لاینر و بوش سیلندرهای موتور
لاینر و بوش سیلندرهای موتور
Cylinder Sleeves and Liners
By Larry Carley
نوشته لاری کارلی
مترجم : محمد توکلی
اغلب ماشین های قدیمی بدنه سیلندرشان از جنس چدن بود و در صورتی که جداره داخلی سیلندر مشکلی بر می داشت و دچار خط و خش می شد کافی بود آن را به تراشکاری بفرستند و درون آن را تراش دهند و در عوض پیستونهای استاندارد STD از پیستونهای سایز بزرگتر = OVERSIZE استفاده می کردند چون جداره سیلندر کلفت بود و این امکان را فراهم می کرد ولی این کار چندین عیب و ایراد داشت :
اولاً بدنه های چدنی موتور سنگین هستند و این روزها از بدنه های آلومینیمی سبک استفاده می شود .
ثانیاً تراش جداره داخلی مستلزم این است که تمامی موتور باز شده و به تراشکاری فرستاده شود و این روشها اکنون منسوخ شده است !
از اینها گذشته تراش سیلندر نیاز به پیستونهای اورسایز دارد و این خود کلی مشکلات دارد !
از این رو این روزها از بوش های سیلندر استفاده می شود که اگر مشکلی پیش آمد بجای پیاده کردن کل سیلندر وتراش آن تنها کافی است که بوش های سیلندر را تعویض کنند .
برای کاهش وزن بسیاری از مدلهای جدید موتور از بدنه های آلومینیمی استفاده می کنند برخی از آنها بدون بوش = اسیلولس = sleeveless هستند و با استفاده از آلیاژهای سیلیکون ساخته شده اند و یا سطح سیلندرهای آنان پوشش های مقاوم در برابر آب دارند . برخی دیگر نیز اسیلوهای خشک استفاده می کنند ( بوش و یا اسلیو خشک به نوعی گفته می شود که در تماس با آب نیست و بطور غیر مستقیم خنک می شوند )
بوش های آهنی دارای سطوح ضد سایشی در سمت رینگ پیستونها هستند . و ضخامت آنها زیاد نیست چون توسط بدنه موتور حمایت می شوند .
در مقایسه بدنه های چدنی نیازی به بوش سیلندر ندارند چون فلز چدن به اندازه کافی سخت هست که در برابر سایش مقاوم باشد.
اگر بدنه سیلندر ترک بردارد یا صدمه ببیند و یا دچار سایش شود که نتوان آن را با سوراخکاری و یا سنگ زدن و به شکل اورسایز در آورد ( چون ریخته گری این بدنه ها ضخامت بالایی دارد ) می توان از یک بوش خشک که با پرس جا زده می شود استفاده کرد و بدین وسیله از تعویض کل بدنه جلوگیری بعمل آورد .
ولی وقتی از بوش های خشک استفاده می شود مشکلات خاصی پیش می آید که نیاز به نگهدارنده هایی دارد که باید بوش ها را از لیز خوردن حفظ کند.
بوش باید همان ضریب انبساطی معادل بدنه سیلندر باشد آلومیمیک ضریب انبساط گرمایی بیشتری نسبت به آهن داردو همین امر سبب می شود که به هنگام گرم شدن موتور بوش از بدن سبلندر بیرون آید
به همین جهت وقتی از بدنه های آلومینیمی استفاده می شود نسبت به وقتی از بدنه چدنی استفاده می شود نصب بوش های خشک با مشکلات بیشتری پیش می آید 003˝ to .004˝.
امروزه از بوش های سیلندر برای طیف گسترده ای از خودروها مورد استفاده است کامیون ها و موتورهای کوچکی که با استفاده از" فرآیند ریخته گری گریز از مرکز "
تهیه می شوند . دارای استحکام بیشتری هستند ( یک سوم بیشتر از چدن های عادی )
and tolerances that are accurate to within one half of one thousandth of an inch. Other new applications include dry sleeves for GM LS1/LS6 (1997-2004), Ford 2.0L (1997 & up), and Lexus 4.3L 3UZFE engines in 2005-2006 models. New heavy duty sleeve applications include CAT 3116 and CAT C-7 3126 engines.
ROUND AND STRAIGHT
Whether a sleeve is being installed in an aluminum or iron block, dimensional accuracy is an absolute must. The cylinders in the block should be machined as round and straight as possible for a good fit. Concentricity of the bores is extremely important to minimize bore distortion. Likewise, the sleeves must be manufactured to exacting tolerances if they are going to fit properly. If the block or sleeves have too much variation, it can distort the cylinder bore and adversely affect ring seating and sealing, compression, blowby, oil consumption and emissions. Severe bore distortion may even cause piston scuffing if the clearance between the piston and wall is insufficient.
A poor fit between the block and a flangeless sleeve also increases the risk of the sleeve coming loose and moving down. A poor interference fit with gaps here and there can also cause an engine to overheat because of reduced thermal conductivity. That’s one of the reasons why GM opted to use cast-in-place dry sleeves rather than press-fit sleeves in their LS engines.
