The Viscosity of a fluid is its resistance to flow. It is generally perceived as “thickness”. The higher the viscosity, the greater the resistance to flow or the “thicker” the fluid. Viscosity changes with temperature – the higher the temperature, the lower the viscosity.  It is therefore always important to specify the temperature at which the viscosity measurement was made. The viscosity of lubricating oil is normally measured at 40˚C and 100˚C.

Viscosity Index (VI) is a measure of the change in viscosity with change in temperature. It is most commonly used to portray the viscosity-temperature behavior of lubricating oils. The lower the VI, the more the viscosity will change with variation in temperature. VI has no units and is calculated using the viscosity of the oil at 40˚C and 100˚C.

Many applications require the lubricant to perform satisfactory across a wide range of temperatures. For instance in automotive engines, an oil with a low viscosity at low temperature, i.e. SAE 15W (“W” for winter), is needed to enable the oil pump to push the oil through the engine during cold morning starts. The oil also needs to be viscous (thick) enough, SAE 40 for example, to protect the engine when it reaches operating temperature. This is when the use of a Viscosity Index Improver (VII) becomes essential.

Viscosity Index Improvers (sometimes referred to as Viscosity Modifiers) are polymers that provide “thickening characteristics” to oil at elevated temperatures. When the oil temperature is low, these polymers curl up into tight balls that flow readily with the oil molecules (Figure 1). As the temperature increases, they expand into large stringy structures that restrict the normal oil flow, which has a thickening effect on the oil. When the oil cools down, the polymers go back to their original shape. The result is that when these polymer additives are blended in the correct proportion with for example SAE15W base oil, the oil flows like an SAE 15W at low temperatures and similar to an SAE 40 oil at high temperatures. The outcome is an SAE 15W40 multigrade oil that will provide adequate protection over a wide temperature range (Figure 2). It should be noted that there is actually no SAE 40 base oil in an SAE 15W40 formulation.

In addition to multigrade engine oils, Viscosity Index Improvers are also used in multigrade gear oils, automatic transmission fluids, power-steering fluids and high viscosity index hydraulic oils.

Unfortunately, viscosity index improvers have some downsides as well. The primary disadvantage is that polymers are susceptible to shearing when subjected to high mechanical stresses in severe service. There are areas in engines, gearboxes, hydraulic pumps, etcetera that have very tight clearances and this can shear the polymers (viscosity index improver molecules) into smaller pieces. This “physical breakage” cannot be reversed when the shear stresses are removed. Consequently, it affects the ability of the polymer molecules to add to the viscosity of the fluid at elevated temperatures.

Various types of polymers are used as VIIs in lubricating oil formulations. These include, amongst others, poly alkyl methacrylates (PMA), olefin copolymers (OCP) and hydrogenated styrene-diene copolymers (SDP). The various polymers have different shear stability characteristics. Higher molecular weight polymers make better thickeners but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear-resistant, but do not improve viscosity as effectively at higher temperatures and must, therefore, be used in larger concentrations. Consequently, different VIIs must be used, for instance, in engine oils and gear applications where very high levels of sliding friction and shearing stresses are encountered.

The viscosity index improvers used in Blue Chip and Q8 lubricants products have been (i) extensively evaluated in laboratory and bench tests, (ii) specifically selected for the intended lubricant application and (iii) proven in field trials and extended service. You can, therefore, rest assured that our lubricants will resist thinning out due to shearing if used in accordance with equipment manufacturers’ recommendations.

If you have any questions concerning this newsletter or any previous ones for that matter, you are most welcome to mail us at info@bcl.co.za. Should you have any lubricant related topics you would like to see published, please do not hesitate to let us know. In fact we are always looking for new ideas. Many of the topics we wrote about in previous issues of OilChat were in response to questions and suggestions from our readers.

In OilChat 33 bore polishing in diesel engines was discussed. Now the question is what is the difference between bore polishing and cylinder bore glazing? Early signs of both bore polishing and cylinder glazing are increased oil consumption (blue exhaust smoke) and loss of combustion pressure. Although the symptoms of the two phenomena are very much the same, their physical appearance and process of development are completely different.

