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What does the SAE Viscosity rating on your Motoroil bottle mean?
How do they come up with this rating . . .really?

Most of the time when viscosity is explained words are used that are too technical for the average person to quickly grasp. This leaves them still wondering what the viscosity numbers really mean on a bottle of motor oil. Simply put, viscosity is the oil’s resistance to flow or, for the layman, an oil’s speed of flow as measured through a device known as a viscometer. The thicker (higher viscosity) of an oil, the slower it will flow. You will see oil viscosity measurement in lube articles stated in kinematic (kv) and absolute (cSt) terms. These are translated into the easier to understand SAE viscosity numbers you see on an oil bottle.

OK . . .What does a 5W-30 do that an SAE 30 won’t?
When you see a W on a viscosity rating it means that this oil viscosity has been tested at a Colder temperature. The numbers without the W are all tested at 210° F or 100° C which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 210° (100° C). The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210° F (100° C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear.

SAE Viscosity Chart (High Temp) 100° C (210° F) SAE Viscosity Kinematic (cSt) 100° C Min Kinematic (cSt) 100° C Max 20 5.6 <9.3 30 9.3 <12.5 40 12.5 <16.3 50 16.3 <21.9 60 21.9 <26.1 Winter or “W” Grades SAE Viscosity Low Temp (°C) Viscosity cP Kinematic (cSt) 100° C Min Cranking Max Pumping Max (NYS) 0W 3,250 @ -30 60,000 @ -40 3.8 5W 3,500 @ -25 60,000 @ -35 3.8 10W 3,500 @ -20 60,000 @ -30 4.1 15W 3,500 @ -15 60,000 @ -25 5.6 20W 4,500 @ -10 60,000 @ -20 5.6 25W 6,000 @ -5 60,000 @ -15 9.3

Obviously, cold temperature or W ratings are tested differently than regular SAE viscosity ratings. Simply put, these tests are done with a different temperature system. There is a scale for the W, or winter viscosity grades and, depending on which grade is selected, testing is done at different temperatures. See the Tables to the right below for more information.

If you look at the table, SAE Viscosity Chart (High Temp) you’ll see that if a measured amount of motor oil flows through the viscometer at 210° F (100° C) faster than 5.6 but less than 9.3 seconds, then it will be considered a SAE 20 viscosity. Consequently, if a motor oil flows through faster than 9.3 and slower than 12.5 seconds, then it will be a SAE 30 viscosity.

Now if you look at the table labeled Winter or “W” Grades, you can get valuable information on how the W or winter grade viscosities are measured. Basically, as shown by the chart, when the oil is reduced to a colder temperature it is measured for performance factors. If it performs like a SAE 0 motor oil at the colder temperature, then it will receive the SAE 0W viscosity grade. Consequently, if the motor oil performs like a SAE 20 motor oil at the reduced temperatures (the scale varies – see the chart), then it will be a SAE 20W motor oil.

If a motor oil passes the cold temperature or W (winter grade) specification for a SAE 15W and at 210° F (100° C) flows through the viscometer like a SAE 40 motor oil, then the label will read 15W-40. Getting the picture? Consequently, if the motor oil performs like a SAE 5 motor oil on the reduced temperature scale and flows like a SAE 20 at 210° F (100° C), then this motor oil’s label will read 5W-20. And so forth and so on!

I can’t tell you how many times I have heard someone, usually an auto mechanic, say that they wouldn’t use a 5W-30 motor oil because it is, “Too thin.” Then they may use a 10W-30 or SAE 30 motor oil. At engine operating temperatures these oils are the same. The only time the 5W-30 oil is “thin” is at cold start up conditions where you need it to be “thin.”

So how do they get a motor oil to flow in the cold when it is a thicker viscosity at 210° F?
The addition of Pour Point Depressant additives (VI) keep the paraffin in petroleum base oils from coalescing together when temperature drops. Pour Point Depressants can keep an oil fluid in extreme cold temperatures, such as in the arctic regions. We will not go into Pour Point Depressing additives at this time except to say they are only used where temperatures are very extreme to keep the motor oil from becoming completely immobilized by the cold temperature extreme. For now we will just discuss the Viscosity Improvers (VI) additives.

Why don’t we just use a SAE 10 motor oil so we can get instant lubrication on engine start up?
The reason is simple: it would be a SAE 10 motor oil at 210° F! The lower the viscosity, the more wear will inevitably occur. This is why it is best to use the proper oil viscosity recommended by the auto manufacturer as it will protect hot and at cold start ups. Obviously a 10W-10 motor oil won’t have the film strength to prevent engine wear at full operating temperature like a 5W-20, 10W-30 or 5W-30 motor oil for example.

The VI additives have the effect of keeping the oil from thinning excessively when heated. The actual mechanics of this system are a little more complex in that these additives are added to a thinner oil so that it will be fluid at a cold temperature. The VI additives then prevent thinning as the oil is heated so that it now can pass the SAE viscosity rating at 210. For example; if you have a SAE 10 motor oil it will flow like a 10W at the colder temperature. But at 210 degrees it will be a SAE 10 giving us a 10W-10 or SAE 10 viscosity rating. Obviously this is good at cold start up, but terrible at engine operating temperature especially in warmer climates. But by adding the VI additives we can prevent the oil from thinning as it is heated to achieve higher viscosity numbers at 210 degrees. This is how they make a petroleum based motor oil function for the 10W-30 rating. The farther the temperature range, like with a 10W-40, then more VI additives are used. With me so far? Good, now for the bad news.

