Motorcycle Lubrication System
Oil is the life blood of any engine and the lubrication system can be considered something similar to our circulatory system. The oil is the blood, the oil pump is the heart and the oil filter is the equivalent of our lungs and kidneys that clean and rejuvenate the oil. The analogy can help us logically conclude that a failure of the lubrication system would be quite like that happening to our circulatory system – death would not be far away. Putting such morbid thoughts of failure and death aside, let’s get up and close with our bike’s engine lubrication system. Knowledge is power and knowing the lubrication system will help us keep it and with that our motorcycle in great riding health.
The Journey of Oil
You’ve emptied that pretty can of oil into your bike’s engine, loving that amber glow of fresh oil as it pours in. The engine would be feeling the same you think. Well, it would and I agree with that. The oil settles down into the sump (most of our bikes here have what is called a ‘wet sump’ lubrication system – which means there’s a tank of oil beneath the engine) and waits for the engine to start. You clean your hands, turn the ignition key and thumb that starter. The engine fires and begins to idle. The oil pump starts sucking in the oil and pumping it up into the engine above. It rises through oil galleries (narrow passages that can be drilled holes, holes with jets like in carburettors, tubes etcetera) and sprays or pours out of holes (orifices for the purists) onto the cam lobes, the cam shaft bearings, the valve stems, the rocker arms. It gets sprayed onto the underside of the piston (in some high performance engines) both to lubricate the small end bearing, the piston pin, the piston rings and also to cool all of the above. The cylinder bore gets its own feed of oil which helps the piston rings form a tight seal with the cylinder walls and keep that compression up. Of course the oil always serves its primary purpose of not allowing metal to metal contact between any two surfaces moving against each other. That and helping keep the engine clean (the detergent properties of oil) and cool (good thermal stability and conductivity of oil ensures this).
The gear box is probably the most mechanically stressful part of the journey of oil through the engine just as the cylinder bore is the most thermally challenging.
The oil’s journey still continues. Most bikes we ride around here have what are called ‘wet clutches’. By now you must have realized that ‘wet’ in context with engines means having loads of oil around and not water. Wet clutches are immersed in oil (and in 4-strokes this is the same as the engine oil) and so require copious lubrication. Clutches produce a lot of heat as there’s a lot of slipping and sliding between the clutch plates and the friction generates heat. Also oil helps form complete plate to plate contact and so helps transfer power efficiently. As if the clutch was not a tough enough environment to work in, the same oil also has to take care of the gear box. The gear box is probably the most mechanically stressful part of the journey of oil through the engine just as the cylinder bore is the most thermally challenging. The gear teeth impose extremely high shear and compressive force loads on the oil film and yet the oil must not fail and allow metal to metal contact between the gear teeth. The old 2-stroke engines had it easier for the gear box oil because the same oil had not already passed through the heated furnace of an engine before being expected to lubricate the gearbox. The gearbox in that case had separate dedicated oil while the engine was fed through a total loss lubrication system.
There’s usually a ‘scavenging pump’ in engines that sucks up this oil once it has served its purpose and sends it back to the oil sump. At times the oil is made to pass through an oil cooler as is the case with quite a few of the present day bikes made in our country. The oil cooler helps to maintain the oil at a steady temperature which allows it to perform all its required functions efficiently and effectively. A controlled thermal environment for the oil also allows it to be used for longer intervals before the need for a change. So the scavenged oil eventually reaches the oil sump and gets pumped back into circulation once again.
Motorcycle oils have a far more difficult life to live and conditions to endure compared to motor oils. They see higher peak engine rpm’s (imagine your car’s engine spinning at 11,000 RPM!) and much higher operating temperatures than car oils. In liquid cooled motorcycle engines the oil runs some 30 degree C hotter than a comparable car engine oil while in case of air cooled engines, the comparable temperatures are twice that – some 60 degree C higher. Adding to the complication is the fact that motorcycle engine oil sumps are a lot smaller than car engine sumps and so there’s lesser quantity to go around. Which means the oil runs more stressed in a bike engine. Also bikes usually have common oil serving the engine, the clutch and the gearbox while car engines have the luxury of totally separate oil for gear boxes and their clutches work dry.
