic compression. The static CR remains the same, but you're putting less air and fuel into the cylinder, so there is less pressure built at TDC and obviously less combustion force [given a complete burn in all instances]. The atkinson cycle design that Toyota uses takes advantage of this principle [and a few others] by leaving the intake valve open late, and thus putting some of the AF mix back into the intake, but at the same time lowering the amount of AF mix to be compressed during that stroke.

In any event, the correlation between intake charge density and bearing load is that higher density = higher pressure on top of the piston which obviously entails higher bearing load.

I briefly touched on this concept in another thread and discussed it with a member [the DIY Warm Air Intake thread], and mentioned a way to do this for a home-modder...

I must point out that at this time, I have NO interest in doing this myself - however, I can help anyone who is wanting to try it out, if there is anyone out there. That said, here is some of the information I remember gathering on the topic; all from memory, as I seldom write down my hare-brained ideas on paper.

A D15 engine from the late 80's to the mid 90's uses a smaller deck height and crank main journal than it's bigger brother, the D16, which is a 1.6 liter 'monster' in terms of efficient engines. IIRC, it also uses smaller rod big-end bearings and an obviously smaller stroke.

My idea was to take a D15 block, bolt the crank girdle on and send it to a machine shop to have it line-bored to accept the D16 crank. While taking measurements, I noticed that there was enough 'meat' there on the caps and block webbing to offset the crank while retaining the same bearing caps and position.

Doing so WILL move the crank centerline closer to one of the main cap bolts than the other, putting a weird stress on the bearing, more than likely.

The D16's longer stroke will compensate for the stroke loss from offsetting a crankshaft while providing a more linear combustion stroke by reducing the arc radius that the rod's big end travels, thus reducing parasitic friction during combustion.

If attempting this, very careful attention must be paid to deck height, rod length, piston crown height, and crank shaft stroke.

For example - doing this ONLY using D15 parts will move the piston down in the bore at TDC and shorten the piston stroke while retaining the same crankshaft's 'stroke' number [the diameter of one full rotation of the rod journal around the main journal]. Thus, doing so with only D15 parts would yield lower static compression ratio and piston stroke length; possibly to the extent that the engine fully assembled would require a deck-adjustment to even start, let alone run efficiently.

The D16 crankshaft has longer stroke than the D15. Some measurement could yield a setup that puts the crown height at TDC the same as the D15's original measurements while retaining the original piston stroke length of the D15 as well.

Small offset should clear the cylinder sleeves and block webbing with no problem... larger offsets may require clearancing similar to the addition of a long-stroke crank or larger rods for performance applications.

In theory, the end result of doing this successfully and retaining OEM operating measurements [piston/deck clearance, stroke length, bore] should gain power and efficiency [through lower frictional losses].

Anyone want to discuss it?

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Last edited by RH77; 02-27-2013 at 02:31 AM.. Reason: Moved from WAI Thread, Offset Crankshaft Thread Inadvertently Re-Started -- Thread started Merged into Post #1.
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Old 02-26-2013, 11:32 PM   #2 (permalink)
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I can see how it would affect bearing friction now, but not the piston rings unless the force on the piston isn't completely parallel to the motion of the piston. But come to think of it, it probably isn't since it would vary slightly depending on the angle of the crank. Is that correct?

So static compression is the number that manufacturer says your engine is rated at? What do you mean by TDC and BDC? Is that short for top dead center and bottom dead center?
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Old 02-26-2013, 11:36 PM   #3 (permalink)
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So dynamic compression is actual amount of air (which changes based on temperature, etc.) divided by the actual volume of the cylinder at the bottom of the stroke? Do I understand that correctly?
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Old 02-26-2013, 11:40 PM   #4 (permalink)
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On most engine designs, the piston is only parallel to force at TDC and BDC [yes, you had those correct] which is where skirt/ring friction come into play, increasing at the beginning of the upward or downward stroke and decreasing at the opposite. Then you have actual engine design specs that can compensate to some extent, but that's a whole other debate.

One instance where this is NOT true is Honda's R18 design [I think that's the one] with the offset crank. Well... it's still partially true, BUT it uses an offset crank so that on the power stroke, the piston is as close to possible as pushing a straight line to make the power stroke's output per millisecond more linear on a graph, which is supposed to result in a smoother power application [I have no idea how it performs in real world testing] - the downside is that the I4 engine ends up more like the slant six Chrysler in days of yore - it wears piston skirts and rings more on one side.

With newer technology, much of this wear is alleviated [and the operating life of the engine as a whole is increased] - but the additional wear is still there, although to a lesser extent than it's closest comparison leaning-bank engine.
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Old 02-26-2013, 11:42 PM   #5 (permalink)
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Free Power! offset cylinders explained | Hell for Leather Hell for Leather

There ya go. Quick [lacking depth] description of the Offset Cylinder design.
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Old 02-26-2013, 11:52 PM   #6 (permalink)
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Interesting. It seems like that offset design (just from looking at the picture) would significantly reduce piston ring friction during the intake and power stroke while increasing it during the compression and exhaust stroke. I guess you would gain the most by decreasing friction during the power stroke, even though it is offset slightly when the piston is going back up.

I appreciate the explanation(s).
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Old 02-27-2013, 12:04 AM   #7 (permalink)
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The primary benefit to that is the reduction in 'crank throw' during the power stroke, mostly.

What happens is that the piston, during the power stroke in a zero offset engine, has to go through an arc of about 180 degrees [2 radians, which I'll be using herein because I'm studying calc/trig and need to use the terms - bare with me]. The more offset the crank has, the lower the ratio of radians per stroke length, which equates to an obviously lower 'arc profile' and creates more linear force over a shorter distance. The end result is that the power stroke occurs over a shorter period of crank travel [in radians] and thus equates to higher /leverage/ on the crank, increasing measured output.

Honda has actually used the design in a whole bunch of stuff ranging from two stroke dirt bikes to marine engines and now cars. They seem to be having success with it, and Honda machines tend to have some of the highest output available in motorsports [or so I've heard] so I'd guess that the longevity of the technology should be a testament to it's gimmick factor [or lack thereof].

At this point, I want to point out that I am BY NO MEANS an expert in the technology. I've read about it, even thought about doing something similar in a more make-shift approach [line-boring a D15 to accept a D16 crankshaft using an offset bore].

Another benefit is the use of a larger stroke-diameter crankshaft with shorter piston stroke, reducing bearing wear [longer travel per revolution on the bearing] and helping to combine benefits of shorter and longer stroke engines.
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Old 02-27-2013, 12:48 AM   #8 (permalink)
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180 degrees [2 radians, which I'll be using herein because I'm studying calc/trig and need to use the terms - bare with me].

180 degrees is not 2 radians, it is pi radians. There are 2pi radians in a circle, therefore 180 degrees is pi radians, i.e. 3.141592653...
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Old 02-27-2013, 01:00 AM   #9 (permalink)
Moderate your Moderation.
 
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Quote:
Originally Posted by 13B_88FC View Post
180 degrees [2 radians, which I'll be using herein because I'm studying calc/trig and need to use the terms - bare with me].

180 degrees is not 2 radians, it is pi radians. There are 2pi radians in a circle, therefore 180 degrees is pi radians, i.e. 3.141592653...
You're right... and THAT is why I need to use those terms. LOL Thank you.
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Old 02-27-2013, 01:19 AM   #10 (permalink)
Moderate your Moderation.
 
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Darin/Admin - Please move the offset crank discussion here:

Offset crankshafts

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Last edited by Christ; 02-27-2013 at 01:33 AM..
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