Cylinder bore glazing (sometimes referred to as internal engine glazing or piling) is characterized by a very smooth, highly polished lacquer- or varnish-like layer on cylinder surfaces. If a glazed cylinder is examined, one will normally find the crosshatch grooves honed into the bore surface, are filled or covered by the glazing layer. (The purpose of the honing pattern is to retain oil to ensure proper lubrication and to form a seal between the piston rings and cylinder bore.)

Glazing in diesel engines is normally the result of prolonged light load, low-speed running and/or extended periods of idling. Typical examples are light trucks in local delivery service and small farm tractors hauling trailers in orchards during the harvesting season. Diesel engines are designed to operate at above 60% of their maximum rated load and ideally closer to 75%. Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since the rings rely on the cylinder pressure to force them against the oil film on the bores to form the seal. When cylinder rings are not sealing properly, hot combustion gases force their way past the rings and flash-bake the oil on the cylinder to form a hard deposit layer, commonly referred to as glazing. Once glazing has occurred, the honing marks in the bore are smoothed over, resulting in an even poorer seal between the piston rings and cylinder bore. The glazing issue becomes a vicious spiral allowing more and more hot combustion gases past the rings to bake further oil deposits on the cylinder.

Various remedies are suggested to cure bore glazing. Many of them involve introducing some sort of abrasive into the engine air inlet to abrade the glazing on the cylinder bores. Any abrasion, however, that occurs, will be along the axis of the cylinder (rather than the original crosshatch grooves), allowing more oil to pass the rings and thereby increasing oil consumption even further. If glazing is detected in its early stages (loss of power, increased oil consumption, and blue exhaust smoke), running the engine on a low-performance oil at maximum load may allow the piston rings to scrape the glazing off the cylinder bores. However, if glazing has been allowed to progress to an advanced stage, this procedure will not have any significant remedial effect. Advanced glazing can only be cured by stripping down the engine, re-boring the cylinders and machining new honing grooves.

Traditionally cylinder glazing was associated with the use of high-performance oils in lightly loaded diesel engines, but modern oil technologies have largely overcome the phenomenon of glazing. New
Q8 Formula Truck 7000 15W-40 is formulated with leading-edge additive chemistry to protect diesel engines against cylinder bore glazing.

To find out more about what Q8 Formula Truck 7000 15W-40 can do for your engines, please visit https://www.facebook.com/BlueChipLubricants/

Q8 Formula Truck 7000 15W40, our new top tier heavy duty diesel engine oil, is designed to protect engines against bore polishing. You may well ask how engine oil can possibly do this. To answer this question one needs to understand what bore polishing is and how it is brought about.

When modern engines are manufactured, the cylinder bores are honed (machined) to produce a “crosshatch” appearance with fine grooves from both directions at about 22 degrees from the horizontal (Photo 1). The crosshatch pattern is required to retain oil to ensure proper lubrication and to form a seal between the piston rings and cylinder bores. Bore polishing is characterized by a clearly defined area of bright mirror-like finish on the cylinder bore where the crosshatch pattern is worn away (Photo 2).

Bore polishing is brought about by a build-up of carbon deposits in the piston top ring land area, i.e. the part of the piston above the top ring (Photo 3). Poor combustion of diesel fuel leads to these hard carbon deposits, which are highly abrasive and scrape away the honing grooves on the cylinder bores. Bore polishing leads to increased oil consumption (blue exhaust smoke) and loss of combustion pressure. This is because the oil film trapped in the honing grooves that maintains the piston ring seal and combustion pressure, is no longer there. Unburned fuel and combustion gases then leak past the piston rings and contaminate the lubricating oil.

The problem is aggravated by the formation of acids in the engine oil resulting from the reaction of these combustion by-products and condensed water. The acidic build-up in the oil causes corrosive wear of engine components. This cycle of degradation results in the engine becoming irreversibly damaged.  The advanced detergent additive system in Q8 Formula Truck 15W-40 protects diesel engines against bore polishing by effectively removing carbon deposits from piston top ring land areas.