Drawbacks of Viscosity Improving additives
Multi-grade motor oils perform a great service not being too thick at cold startup to prevent engine wear by providing more instantaneous oil flow to critical engine parts. However, there is a draw back. These additives shear back in high heat or during high shear force operation and break down causing some sludging. What’s worse is once the additive begins to be depleted the motor oil no long resists thinning so now you have a thinner motor oil at 210 degrees. Your 10W-30 motor oil can easily become a 10W-20 or even a SAE 10 (10W-10) motor oil. I don’t have to tell you why that is bad. The more VI additives the worse the problem which is why auto manufacturers decided to steer car owners away from motor oils loaded with VI additives like the 10W-40 and 20W-50 viscosities.

The less change a motor oil has from high to low temperatures gives it a high Viscosity Index. Synthetic motor oils that are made from Group IV (4) PAO base stocks have Viscosity Indexes of more than 150 because they are manufactured to be a lubricant and don’t have the paraffin that causes the thickening as they cool. But petroleum based motor oils (Group I (1) & II (2)) usually have Viscosity Indexes of less than 140 because they tend to thicken more at the colder temperature due to the paraffin despite the addition of Viscosity Improving additives. The higher the Viscosity Index number the less thinning and thickening the motor oil has. In other words, high number good, low number bad. Low numbers thicken more as they cool and thin more hot. You see these Viscosity Index ratings posted on data sheets of motor oils provided by the manufacturer.

As already mentioned, VI improving additives can shear back under pressure and high heat conditions leaving the motor oil unable to protect the engine properly under high heat conditions and cause sludging. Also there is a limit to how much viscosity improving additives can be added without affecting the rest of the motor oil’s chemistry. Auto manufacturers have moved away from some motor oils that require a lot of viscosity improving additives, like the 10W-40 and 20W-50 motor oils, to blends that require less viscosity additives like the 5W-20, 5W-30 and 10W-30 motor oils. Because stress loads on multi viscosity motor oils can also cause thinning many racers choose to use a straight weight petroleum racing motor oil or a PAO bases Synthetic which do not have the VI additives. But only the Group IV (4) PAO based synthetics don’t need VI additives. Read on to learn why:

What about synthetic motor oils? Do they need Viscosity Additives?
Group IV (4) and Group V (5) base oil (synthetics) are chemically made from uniform molecules with no paraffin and don’t need Viscosity Additives. However, in recent years Group III (3) based oils have been labeled “synthetic” through a legal loophole. These are petroleum based Group II (2) oils that have had the sulfur refined out making them more pure and longer lasting. Group III (3) “synthetic” motor oils must employ Viscosity Additives being petroleum based.

Group V (5) based synthetics are usually not compatible with petroleum or petroleum fuels and have poor seal swell. These are used for air compressors, hydraulics, etc. It’s the Group IV (4) PAO based synthetics that make the best motor oils. They are compatible with petroleum based oils and fuels plus they have better seal swell than petroleum. Typically PAO based motor oils use no Viscosity Additives yet pass the multi-grade viscosity requirements as a straight weight! This makes them ideal under a greater temperature range. One advantage of not having to employ Viscosity Improving additives is having a more pure undiluted lubricant that can be loaded with more longevity and performance additives to keep the oil cleaner longer with better mileage/horsepower.

How do I know what motor oil is a Group IV (4) based PAO synthetic motor oil?
As more and more large oil companies switched their “synthetic” motor oils to the less expensive/more profitable Group III (3) base stocks it has become much easier to identify which are PAO based true synthetic. Of the large oil companies, only Mobil 1, as of this writing (12-15-2007), is still a PAO based true synthetic. The rest, including Castrol Syntec, have switched to the cheaper/more profitable Group III (3) petroleum based “synthetic” motor oil. AMSOIL Synthetic Motor Oils are PAO based true synthetic motor oils with the exception of the short oil drain XL-7500 synthetic motor oils sold at some Auto Parts Stores and Quick Oil Change Centers. This leaves more than 20 PAO based true synthetic motor oils manufactured and marketed by AMSOIL with only 4 Group III (3) based synthetic motor oils identified by the “XL-7500″ product name.

So as you can see, the average performance of motor oils can be affected by how they change during their service life. Multi grade petroleum can lose viscosity and thin causing accelerated wear as the VI additives shear back. Straight weight petroleum (i.e. SAE 30, SAE 40) thicken a lot as they cool meaning longer time before lubricant reaches critical parts on cold starts, but have no VI additives so they resists thinning. However, they can degrade and thicken as heat and by products of combustion affect the unsaturated chemistry. Group III (3) synthetics resists this degradation much better, but being petroleum based employ some VI additives which is a negative and typically don’t have as good performance in the volatility viscosity retention areas. Only the Group IV (4) PAO base synthetics have the saturated chemistry to resist degrading when exposed to the by products of combustion and heat, plus typically employ no VI additives making them very thermally stable for longer periods. For this reason the Group IV (4) synthetics maintain peak mileage and power throughout their service life

Modern motor oils are a marvel of chemistry to be sure. There are a lot more additives in play than the few mentioned here. The API (American Petroleum Institute – sets oil standards in the U.S.), ILSAC (International Lubricants Standardization and Approval Committee – U.S. & Japanese auto/truck manufacturers standards for motor oil) and ACEA (Association des Constructeurs Europeens d’Automobiles – European auto/truck manufacturer oil standards) are some of the different organizations you will see providing rating information on the service grades of different motor oils. Plus there are some auto manufacturers like Mercedes, BMW and Volkswagen that have unique oil standards for their cars. You need to read your owner’s manual clearly to be sure you are using the proper oil for your application.