So motorcycle engine oil needs to handle extreme mechanical loading, high operating temperatures, higher piston speeds and must have a quicker response time in starting to fully lubricate the engine. From the view point of the wet clutch, the oil needs just the right balance of properties giving slip and grip. And the gearbox usage means extreme mechanical and shear strength that remains intact at high RPMs and high temperatures. The motorcycle engine oils and the entire lubrication system has its work cut out with greater difficulties than is usually the case with other motor oils. Let’s get to know the lubrication system as well as the oil that it circulates a little better further in this article.
So motorcycle engine oil needs to handle extreme mechanical loading, high operating temperatures, higher piston speeds and must have a quicker response time in starting to fully lubricate the engine.
The very first engines used both in automobiles and motorcycles did carry oil inside them for their lubrication needs but it remained at the bottom of the crankcase and was splashed around by the spinning crankshaft to lubricate the piston rings, the cylinder bore and the small and big end bearings. For the top end of the engine the rider supplied all that oil from the outside while on the ride! Yes, that’s how it was done. A squirt of oil on the engine head which had the cam and rockers operating in the open. He would carry an oiling can and squirt a helping of oil on the rockers and cam every 10 miles or so. Must have been sort of fun on a weekend ride but a major pain in the neck when you wanted to get someplace in a hurry. But those were times when no one really hurried.
The idea of carrying the oil at the bottom of the engine remained since it was not only convenient, designers were used to the idea and it also made maintenance easier. Pressure feeding meant the need for an oil pump to feed oil to all parts of the engine.
Wet Sump – Things gradually refined in the lubrication department as more performance was extracted from the engine. Which not only meant greater demands for lubrication but also 10 miles would be over within a few minutes at most! And our poor brother rider would be left holding the oil can in one hand and the throttle in the other. On a serious note though, the lubrication demands increased manifold with increase in engine power output and so forced lubrication was a logical evolutionary step in engines. The idea of carrying the oil at the bottom of the engine remained since it was not only convenient, designers were used to the idea and it also made maintenance easier. Pressure feeding meant the need for an oil pump to feed oil to all parts of the engine. The initial pumps were piston type which gradually gave way to gear pumps but were later almost universally replaced by Trochoidal (or Gerotor pumps – more on pumps later). Wet sump engines have a removable oil pan under the crankcase (the ‘ease of maintenance’ thing) and they also have a metal plate separating the crankcase from the oil sump – this is to avoid power losses arising from oil being churned by the crankshaft as it spins and also causing frothing (mixing air with oil) which will make it difficult if not impossible to pump. The sump is a part of a closed loop oil supply and recovery system. The feed pump’s work is usually balanced by gravity assisted return of the oil back to the sump. Some powerful modern day wet sump engines require faster and more voluminous circulation of oil and so pump assisted scavenging has been experiment with but usually the returning oil depends on gravity to accumulate back in the sump.
The wet sump system is a lot simpler to design, run and maintain compared to the dry sump explained below. The sump design though becomes critical when sharp acceleration/ braking and sustained high lean angle cornering (during a MotoGP race for example) is brought into the picture. Imagine a flat sump filled with oil and the pump sucking oil through an inlet tube at some point in the sump. If the rider accelerates hard, the oil will tend to pile up towards the rear of the sump and in case the inlet pipe in placed up front the engine will be starved of oil when it needs it the most! Place the inlet at the rear and the same problem occurs while braking hard. The remedy is to design a deep part in the sump that does not get so acutely affected by acceleration and braking and so there’s enough oil around the pump intake to keep feeding the engine. The only disadvantage here is to make the sump taller downwards and so effectively reducing the ground clearance beneath it. Another possible solution is to have a movable oil pick-up pipe that also responds to the acceleration/braking as the rest of the oil. An added issue with wet sump system is the excessive splashing of oil (called ‘windage’) around the engine when revved hard with the oil taking a lot longer in returning to the sump thereby reducing effective lubrication. Increasing the volume of the sump usually is the solution for this.
