crankpin can send transverse forces to the connecting rod?

wrote:


If you assume an average auto speed of 35 MPH it is about equal to 10 Months steaming.
Bruce in Bangkok (brucepaigeATgmailDOTcom)
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snipped-for-privacy@gmail.com wrote:

On the other hand, container ship engines operate in an almost IDEAL environment- constant power output for weeks on end without throttle changes, constant RPM, almost no vibration, a prop in water providing an almost perfectly constant load on the engine, abundant cooling to seawater, etc.
Whereas the car engine operates under constantly changing loads, constantly changing speeds, constantly changing G-loads from road vibration and handling of the car, varying ambient temperatures, etc. etc. etc.
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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.
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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
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Jerry wrote:

Nah, offset pins ARE all about reducing piston slap. Which is why high performance pistons have centered pins.
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Zackly, and why all regular vehicles have offsets. NVH is the primary driver.
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Anthony

You can't 'idiot proof' anything....every time you try, they just make
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On Aug 30, 4:27 pm, dances_with snipped-for-privacy@yahoo.com wrote:

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.
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Don Stauffer in Minnesota wrote:

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...
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Tim Wescott
Wescott Design Services
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Here's a link to a huge crosshead engine, single-acting cylinders, still in production. The large and slow side-forces likely mandate the crosshead design. http://people.bath.ac.uk/ccsshb/12cyl /
Dan
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On Sep 1, 1:49 am, Dan_Thomas snipped-for-privacy@yahoo.com wrote:

Nice one, thanks.
This one isn't a crosshead design but it's an interesting oldie from WWII submarines that is still in production.
http://www.fairbanksmorse.com/engine_opposed_piston_model_38.php?return=marine_power.php
jw
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And an engine with really offset rods: http://www.zpipedragon.com/Home/Automotive/Engines/Scuderi/Scuderi.htm
Dan
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wrote:

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.
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Why does a non-harmonic motion change the efficiency of burning? I don't see that. My understanding is that few Diesels are truly constant pressure burn anyway- they cannot control the injection well enough. Certainly high speed car and truck Diesels are not.
And since the valves are closed during the entire burn time, I fail to see how the crosshead geometry would affect the valve timing.
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Don Stauffer in Minnesota wrote:

I would say that it changes the efficiency of the coupling of the burn pressure to the reciprocating assembly more than it affects the burn itself. I

The ideal diesel is a constant pressure burn (continuous burn as the piston moves down), the ideal Otto cycle is a constant volume burn (instantaneous burn at TDC followed by adiabatic expansion). In the real world, neither one follows that ideal cycle, but the diesel is CLOSER to constant pressure and the Otto is CLOSER to constant volume than vice-versa.

The valves aren't actually closed during the whole burn time, really. Exhaust valves, for example, tend to open before the burn is complete, because you get a bigger gain in efficiency from the extra time to purge the cylinder than the last few joules of energy out of the last little bit of expansion.
But I agree, any time the rod length/stroke ratio is greater than about 1.7 or 1.8:1, the motion is close enough to ideal that it doesn't matter. Some engines work with a shorter rod, the biggest small-block Chevies, for example had a rod ratio of something really sucky like 1.5:1, but rod ratios that bad are the exception more than the rule.
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I agree that the valve is not closed for the whole EXPANSION STROKE, but the fuel in either a Diesel or SI engine does not normally burn during the whole expansion stroke. SI engines burn only a few degrees of crank rotation, not 180 degrees. Diesels burn during injection time and a few degrees after (especially in high speed or truck engines), but not THAT many degrees after end of injection. Expansion cooling is freezing the process.
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wrote:

The design I refered to uses more non-harmonic motion than normal, IOW a shorter than normal rod. Now, with a short rod, you would have excessive piston dwell at BDC, and high piston speeds at TDC. By adding a cross head, and inverting the rod, you get longer dwell at TDC, and faster psiton motion at BDC. That is the basis for the invention. He was able to achieve a better timing relationship for a higher rpm diesel, and thus more power per displacement/size tradeoff. Normal diesels are speed limited, due to timing considerations. You don't see a lot of 7 liter diesels turing a lot of rpm do you? The way ti was explained to me, was that getting the diesel to develop power and to start at low rpm limited it's rpm range. The inversion of "piston dwell" timing allowed a faster rpm, by allowing more time at TDC. FWIW.
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