Some of these organizations, such as the API and ILSAC, have reduced friction modifier amounts in order to extend the life of catalytic converters and reduce pollution. These will increase wear but will be still within the “acceptable wear” range. Because of the increased wear and expense of licensing these oils some companies will not certify for API & ILSAC in order to achieve a higher level of performance. People with older engines that do not have roller cams find these oils especially attractive to maintain a reduced level of engine wear. AMSOIL only has 5 motor oils certified for the API & ILSAC for this reason (the four XL-7500 Branded motor oils and the semi-synthetic 15W-40 PCO). The rest of the nearly 30 synthetic motor oils are not certified in order to maintain the higher levels of friction modifier to maintain the enhanced level of performance necessary for their targeted market. In other words, the less expensive motor oils made by AMSOIL are API & ILSAC certified while the high end more expensive performance motor oils are not. One reason companies like AMSOIL and Mobil are at odds with the reduced friction modifier standards is they don’t take into consideration the reduced volatility of PAO based motor oils which leads to much less pollution and thereby less problems for the catalytic converter. Even with the full wear preventing additives these oils do not produce the pollution of petroleum motor oils. For this reason AMSOIL has left the friction modifier levels high and skips certification for these higher performing motor oils.

From Amsoil……

Akhir-akhir ini seringkali kita dengar, ada banyak pihak yang telah berhasil membuat mobil dan dianggap sebagai cikal bakal mobil nasional Indonesia, walaupun hanya sebatas protitipe sederhana dan jauh dari kualitas yang sekarang bisa dinikmati oleh para konsumer lewat mobil mobil jepang, korea, dsb. Namun walaupun demikian, hal itu merupakan hal baik dan seharusnya pemerintah cukup jeli untuk memfasilitasi kearah perkembangan lebih lanjut.

Dimanapun di dunia ini, Ekonomi berkembang dan masyarakat menjadi makmur, karena adanya proses perpindahan orang dan barang dari satu tempat ketempat lainnya. Dan mobil lah yang saat ini merupakan pilihan paling tepat untuk menfasilitasi tujuan tersebut. Amerika sebagai moyangnya Industri mobil, telah berhasil membangun industri ini menjadi sekian besarnya dan pertahun nilai total bisnis ini mencapai $338 Billion dan memperkerjakan hampir 1juta orang. Jadi bisa dibayangkan betapa motorization telah berhasil mengubah wajah sebuah negara dan memberikan kontribusi yang besar terhadap perbaikan ekonomi masyarakatnya. Terbukti Jepang, Korea, China dan akhir-akhir ini India….Dengan komitmen dan kerja keras mereka berhasil membangun industri ini menjadi faktor penting dalam struktur industri mereka.

Memang untuk masuk ke Industri ini bukan hal yang mudah, karena entry barrier yang harus dilewati sangat-sangat susah dan butuh modal yang sangat besar. Gaikindo pernah menghitung paling tidak dibutuhkan dana triliunan rupiah untuk bisa menggarap sebuah mobil nasional yang layak jual dan bisa bersaing dengan mobil mobil dari perusahaan yang sudah mapan saat ini, seperti Toyota, Honda, Nissan, etc. Terlebih diera globalisasi ini, produk Automotive sudah melakukan harmonisasi dan untuk bisa dijual maka sebuah mobil harus memenuhi standar-standar International, seperti emisi, safety, dan lainnya, yang pada akhirnya membuat ongkos development menjadi sangat besar dan untuk negara yang masih kembang-kempis seperti Indonesia ini, hal ini menjadi tidak feasible.

Lantas apakah masih pantas kita punya Mimpi punya MOBNAS??? Atau apakah tidak lebih baik kita mendorong Industri yang sudah ada saat ini untuk bisa lebih kompetitif dan pada akhirnya bisa menghasilkan produk yang bisa lebih murah dan mampu dinikmati oleh sebagian besar rakyat dengan daya beli yg sangat terbatas ini? Jawabannya bisa pantas atau bisa juga tidak. Kalau memang pemerintah punya niatan yang kuat (seperti China diera 25 tahun yang lalu mencanangkan dirinya untuk menjadi salah satu kekuatan otomotif dunia), maka hal ini mungkin. Kita bisa menempuh jalan lisensi seperti Proton, dan secara bertahap mengembangkannya. Tentunya scale of economic tetap menjadi pilihan utama dan harus disertai juga dengan visi bahwa untuk maju perlu R&D. Jadi segala sesuatu harus tetap berpegang bahwa teknologi harus direbut dan untuk itu perlu dana dan usaha yang keras.

