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Tinkering under the hood is no longer part of the car owner’s unwritten contract with automakers. Warranties and 100,000-mile tune ups are. Automotive manufacturers have to make their components work harder and last longer than ever before or fix them free of charge. At first, better metals, plastics and rubbers were developed to extend the life of the product, but that wasn’t enough. The petroleum-based lubricants could not keep up with the broader temperature requirements. They’d fail due to oxidation or thermal degradation and would hardly last the tens of thousands of cycles required to pass today’s service life tests. So in the early 1980s, automotive component engineers began to leave their grandfathers’ greases behind and turn to synthetic lubricants to ensure performance and reliability.
Synthetic oils had been around for quite some time. Esters were developed in the 1940s and 1950s for the fast-growing aviation industry, where lubricants for components in jets had to withstand freezing, high-altitude temperatures, as well as the heat from jet engines. The next two decades widened temperature requirements even further in the aerospace industry, which gave rise to new classes of synthetics lubricants: polyphenylethers and perfluoropolyethers. Not counting noncommercial experimental synthetic oils, there are currently six basic families of synthetic lubricants: synthetic hydrocarbons, polyglycols, esters, silicones, fluoroethers and polyphenylethers. Together, they extend the operating temperature range of lubricants from 90°C to 250°C, a quantum improvement over what was once known as black gold!
Sensors were one of the first automotive components that required something better than petroleum grease. In the 1980s, throttle position sensors (TPS) and exhaust gas recirculating (EGR) sensors were introduced to monitor electronic fuel injection and exhaust emissions. These potentiometers, mounted on the air intake and exhaust valves, sent electrical signals to a computer, which used the data about valve position to optimize performance. The signals had to be accurate, despite the fact that the sensor had to operate in wide temperature ranges and were exposed to fuel and exhaust vapors. Greases were applied to the resistive elements because a small amount of wear on the ink may change the electrical signal that was sent to the computer. If the grease dried, varnished, froze or was dissolved by fumes, the potentiometer was doomed, and so was the performance of the engine. Synthetic esters, silicones and fluorinated oils helped ensure the reliability of these sensors.
A Word to the Wise
Choosing the right synthetic base oil is the key to choosing the right synthetic lubricant. All oils - synthetic, animal, vegetable and mineral - are liquids, which are subject to freezing and evaporation. In either state, the oil cannot lubricate and the component fails (Figure 1).
Figure 1. Lubricant Temperature Ranges °C
So matching the temperature of an oil to the temperature extremes in which a component must operate is essential to selecting the right lubricant for an application.
Synthetic lubricants withstand much broader temperature extremes than mineral-based lubricants. It’s important to pick a base oil whose temperature range matches the operating environment of the component (Figure 1).
When an original equipment manufacturer recommends a lithium-based grease, most people are comfortable using any general-purpose lithium-based product. However, knowing you have a lithium grease tells you little about the lubricant’s performance characteristics. Greases are made by mixing a solid material, called a thickener, with a base oil; but it’s the oil that lubricates. The grease thickener can be thought of as a sponge saturated with oil. Moving parts squeeze the oil out of the grease to reduce friction and wear. Because the thickener is only the sponge, it will behave differently depending on what oil is mixed with it.
During the last 15 years, the use of automotive wire harnesses has grown at an exponential rate. Not only did sensors need to be plugged into computers, but the growth of power features, switches and lighting requirements has also helped multiply the number of wire harnesses in today’s vehicle. As the number of harnesses grew, the need for cost-effective metals, smaller terminals, and more terminals per connector also grew. These changes, combined with wider operating temperatures, multiple low-current signals and higher reliability requirements presented connector engineers with new challenges. Synthetic greases were developed to meet those challenges by offering improved connection protection.
Because connectors operate in such a wide temperature range, the lubricant must also. If the grease fails, connectors can corrode causing degradation of the electrical signal. Connectors are also subject to fretting corrosion caused by vibration. The use of gold or other nonoxidizing metals, or increasing the contact pressure on each mated terminal, can minimize fretting corrosion; but those are costly solutions, generally reserved for only critical circuits, like the ones in air bags. However, synthetic hydrocarbon and silicone greases can achieve the same results at a significantly lower cost. They also reduce the force needed for mating and unmating connections and prevent corrosion of the metal surfaces.
Most people feel comfortable using grease on bearings, gears, slides and other mechanical devices. However, many don’t recognize how a grease can also extend the life of electrical switches. Years ago, electrical relays with butt contacts were used to turn many devices in a car on and off. The need to reduce cost and the number of parts on a car pushed electrical engineers to make a better switch. While some relays are still used, most control is handled by the switch.
Some switches are designed to handle low electrical currents to send signals to relays or computers. Some are designed to carry high electrical loads (20 to 60 amps) to starters, headlights and turn signals. More and more switches are expected to do both and to do it across a wide temperature range, and for a safety margin, for three times the forecasted life. This might involve hundreds of thousands of cycles.
Low-current switches don’t require much metal to do the job, so they don’t have much contact force. The proper contact grease can make all the difference. It has to be light enough to allow good electrical contact, not freeze in subzero environments, and it can’t dry or varnish, causing poor conductivity. Light viscosity synthetic hydrocarbons and esters, which perform well at 40°C and below, have been successful lubricants in low-current switches.