A number of engine tests have been developed to evaluate the bore polishing tendency of diesel engine oils. One such test is the CEC L-101-08 procedure using a Mercedes Benz OM501LA engine. In addition to bore polishing, the test also evaluates piston cleanliness, oil consumption, and engine sludge.  Q8 Formula Truck 7000 15W-40 exceeds the requirement of the CEC L-101-08 test protocol by far.

To find out more about what Q8 Formula Truck 7000 15W-40 can do for your engines, please visit https://www.facebook.com/BlueChipLubricants/

Total Base Number (TBN), sometimes referred to as Base Number (BN), is an important property of engine oil. TBN is a measurement of the alkalinity of the oil expressed in terms of the equivalent number of milligrams of potassium hydroxide (an alkali) per gram of oil (mg KOH/g). Unfortunately this tells us little about what TBN does in engine oil, or how much we need for effective oil performance and engine protection.

The prime functions of motor oil are to lubricate, clean, protect and cool the engine. Various additives are added to the oil to enhance these functions. Detergent additives in engine oil have two basic functions:

• Control deposits that accumulate in the engine.
• Neutralize acidic products that contaminate the oil.

To do this the oil needs to be alkaline. TBN is a measure of the alkalinity additives in the oil.  Generally speaking, the higher the alkalinity (TBN) of the oil, the better is its ability to neutralize contaminants such as combustion by-products and acidic materials. Higher TBN oils are believed to be capable of neutralizing greater amounts of acidic materials. This results in improved protection against corrosive reactions and longer oil life. TBN levels are optimized for the intended application. For example, petrol engine oils typically have lower TBN values, while diesel oils must manage higher contaminant-loading from soot and sulfur, and therefore normally have a higher TBN. Modern high performance diesel engine oils typically have a TBN of 8 or more. Traditionally oils formulated specifically for extended drain intervals, displayed higher TBN levels to ensure proper corrosion protection for the duration of the extended interval.

TBN levels decrease as the oil neutralizes acidic contaminants in service. When the level reaches a point where it can no longer protect against corrosion effectively, the oil must be changed. Engine manufacturers’ maintenance philosophies vary as to when TBN should trigger a lube change. Some manufacturers recommend that when the TBN reaches 50% of the initial TBN, the oil should be drained, e.g. new oil TBN 10, drain at 5. Other manufacturers specify minimum TBN warning limits. Cummins for instance, stipulates the base number should not be allowed to drop below 2.5 mg KOH/g.
There is great controversy over when oil should be drained in mixed fleet applications. General advice is that this is not a real problem when engine oil with appropriate TBN is used for the fuel sulphur level.  Engine oil base number is not generally a reason to change oil in applications where fuel sulphur levels are low, e.g. on highway truck engines operated on low sulphur diesel. However the TBN should never ever be allowed to drop below 2 mg KOH/g.
Nowadays the main driving force for the development of new diesel engine oils is concern over the environmental impact of diesel engine emissions. New generation engine oils must provide optimum exhaust gas emission control system durability, while still offering peak engine protection. To protect emission control after-treatment devices, modern engine oils must contain lower Sulphated Ash, Phosphorus and Sulphur (SAPS) levels.  Although SAPS can poison exhaust gas after-treatment devices, it contributes significantly to oil alkalinity as well as oil performance. The reduction in oil SAPS limits has resulted in a shift from traditional engine oil technology to alternative additive chemistries.