Atau kita juga bisa seperti Thailand, mereka tidak mimpi untuk punya mobil nasional. Tapi mereka punya visi menjadikan negaranya sebagai Detroit of the East dan memberikan fasilitas sebesar-besarnya untuk para Industri Mobil dari negara luar untuk menanamkan modalnya di Thailand dan pada akhirnya mereka bisa menjadi pusat produksi, baik kendaraan utuh dan komponen pendukungnya. Efeknya tentu saja harga mobil bisa lebih murah dan mereka menjadi basis ekspor kendaraan CBU dan devisa mengalir ke dalam kantong kas negara. Peraturan dibuat pemerintah sedemikian rupa untuk menstimulus terjadinya alih teknologi dan pengembangan energi alternatif, misalnya pemotongan excise tax untuk kendaraan yang menggunakan alternatif energi (CNG, Ethanol, Methanol, Bio-Diesel, Hybrid), sehingga banyak Perusahaan mobil berlomba-lomba menanamkan new investasi dan meng-introduce banyak model dipasar Thailand dan sekaligus menjadi basis ekspor untuk negara negara lain disekitar ASEAN, termasuk Indonesia.

Keduanya adalah Pilihan, dan ketika kita memilih pasti akan timbul konsekuensi. Terus terang dengan kondisi pemerintah saat ini, mustahil Mobnas itu bisa terwujud. Koordinasi antar Departemen yang kurang jelas dan aturan yang tumpang tindih akan sangat memberatkan hal itu terwujud. Sebagai contoh nyata sampai saat ini belon ada peraturan In-used Vehicle Maintenance yang benar benar bisa dijalankan dengan mulus dinegeri sebesar Indonesia ini. Uji Emisi dilakukan oleh Kementrian Lingkungan Hidup, Kir oleh Hubdar, STNK oleh Polisi….masing masing punya porsi sendiri dan tidak bisa jalan in-line. Dan akhirnya berapa jumlah CO2 yang kita buang ke Langit Indonesia, karena ketidakberesan penegakkan aturan emisi gas buang. Uji emisi pun saat ini hanya berupa Idle Test, yang tentunya sangat tidak mencerminkan kondisi gas buang apabila kendaraan tersebut berjalan dijalan raya.

Untuk itu, marilah kita berharap, agar Pemerintahan yang akan dipilih dalam beberapa bulan kedepan ini, adalah pemerintahan yang cukup kompeten dan profesional. Meletakkan kepentingan masyarakat diatas kepentingan Golongan maupun Individu-individu penguasa. Sehingga Mimpi ini tidak hanya ada diawan dan sedikit demi sedikit bisa turun ke bumi Indonesia yang Kaya Raya ini………………..Akhirnya Tujuan UUD 45-Masyarakat Adil dan Makmur bisa segera kita wujudkan dan tidak perlu menyalahkan “BELANDA” yang sudah menjajah kita selama 350th…………….

Merdeka,

It’s hard to believe the reciprocating piston engine has been around for 137 years. Nicholaus August Otto invented the first such engine in 1866, one year after the Civil War ended. Given that much time, you’d think the pistons inside today’s engines would be radically different from those of their ancestors. Piston materials and designs have evolved over the years and will continue to do so until fuel cells, exotic batteries or something else makes the internal combustion engine obsolete. But until that happens, pistons will continue to power the vehicles we drive.

One thing that has not changed over the years is the basic function of a piston. The piston forms the bottom half of the combustion chamber and transmits the force of combustion through the wrist pin and connecting rod to the crankshaft. The basic design of the piston is still pretty much the same, too. It’s a round slug of metal that slides up and down in a cylinder. Rings are still used to seal compression, minimize blowby and control oil. So what has changed? The operating environment. Today’s engines run cleaner, work harder and run hotter than ever before. At the same time, engines are expected to last longer than ever before, too: up to 150,000 miles or more – and with minimal maintenance. Consequently, heat management is the key to survival of the fittest.

“Piston design used to be a process of trial and error.” says Kent Fullerton, an engineer with Zollner Pistons. “You’d make and test a new design three or four times before you got it right. Today, everything is modeled in 3D on a computer, then evaluated with finite element analysis software before anything is made. That speeds up the design and testing process, reduces the lead time to create new piston designs, and produces a better product.

” According to a book produced by Mahle Inc. called Pistons for Internal Combustion Engines, engineers use two methods to evaluate new piston designs before they are actually produced for engine dyno testing: finite analysis and photoelastic stress analysis. The idea behind finite analysis is to divide a model piston into a fixed (finite) number of elements. The resulting grid forms lines that intersect and connect. Computer software generates equations for each individual element and predicts the overall stiffness of the entire piston.

Analyzing the data shows how the piston will behave in a real engine and allow the engineer to see where loads and temperatures will be greatest and how the piston will react. With photoelastic stress analysis, a 3D transparent resin model is cast of a piston. When the model piston is subjected to loads, the refractive properties of the plastic change causing polarized light passing through the piston to change colors. This reveals how the piston deforms under load and the areas where it is experiencing the greatest stress.

Hot Pistons

The most critical area for heat management is the top ring area. One of the “tricks” engine designers came up with to reduce emissions was to move the top compression ring up closer to the top of the piston. A decade ago, the land width between the top ring groove and piston crown was typically 7.5 to 8.0 mm. Today that distance has decreased to only 3.0 to 3.5 mm in many engines.

The little crevice around the top of the piston between the crown and top ring creates a dead zone for the air/fuel mixture. When ignition occurs, this area often does not burn completely leaving unburned fuel in the combustion chamber. The amount isn’t much, but when you multiply the residual fuel in each cylinder by the number of cylinders in the engine times engine speed, it can add up to a significant portion of the engine’s overall hydrocarbon (HC) emissions.

