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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / November 2007crankpin can send transverse forces to the connecting rod?
they say that the crosshead type of engine was invented to overcome the problem of, crankpin originating transverse forces up the connecting rod to the piston, causing ovalization of the bore. I can't visualize how a properly toleranced & fitted crankshaft/ crankpin is originating sideways movement. what am I missing? On Aug 30, 5:27 pm, dances_with_barka...@yahoo.com wrote: > they say that the crosshead type of engine was invented to overcome > the problem of, crankpin originating transverse forces up the [quoted text clipped - 4 lines] > > what am I missing? 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 On Aug 30, 5:27 pm, dances_with_barka...@yahoo.com wrote: > they say that the crosshead type of engine was invented to overcome > the problem of, crankpin originating transverse forces up the [quoted text clipped - 4 lines] > > what am I missing? 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 > they say that the crosshead type of engine was invented to overcome > the problem of, crankpin originating transverse forces up the [quoted text clipped - 4 lines] > > what am I missing? 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. > dances_with_barka...@yahoo.com writes: > > they say that the crosshead type of engine was invented to overcome [quoted text clipped - 10 lines] > the other component of the force vector has to be supplied by the > cylinder wall. 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. >> they say that the crosshead type of engine was invented to overcome >> the problem of, crankpin originating transverse forces up the [quoted text clipped - 9 lines] > the other component of the force vector has to be supplied by the > cylinder wall. 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.  SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html > >> they say that the crosshead type of engine was invented to overcome > >> the problem of, crankpin originating transverse forces up the [quoted text clipped - 18 lines] > rod becomes important, and then the only additional force is that > needed to accelerate the ends of the con-rod. Thanks for the clarification. That was implicit in what I wrote (the first "has to be"), but I didn't say it explicitly. > they say that the crosshead type of engine was invented to overcome > the problem of, crankpin originating transverse forces up the [quoted text clipped - 4 lines] > > what am I missing? The con rod bushing on a steam engine? dances_with_barkadas@yahoo.com wrote in news:1188509239.650892.148790 @e9g2000prf.googlegroups.com: > they say that the crosshead type of engine was invented to overcome > the problem of, crankpin originating transverse forces up the [quoted text clipped - 4 lines] > > what am I missing? 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... SignatureTegger > dances_with_barkadas@yahoo.com wrote in news:1188509239.650892.148790 > @e9g2000prf.googlegroups.com: [quoted text clipped - 15 lines] > an attempt at preventing just this sort of sideways thrust. I've got > an article on that somewhere... 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 <http://www.marinediesels.info/Basics/the_2_stroke_crosshead_engine_explanation.htm> 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. SignatureTegger > I just looked up "crosshead". Pretty cool engine. > [quoted text clipped - 10 lines] > connecting rod has passed TDC, when it is therefore on an angle, > and will impart angular force on the piston. Now HERE's a diesel 2-stroke! <http://www.sa.hillman.org.au/TS3.htm> The animated graphic sucks, but look at the static ones. SignatureTegger > Now HERE's a diesel 2-stroke! > <http://www.sa.hillman.org.au/TS3.htm> > > The animated graphic sucks, but look at the static ones. Wow. What a hard way to turn fuel into motion. Must have been one of the last opposed piston two-stroke diesels ever designed, two cranks was a much more common approach and probably worked better. I posted a link for the new 6V53 Detroit in rec.crafts.metalworking recently: HP:displacement in cubic inches is over 1. Pretty mighty two-stroke diesel.. John >> Now HERE's a diesel 2-stroke! >> <http://www.sa.hillman.org.au/TS3.htm> [quoted text clipped - 4 lines] > last opposed piston two-stroke diesels ever designed, two cranks was a > much more common approach and probably worked better. Given the huge success and continued production of the Fairbanks-Morse engine 38 series, I'd say that two cranks did work better. http://www.fairbanksmorse.com/engine_opposed_piston_model_38.php?return=marine_p ower.php The same could be said for THREE cranks, ala the Napier Deltic... with the sole exception that it is no longer in production. http://en.wikipedia.org/wiki/Delta_engine http://www.thedps.co.uk/links.php?category=Napier+Interest >>> Now HERE's a diesel 2-stroke! >>> <http://www.sa.hillman.org.au/TS3.htm> [quoted text clipped - 9 lines] > > http://www.fairbanksmorse.com/engine_opposed_piston_model_38.php?return=marine_p ower.php Hey, I didn't know they were still in production, cool. > The same could be said for THREE cranks, ala the Napier Deltic... with > the sole exception that it is no longer in production. > > http://en.wikipedia.org/wiki/Delta_engine > > http://www.thedps.co.uk/links.php?category=Napier+Interest Great stuff, thanks for the links. John > > dances_with_barka...@yahoo.com wrote in news:1188509239.650892.148790 > > @e9g2000prf.googlegroups.com: [quoted text clipped - 33 lines] > -- > Tegger 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 <snip> >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. 