Piston wrist pin offset
12-06-2009, 02:02 PM
#61
Rennlist Member
But, with more of a chance of piston slap? 
Anyway, i thought when porsche got it right with the symentrical valve cut piston layouts, that they oriented the pistons this way. sounds like they did the opposite if those drawings are correct. If so, if anyone ever takes the S4 engine apart, they should reverse the orientation of the pistons for better power?
interesting.
mk
Anyway, i thought when porsche got it right with the symentrical valve cut piston layouts, that they oriented the pistons this way. sounds like they did the opposite if those drawings are correct. If so, if anyone ever takes the S4 engine apart, they should reverse the orientation of the pistons for better power?
interesting.
mk
01-08-2013, 09:48 PM
#62
Nordschleife Master
If this really is an important issue, why not design the block in a way that the crankshaft is offset and producing a shallow rod angle during the power stroke? A V engine would look a little like left-tilted L engine, where the left bank is taller and the crankshaft is offset to left? Assuming the crankshaft rotates clockwise.
http://en.wikipedia.org/wiki/Desaxe
A desaxe engine, in engineering (usually automotive, but also steam engines), means one in which each cylinder is not positioned with its exact center (the bore axis) directly above the centerline of the crankshaft; instead, the cylinder is offset slightly. "Désaxé", in French, means "unbalanced"; it is not (despite remarks to the contrary) the name of the inventor of French origin.
If the offset is in the direction of rotation, it has the effect of increasing the leverage applied to the crankshaft during the "power" stroke, and reducing thrust wasted against the cylinder wall.
It also distorts the duration of the four strokes, or cycles of combustion. In a conventional engine, each of the strokes (intake, compression, power, exhaust) is nominally 180°, totaling 720° for the complete 4-stroke combustion cycle. A desaxe engine adds to the duration of the two downward strokes (intake and power), and subtracts the same amount from the two upward strokes (compression and exhaust), with the total remaining 720°. A typical desaxe engine will have events timed 185° - 175° - 185° - 175°, etc., with the differential roughly (but not directly) proportionate to the percentage of offset distance to stroke length.
The relative proportion of offset distance vs. stroke length ranges from very small to almost 20%; viz. an engine with an 80mm stroke may have a cylinder offset of 20mm.
Use of Desaxe is now becoming much more common.
VW VR6 required desaxe cylinders, with an offset of 12.5mm. The front bank are offset forwards of the crank while the rear are offset rearwards.
Toyota Prius has Desaxe cylinder bores offset by 13mm.
All single crank split singles such as DKW and Puch had by necessity highly desaxe cylinders.
The Scuderi Engine as shown has highly desaxe cylinders. The compressor (blue) is offset a greater amount than the power cylinder (yellow).
It's benefit is to give a better line of thrust from piston to crank though con-rod during downward power stroke but it has disadvantages in vibration and bore wall thrust. http://papers.sae.org/2004-01-0606/
If the offset is in the direction of rotation, it has the effect of increasing the leverage applied to the crankshaft during the "power" stroke, and reducing thrust wasted against the cylinder wall.
It also distorts the duration of the four strokes, or cycles of combustion. In a conventional engine, each of the strokes (intake, compression, power, exhaust) is nominally 180°, totaling 720° for the complete 4-stroke combustion cycle. A desaxe engine adds to the duration of the two downward strokes (intake and power), and subtracts the same amount from the two upward strokes (compression and exhaust), with the total remaining 720°. A typical desaxe engine will have events timed 185° - 175° - 185° - 175°, etc., with the differential roughly (but not directly) proportionate to the percentage of offset distance to stroke length.
The relative proportion of offset distance vs. stroke length ranges from very small to almost 20%; viz. an engine with an 80mm stroke may have a cylinder offset of 20mm.
Use of Desaxe is now becoming much more common.
VW VR6 required desaxe cylinders, with an offset of 12.5mm. The front bank are offset forwards of the crank while the rear are offset rearwards.
Toyota Prius has Desaxe cylinder bores offset by 13mm.
All single crank split singles such as DKW and Puch had by necessity highly desaxe cylinders.
The Scuderi Engine as shown has highly desaxe cylinders. The compressor (blue) is offset a greater amount than the power cylinder (yellow).