One of the consequences of relocating the top ring closer to the top of the piston is that it exposes the ring and top ring groove to higher operating temperatures. The top rings on many engines today run at close to 600° F, while the second ring sees temperatures of 300° F or less. These extreme temperatures can soften the metal and increase the danger of ring groove distortion, microwelding and pound-out failure. The reduced thickness of the land area between the top of the piston and top ring also increases the risk of cracking and land failure.

The evolutionary advances that enable today’s pistons to handle this kind of environment include changes in piston geometry, stronger alloys, anodizing the top ring groove and using tougher ring materials. Ordinary cast iron top compression rings that work great in a stock 350 Chevy V8 can’t take the kind of heat that’s common in many late model engines. That’s why ductile iron or steel top rings are used in some of these engines. Anodizing has become a popular method of improving the durability of the top ring groove and is now used in many late model engines. Anodizing reduces microwelding between the ring and piston to significantly improve durability. But it can’t work miracles: an anodized piston can still fail if it gets too hot.

Anodizing is done by treating the ring groove with sulfuric acid. The acid reacts with the metal to form a tough layer of aluminum oxide, which is very hard and wear-resistant. Part of the layer is below the surface of the metal and part is above. On average, the layer is about 20 microns (.001˝) thick so the piston manufacturer compensates for the added thickness when the top ring groove is machined.

Another approach some piston manufacturers have used to improve top ring durability is to weld nickel alloy into the top ring groove. This was the approach used for the OEM pistons in Saturn 1.9L engines made from 1991 to 2001. The 2002-’03 Saturn engine uses an anodized top ring groove.

Low Tension Rings

To further complicate the problem of heat management, rings have been getting smaller. Starting in the 1980s, “low tension” piston rings began to appear in many engines. Typical ring sizes today are 1.2 mm for the top compression ring, 1.5 mm for the second ring, and 3.0 mm for the oil ring. Some are even thinner. A few engines have top compression rings only 1.0 mm thick, and the current Buick 3800 V6 uses a narrow 2.0 mm thick oil ring.

The OEMs went to thinner, shallower rings to improve fuel economy because the rings account for up to 40 percent of an engine’s internal friction losses. Thinner rings produce less drag and friction against the cylinder walls. But the downside is they also reduce heat transfer between the piston and cylinder because of the smaller area of contact between the two. Consequently, pistons with low tension rings run hotter than pistons with larger rings.

Low tension rings also present another problem. They are less able to handle bore distortion. To maximize compression and minimize blowby, the cylinder must be as round as possible. This often requires the use of a torque plate when honing to simulate the bore distortion that is produced by the cylinder head.

Piston Geometry

Changes in piston geometry have also been made to improve their ability to survive at higher temperatures. Russ Hayes, an engineer with Federal Mogul/Sealed Power, said piston manufacturers used to grind most pistons with a straight taper profile. When the piston got too hot, it would contact the cylinder along a narrow area producing a thin “wear strip” pattern on the side of the piston. “Now we use CNC machining to do a barrel profile on our pistons. The diameter of the piston in the upper land area is smaller to allow for more thermal expansion and to spread any wall contact over a larger area.

” Pistons are getting shorter and lighter. In the 1970s, a typical 350 small block Chevy piston and pin assembly weighed around 750 grams. The same parts in a late model Chevy LS1 engine weigh only about 600 grams.

Part of the weight reduction has been achieved by reducing piston height and using shorter skirts. The distance from center of the wrist pin to the top of the piston (called “compression height”) used to be 1.5˝ to 1.7˝ back in the 1970s, said Hayes. Today, wrist pins are located higher up. On Ford 4.6L engines, the compression height is 1.2˝, and it’s 1.3˝ on small block Chevys.

Moving the location of the wrist pin higher up on the piston also allows the use of longer connecting rods, which improve torque and make life easier on the bearings and rings.

Some aftermarket pistons are now available with wrist pins that have been relocated upward slightly to compensate for resurfacing on the block and heads. The other alternative is to shave the top of the piston if the block has been resurfaced, but this reduces the depth of the valve reliefs which may increase the risk of detonation and/or valve damage.

Pistons used to have long tail skirts (which sometimes cracked or broke off). Now most pistons have “mini-skirts.” Instead of a 2.5˝ skirt length, the piston may only have 1.5˝ skirt. Shorter skirts reduce weight but also require a tighter fit between the piston and cylinder bore to minimize piston rocking and noise. Consequently, today’s piston clearances are much less than before (typically .001˝ to .0005˝ or less). Some have a zero clearance fit or even a slight interference fit (made possible by special low friction coatings).

Piston Materials

The alloy from which a piston is made not only determines its strength and wear characteristics, but also its thermal expansion characteristics. Hotter engines require more stable alloys to maintain close tolerances without scuffing.

Many pistons used to be made from “hypoeutectic” aluminum alloys like SAE 332 which contains 8-1/2 to 10-1/2 percent silicone. Today we see more “eutectic” alloy pistons which have 11 to 12 percent silicone, and “hypereutectic” alloys that have 12-1/2 to over 16 percent silicone.

Silicone improves high heat strength and reduces the coefficient of expansion so tighter tolerances can be held as temperatures change. Hypereutectic pistons have a coefficient of thermal expansion that is about 15 percent less than that for standard F-132 alloy pistons. Because of this, the pistons can be installed with a much tighter fit – up to .0005˝ less clearance may be needed depending on the application.

