In our last article we looked at how oils and lubricants are blended and how very tight production controls and extremely high quality standards are necessary to produce modern, premium lubricants capable of keeping a well-designed engine or gearbox running for the hundreds of thousands of miles we have come to expect. In fact you may not realise it, but it’s the revolution in Lubricant technologies that is the biggest single factor extending the life of today’s hard-pressed engines and drive trains.
But when it comes to actually choosing and using an oil, what do all those numbers and descriptions actually mean? Have you ever wondered why some engines have different recommended oils to others? Or why identical engines use different grades of oil, depending on local climate and usage? And why do some manual gearboxes use engine oil or even automatic transmission fluid instead of heavy gear oil?
Well it had us scratching our heads so we called the Chief Technical Manager at Brett Oils, and asked if he could help with a few answers. A good starting place, we were told, would be to understand the different qualities that oils can exhibit, and why this makes them more suited to certain jobs as opposed to others. With this in mind then, this month we are going to take a closer look at Engine oils.
Engine Oils – the toughest job of all
Unlike other oils, engine oils probably have the toughest job of all lubricants. Not only do they have to withstand extremes of temperature and still stop wear and tear, they also have to cope with all the combustion by products thrown at them as fuel is burnt, sometimes badly, in the engine itself.
Different fuels produce different oil polluting by products, and different engines have different stresses, strains and heat distribution. To complicate things further, different climates, engine tune, engine wear and even different driving techniques all vary the pollutants, stresses, strains and temperatures in those same engines. This is why there is no such thing as a Universal oil for all engines, despite what some manufacturers may claim. In fact, there’s not necessarily a Universal oil even for one engine, one country and one driver, because the all those factors outlined will make different demands on the engine throughout its working life, more on this later.
Summarising these points, it seems that engine oils have to:
- Perform in High and low temperatures, exaggerated by local climates
- Cope with being polluted by products from combustion of fuel
- Minimise mechanical wear in stressed, sometimes highly stressed, and the dissimilar materials that make up an engine
OK then, lets take each factor in turn and try to relate it to how an oil is ‘designed’ and blended.
Temperature
On most vehicle engines, the oil sits in a reservoir underneath called a sump. From this sump it is sucked up by a pump and pushed through a filter, before continuing its journey through oilways and galleries to the rest of the engine components where it is needed.
When an engine is at its normal running temperature, all these components have heated up and expanded to assume their normal running tolerances, but on a cold engine these tolerances are not so well matched and so mechanical distortions are greater.
Most of us know too that when oils are cold they are also more viscous (thicker). As you’d expect, the more viscous they become the less good they are at flowing and splashing around the narrow oilways, galleries and chambers within the engine itself. This is especially true at the top end of the engine, where the engine oil pump is having to overcome gravity as well as the oils reluctance to flow.
Couple the greater mechanical distortions with the lack of a fast flowing oil film (because the oil is still cold and relatively viscous) and you can understand why the majority of engine wear occurs in the first few minutes of running, as engine and oil struggle to reach ideal operating temperatures. What this tells us, is that it absolutely vital for engine oils to maintain easy flowing properties especially on those cold winter mornings.
Now all oils are manufactured using ‘base stocks’ and of these the simplest and most straightforward are so called ‘Mineral’ base stocks. Mineral base stocks are derived from the crude oil that is extracted from oilfields around the world. The crude oil is put through a process that in simple terms splits the crude into different viscosity grades. This is done by literally boiling the crude oil in huge, tall vessels and as the gasses rise up the vessel they cool to become liquids that can be collected. The further up the vessel you go the cooler it gets, so the heavier, thicker elements (or fractions) of the oil liquefy first and the lighter, thinner fractions travel further up to liquefy at cooler temperatures. This produces a range of ‘fractions’ with decreasing viscosities. Each fraction is then refined (to remove any impurities) into a series of mineral base stocks and combinations or ‘blends’ of these different viscosity grade, mineral base stocks are used to produce the required viscosity of the finished product.
The problem with very simple mineral base stock blends though, is that they can still only maintain the correct, easy flowing viscosities over relatively narrow bands of temperature. So a mineral oil with a viscosity that’s fine for a cold start at –5 degrees C will have become far too thin when it and the engine are up to full operating temperature (which can be in excess of 130ºC). Oil that is too thin is just as bad as oil that is too thick since it leaks past the oil pump gears and bearing surfaces alike, lowering oil pressure and again starving vital components of much needed lubrication. As if all this wasn’t enough, engine oils also serve to cool and dissipate heat, often in critical engine components such as turbochargers, so any lack of oil flow on these would lead to disastrous overheating and wear.