The LS aluminum block is cast around the sleeves. This makes the sleeves an integral part of the block and eliminates any concerns over fitment, movement and thermal conductivity. The trade-off is that the sleeves can’t be replaced like a press-fit dry sleeve or flanged sleeve if the cylinders are worn out or damaged. If the original sleeve is machined out, there’s not much left to support a new sleeve. So one alternative is to machine out the cylinders entirely and convert the block to a wet sleeve configuration. This has become a popular technique for racers who want to increase the bore size, displacement and thermal conductivity of dry sleeve aluminum blocks.
In 2004, Darton Sleeves was awarded a patent for their “Modular Integrated Deck” system for converting dry sleeve aluminum blocks (such as Honda sport compact engines) to wet sleeves. This same technology has since been adapted for converting GM LS engines and various other engines for extreme performance.
Converting a GM LS engine to a wet sleeve configuration takes about six hours of machine work, and should be done with CNC equipment, says Dave Clinton of Darton. But the results are well worth the effort. The wet sleeve configuration can handle significantly more power while improving reliability in high output engines. Clinton says their MID wet sleeve conversion kit for Nissan VR V6 blocks allows that engine to handle up to 2,600 horsepower from 3.0L of displacement!
“GM has had cylinder cracking problems with the stock liners in their supercharged Corvette LS7 engines. The factory sleeves can’t take the heat, but our wet sleeves can,” explains Clinton. “We use a high nickel alloy cast iron or ductile iron in our sleeves, depending on the application. The strength standards typically exceed ASTM standards by 20 to 30 percent depending on the grade of material used. We produce wet sleeves for 80 to 90 percent of the Top Fuel drag racers, and about a third of our business is custom sleeves.”
Another company that produces custom sleeves for performance applications is EZ-Slider Cylinder Liners, a division of Quaker City Castings. Brent Boyle says custom sleeves represent about 90 percent of their current business. “We make all kinds of sleeves for all kinds of racing, including truck and tractor pulling. We have our own foundry and make our own sleeves here in Salem, OH. Ductile iron is our most popular sleeve material, but we also have a couple of customers who want sleeves made of compacted graphite for use with compacted graphite blocks.”
Boyle says they key to making performance sleeves is keeping the tolerances extremely tight, especially with flange style sleeves. “Our customers tell us what they want, and we build the sleeve to their exact specifications. Turn around time is usually less than a week, and sometimes we can ship them finished sleeves within a couple of days.”
Dave Metchkoff with LA Sleeves says his company’s focus is also on the performance market. “In the 1970’s, the repair sleeve business was severely impacted by inexpensive imported sleeves. In the 1990s, another invasion of cheap sleeves nearly wiped out everybody who was making sleeves in the U.S. But tolerances were an issue with many of these imported sleeves. So our company made a comeback by producing high quality sleeves for the performance market.”
Metchkoff admits his company’s sleeve prices are higher than most importers due to their high quality materials and tolerances. “Instead of using nickel in our cast and ductile iron, we add more chromium to improve wear resistance and strength. We also offer surface coatings and cryogenic treatments if a customer wants it.”
Metchkoff says that one of the myths of installing dry sleeves in blocks is that you have to use CNC equipment to machine the block. “That’s not true – unless you are converting a block to wet liners. We used to make wet liners for some sport compact applications, but have moved out of that market because we feel there are too many installation issues. The sleeves can leak, and in sand rail racing, block flex pulls the sleeves apart from the block. We now make a stepped dry sleeve that works great in these high horsepower engines.”
Wet sleeves, or liners as they are often called in heavy-duty engines, are different than dry sleeves. A wet sleeve is essentially a stand alone cylinder, supported at the top and bottom by the block, and surrounded by the water jacket. The coolant is in direct contact with the outside of the sleeve. There is no supporting bore structure around the sleeve, so the sleeve has to be thick enough and strong enough to withstand the forces of combustion all by itself.
The main advantage of a wet sleeve is that it allows any or all of the cylinders to be easily replaced if one or more cylinders are worn out or damaged, which greatly extends the potential service life of the engine. Wet sleeves also allow engines to handle higher horsepower loads without overheating because the coolant is in direct contact with the sleeve. Replaceable sleeves also make it possible to change the bore diameter and displacement of the engine (within limits) more easily than with a conventional one-piece block.
In large (and expensive) heavy-duty diesel engines, wet sleeves make sense because they allow a block to be rebuilt over and over again.
Russ Nardi of Federal-Mogul says his company still sees a lot of demand for cylinder liners to fit diesel engines that are 60 to 70 years old. “We still sell parts for old Detroit Diesel two-stroke engines that were first built back in the 1940s.”
Nardi says that the service life of a late model Class 8 heavy-duty truck diesel engine today is upwards of 500,000 miles.