We mentioned earlier that in traditional terms, higher TBN values are viewed as having the ability to neutralize more acidic contaminants than lower TBN products. This lead to the assumption that higher TBN products always allowed extended drain intervals. This, however, does not take into account TBN Retention of engine oil. The majority of motor oils currently on the market use a detergent package based on calcium, magnesium or a mixture of the two additives as their detergent package. It has now been proven that all detergent packages do not have the same ability to neutralize acidic contaminants in the long term. This has been proven in various laboratory tests and field trials. The significance of TBN Retention was once again demonstrated by a recent laboratory simulation, using sulfuric acid (H₂SO₄) to replicate acid build up during a drain interval from oil contaminants. The simulated test evaluated two oils with the following formulations:

• Oil 1: Traditional chemistry with initial TBN 11
• Oil 2: Modern technology with starting TBN 10

The graphs below show a visual representation of the test results:

TBN (mg HOH/g)

On completion of the 20,000 mile service simulation Oil 1 (traditional chemistry) dropped considerably more in TBN than Oil 2 (new technology). After about 5,000 miles the TBN of Oil 1 plunged lower than that of Oil 2. At 15,000 miles the TBN of Oil 1 tumbled below the 2 mg KOH/g warning limit. The TBN of Oil 2 never reached this threshold during the test. This laboratory simulation once again demonstrates the positive attributes of new developments in oil chemistry.

Q8 Formula Truck 7000 15W40, our new top tier heavy duty diesel engine oil, is formulated using such technology, making it particularly suitable for extended drain intervals as recommended by 

engine manufacturers. To find out more about this exciting new product, please visit and like our Facebook Page

https://www.facebook.com/BlueChipLubricants/

The two primary considerations when selecting a hydraulic fluid are the viscosity grade and the hydraulic oil type. These are typically determined by the design of the hydraulic pump employed in the system and the operating temperatures and pressures. Further items for consideration are overall lubricant quality, performance requirements and base oil type. The three common varieties of hydraulic fluids found on the market today are oil-based, water-based and synthetics.

The International Standards Organization (ISO) established the ISO 6743-4 and ISO 11158 classifications of hydraulic fluids. These classification systems do not include automotive brake fluids or aircraft hydraulic fluids and generally apply to the following three primary classes of hydraulic fluids:

• Mineral Hydraulic Fluids
• Biodegradable Hydraulic Fluids
• Fire Resistant Hydraulic Fluids

Discussions in this publication will be restricted to Mineral Hydraulic Fluids and we will endeavour to provide a summary of the ISO classifications that you will find useful in understanding the most common hydraulic fluid categories. We operate in a global marketplace with equipment that is manufactured in countries from all over the world and it is very likely you may see hydraulic fluid specifications other than the ISO designations. For example, while the classifications of hydraulic fluid are set out in ISO with the designations HL, HM, HV, etc. in Germany the designations HL, HLP, HVLP are standard and frequently used in accordance with DIN 51524.

ISO and DIN are the most commonly used industry classification systems and their correlation is shown in the table below:

Following is a short discussion of the different hydraulic oil types covered by the above classifications:

 Uninhibited Hydraulic Oil

These fluids are refined mineral oils with no active ingredients (additives). They are the most basic hydraulic oils and have a relatively short service life as they are not oxidation-stable and have very limited use. Uninhibited products are in accordance with ISO HH and DIN H.

Rust and Oxidation Inhibited Hydraulic Oil

Formulated with active ingredients to increase corrosion protection and resistance to oxidation which helps the system to be protected from chemical attack and water contamination. They are used in low-pressure hydraulic systems (with no specific anti-wear requirements) in which temperatures of around 50°C are to be expected. These oils conform to ISO HL and DIN HL.

Anti-Wear Hydraulic Oil

Fluids in this category contain additives to inhibit oxidation and corrosion, as well as additives that reduce wear and/or improve the high-pressure properties of the oil. This is the most widely used type of hydraulic oil. Zinc dialkyl dithiophosphate (ZDDP) is largely employed as anti-wear additive. The presence of ZDDP, however, is not always seen as a positive, since it can attack certain metals found in some hydraulic pumps, such as silver. Furthermore, ZDDP can break down in the presence of moisture, heat and mechanical stress to form deposits. Some severe duty hydraulic pumps and other sensitive hydraulic system components (such as close clearance servo-valves and high accuracy numerically controlled machine tools) are intolerant to these deposits.  Zinc-free (ashless) anti-wear hydraulic oils are recommended for sensitive hydraulic systems. Modern ashless hydraulic oils offer excellent wear protection and exhibit outstanding oxidation and thermal stability to extend oil and filter life. They also provide effective corrosion protection for copper alloys and silver pump components. Anti-wear hydraulic oils are in accordance with ISO HM and DIN HLP.