Hypereutectic alloys are also slightly lighter (about 2 percent) than standard alloys. But the castings are often made thinner because the alloy is stronger, resulting in a net reduction of up to 10 percent in the piston’s total weight. Hypereutectic alloys are more difficult to cast because the silicon must be kept evenly dispersed throughout the aluminum as the metal cools. Particle size must also be carefully controlled so the piston does not become brittle or develop hard spots making it difficult to machine. Some pistons also receive a special heat treatment to further modify and improve the grain structure for added strength and durability. A “T-6″ heat treatment, which is often used on performance pistons, increases strength up to 30 percent.

Machining hypereutectic pistons is also more difficult because of the harder alloy. Consequently, hypereutectic pistons typically cost several dollars more than standard alloy pistons. That’s why most OEMs (except Ford) have gone back to eutectic alloy pistons in their late model engines. High copper eutectic alloys offer most of the advantages of hypereutectic alloys without as much cost.

Piston Coatings

Survival of the fittest also requires a high degree of scuff resistance. Cold starts without adequate lubrication can cause piston scuffing. The same thing can happen if the engine overheats. Piston-to-cylinder clearances close up and the piston scuffs against the bore. The initial start-up of a freshly built engine is also a risky time for scuffing and is of special concern to engine builders because that’s when many warranty problems occur.

Applying a permanent low friction coating to the sides of the pistons provides a layer of protection against scuffing. Many rebuilders have found that using coated pistons has virtually eliminated warranty problems due to scuffing.

Many late model OEM engines including Ford 4.6L V8, Chrysler 3.2L, 3.5L, 3.8L and 4.0L, and GM 3.1L use pistons with graphite moly-disulfide coatings on the piston skirt to improve scuff resistance. Most aftermarket piston manufacturers also offer some type of coated replacement pistons to rebuilders who want them. Coatings typically add about a buck to the price of a replacement piston, but the added scuff protection and reduction in warranty claims more than offsets the higher cost say many engine builders who use them.

“Thermal barrier” ceramic-metallic coatings for the tops of pistons are another type of coating that have been used on some diesel pistons and performance pistons. Improving heat retention in the combustion chamber improves thermal efficiency and makes more power. It also helps the piston run cooler. But too much heat in the combustion chamber also increases the risk of detonation and preignition, which is not a problem with diesels but is with gasoline engines. So when a coating is used, ignition timing must usually be retarded several degrees to reduce the risk of detonation.

Piston Crowns

The shape and finish on the tops of pistons has also been changing. Flat top pistons have been replaced by dished pistons, domed pistons and pistons with intricate contours to swirl the fuel mixture and promote better fuel atomization.

Some piston crown designs can be very complex because they are designed to produce the lowest possible emissions with the best overall fuel efficiency. The shape of the crown controls the movement of air and fuel as the piston comes up on the compression stroke. This, in turn, affects the burn rate and what happens inside the combustion chamber. Replacement pistons for stock engines with complex piston designs should be the same as the original to maintain the same emissions and performance characteristics.

With performance pistons, designs can be even more specialized. Manufacturers have developed special “fast burn” configurations that allow engines to safely handle more compression without detonating.

John Erb of United Engine & Machine (Silvolite and KB Pistons) said an “Attenuator-Groove” is used on some KB pistons to enhance the valve reliefs. The groove removes two potential hot spots in the combustion chamber and improves airflow and wet flow atomization.

Another unique design feature, said Erb, is the “Mini-Grooves” machined into the top ring land on KB performance pistons. If the piston gets too hot, the top of the piston swells causing the Mini-Grooves to contact the cylinder. This momentary contact helps cool the piston to reduce the danger of detonation and piston destruction.

Piston Pins

Zollner’s Fullerton says piston pin holes have also been changing. “Rather than being round and straight, pin bores are taking on new shapes. Some are oval and some are trumpet-shaped, flaring out toward the inside edges of the pin bosses. The reason for these shapes is to accommodate wrist pin bending and ovalization. These variances from straight and round are quite small, measured in tenths of a thousandth, but have proven to extend piston life.

Down The Road

Pistons may continue to get shorter and lighter, but most engineers believe rings can’t get much smaller than they are today. Some do think, though, that the two ring piston may not be too far away. Some Indy racing motors are already running two ring pistons quite successfully.

Other design innovations that may shape the direction of future piston development include lightweight alloy wrist pins, more anodizing and/or the use of ceramic coatings on the tops of pistons and upper ring groove to improve heat resistance and wear, and maybe top rings with no end gaps.

One engineer mentioned a new piston design he’s working on for an undisclosed performance application that has only a one-inch compression height.

The best indication of what’s coming down the road is to look at today’s state-of-the-art racing pistons: super lightweight designs with almost no skirts, holes machined into the sides to reduce weight, and various design tricks to control thermal expansion and detonation under high load.

We may see some exotic graphite reinforced pistons for certain high output engines similar to ones that are now being used in diesel engines. The development of direct injection gasoline engines in the U.S. market will likely require complex fuel bowls in the tops of pistons similar to those now used in many diesel engines. Direct injection, which is starting to come on strong in Europe, allows extremely lean air/fuel mixtures (up to 40:1) and much better fuel economy. But it also requires precise control of airflow in the combustion chamber for reliable ignition and complete combustion.

If hybrid gasoline/electric or diesel/electric vehicles become more common in the not-too-distant future (which many predict will happen), no big changes in piston design will be needed because most such systems use the same basic engine designs as today.

The biggest change in piston design will occur if and when fuel cells become a competitive power source for automotive applications. In that case, there will be no need for pistons and they’ll be on the endangered species list.