It's benefit is to give a better line of thrust from piston to crank though con-rod during downward power stroke but it has disadvantages in vibration and bore wall thrust. http://papers.sae.org/2004-01-0606/
11-15-2015, 11:57 AM
#63
Nordschleife Master
Here's a Mahle document on piston design, inlcuding piston wrist pin offset. Ake posted it in another thread. There's one section of friction and another section on noise. https://www.fsb.unizg.hr/miv/MSUI/Ko...ubner-2012.pdf
Looking at the engine from the front, the crankshaft rotates clockwise and the thrust side is on the left side of the piston and the anti-thrust side is on the right hands side. On the passenger side, the thrust side is the exhaust side and, on the driver side, the thrust side is the intake side.
The S4 pistons are offset to the thrust side, such that looking from the front of the engine the pin is closer to the left hand side of the cylinder wall. The arrow on the pistons that is supposed to point to the front of the engine is offset in the same direction as the wrist pin.
I've been thinking about the "tests" on piston pin offset direction. I think most published, simple dyno tests are flawed because they don't subsequently adjust the ignition timing. Suppose that the ignition advance is held constant relative to the crankshaft angle. If the wrist pin is offset to the thrust side (as in the stock S4), the piston reaches its highest position earlier; if it's offset to the anti-thrust side, the piston reaches its highest position later. The combustion speed really speeds up with the squish, maximum compression, and reverse-squish. Therefore, switching the direction of the pistons from being offset to the thrust side to being offset to the anti-thrust side effectively advances the ignition timing relative to the critical event of piston reaching its highest position. I am guessing that the switching the piston orientation has a same effect as advancing ignition timing, which usually increases power somewhat in knock limited engines that have some margin of safety. If the dyno experiment of testing the different piston orientations doesn't reoptimize the ignition timing, the experiment is in my opinion invalid.
Section 7.3.3.3 of the Mahle document talks about friction forces/losses. Friction losses are important, because when the charge is burned, basically the main ways in which energy is wasted is thru friction, out of the radiator, and out of the tail pipe. Friction losses are also relevant for reliability. The right hand side of figure 7.39 is the more relevant to my purposes as it considers the engine operating on its own and not motored with an external power source. Figure 7.39 picks a single load level and shows that a modest offset (0.5mm in that example, but the actual numerical value should of course be interpreted relative to the bore size) to the thrust side (the same direction as in the stock S4) gives the lowest friction losses. From the friction loss perspective, therefore, I'd keep the stock S4 piston orientation.
The document also goes thru all types of piston noises, ("rattling", "croaking", "pin ticking", etc.) and evaluates the impact of pin offset on those types of noises. The general conclusion is that a very small offset in the thrust-side direction minimizes the noise.
On a somewhat related note, the new JE asymmetrical pistons have a larger skirt on the thrust side than on the anti-thrust side. This makes sense as the thrust-side forces are typically larger, especially on turbo engines. For the same weight, one is better off moving material from the anti-thrust side to the thrust side. They combine the asymmetric design with a piston wrist pin offset towards the thrust side, which is in the same direction as in the stock S4. (http://www.jepistons.com/Articles/10...d-Pistons.aspx ; http://www.dragzine.com/tech-stories...or-ls-engines/)
Some food for thought, that's all.
Looking at the engine from the front, the crankshaft rotates clockwise and the thrust side is on the left side of the piston and the anti-thrust side is on the right hands side. On the passenger side, the thrust side is the exhaust side and, on the driver side, the thrust side is the intake side.
The S4 pistons are offset to the thrust side, such that looking from the front of the engine the pin is closer to the left hand side of the cylinder wall. The arrow on the pistons that is supposed to point to the front of the engine is offset in the same direction as the wrist pin.
I've been thinking about the "tests" on piston pin offset direction. I think most published, simple dyno tests are flawed because they don't subsequently adjust the ignition timing. Suppose that the ignition advance is held constant relative to the crankshaft angle. If the wrist pin is offset to the thrust side (as in the stock S4), the piston reaches its highest position earlier; if it's offset to the anti-thrust side, the piston reaches its highest position later. The combustion speed really speeds up with the squish, maximum compression, and reverse-squish. Therefore, switching the direction of the pistons from being offset to the thrust side to being offset to the anti-thrust side effectively advances the ignition timing relative to the critical event of piston reaching its highest position. I am guessing that the switching the piston orientation has a same effect as advancing ignition timing, which usually increases power somewhat in knock limited engines that have some margin of safety. If the dyno experiment of testing the different piston orientations doesn't reoptimize the ignition timing, the experiment is in my opinion invalid.