Detergent Hydraulic Oil

These fluids contain detergent additives (cleansing agents) in addition to the additives in anti-wear oils. The use of detergent hydraulic oils is approved by several hydraulic component manufacturers. They can be advantageous in many applications, such as mobile equipment, to prevent a build-up of sludge and varnish deposits, which can lead to valve sticking and other reliability problems. The main caution with these fluids is that they have water emulsifying ability, which means that water is not separated out of the fluid. Emulsified water not only reduces lubricity and filterability, but can also cause corrosion and cavitation, and reduce the life of the oil. These problems can be avoided by maintaining water content below 0.1% – which is not a low water content target for any high-performance hydraulic system. A hydraulic fluid that has the ability to emulsify small amounts of water can be beneficial in mobile equipment applications. Caterpillar, for instance, maintain that separated water drawn through the hydraulic system can damage pumps and other components. If this water freezes, it can also cause serious damage to hydraulic systems. Detergent hydraulic oils are in accordance with DIN HLPD

High Viscosity Index Hydraulic Oil

Oils which, in addition to additives that inhibit oxidation, corrosion, and wear, also contain additives that improve viscosity index (VI). These oils have a VI of higher than 140 and therefore have good viscosity/temperature characteristics. Other hydraulic oils generally have a viscosity index of around 100. The high viscosity index is achieved through the addition of VI improvers and/or by using oils with a naturally high VI. Base oil with a naturally high VI is preferable because this avoids shear-losses. If a VI improving additive is used, it is important that it has a high mechanical stability to prevent shear-losses, which would lead to a decrease in viscosity. Shear stability is a measure of the ability of the oil to withstand viscosity drop due to the breaking down of the VI improver. These oils also contain a pour point depressant to improve low-temperature performance. High VI oils are used in extreme temperature conditions (e.g. mobile hydraulics and critical systems such as CNC machine tools). High viscosity index hydraulic oils conform to ISO HV and DIN HVLP.

Anti-Stick-Slip Hydraulic Oil

Fluids falling in this category have additives to improve their stick-slip properties. Such additives prevent jerky movements, which can arise in the event of very low sliding speeds and high loads. Stick-slip resisting hydraulic oils are in accordance with ISO HG and are for example used in hydraulic elevators and cranes.

Other oil requirements you may find on a hydraulic system or in the service manual may well include one of the following specifications:

To ensure optimum system performance, it is very important to follow hydraulic equipment manufacturers’ oil recommendations. Simply compare the manufacturer’s requirements with the specifications on the hydraulic oil label or product data sheet. If you are still in doubt our experts are at your disposal and ready to provide you with advice and answer any questions you may have. For more information please call 011 462 1829 or email lethabo@p05.4d8.myftpupload.com

The topic of this newsletter was triggered by a noteworthy question that was asked by a visitor to our stand at the recent Automechanika Expo at Nasrec, Johannesburg.  The question was about Borderline Pumping Temperature (BPT) of engine oil.

The viscosity of lubricating oil becomes progressively higher as the temperature of the oil is lowered until it becomes too thick or viscous to flow. The Pour Point of lubricating oil is the lowest temperature at which the lubricant will flow under specified laboratory conditions. It is often believed that Pour Point is the lowest ambient temperature at which oil can be used in a lubricating system, but this is a misconception.

In a system where the pump is positioned higher than the oil sump, such as an automotive engine, this will present a serious problem. We will endeavour to explain this using honey as an example. At normal room temperature honey will be above its Pour Point. When you open a jar of honey and turn it upside down, the honey will flow out under the force of gravity. Yet at the same temperature, it will be impossible to suck the honey out of the jar with a straw although the honey is still above its Pour Point. Now compare this with the engine oil circulating system on the right.