Most experts believe fuel cell technology is still years away. And when it does go into production, volumes will be very limited because of high costs. Eventually the cost will come down.

But even if fuel cells do eventually take over, many experts believe piston engines will continue to be produced for smaller, economy vehicles as well as heavy-duty vehicles.

There will also be an ongoing replacement market for pistons as long as piston-powered vehicles remain on the road.

by Larry Carley, Technical Editor

So why do the power and torque figures that are quoted in the technical specifications for engines and motors matter so much in vehicle design?

Torque is the rotational version of force. The more torque an engine produces, the more force it can exert at the rim of a flywheel of a given radius.

Power is force multiplied by speed. The more power an engine generates, the more work it can do in a given time.

A typical automobile engine will run evenly from somewhere around idle speed (about 800 revolutions per minute or rpm) to its “red line” which might be anywhere from 4000 rpm for an older engine to 12000 rpm for a Formula One engine. The power and torque will vary through this range.

Torque increases as rotational speed increase from idle to a certain figure and then falls as the rotational speed increase above this figure. Acceleration is proportional to the amount of force pushing the vehicle forward; so maximum acceleration in a given gear is obtained when maximum torque is obtained.

Power is force (~torque) multiplied by speed (~rotational speed) so power increases with rotational speed up to and past the point of maximum torque. However, at still higher rotational speed the engine starts to be limited by the amount of air that it can draw in (4 valves per cylinder help) and torque then decreases more rapidly than the rotational speed increase, and therefore power also decreases. (Electric motors, though their torque and horsepower behavior differ considerably from that of internal combustion engines, experience a similar loss of torque and power at high rotational speeds due to electromagnetic effects.)

Power is force multiplied by speed, and maximum acceleration is obtained by having maximum propulsive force at the wheels. Use of a low gear ratio multiplies the engine torque at the wheel (at the price of having the engine rotate more quickly). Maximum acceleration at a given speed is obtained by having the engine operate at maximum power.

Driving a car is easier and more relaxed if it has a flexible engine. Flexibility even becomes a safety issue in four-wheel drive vehicles. Maximum torque is obtained at a certain rotational speed, and maximum power at a higher rotational speed. An engine is flexible if these maximums occur at widely different rotational speeds, say in the ratio 1:2 or more.

A vehicle engine operating at a rotational speed above its maximum-torque point is in a “stable” speed regime. If it slows down by a small amount (due to the vehicle encountering an incline, head-wind, etc.) engine torque will tend to increase and resist the slowing. Conversely, if it speeds up by a small amount, torque will tend to decrease and discourage a further increase in speed.

A vehicle engine operating at a rotational speed below its maximum-torque point is in an unstable speed regime. If it slows down by a small amount, the torque decreases and its speed will fall further. The driver can compensate by opening the throttle. Conversely, if the speed increases then the torque increases and the speed increases even more. The driver can compensate by closing the throttle (or risk a speeding ticket). The driver has to actively compensate for these variations (or has to rely on automatic cruise control), so the car is considerably less fun and relaxing to drive.

The driver cannot correct for a fall in engine rotational speed and loss of torque if the throttle is already wide open, except by changing into a lower gear. This can be a safety matter on a steep road. If you start up a steep hill in too high a gear, or have to slow down due to obstacles, for example, the engine may fall below its maximum-torque point and be unable to recover from a downward speed spiral. Changing gears causes a loss of more speed and, additionally, traction may be lost as the clutch engages. Inaction on the part of the driver may lead to a stalled engine and a forced restart on a dangerous slope. These risks are minimized if maximum engine torque is designed to occur at low engine rotational speed.

So, if you are into racing, particularly Formula One racing, you want an engine with the highest possible power output and this is best achieved by generating maximum power at very high rpm. Such engines also tend to generate maximum torque at very high rpm and are inflexible and difficult to drive — but that is what the driving aces are paid big bucks for!

If you are into four wheel driving or drag racing, you want a very flexible engine that generates maximum torque, and lots of it, at low rotational speed and generates maximum power at a higher rotational speed – a ratio of 1:2 is good. A ratio of 1:3 (e.g. 1500rpm and 4500rpm) is excellent. As a side benefit, which is especially valuable for passenger vehicles, engine wear is largely determined by piston speeds and producing high torque at low rpm allows you to use a high “over-drive” top gear for quiet highway cruising and for long engine life.

[This information was adapted from an article on Power and Torque appearing on the 4WDOnline.com web site.]

Graphing Torque and Horsepower for an Internal Combustion Engine

If you plot the torque and the horsepower versus the rpm values for an engine, what you end up with are torque and horsepower curves for that engine. This is what a machine called a dynamometer does. Typical torque and horsepower curves for a high-performance engine might look like those in the graph below (these happen to be the curves for the 300-horsepower engine in the Mitsubishi 3000 twin-turbo). The horizontal axis is rotational speed (in rpm) and the vertical axis is horsepower (for the horsepower curve) or torque (in ft-lbs for the torque curve).

Notice the very “flat” torque curve, with a maximum at 2500 rpm and only a gradual drop off in torque at the high end. Notice that the horsepower increases almost linearly until it peaks at about 6500 rpm. The ratio here is 2.6/1. This makes for a very “sweet” ride!

[This information is adapted from an article on the HowThingsWork.com web site.]