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 > <snip> >>When our cars run for 250,000 miles these days, what would we gain by [quoted text clipped - 9 lines] > Mark Rand > RTFM Well you lost me there. How does 250,000 miles on a car equate to hours on a ship engine? >> <snip> >>>When our cars run for 250,000 miles these days, what would we gain by [quoted text clipped - 12 lines] >Well you lost me there. How does 250,000 miles on a car equate to hours on a >ship engine? Assume 50mph average speed... Mark Rand RTFM >>> <snip> >>>>When our cars run for 250,000 miles these days, what would we gain by [quoted text clipped - 19 lines] > Mark Rand > RTFM The ship engine turns 100 rpm vs 2-3000 for the car engine. I'd say things are equal. IOn Fri, 31 Aug 2007 20:48:28 -0400, "Chas Hurst" <hurst1@comcast.not> wrote: >> <snip> >>>When our cars run for 250,000 miles these days, what would we gain by [quoted text clipped - 12 lines] >Well you lost me there. How does 250,000 miles on a car equate to hours on a >ship engine? If you assume an average auto speed of 35 MPH it is about equal to 10 Months steaming. Bruce in Bangkok (brucepaigeATgmailDOTcom) > IOn Fri, 31 Aug 2007 20:48:28 -0400, "Chas Hurst" <hurst1@comcast.not> > wrote: [quoted text clipped - 19 lines] > If you assume an average auto speed of 35 MPH it is about equal to 10 > Months steaming. 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. > What are you missing? Obviously, the reason automakers tend to offset > the connecting rod pin from the piston's axial centerline. 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. > > What are you missing? Obviously, the reason automakers tend to offset > > the connecting rod pin from the piston's axial centerline. [quoted text clipped - 6 lines] > 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 >> > What are you missing? Obviously, the reason automakers tend to offset >> > the connecting rod pin from the piston's axial centerline. [quoted text clipped - 12 lines] > >Dave Several of the MOPAR tech bulletins used to suggest installing pistons with offset reversed from the quiet direction for drag racing for a small power boost >>What are you missing? Obviously, the reason automakers tend to offset >>the connecting rod pin from the piston's axial centerline. [quoted text clipped - 6 lines] > loading, permitting lighter more power-efficient parts to be used, and the > engines to be capable of higher rpm. Nah, offset pins ARE all about reducing piston slap. Which is why high performance pistons have centered pins. > Nah, offset pins ARE all about reducing piston slap. Which is why high > performance pistons have centered pins. Zackly, and why all regular vehicles have offsets. NVH is the primary driver. SignatureAnthony You can't 'idiot proof' anything....every time you try, they just make better idiots. Remove sp to reply via email On Aug 30, 4:27 pm, dances_with_barka...@yahoo.com wrote: > they say that the crosshead type of engine was invented to overcome > the problem of, crankpin originating transverse forces up the [quoted text clipped - 4 lines] > > what am I missing? 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. > On Aug 30, 4:27 pm, dances_with_barka...@yahoo.com wrote: >> they say that the crosshead type of engine was invented to overcome [quoted text clipped - 16 lines] > I was under the impression that was the main reason for crosshead > engines. 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... SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html > > There was another big reason for the crosshead. The double acting > > steam engine came along not long after the Watt improvements. The [quoted text clipped - 14 lines] > > I wish I had the link, I'd post it... 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 On Sep 1, 1:49 am, Dan_Thomas_nos...@yahoo.com wrote: > ... 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- 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_p ower.php jw > On Sep 1, 1:49 am, Dan_Thomas_nos...@yahoo.com wrote: > [quoted text clipped - 12 lines] > > jw And an engine with really offset rods: http://www.zpipedragon.com/Home/Automotive/Engines/Scuderi/Scuderi.htm Dan On Aug 31, 8:44 am, Don Stauffer in Minnesota <stauf...@usfamily.net> wrote: > On Aug 30, 4:27 pm, dances_with_barka...@yahoo.com wrote: > [quoted text clipped - 17 lines] > I was under the impression that was the main reason for crosshead > engines. 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. > On Aug 31, 8:44 am, Don Stauffer in Minnesota <stauf...@usfamily.net> > wrote: [quoted text clipped - 33 lines] > more rpm, and higher specific outputs per displacement. IIRC ther was > some potential improvements in emissions also. 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. >>On Aug 31, 8:44 am, Don Stauffer in Minnesota <stauf...@usfamily.net> >>wrote: [quoted text clipped - 35 lines] > > Why does a non-harmonic motion change the efficiency of burning? 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 > 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. 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. > And since the valves are closed during the entire burn time, I fail to > see how the crosshead geometry would affect the valve timing. 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. > The valves aren't actually closed during the whole burn time, really. > Exhaust valves, for example, tend to open before the burn is complete, [quoted text clipped - 7 lines] > 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. 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. On Sep 2, 12:46 pm, Don Stauffer in Minnesota <stauf...@usfamily.net> wrote: > > The valves aren't actually closed during the whole burn time, really. > > Exhaust valves, for example, tend to open before the burn is complete, [quoted text clipped - 15 lines] > but not THAT many degrees after end of injection. Expansion cooling > is freezing the process. 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. > On Sep 2, 12:46 pm, Don Stauffer in Minnesota <stauf...@usfamily.net> > wrote: [quoted text clipped - 35 lines] > > - Show quoted text - OK.. Here is the thread where the inventor describes the advantage of the non-harmonic motion for a high speed diesel motor. http://www.practicalmachinist.com/ubb/ultimatebb.php/topic/15/928.html |
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