crankpin can send transverse forces to the connecting rod?

Anytime the crank rod does not line up with the bore (rod big end, piston pin in a line) and the piston is pushing on the crank, or the crank is pushing on the piston, there is a side-force.

Dave

Which sideways direction isn't obvious? Without a crosshead, when the crank throw of a horizontal engine is straight up, the angled connecting rod forces the piston hard against the lower cylinder wall. The crosshead also facilitates double-acting engines. jw

The piston is pushing down on the connecting rod, which is at an angle. The force on the con rod has to be along the axis of the rod; the other component of the force vector has to be supplied by the cylinder wall.

The con rod bushing on a steam engine?

dances_with snipped-for-privacy@yahoo.com wrote in news:1188509239.650892.148790 @e9g2000prf.googlegroups.com:

What are you missing? Obviously, the reason automakers tend to offset the connecting rod pin from the piston's axial centerline.

There was even an early-20th century make (the obscure and extremely short- lived Canadian "Lavoie") that had pistons with angled tops, in an attempt at preventing just this sort of sideways thrust. I've got an article on that somewhere...

Tegger wrote in news:Xns999CD57F464AEtegger@207.14.116.130:

I just looked up "crosshead". Pretty cool engine.

Evidently they are putting the side thrust in a location away from the combustion chamber, which is good. Especially when your piston probably weighs 500 lbs.

It should be obvious from the animated graphic here

why they've done this.

In a regular car engine without the additional "crosshead" link, combustion pressure is brought to bear on the piston after the connecting rod has passed TDC, when it is therefore on an angle, and will impart angular force on the piston.

Tegger wrote in news:Xns999CD8031C808tegger@207.14.116.130:

Now HERE's a diesel 2-stroke!

The animated graphic sucks, but look at the static ones.

And yet the bores of worn-out auto engines aren't all that oval. The speed of the piston versus the sideways force it exerts on the wall work together to cause wear in some engines. A slow-moving piston will wear the wall more if there's a lot of pressure on it just because the oil has time to squeeze out of the clearances. The auto engine piston moves rather fast and doesn't have this problem. So crosshead pistons might be found in large, slow-turning engines but they represent unnecessary weight and complexity in an auto engine. When our cars run for 250,000 miles these days, what would we gain by making the engines more complex? A higher purchase price, that's what. We're already paying for techno-gimmicks that we don't need just so our friends are impressed. When I was a kid there was a popular myth that horizontal engines like the VW or angled engines like the Dodge Slant-Six would have more wear on the bottoms of their cylinders because of the weight of the piston. Yet the piston's weight is tiny compared to the side forces generated by combustion pressures, and those engines didn't wear any faster than vertical engines. The horizontally-opposed aircraft engines I work on don't have unusual wear patterns, either.

Dan

Tegger wrote in news:Xns999CD57F464AEtegger@207.14.116.130:

While the offset piston pin can reduce piston slap thats not the prime reason its done. An offset pin causes the piston to reach top dead center at a different time than the pistoin end of 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.

Hmmm, seems this would also allow more combustion time with the piston near the top (smaller volume, higher pressure). Perhaps this gives a small boost to efficiency, or power?

Dave

There was another big reason for the crosshead. The double acting steam engine came along not long after the Watt improvements. The double acting engine requires a way to seal that half of the cylinder. It would be almost impossible to seal against a connecting rod. By having a cylindrical rod passing through the end of the cylinder, and holding it axially with a crosshead, it was easy to pack the seal in the cylinder head.

I was under the impression that was the main reason for crosshead engines.

It is important to note, however, that there is NOT a big energy loss due to this side force. While there is SOME friction loss, even the total friction loss on an engine delivering an appreciable fraction of max power.

Many inventors have come up with complex schemes to eliminate this supposed inefficiency. While the connecting rod does not create a pure sinusoidal force on the crank, the pressure/enthalpy in the cylinder is mostly conserved, and merely changes the positions at which the energy is transferred from the working fluid to the crank motion.

Nah, offset pins ARE all about reducing piston slap. Which is why high performance pistons have centered pins.

Allow me to expand on this a bit:

_Any_ time you have anything with a link that freely pivots at both ends, the force is _always_ straight between the centers of rotation of the pivots -- it can't be anything else. The only exceptions to this is when there's friction or spring force in the pivot, or when the assembly is moving fast enough that the inertia of the connecting rod becomes important, and then the only additional force is that needed to accelerate the ends of the con-rod.

I've seen at least one set of pictures of a single-acting gas engine with a crosshead. This was from the late 1800's, early 1900's era when folks may not have had a firm grasp of why things were done the way they were, and there were more engine manufacturers than you could shake a stick at and a bewildering variety of design features on engines.

I wish I had the link, I'd post it...

There was someone in Italy working on a modern Diesel design, with a crosshead, it used a connecting pointing away from the piston. Since a connecting rod and especially a short connecting rod produces non- hamronic motion, he was able to optimze the combustion burn vs piston motion. Basically a short con rod will dwell the piston at the bottom of the strke, and increase the speed at the top of the stroke. By using a crosshead to invert the con rod, you can get more burn time vs rpm. And more favorable valve timing. This allows more rpm from a diesel, which is compromised from breathing at higher rpm. This then allows more rpm, and higher specific outputs per displacement. IIRC ther was some potential improvements in emissions also.

Thanks for the clarification. That was implicit in what I wrote (the first "has to be"), but I didn't say it explicitly.

Steve wrote in news:H-KdnZRXudJe9kXbnZ2dnUVZ snipped-for-privacy@texas.net:

Zackly, and why all regular vehicles have offsets. NVH is the primary driver.

Note that a 250,000 mile life on a care engine is about the same number of hours that a container ship engine will run in one year. They need 15-20 year working life's.

Mark Rand RTFM