12 replies to this topic

[Chris Wilson]

Do the later F1 engines (late eighties onwards) run piston pin offset? Thanks.

[Greg Locock]

Can you think of a reason why they would not?

[Wolf]

Greg, wouldn't the crankshaft offset be a wee better option? The way I figure it pin offset could have a slight disadvantage re. piston load/wear because load would (presumably) have both force and momentum components around the pin, which would not be the case if crankshaft was offset instead...

[Greg Locock]

I'd have guessed they use both. It just seemed to me that it is dead easy to adjust gudgeon pin offset, so you would always keep that in your bag of tricks.

[Powersteer]

I wonder how it would go together with piston skirt offset ;)

[Chris Wilson]

Doesn't offset increase piston skirt frictional loadings? I know offset is used to effectively cushion the shock of the piston and rod "stopping" at TDC, but just how necessary is this, and are materials up to doing without this, in the interests of less friction?

[Stian1979]

Originally posted by Chris Wilson
Doesn't offset increase piston skirt frictional loadings? I know offset is used to effectively cushion the shock of the piston and rod "stopping" at TDC, but just how necessary is this, and are materials up to doing without this, in the interests of less friction?

I think offset is to decrease piston skirt friction.

If the bolt is in center off the piston the piston will tilt to one side when the crant is at anny other position than dead center.

I had it explained at school by a teatcher that was a engineer on marine diesels, but the same guy also claimed that all diesels are two strokes

[Powersteer]

Originally posted by Wolf
Greg, wouldn't the crankshaft offset be a wee better option?

Probably not for a vee engine.

[desmo]

From the X-section 049 engine drawing in Wright's Ferrari F1 book (FWIW), it looks to me there's no significant offset.

[m9a3r5i7o2n]


quote:

Originally posted by Wolf
Greg, wouldn't the crankshaft offset be a wee better option?

Probably not for a vee engine


I just wonder just how much offset piston pins would affect engine balance. I once made a sketch of the crankshaft layout with deSaxe and I remember it seems to affect engine balance with a 180 degree crankshaft. And I believe all these engines have that type of crankshaft. But I never went so far as to put in offset pistons.

M.L. Anderson

[Greg Locock]

Desmo, having just got the coffee table book, look at the engine on p28. They've offset both banks downwards , so on one bank they've got positive offset, on the other negative. Maybe offset isn't that important?

[malbear]

I have delved into my engine docs library and have come up with the following quotes.

"Offset Pins

Some of our pistons are supplied with offset gudgeon pins. This is to help reduce piston “slap” and have a pin that is offset to the thrust side. Although all piston crowns are marked accordingly, additional machining processes can inadvertently remove these markings. Offset pin pistons fitted incorrectly will result in mechanical noise and reduce efficiency. Race pistons defer almost exclusively to a central pin design, a necessity for engine speeds over 10,000 rpm.
Accralite Performance Forged Pistons "

"Most engines today utilize pistons with an offset pin bore. That is, the pin bore is "moved" a specific distance from the centerline of the piston. In gas engines the offset is always to the major thrust side of the piston. The piston thrust side is the part of the piston perpendicular to the pin bore that carries the majority of side loading during the power stroke.
The primary reason for pin offset is to prevent the piston from slamming into the cylinder bore after the connecting rod journal passes top dead center. This problem is referred to as piston slap. The desired scenario for piston movement is to gently rock from side to side within the bore. This rocking motion eliminates slap. " by Hunter Betts

"
Modern powersports engine pistons are not mounted on their connecting rods exactly on center. They are offset radially, that is, perpendicular to their rotational axis. In other words, in a vertical, single-cylinder engine, the piston is offset toward the exhaust side of the cylinder. That is, the piston pin hole is bored off center toward the intake side, usually less than 1 millimeter. It is frequently stated with a sort of bored complacency that the purpose of this offset is to reduce piston slap. The truth is, while piston slap is reduced through piston pin offset, but that is not the purpose of the technology.
The piston engine has three major parts: crankshaft, connecting rod, and piston. Each has a different job to do. The reciprocating part -- the piston -- makes the crankshaft -- the rotating part, uhh, rotate. The connecting rod is simply the part in the middle. It translates the piston's recip motion into the crankshaft rotary motion. The neat thing is that in the process, it shares in the motion of both. That is, the connecting rod is both a reciprocating part and a rotational part, at the same time. (In fact, when balancing an engine, it is common to divide the connecting rod's weight in two, thus permitting half its weight to be calculated as recip and half as rotation.) The point is, the upper half of the connecting rod reciprocates with the piston, the lower half rotates with the crankshaft, and this is important to understanding the stresses on all three parts, but especially those on the connecting rod.
The piston and its half of the connecting rod stops twice per crankshaft revolution, even though the crankshaft continues to turn. This means the piston and top of the rod also start back up twice. This stopping and starting imposes stresses on all three of the parts, stresses that increase with crankshaft rpm. To reduce these loads, the piston is mounted to the connecting rod slightly offset. This causes the piston to reach top dead center at a different time than the connecting rod,
effectively spreading the shock loading over a greater number of crankshaft degrees. In short, the real reason for piston pin offset is that it softens reciprocal loading, permitting lighter more power-efficient parts to be used, and the engines to be capable of higher rpm.
However, there is another phenomenon at work also, a kind of side benefit. Because the connecting rod spends most of its time in the engine at an angle, the piston engine has what is called minor and major thrust. Major thrust refers to the downward-stroking piston's force against the cylinder wall during combustion, due to the rod being angled in that direction. Minor thrust is the piston's thrust against the opposite cylinder wall during compression, because the rod's angle is opposite also. These thrust forces push the piston firmly against the cylinder wall. The important thing is that at TDC, they flip-flop. Major thrust turns into minor thrust, and visa-versa. In older engines, this flip-flop caused the piston smack the cylinder, resulting in a noise. Fortunately, the piston pin offset in today's engines, besides reducing inertia stresses, does two things that reduce this noise. First, because the piston is mounted off center, the transition from major to minor thrust is less sudden. There is less impact. Second, instead of a sudden lateral shift, the piston actually rolls from major to minor thrust. That is, the piston shifts first at the skirt, then gradually the rest of the piston makes contact, instead of all of the piston at once.
To summarize, piston pin offset is the manufacturer's way of reducing stress on reciprocating parts. It permits these parts to be lighter, which results in more efficient manufacture and less power loss in the engine, as well as higher rpm. A complementary result of piston pin offset is reduced piston slap due to the more gradual shift from major to minor thrust. "Mike Nixon

In my oppinion
F1 engines would not run offset gudgeon or offset crank because these designs would place a lower rev limit to the engine as this would increase piston accellerations on the upstroke and also around TDC because of the changed conrod angles

[Chris Wilson]

Thanks for the replies, I wonder if Fred at Omega would shed any light on this?