The torque and horsepower curves for a typical small dc motor differ a bit from the internal combustion engine. The torque curve is very linear with maximum torque developed at zero rotational speed, and the horsepower curve peaks near the middle of the range. The following MATLAB plot for the small VEX dc motor illustrates these characteristics.

Salah satu latest innovation dalam hal combustion mesin bensin adalah teknologi Direct Injection. Kalau selama ini mesin bensin menggunakan sistem port injection, dimana bahan bakar/fuel di semprotkan pada intake manifold, maka pada teknologi GDI bahan bakar langsung di inject kedalam ruang bakar seperti halnya mesin Diesel. Maka tidak salah kalau ada yang mengatakan GDI adalah hybrid antara mesin bensin dan diesel. So apa dibalik penemuan ini???Tentunya dorongan untuk menghasilkan sistem pembakaran mesin bensin yang lebih efisien, powerful dan ramah lingkungan. Bagaimana ketiga hal itu bisa dicapai oleh Sistem ini???

BMW DI System

Karena menggunakan direct injection, maka suhu didalam ruang bakar bisa lebih rendah dan efeknya kompressi ratio bisa ditingkatkan menjadi 1:12 tanpa harus mengalami gejala knocking.  Jadi dengan fuel beroktan rendah pun (87), mesin GDI bisa menghasilkan pembakaran yang lebih efficient (volumetric efficiency) dan juga increase power. Selain itu juga karena sifatnya yang unik ini, gas buang hasil pembakaran juga relatif lebih bersih sehingga ramah bagi lingkungan. Untuk menjalankan sistem ini maka dibutuhkan kontrol sistem yang reliable, dimana sensor dan ECU berperan sangat besar dalam menentukan kondisi pembakaran yang paling sempurna. Injector yang digunakan adalah injector yang dirancang khusus untuk mampu menyemprotkan bahan bakar pada tekanan 120 Bar agar fuel bisa teratomisasi dengan baik dan pembakaran bisa berlangsung dengan sempurna.

Pada sistem yang didevelop oleh Bosch, ada 2 macam pembakaran yaitu Homogeneous dan Stratified, dan ini disesuaikan dengan kebutuhan driver. Pada saat butuh power, maka sistem akan memerintahkan injector untuk menyemprotkan fuel secara merata kedalam ruang bakar sehingga terbentuk homogeneous combustion dan sebaliknya pada low load, sistem bisa mensetting untuk terjadinya stratified combustion dimana hanya ada sebagian fuel yang diinjectkan disekitar spark plug dan terjadi lean combustion.

Saat ini sistem ini sudah mulai dijumpai dibeberapa model yang dikeluarkan oleh pabrikan, dan untuk Indonesia LEXUS LS yang sudah mengadoptnya. So, kita tunggu kiprahnya untuk mobil mobil yang lain………….terutama mobil yang mampu dibeli tanpa harus korupsi………………hehehehehhehe.

Banyak cara dilakukan oleh Auto Engineer untuk meningkatkan efisiensi pemakaian bahan bakar dan salah satunya yang terakhir banyak diperbincangkan adalah penerapan sistem micro hybrid pada kendaraan. Sistem ini intinya mampu membuat mesin untuk hidup/mati secara otomatis apabila sistem mendeteksi mesin dalam keadaan idle. Dengan cara ini , pemakaian bahan bakar bisa dihemat sebesar 5-10%. Apalagi kalu diterapkan pada kondisi berkendara pada saat jalan macet (banyak stop-go) dan lampu merah yang lamanya bukan kepalang………….(ampe ampe ada yang 3 menit lebih..). BMW adalah leader pada teknologi ini, walaupun FIAT sudah menerapkan pada modelnya sejak dua dekade yang lalu (80`an). Pada salah satu modelnya,  sistem BMW akan mematikan mesin pada saat driver menginjak rem dan kecepatan kendaraan <8km/h dan akan otomatis start pada saat driver melepaskan pedal brake.

from wikipedia

Untuk bisa berfungsi perubahan utama adalah pada motor stater, dimana dibutuhkan motor stater yang mempunyai daya tahan lebih lama dalam kondisi on/off dan didukung oleh control sistem lewat sensor yang lain. Ada beberapa vendor yang mengembangkan sistem ini bagi pabrikan, diantaranya Bosch, Denso dan Valeo dan untuk valeo mereka beri nama STARs. Dalam sistem ini ada fungsi tambahan untuk memanfaatkan tenaga dari proses braking(regenerative braking)

Valeo Stars System

Produsen India (Mahindra-mahindra)  juga telah mengintroduce sistem ini pada line-up mereka (scorpio & balero) dan hanya menambah cost Rs:3,800 dan tentunya sangat menarik buat pasar India. Seandainya mobil dijakarta dapat dilengkapi dengan sistem ini, maka bisa kita bayangkan berapa banyak fuel yang bisa kita saving dan pada akhirnya bisa mengurangi beban pemerintah untuk mensubsidi BBM.

Dari sini apa yang bisa kita petik, bahwa untuk mendapatkan efisiensi 5% saja, butuh effort yang sangat besar, jadi bisa dibayangkan kalau ada produk dipasaran yang mengklaim bisa menghemat sampe 40% tanpa perubahan yang radikal pada sistem power train kendaraan, maka sangat bisa jadi itu adalah hanya klaim semata (Good to be true……….Kata orang bulek). So mungkin ada yang berminat untuk research mengenai start stop sistem ini??? Peluang terbuka besar……..