THE SLIPPERY STORY OF OILS

By Dr. Horizon Gitano CBT

One of the questions I get asked most often is which is the best motor oil to use. When you go to a shop to buy oil for your vehicle, you are confronted with a wide variety of oils to choose from, ranging from expensive “synthetics” to cheap “brand X” oils. Which oil is best for you depends a lot on the vehicle and driving pattern and how regularly the oil is changed. Motor oils fulfill a number of roles in an internal combustion, some of which are engine type specific. This is why there are so many different kinds of oils available.

We all know that the primary function of an oil is lubrication: there are rolling and rubbing contacts in the engine which would have high friction and wear without an oil film to lubricate the interface.

One of the other main functions engine oil fulfills is heat dissipation. While there is a water or air circulation system to remove heat from the head and cylinder, it is more difficult to remove heat from the piston, making it one of the hottest components in the engine after the exhaust valve and spark plug. Oil is delivered to the piston, either by being pumped through the connecting rod, sprayed from a nozzle or simply splashed on from the motion of the crankshaft. Heat is conducted from the piston to the oil which subsequently flows back down to the “sump” or oil pan at the bottom of the engine where it looses heat to the cooler surfaces of the engine. It is important to keep the oil from overheating, as this will “singe” or partially burn the oil, degrading its performance. Overheating is one of the main reasons oils darken with age as they are used. Many engines have a dedicated oil cooler to remove heat from the engine oil. The old air-cooled Volkswagen engines had a large oil cooler hidden away in their fan housing, in effect making them “oil cooled” engines. Some modern engines have a combined radiator and oil cooler with “double pipe” circulation: the oil is circulated through the inner pipes in the radiator while the water is circulated in between the inner oil pipe and the outer pipe.

Engine oil also serves to trap and accumulate particles from wear, dust and soot. Each time the pistons rise in the cylinders, they create a low pressure in the crankcase, and as the piston descends the crank case pressure rises. This creates a pumping flow of air into and out of the crank case. Air is supposed to be released from the crank case via the Pressure Control Valve (or Positive Crankcase Ventilation system) into the air cleaner. This is the hose usually running from the valve cover into the air cleaner box. Inevitably some dust will make its way into the engine, where it will become trapped in the circulating motor oil. This, along with fine metal particles from normal engine wear, will be transported through the oil system and should become tapped in the oil filter. As the filter becomes loaded up with contaminants the pressure across the filter increases, which can adversely affect the oil flow, or simply result in more energy required to circulate the oil. As the oil becomes loaded with contaminants it will start to turn black.

While most engines rely on paper element oil filters, a few alternative designs exist. In older motorcycles oil was often circulated through a rotating drum. As the drum rotates the, heavier components, ie. the particles, are flung outwards where they are trapped on ribs in the drums wall. These filters need to be periodically opened up and cleaned. Still other engines had a “gravimetric” particle separation system where a cavity was placed below the oil pickup point in the sump. Oil and particles would flow towards the oil pickup (ie. the inlet pipe to the pump). The heavier particles would drop down into the cavity while the lighter oil would be sucked up into the inlet pipe and circulated through the engine.

Chemically oils perform a few useful tasks as well. Engine oils help prevent oxidation of internal components such as the crank shaft, cam shaft and etc. which would slowly oxidize creating rust particles and pitting the surfaces. Oils also help neutralize acids which likewise would etch away important internal components.

One final function engine oil performs is that of helping seal the combustion chamber. The clearances between the piston groves and rings, and between the rings and cylinder wall are very small, but some of the pressure in the combustion chamber leaks past and around the rings and into the crank case. A thin layer of oil helps seal these passages off, reducing the amount of this “blow by”.

Engine oils can be separated into two main categories: mineral oil and synthetic based oils. Typical motor oils are mineral based, meaning that they come straight from refined crude oil, and consist of a large variety of different molecules, much as gasoline is not a single chemical compound, but a mixture of hundreds of relatively short hydrocarbon chains. In the case of gasoline the hydrocarbon chains are typically around 8 carbon atoms long, while motor oils tend to be around 20 to 30 atoms long. Synthetic oils are produced from various petroleum feed stocks, including methane, but are chemically assembled into a very narrow range of chemical compounds. This gives them much greater uniformity of their properties, but also a much higher price tag.

One of the most important properties of an engine oil is its viscosity, or how “thick” it is. In general oils tend to thin out as they heat up. A certain amount of viscosity is required for good lubrication: too thick and the oil will be “scrapped” off of the surface like grease, leaving it raw, too thin and it will flow off of the surface. This is one of the major differences between mineral based and synthetic oils. At low temperatures the larger molecules in the mineral oils tend to “gel”, causing the oil to thicken dramatically (once in the snowy mountains of Colorado I had to wait several minutes to get a bottle of 20-50 weight oil to begin to flow). At higher temperatures the mineral oils can thin out so much that they lose their lubricating ability. As synthetic oils are a more homogenous chemical mixture, their viscosity is much more stable over wide temperature swings. Synthetic blends or “semi synthetics” are simply a mixture of synthetic and mineral based oils and correspondingly have properties between pure synthetics and pure mineral based oils.

Apart from the base oil engine oils have a lot of additives to enhance their properties. Common additives are special friction reducing compounds, corrosion resisting compounds and additives to prevent “foaming” (entrainment of air causing foam buildup and reducing the effectiveness) and detergents to prevent sludge buildup. This brings us to another important difference in oils. Most small motorcycles have a “wet clutch” system where the engine power is coupled to the transmission through a series of clutch plates which is soaking in the engine oil. Automobiles have their clutches outside of the crankcase where they are not wetted by oil. Here’s the problem: for a clutch to work it must have high friction, but we want low friction in side the engine. Automobile oils have friction reducing additives to improve efficiency, while these same additives would cause clutch slip on motorcycles. One of the biggest differences between car and motorcycle oils is that motorcycle oils don’t have these friction reducing compounds to prevent clutch slip. I usually run automotive engine oils in my motorcycles and maintain the clutch plates in top condition. The clutch may slip a little, but the engine runs smooth. Another difference in car and bike oils is that the air cooled motorcycles tend to run hotter than the water cooled cars. Within motorcycle oils there is a great difference between 2-stroke and 4-stroke oils. Two stroke oils are designed specifically to lubricate the piston-bore interface, and burn with little residual and smoke. Using four stroke oil instead of two stroke oil will greatly shorten the life of the engine (by as much at 75%), and result in more deposits and spark plug fouling.

What is the difference between Diesel and gasoline engine oils? Again this gets back mostly to operating temperature. Peak oil temperatures in a gasoline engine may be around 150C, while Diesels will get up over 350C. Diesels therefore require a more viscous oil (at room temperatures) than cars. This is one of the things that makes it harder to start cold diesel engines. 

So how often do you really need to change your oil? Again the answer depens on your car, and how and where you drive. Oils degrade with heat, blow by, humidity, and dust. Synthetics hold up better than mineral based oils, and newer cars require less frequent oil changes than older cars. Newer cars can easily go over 10,000 km before requiring an oil change with a mineral based oil, while synthetics can go over 20,000km between changes. With older cars that are run hot on dusty roads you might want to be careful and change more frequently just to be safe. The most important this to remember is to get an oil that matches your vehicles needs, and change it on a regular basis. Generally the vehicle manufacturer has good information on which oil to use and how often it should be changed.