5 replies to this topic

[marion5drsn]

In a discussion about camshafts a side issues came up about conrod length.
Ricardo in his presentation of this feature presented the ratio as being a ratio of crank arm length to conrod length as being appropriate. This would be the stroke divided by two equaling the letter “ r”. The upper case “L” as the length of the con rod. Or r/L = the optimum length of the conrod. The figure he used was 3.8 times the radius of the crank arm.
If one used 3.8 times “r” the conrod would be only be 22.8mm times 3.8 =86.64 mm (3.411”). So therefore the conrod is very long, 110mm. Using this ratio, the resulting 4.956 ratio is long even using this rule of Ricardo. Another person in 1946 was figuring the optimum ratio and used the figure of 3.78 times the radius of the crank. Still very close to Ricardo’s figure.
This in conjugation with the very short stroke maybe another reason of the why these engines achieve such remarkable RPM.
The figure of 1 divided by 4.956 gives a max conrod angle of 11 degrees and 40 minutes. Marion L. Anderson
[p][Edited by marion5drsn on 12-03-2000]

[desmo]

In the 70s we used the term L/D L being rod legth c to c and D being the diameter described by the crankpin as it rotates, in other words stroke. Rod ratio seems to be the common term in use now. I prefer L/D just for brevity's sake. The L/Ds in F1 engines, apparently close to 2.5, are extraordinarily large. You increase the dwell time around TDC, and at 18K you probably need all the dwell time for combustion you can find. As well the long rods reduce the maximum piston accelerations, no small onsideration at max revs in an F1 engine. Large L/Ds also lower port velocity which is another characteristic that suits high-rpm operation.

[Wolf]

With two of experts around, I'd like to raise another issue here. Do they still use excentrically placed piston bolts in F1? I'd think it more suitable (and efficient) to excentrically place the crankshaft. That way they'd get same advantages, without unwanted sideeffects (since bolt 'should' be placed on a diameter of the piston to get force without momentum on the conrod).

[marion5drsn]

Wolf; I’m going to take a shot at this, but first I must clarify your term of “bolt”. I’m going to assume that you mean a piston pin, wrist pin or gudgeon pin.

The point you make about offset crankshaft main bearing is well taken as this was done on engines previous to World War One. One of the engines using this feature was the old Cadillac four cylinder. After they dropped the four cylinder and went to the V-8 it was dropped.
The only other engine that used this feature was a British engine of WW-1,the Beardmore.This engine was copy of the Austro-Daimler straight six. Which had a bore of 144 mm and a stroke of 175 mm. You may want to read about these engines in the book,“Jane’s Fighting Aircraft of World War 1”. Page 269.
You might also note the use of the word,“de Saxe” Which was the phrase used to define this feature.
Also the offset was 18mm.
I have in a time past made sketches of the feature and have come to the conclusion that the feature was not retained due to production difficulties and cost. Also at that time the offset piston pin was not in vogue. My sketches seemed to indicate that if you used one it was a necessity to use the both de Saxe and piston pin offset. But that is only an observation with no testing background.
I also made sketches of doing this on a Vee type engine and found it not to be a large difficulty. I would not have any idea if the auto manufacturers have even tried this to the testing stage. Marion L. Anderson

[Richard Border]

One of the engines using this feature was the old Cadillac four cylinder. After they dropped the four cylinder and went to the V-8 it was dropped. The only other engine that used this feature was a British engine of WW-1,the Beardmore.

MOPAR used offset piston pins in some of their engines. They used it to help eliminate piston slap after a cold start. I remember that the "trick" thing to do was to put the pistons in backwards, this made them noisy, but was reported to give a slight HP increase due to a "better" angle to the crankshaft.

[Ray Bell]

Offset pins are not uncommon. In V8s, in particular, but also in other engines. And turning them around is the first 'hot-up' trick many learn.
But I thought it was for a different reason, something other than piston slap, more for a protection against wear, which is forgotten when you want more power.
As far as rod length goes, I believe that the common failures in the Peugeot 605 V6 may be caused by under-length rods, although I am just expressing a feeling, and know nothing about the reality of the situation.
Anyone got figures on this engine?