High-current switches have large metal contacts to carry the electrical load. When the contacts mate and break there may be a fair amount of arcing. This arcing super-heats the grease. It actually burns many greases and heats the metal contacts. Glycol, ester and perfluoropolyether (PFPE) oils are able to resist degradation at these extreme temperatures.
Switches that carry both high and low current often require different contact greases one for the low current and one for the high. Many switch designs don’t allow for two greases, but the flexibility of synthetics often allows one grease to do both.
More than a dozen cables can be found on most cars. They vary in length, load-carrying capability and duty cycle. However, they have a few things in common. They are hand operated, have long stranded bundles of wire inside a plastic sleeve, and if they don’t work properly, the car owner gets greatly annoyed.
The beauty of the cable is its flexibility. It can be wrapped through and around all kinds of obstructions in the car. This erratic path causes a great deal of friction between the stranded wire and the plastic. The right lubricant is critical to keep friction at a minimum. The right lubricant should get in between those parts quickly, particularly on frequently used accelerator, brake, clutch and shifter control cables which often have to pass one million stroking cycles before a design is accepted.
The near universal solution for cable lubrication is silicone. Silicone oils and greases offer wide temperature and good surface wetting characteristics. While cable lubricants are not exclusively silicone-based, it is common to find some type of synthetic lubricant on these popular parts.
The Need for Power
Electric motors have been used for years on cars to power both essential and luxury components. Some of today’s cars have more than 60 electric motors! Over the years, these motors have been designed smaller. A smaller motor not only reduces the weight and cost of the component, but it also provides a more efficient use of electricity by causing less drain on the rest of the electrical system.
Starter motors are a good example of high output motors that have undergone significant size reduction in recent years. Yet they still need to start six-, eight- or even 10-cylinder engines. In addition, they are exposed to road splash and grime, and 150°C exhaust pipes are routed close by. Despite the miserable operating environment, engineers must make sure these motors last at least 10 years.
One System, Five Lubricants
There are often several components in one system. This driver’s seat system contains cables, gears, sintered metal bearings, slides and lead screws (Figure 2). Different synthetic lubricants are available to meet the operating conditions of each component.
Other under-the-hood motors strapped with the same environment and reliability issues include motors for the exhaust pump, cooling fan, antilock braking system, traction control, and windshield wipers. An ordinary grease won’t cut it, but ester and fluorinated greases and oils usually do the trick.
While extreme temperatures and road grime aren’t of great concern to passengers, the weight, power output, long life and low noise are. Windowlift motors are a good example. The trend over the years has been to make the inside of the car quieter. One measure was to make the rubber window seals fit tighter against the glass to reduce outside noise. This requires the windowlift motor to have much higher output without changing its size. Synthetic greases and oils based on synthetic hydrocarbons and esters with the proper low-friction and noise-reducing additives, have done a nice job providing power output efficiency and low noise on all interior motors.
Synthetic Lubricants for Automotive Components
Lubricant engineering is both an art and a science. Today, if lubricant engineers understand an application thoroughly, they can custom design a lubricant to match the operating conditions of a specific component. Many of the automotive components for which synthetic lubricants have already been developed are mentioned above.
From Engine Oil To Power Train
While the general public is starting to accept synthetic engine oil and transmission fluid because they last longer and work better in subzero conditions than petroleum-based lubricants, there are a wide variety of other parts involved in a powertrain system that can also benefit from synthetic lubricants. If greases in under-the-hood components such as alternators, condensers and water pumps minimize frictional drag, then fuel economy is optimized. Clutch and brake systems, fuel and air controls, and even superchargers and turbochargers - all tested at 150°C and higher for long periods of time will take the heat much longer with synthetic lubricants. Generally esters, silicones and fluorinated greases are used to meet the performance targets. In addition, synthetic hydrocarbon greases are significantly extending the life and improving the efficiency of CV joints, U joints in rear axles, and wheel bearings.
A Bumpy Road Ahead
Steering and suspension systems have become more complex in recent years. Active suspension systems that accurately adjust the flow of shock absorber fluid to change the response and feel of the car must use oils that are fluid at 40°C and below. Traditional power steering systems that use hydraulic fluid are starting to be replaced with electric motor-driven systems that must actuate quickly, responsively and quietly. Synthetic hydrocarbon lubricants support those design objectives.
The ball bearings and gears in these systems place a high demand on the lubricants. Ball joints in the front-end suspension systems used to have grease fittings on them. When a car got an oil change it also got a chassis lube, which entailed pumping these joints with fresh grease because the original grease was used up. Today, the grease fittings are gone and these parts are lubed for life, for 10 years or longer. Synthetic hydrocarbons, esters, silicones and fluorinated lubricants have allowed this new maintenance-free ability.
Cars of the next millennium will certainly require more efficient use of electricity from all of the power-consuming components, especially with the battery-powered vehicles. They will feature an increased number of sensors, wire harnesses, computers and screens, creature comforts that must feel and sound good, longer life requirements, longer warranties, hotter under-the-hood temperatures as the engine compartment gets smaller, perhaps cold temperature requirements lower than 40°C, and improved weight reduction and fuel economy. Add technologies like the fiber optics, which also use synthetic oil-based gels to improve signal transmission at the end of each fiber. Throughout the tightening of quality standards, synthetic lubricants will continue to play an ever-more important role. They’ll help engineers meet performance requirements of every moving part and some nonmoving parts. And like they have for the last decade, they will play an increasingly important role in keeping the auto industry and the vehicles it produces moving.
A version of this article appeared in Machine Design, January 1999.