The Aerodrome Forum - View Single Post

David Paule · 18 September 2009, 09:38 AM

Craig wrote this episode -

Pistons and heat transfer

We spent the afternoon of the 16th looking at pistons and wrist pins. We had previously measured all the pistons and cylinders and found that both had wide variation in dimensions. Of the good pistons we selected the six with the greatest skirt diameter for use. The final selection included four used pistons and two new ones. We then matched the biggest piston with the biggest bore, etc.

The resulting clearance between piston skirt and cylinder wall is about .010 inch (.25 mm). This is terribly large by modern standards (.003 inch today would be normal) but this engine has several differences from a modern one. Most important is the terrible cooling of the pistons. First of all, the pistons are iron and thus are poor conductors of heat so that the piston crowns don’t transfer the heat down to the skirts well.

Secondly, the diameter at the crown end is the same as at the skirt end (unlike modern engines) so this is where seizure would likely happen. To prevent seizure at the crown end, the entire piston is given large clearance. The large clearance at the skirt end results in poor heat transfer between the skirt and cylinder wall.

Finally, the Anzani has a total loss oil system. This necessitates a low oil flow to give a significant range to the aircraft. Modern engines have much higher oil flow and depend on the oil to cool the underside of the piston and especially count on the oil film between the aluminum piston skirt and the cylinder wall to conduct heat away from the piston.

Note also that aluminum has three times the thermal expansion of iron. This makes the .010 clearance even more extreme.

So why did we decide to run with these pistons and bores? First of all, it seems to be how it was designed and it did work (although with poor reliability). Secondly, to change its design to modern practice would entail extensive changes. Finally, we can always change it later after we see how it ran in its time.

Piston pins

We examined the wrist pins of the used pistons and discovered they had about .002 inch (.05 mm) of wear where they ran in the connecting rod bushings. This is a very high amount of wear by modern standards. We have several ideas for what might have caused it. First, the wrist pins are held in position in the pistons by a bolt screwed into one of the bosses in the piston and extending into a hole in the wrist pin. This keeps the pin from turning and concentrates the wear on the top (piston crown) side and bottom sides of the pin where it runs against the rod bushing.

Then there is the marginal oiling. This is exacerbated by three holes in the rod bushing. These holes are on the axial center of the bushing and at about 120 degrees from each other with one hole at the top of the rod. The problem with these holes is that a plain bushing generates the maximum oil pressure at its axial center and these holes are strategically placed to bleed that off, especially the one at the top of the rod. Modern engines have a single angled hole in the rod at the bottom (opposite the piston crown) angled toward, but not reaching, the axial center of the pin.

Another problem with the wrist pins is the tolerances on their diameter. The pins out of the new pistons showed a variance of about .001 inch (.025 mm). This might seem like precision machining, but in a modern engine, a .001 inch clearance at the wrist pin bushing is considered worn out. The reason for such tight clearances is that there is very little rotation and the load is hammering. Significant clearance allows the oil to flow around the pin (and out the holes in the Anzani bushings) and concentrates the load in the top and bottom of the bushing. Another peculiarity of the pins is that they are tapered about .001 inch from one end to the other, with the bolt-hole end being bigger. I suspect this is from holding the pin by one end when grinding, rather than grinding between centers. It could also be from worn-out equipment or even intentional.

When we attempted to put the new pins into the used pistons we discovered another problem. The pin holes in the pistons (even the new ones) are not aligned. We didn’t measure the misalignment, but it appeared to be as much as .010 inch (.25 mm). This lead to some debate as to whether we should hone the holes to a larger and aligned state and make new wrist pins (we could then hone and use the existing rod bushings) or just force the pins as was done with the original equipment and make new rod bushings. We opted for the simpler "force the pins" and new rod bushings because the pins don’t turn in the piston anyway, and the pistons have plenty of clearance so that a little distortion isn’t going to hurt anything.

18 September 2009, 09:38 AM	#9 (permalink)
David Paule Observer Join Date: Feb 2008 Posts: 92	Craig wrote this episode - Pistons and heat transfer We spent the afternoon of the 16th looking at pistons and wrist pins. We had previously measured all the pistons and cylinders and found that both had wide variation in dimensions. Of the good pistons we selected the six with the greatest skirt diameter for use. The final selection included four used pistons and two new ones. We then matched the biggest piston with the biggest bore, etc. The resulting clearance between piston skirt and cylinder wall is about .010 inch (.25 mm). This is terribly large by modern standards (.003 inch today would be normal) but this engine has several differences from a modern one. Most important is the terrible cooling of the pistons. First of all, the pistons are iron and thus are poor conductors of heat so that the piston crowns don’t transfer the heat down to the skirts well. Secondly, the diameter at the crown end is the same as at the skirt end (unlike modern engines) so this is where seizure would likely happen. To prevent seizure at the crown end, the entire piston is given large clearance. The large clearance at the skirt end results in poor heat transfer between the skirt and cylinder wall. Finally, the Anzani has a total loss oil system. This necessitates a low oil flow to give a significant range to the aircraft. Modern engines have much higher oil flow and depend on the oil to cool the underside of the piston and especially count on the oil film between the aluminum piston skirt and the cylinder wall to conduct heat away from the piston. Note also that aluminum has three times the thermal expansion of iron. This makes the .010 clearance even more extreme. So why did we decide to run with these pistons and bores? First of all, it seems to be how it was designed and it did work (although with poor reliability). Secondly, to change its design to modern practice would entail extensive changes. Finally, we can always change it later after we see how it ran in its time. Piston pins We examined the wrist pins of the used pistons and discovered they had about .002 inch (.05 mm) of wear where they ran in the connecting rod bushings. This is a very high amount of wear by modern standards. We have several ideas for what might have caused it. First, the wrist pins are held in position in the pistons by a bolt screwed into one of the bosses in the piston and extending into a hole in the wrist pin. This keeps the pin from turning and concentrates the wear on the top (piston crown) side and bottom sides of the pin where it runs against the rod bushing. Then there is the marginal oiling. This is exacerbated by three holes in the rod bushing. These holes are on the axial center of the bushing and at about 120 degrees from each other with one hole at the top of the rod. The problem with these holes is that a plain bushing generates the maximum oil pressure at its axial center and these holes are strategically placed to bleed that off, especially the one at the top of the rod. Modern engines have a single angled hole in the rod at the bottom (opposite the piston crown) angled toward, but not reaching, the axial center of the pin. Another problem with the wrist pins is the tolerances on their diameter. The pins out of the new pistons showed a variance of about .001 inch (.025 mm). This might seem like precision machining, but in a modern engine, a .001 inch clearance at the wrist pin bushing is considered worn out. The reason for such tight clearances is that there is very little rotation and the load is hammering. Significant clearance allows the oil to flow around the pin (and out the holes in the Anzani bushings) and concentrates the load in the top and bottom of the bushing. Another peculiarity of the pins is that they are tapered about .001 inch from one end to the other, with the bolt-hole end being bigger. I suspect this is from holding the pin by one end when grinding, rather than grinding between centers. It could also be from worn-out equipment or even intentional. When we attempted to put the new pins into the used pistons we discovered another problem. The pin holes in the pistons (even the new ones) are not aligned. We didn’t measure the misalignment, but it appeared to be as much as .010 inch (.25 mm). This lead to some debate as to whether we should hone the holes to a larger and aligned state and make new wrist pins (we could then hone and use the existing rod bushings) or just force the pins as was done with the original equipment and make new rod bushings. We opted for the simpler "force the pins" and new rod bushings because the pins don’t turn in the piston anyway, and the pistons have plenty of clearance so that a little distortion isn’t going to hurt anything.