These viscosity problems are the reason that many years ago, different grades of mineral oil were used, depending on whether it was winter (low ambient temperatures) or summer (high ambient temperatures). These grades of mineral oil, because of their wide, temperature related viscosity changes, were called monograde oils.
Chemical engineers however, soon discovered new ways to artificially limit these viscosity variations and devised blends that would keep the oil as near as possible to the optimum viscosity, regardless of the temperature of the oil itself. Firstly they perfected special additives called Viscosity Improvers (VI) that are blended with the mineral base oil stocks. These especially target those winter mornings and prevent the oil thickening before those critical cold starts. These cold start additives are added at the blending stage and are carefully designed to meet the specifications determined by vehicle manufacturers and Mother Nature! Because a single version of these additive treated lubricants could now replace both winter and summer monogrades, they were christened Multigrade Oils.
These Multigrade Mineral Oils ruled the roost for many years and largely superseded monogrades. But they did not solve all the problems caused by large variations in engine temperature. This is because over time, in hot engines, the lighter, thinner mineral base stocks used in the multigrade blends actually evaporate away. This evaporation leaves behind the ‘heavier’ base stocks of the blend, thickening the remaining oil as well as concentrating the other pollutants and wear particles carried within it. As can be seen, this not only increases oil consumption, it can also lead to the formation of sludge resulting in yet more engine wear and even seizing.
In recent years this situation was made even worse as a new generation of lean burning engines was created using powerful computers, new and improved materials and more precise manufacturing techniques, all leading to far greater demands on the engine lubricant. As fuel economy became increasingly important so did the need for yet thinner oils since these have the added advantage of reducing friction. Add to this the dramatic increase in road development, especially Motorways with their sustained higher speeds and prolonged driving periods at higher engine temperatures, and you can see why lubricant companies were forced to rethink and redesign their multigrade, mineral oil products to cope with this technological evolution.
Once of the most significant outcomes of all this development was the formulation of so called Semi Synthetic and Fully Synthetic multigrade oils manufactured from a new ‘breed’ of synthetic base stocks. In fact there is an enormous range of synthetic base stocks produced but in vehicle lubricants, two types are commonly used
- Hydrocracked, or Group III base oils.
These are made when ordinary mineral base stocks are subjected to a further refining process that ‘streamlines’ the molecules (removing all of the molecular ‘side chains’ for all you chemists out there).
- Polyalphaolyfins (P.A.O.)
Unlike the hydrocracked base stock detailed above, PAO’s are not manufactured from existing mineral oil stocks. They are manufactured from selected raw materials to produce a range of PAO’s that are chemically engineered to give enhanced properties.
What all this complicated Chemistry means is that Synthetic base stocks give distinctive advantages over mineral base stocks:
- They have far better low temperature properties, even maintaining their ability to flow at temperatures where mineral oils would become solid!
- Their actual viscosity is far more stable over wider temperature ranges, meaning optimum, lower viscosities can be maintained whether hot or cold, so reducing friction in the engine and improving fuel economy
- They are far less volatile at high temperatures, so they don’t evaporate away.
This allowed lubricant companies to design oils that maintain much thinner, ideal viscosities at very low or very high temperatures but which do not evaporate away.
The amount of synthetic base stocks used in the formulation of an engine oil determines whether the oil is marketed as full synthetic, semi synthetic, part synthetic or synthetically modified
- Fully synthetic oils are manufactured using only synthetic base stocks. They contain no mineral base stock at all.
- Semi synthetic, part synthetic or synthetically modified oils contain a mixture of synthetic and mineral base stocks. The amount of synthetics used depends upon the specification and viscosity the manufacturer is trying to meet.