The heart of the lubrication system of an engine is the oil pump. Its function is to suck oil up from the sump (via the oil screen and oil pickup tube), and push it through the filter and into the engine to lubricate moving components. Oil pressure is created by a fluid flow restriction (orifice) in the outlet line of the pump. If for any reason, the oil pump can’t deliver its normal dose of oil, it is bad news for the engine. An oil pump failing to deliver oil to the engine is just as bad as cardiac arrest since the results are often fatal. Loss of oil pressure means loss of the protective oil film between moving engine components. With no oil to keep the surfaces apart, the engine will fail. It is therefore vital that even at very low startup temperatures, the oil must remain sufficiently fluid to enable the oil pump to suck it up and deliver it to the engine. It is crucial that adequate oil must flow from the sump through the oil screen and pickup tube to the oil pump.

When oil is cooled down, the viscosity of the oil increases exponentially with decreasing temperature. This may well result in the oil pump not being able to suck oil in from the sump, even before the Pour Point of the oil is reached. For this reason other test methods are also used to evaluate the cold temperature behaviour of engine oil, particularly lower viscosity oils that are formulated for low temperature applications. One such procedure is the ASTM D3829 Borderline Pumping Temperature of Engine Oil – a measure of the lowest temperature at which an engine oil can be supplied to the oil pump inlet of an automotive engine. BPT is normally measured using a mini-rotary viscometer (MRV).

However, actual operational tests in Cummins diesel engines suggest that values derived by this test method may be quite misleading. First, there is a considerable difference between the actual pumpability of two oils that are identical in every way except in the nature of the viscosity index improver (VII) additive. This BPT difference may be as much as 10°C. Secondly, the values obtained using the MRV showed virtually no difference between these oils and gave values over 20°C lower than the actual BPT in the operational tests. In addition, individual engines

differ widely in the design of their oil distribution systems, which strongly affects their low-temperature performance. For example, in one system with a restriction orifice, the size of the orifice strongly influenced the time it took for the oil to reach the bearings. At -25°C this took 90 seconds with a 1.5mm orifice (and one test engine seized during the test), while it took less than 40 seconds with a 2.0mm orifice. Other influential factors are the oil screen design as well as the diameter and length of the oil pickup tube. Oil with pumping characteristics that are satisfactory in one engine may therefore not be suitable for another at very low temperatures.

With all this in mind, we suggested in OilChat # 23 (where we discussed the Pour Point of lubricating oil in more detail) a good rule of thumb is that the Pour Point of the oil should be at least 10°C below the lowest anticipated ambient temperature. This will ensure dependable lubrication and better reliability in low-temperature applications.

Modern Automatic Transmission Fluids (ATF’s) are formulated with the most complex chemistry of all lubricating fluids. During the late 1930’s General Motors developed the first truly automatic transmission that used hydraulic fluid to change gears. It was introduced as the Hydra-Matic transmission in their 1940 Oldsmobile range. Take a trip down memory lane and experience the introduction of the Hydra-Matic auto box by visiting www.youtube.com/watch?v=8vv400bysiM

Today’s automatic transmissions are worlds apart from the original designs with only two forward gears which were used during the roaring forties of the previous century when all cars would run quite well using the same ATF. The first major change came about in the 1950’s when ATF became available in two variants: ATF Types A and F. General Motors specified Type A whilst Type F was developed for Ford ATF’s. These specifications have been revised and improved repeatedly since then to bring about the current General Motors DEXRON and Ford MERCON transmission fluids. In addition, most other manufacturers have also developed their own proprietary ATF specifications.

Automatic transmissions used in present-day vehicles are nothing short of mechanical marvels. Many vehicle manufacturers are using six- and seven-speed automatic transmissions to improve fuel efficiency, performance, and drivability. Various top of the range luxury cars are now available with eight-, nine- and even ten-speed auto boxes. These transmissions are incredibly sophisticated with many of them requiring their own specific fluid formulations, such as the Mercedes-Benz 9G-Tronic transmission on the right.