Section 7.3.3.3 of the Mahle document talks about friction forces/losses. Friction losses are important, because when the charge is burned, basically the main ways in which energy is wasted is thru friction, out of the radiator, and out of the tail pipe. Friction losses are also relevant for reliability. The right hand side of figure 7.39 is the more relevant to my purposes as it considers the engine operating on its own and not motored with an external power source. Figure 7.39 picks a single load level and shows that a modest offset (0.5mm in that example, but the actual numerical value should of course be interpreted relative to the bore size) to the thrust side (the same direction as in the stock S4) gives the lowest friction losses. From the friction loss perspective, therefore, I'd keep the stock S4 piston orientation.
The document also goes thru all types of piston noises, ("rattling", "croaking", "pin ticking", etc.) and evaluates the impact of pin offset on those types of noises. The general conclusion is that a very small offset in the thrust-side direction minimizes the noise.
On a somewhat related note, the new JE asymmetrical pistons have a larger skirt on the thrust side than on the anti-thrust side. This makes sense as the thrust-side forces are typically larger, especially on turbo engines. For the same weight, one is better off moving material from the anti-thrust side to the thrust side. They combine the asymmetric design with a piston wrist pin offset towards the thrust side, which is in the same direction as in the stock S4. (http://www.jepistons.com/Articles/10...d-Pistons.aspx ; http://www.dragzine.com/tech-stories...or-ls-engines/)
Some food for thought, that's all.
06-24-2022, 10:58 PM
#64
Rennlist Member
Tonight's episode of Engine Masters covered piston wrist pin offset.
Turns out there was no HP gain when flopping the pistons.
KB
Turns out there was no HP gain when flopping the pistons.
KB
06-25-2022, 03:04 AM
#65
Rennlist Member
Join Date: Feb 2011
Location: Mostly in my workshop located in Sweden.
Posts: 2,230
Received 463 Likes
on
248 Posts
From Wiseco:
By offsetting the piston pins, the engine runs quieter and produces more horsepower due to lower friction during the combustion event. The amount by which the cylinder axes need to be offset depends on many variables, but the main ones are the ratio of crank throw to con rod length and the angle after TDC at which maximum cylinder pressure occurs. Moving the pin bore is more practical Wiseco Pistons have determined that a pin bore offset of 0.050-inch is enough for most applications. They are also aware of the effect on engine stroke length due to offsetting the pin bore. Offsetting the pin bore slightly increases the stroke which may be enough to throw the engine out of spec for some racing classes that limit displacement visa bore or stroke.
Moving the pin bore enjoys a similar benefit to adding rod length in that it slows the piston speed at TDC thus allowing a longer period for flame propagation to build cylinder pressure before the power stroke really gets under way. The tradeoff is higher velocity approaching TDC and lower velocity departing TDC which may affect fuel octane requirements. While not as beneficial on short track applications, it does offer an advantage on longer tracks or in drag racing and Bonneville applications. The improved leverage angle while maintaining TDC is still important. Delaying pressure while the crank moves to a better leverage angle will help increase torque particularly at low rpm. It should be noted that the effects are minimal but nonetheless found to be beneficial when they are applied in max effort applications.
By offsetting the piston pins, the engine runs quieter and produces more horsepower due to lower friction during the combustion event. The amount by which the cylinder axes need to be offset depends on many variables, but the main ones are the ratio of crank throw to con rod length and the angle after TDC at which maximum cylinder pressure occurs. Moving the pin bore is more practical Wiseco Pistons have determined that a pin bore offset of 0.050-inch is enough for most applications. They are also aware of the effect on engine stroke length due to offsetting the pin bore. Offsetting the pin bore slightly increases the stroke which may be enough to throw the engine out of spec for some racing classes that limit displacement visa bore or stroke.
Moving the pin bore enjoys a similar benefit to adding rod length in that it slows the piston speed at TDC thus allowing a longer period for flame propagation to build cylinder pressure before the power stroke really gets under way. The tradeoff is higher velocity approaching TDC and lower velocity departing TDC which may affect fuel octane requirements. While not as beneficial on short track applications, it does offer an advantage on longer tracks or in drag racing and Bonneville applications. The improved leverage angle while maintaining TDC is still important. Delaying pressure while the crank moves to a better leverage angle will help increase torque particularly at low rpm. It should be noted that the effects are minimal but nonetheless found to be beneficial when they are applied in max effort applications.
06-25-2022, 09:10 AM
#66
Rennlist Member
Season 6 episode 24. Not on YouTube yet.