Combustion by products
If you have ever followed an old banger down the high street, you’ll know that some engines create an awful lot of smoke and pollution. The truth is though, that all engines produce unwanted waste products as they burn fuel and whilst much of this is pushed out of the exhaust, a significant amount is picked up in the oil of the vehicle. These waste products take the form of sooty, acidic particles and gums and are especially prevalent in Diesel engines. The oil too breaks down over time, largely due to oxidation, and can form sludge and varnish like coatings. Added to these are the tiny (and sometimes not so tiny) metal particles resulting from actual engine wear itself as well as minute particles of ‘dust’ that make it past the air filter. It’s worth mentioning here that standard Oil and Air filters are by no means perfect filters. They do trap the majority of unwanted particles but cannot stop the extremely tiny particles that are too small to be filtered and that ultimately build up over time, promoting wear.
As you can imagine, this is quite a cocktail of nasties and so oils, whether mineral, semi synthetic or fully synthetic, contain yet more special additives that give it two further qualities:
- High detergency
- used to keep engines clean
- soaks up and neutralises the acids formed during the combustion process which would otherwise lead to corrosion.
- High dispersancy
- used to keep solid contaminants (soot) in suspension.
- prevents the build up of sludge and varnish on engine parts
- prevents soot particles combining to form larger particles that could lead to engine wear, particularly in the cylinder wall region.
Minimise mechanical wear
We’ve already talked about how oils are affected by temperature and how this can prevent them from reaching those critical engine parts that rotate or reciprocate against each other, gradually wearing away as the miles clock up. But even when the oil is perfectly distributed throughout the engine, its actual ability to lubricate and so reduce wear, is limited by its own physical properties. Oil is naturally ‘slippy’ and so good at reducing friction and wear. But in areas where very high pressures exist between two surfaces, oil alone is just not good enough at preventing wear. Yet again those all-important additives come to the rescue and specially formulated ones are added to the oil to form and maintain a thin. Lubricating film at all times in highly loaded areas.
When the best isn’t good enough!
Well, it seems from all that’s been said so far, if you want your engine to last and last then you simply go out and buy the most expensive, fully synthetic, highest specification oil that money can buy.
Well actually, nothing could be further from the truth! You see the best oil for your particular vehicle is determined by lots of different factors such as:
- The design of the engine
- The existing engine wear
- The fuel quality
- The frequency of servicing (hence oil changing)
- The way you drive
- The country (and hence climate) you live in
For instance, if you put a low viscosity (thin), modern, fully synthetic oil with all the bells and whistles into an engine that either has an old design, or is perhaps halfway through its life (and hence showing some wear and tear) then like as not your oil pressure will be too low (because the oil is too thin for the oil pump) and you will increase oil consumption albeit through increased leakage and unwanted combustion (as the thin oil pushes past old gaskets, oil seals and piston rings). Equally, if you live in a part of the world where winter can see temperatures below -15ºC then a fully synthetic, ultra thin oil will be ideal for those winter months. But, even in a relatively modern engine, it may become too thin when summer temperatures rise above 25ºC.
There’s also the question of economy too, not just that of the fuel but also that of the oil. Modern highly specified lubricants are expensive and rightly so because they contain all the sophisticated chemicals and base stocks we have mentioned. But they also have a lifespan that is matched to the recommended service interval of the vehicle for which they are best suited. This lifespan can be as much as 45,000 miles between oil changes and in modern cars 12,000 miles is common.
But if you put such oils into vehicles that have the oils changed every 6,000 miles (because that’s what the book says) then at every service you are effectively throwing away oil that is still perfectly good. It’s like buying a set of tyres and changing them when only half the tread has worn away. Of course there’s nothing mechanically wrong with doing this but it simply costs you money unnecessarily.
On a different tack, putting a highly detergent, highly dispersant oil into an old engine can also give rise to problems. It will search and dissolve gums, varnishes and deposits but these may have been taking up wear on some surfaces and once gone, may lead to increased leakage or mechanical distortion. Worse still, if any large, accumulated particles are freed then these could block narrow oilways and galleries leading to oil starvation.
Luckily, vehicle manufacturers consider all these factors when recommending oils and lubricants for their vehicles. They work with lubricant companies and the highly specialised additive manufacturers to ensure that oils are available that match the vehicles requirements. In addition, all oils are graded according to a particular set of standards and it is these standards that allow you to compare different oils. Whenever you chose and use oil for your vehicle you MUST ALWAYS ensure that at the very least it meets the standard recommended by the vehicle manufacturer. You must also carefully consider the consequences choosing oils that exceed these standards, since all that glitters is most definitely not gold in the case of vehicle engines.
Coming next - we will consider the various recommended specifications for engine oil, what they all mean and how they are defined.