An ATF has various functions to fulfil. Not only does it have to reduce friction to prevent wear like all other lubricants, it also has to allow a certain level of friction to enable the transmission’s internal clutch materials to engage. Since most manufacturers use proprietary frictional materials, virtually every ATF is manufacturer specific. In some cases, they are transmission-specific. A typical example is the Mercedes-Benz oil specification MB 236.17 that was specifically developed for the Mercedes-Benz 9G-Tronic nine-speed automatic transmission. This oil is not suitable for use in older Mercedes five- and seven-speed auto boxes. ATF’s must also be compatible with all transmission components, they have to transmit power and act as a hydraulic medium, operate at both low and high-temperature extremes, and maintain constant performance for extended periods of time. In addition, they must also control sludge and varnish, resist oxidation and prevent rust and corrosion. To fulfill all these complex tasks, a typically ATF formulation will contain the following additive components:

Antiwear Agents

Friction Modifiers

Viscosity Modifiers

Corrosion Inhibitors

Dispersants

Antioxidants

Pour Point Depressant

Seal Swell Agents

Foam Inhibitors

Dyes are also added to ATF’s to distinguish them from other fluids such as engine oil, brake fluid, and antifreeze. Traditionally all ATF’s were dyed red, but nowadays ATF’s are available in other colours, such as blue, green and yellow, depending on what is specified by the transmission manufacturer.

One may well ask whether having an automatic transmission with so many gears is really better and, if so, what the limit is. With more gears in modern automatic transmissions, they can match the engine’s optimum torque and power curve with what is needed to propel a vehicle better under all driving conditions. Simply put, extra gears allow an engine to operate more efficiently and economically, regardless of the type of operation. The downside is that more gear ratios come with some specific disadvantages. These include transmission size and weight, complexity, possible reliability issues and, last but not least, more frictional losses. As a result, you lose the efficiency benefits of more gear ratios. It is, therefore, possible that we may have reached “ultimate” auto boxes where having more and more gears will begin to see diminishing returns. In fact, some manufacturers are now focusing on Continuously Variable Transmissions (CVT’s) that can change seamlessly through an unlimited range of gear ratios.

Continuously Variable Transmissions are not a new concept. For many years motor scooters have been fitted with CVT’s, usually the rubber belt with variable pulley variety, commonly known as twist-and-go transmissions. These transmissions consist of two variable-diameter pulleys, each shaped like a pair of opposing cones, with a rubber belt running between them. One pulley is connected to the engine and the other to the rear wheel. The halves of each pulley are movable. As the pulley halves come closer together, the belt is forced to ride higher on the pulley, effectively making the diameter of the pulley larger. Changing the diameter of the pulleys varies the ratio of the transmission. Making the input pulley smaller and the output pulley larger gives a low ratio for better low-speed performance. As the scooter accelerates, the pulleys vary their diameter to lower the engine speed.

In CVT’s fitted to cars the rubber belt is replaced with a metal belt or chain running between the variable-diameter pulleys. This poses a unique set of different challenges as opposed to traditional ATF’s such as requiring higher shear stability and maintaining the appropriate amount of metal-to-metal friction while having enhanced anti-shudder performance. As in the case of ATF’s, there is not one universal CVT fluid that is suitable for all Continuously Variable Transmissions.

There is, however, a downside to CVT’s as well. CVT’s generally perform well in combination with smaller displacement engines, but engines developing more horsepower and torque exceed the (current) capacity of CVT’s. For this very reason CVT’s are presently not used in larger vehicles and some major manufacturers, including Chrysler and Ford, have in fact dropped CVT’s from their line-up. Other disadvantages associated with CVT’s are driver acceptance (changes in engine speed sounds like a slipping transmission), belt noise and durability (slipping CVT belts in particular).

It is, therefore, safe to assume that conventional automatic transmissions will still be with us for quite some time while other technologies are being refined. The only question is the maximum number of gear ratios that will be engineered into conventional auto boxes. Different automakers commit to transmission technologies for any number of reasons such as cost, durability, branding, experience, and drivability. In fact, because of the different advantages and disadvantages, it is hard to say that any one technology is best.