What’s more, many of these engines are developing significantly more horsepower from the same displacement, which places even greater mechanical and thermal loads on the cylinder liners. Consequently, it is critical to design replacement liners that equal or exceed the performance requirements of the OEM liners. The surface finish of the liners must also meet the OEM specifications and match the type of pistons and rings that are used in the cylinders.
“A keystone chrome ring will require a different kind of bore finish than a rectangular ring with moly faced rings. We look at all of the OEM surface finish requirements, not just the Ra (roughness average) numbers. This includes the Rk bearing area that exists after the rings are seated, and the Rpk peak height and RvK valley depth of the surface when the bore is finished,” says Hoover Oliver, Federal-Mogul’s engineer for commercial liners.
Oliver says liner hardness alone does not determine wear resistance. It also depends on the size and distribution of the graphite flakes in the metal matrix. Consequently, different materials may be required for different applications. One of the best wearing materials is a form of cast iron with a bainite crystal structure. It’s strong, tough and easy to machine.
Nardi said that one of the advantages of being an aftermarket supplier of cylinder liners is that improvements can be made in a liner design if an OEM liner turns out to have a history of problems. “We don’t see a demand for replacement liners until the engine is out of warranty, so we have a number of years to watch for any problems and make any modifications that may be needed.”
One of the problems with wet liners in diesel engines is cavitation erosion on the outside of the liners. Harmonic vibrations produced by combustion inside the cylinders cause tiny air bubbles to form in the coolant on the outer surface of the liners. When the bubbles collapse, the implosions create shock waves that chip away at the metal. Over time, this can lead to severe erosion and surface pitting that may eventually cause the liner to leak or fail.
Cavitation damage can be reduced or avoided by eliminating operating conditions that cause unwanted engine harmonics. This includes making sure fuel injection timing is correct, and that engine speed is kept within the specified rpm range. Cavitation damage can also be mitigated by using supplementary coolant additives, as specified by the engine manufacturer. Following the OEM coolant recommendations is important for long liner life.
Evans Cooling Systems sells a waterless engine coolant that has a boiling point of 375° F. The company says its coolant can handle higher engine temperatures and vibrations without forming vapor bubbles that can cause cavitation erosion.
Results from the John Deere Engine Cavitation Test (ASTM Test Procedure D-7583) showed that Evans Heavy Duty Thermal Coolant (HDTC) performed 70 percent better than the second best fluid in reducing wet liner cavitation. The coolant also creates almost no vapor pressure inside the cooling system (because there are no steam bubbles). The coolant’s higher boiling point also helps fuel economy by allowing the engine to safely operate at a higher temperature.
Steve Scott of IPD says another problem that is sometimes overlooked when replacing wet liners is wear in the lower receiver bore area of the engine block. This area is critical because it supports the liner. If damaged, it needs to be repaired before a new liner is installed.
The process for repairing a damaged lower receiver bore is to step bore the lower block area. To do this, the lower receive bore is machined to a specific oversize diameter and to a specific depth that matches the repair sleeve.
Something else to keep in mind is the end gap on the piston rings when liners are replaced. The ring end gap is specified by the engine manufacturer and depends on the bore diameter. Ring end gaps can be checked by placing the rings in the liner and measuring the end gap with a feeler gauge. On a 5.400˝ bore liner, the end gap on the piston rings will change approximately .003˝ (0.08 mm) for every .001˝ (0.03 mm) change in bore diameter.
Scott says checking ring end gaps as well as piston clearances is especially important on late model high output diesel engines because of the higher loads and temperatures at which these engines operate. “Some of these engines are running steel pistons that are installed with much less clearance than aluminum pistons,” he says.
Liner protrusion at the top of the block is also important for proper head gasket sealing. If the amount of protrusion is not equal cylinder-to-cylinder on engines that use a single head, and within specifications, the engine will eventually blow the head gasket.
Scott says liner fractures can be caused by one of two things. Vertical fractures are usually due to impact damage, while horizontal fractures are due to fitment issues – or weakness or defects in the liner metal. “Because newer diesel engines are running with much higher cylinder pressures these days, liner quality is more important than ever.”
Joe Kercher of Interstate McBee says many OEMs are now using hardened wet sleeves in their diesel engines to handle the higher loads. “Many are using induction hardened cast iron or ductile iron alloys for improved wear resistance and strength.
As for installation issues, Kercher says he gets a lot of calls on Navistar DT466E engines. “Originally, these engines used a pair of O-rings to seal the bottom of the liners. But after model year 2000, the O-rings were eliminated. So there are no O-rings to replace on these liners. On applications that do use O-rings, it’s important to lubricate them before they are installed. Also, liners should be washed with hot soapy water and scrubbed with a brush to remove any honing residue before they are installed.”
................مقدمه و ترجمه از محمد توکلی – ممکو bipc خوارزمی – 22 آذرماه 1390
¤ نوشته شده در ساعت ۱٢:٤۳ ب.ظ توسط محمد توکلی