Dry Sumps – The other way of providing a reservoir for oil and feeding the engine lubrication needs is by having the oil sump located away from the engine. Sometimes it is a separate tank or at times the designers use the frame itself as an oil tank! Ingenious. This type of lubrication system necessarily needs two pumps – the feed and the scavenge pumps. The feed pump force feeds oil to various parts of the engine and the oil thus fed drops down towards the bottom of the engine where it collects in a smaller sump and encounters the scavenge pump that picks up this oil and sends it back to the oil tank. Scavenge pumps need to be bigger and higher performing than feed pumps since they are to force collect the returning oil and push it back to the reservoir. Also at times the scavenge pumps also force lubricate the gear box while pumping the returning oil.
Dry sump systems are complex to build and maintain and so are also expensive. In addition the complexity does increase the number of things that can go wrong. But there sure are some good advantages that come with this system. The foremost is that you do not need a voluminous oil pan under the engine and so the entire engine can be placed lower without seriously affecting the ground clearance. A lower centre of gravity is always helpful in improving the handling dynamics of the bike. A mere 4 or 5 inches might not seem very dramatic but keeping in mind that the engine is the one single heaviest part in a motorcycle, even such measly numbers can do wonders to a bike’s flickability and stability together. At the other end of the spectrum, off-road bikes can benefit from the additional ground clearance that comes from a motor that hangs less beneath the frame. Then there’s no possibility of slosh induced oil starvation of the engine when the bike accelerates or brakes hard. And oil capacity can be as large as the engine needs dictate without worrying about a large oil pan reducing ground clearance. And last but not the least there’s no possibility of the crank being dragged through oil, something that can happen in wet sumps despite the baffles and separator plates. All said and done though wet sump systems are still preferred over dry sump types for the former’s simplicity and low cost against the latter’s complexity and high cost.
Pumping the Lifeblood!
With those early engines almost a century ago, life was simple with manual lubrication. It became a trifle complex with the first plunger type pumps that ran off a cam driven by the engine. The plunger pumps were limited in the amount of pressure they could produce as also by the volume of oil they could handle. And reciprocating machines in any case are less efficient than rotary ones. So the inevitable evolution was towards rotary pumps and gear pumps were the first of the type. A majority of lubrication pumps are driven mechanically inside the engine itself. Usually it is the cam-shaft that provides the drive and at times it is the cam-shaft drive belt that has a pulley driving the pump. Smaller engines have gear pumps directly mounted on the crankshaft.
Gear pumps work by circulating oil through a set of closely meshed gears. Since the gears rotate in one direction, whatever oil enters between the gear teeth and the surrounding pump body has no way of going back and so gets pushed out under pressure. Gear pumps are ‘fixed displacement’ pumps which means they pump out a fixed amount of oil for every revolution irrespective of how fast that revolution is happening. So the output of a gear pump increases with an increase in its operating rpm. They have small mechanical clearances (as low as 10 micro meter or one-thousandth of a meter) that helps maintain output consistency.
In fact there’s a need to digress a bit and clarify about the concept of pump pressure here. The pump as is does not produce pressure by itself. It just produces flow but has the ability to maintain that flow even in the face of resistance to it. The pressure builds up only when the pump has to push the oil against some resistance. And this resistance comes from the small clearances between the engine components through which the oil is pushed. This is why in a worn out engine there is a drop in oil pressure even though the pump is fully fit and functional. It happens because increased space between the components lets the oil flow more easily and thus causing a drop in pressure!
Gear pumps are still preferred in low capacity engines since they have low volumetric demands on the lubrication system. But high performance outputs even from small engines entail the use of Trochoidal pumps.
Trochoid pumps or Gerotor pumps (a name more familiar to Harley owners because that’s what Harley Davidson refers to their trochoid pumps as) are also positive displacement pumps quite like gear pumps and also have gear-like rotors. But that is where all similarities between the two end. The pump consists of two rotors spinning in the same direction. The inner rotor has a star shape while the outer rotor has a star shaped cavity within which the inner rotor rotates. The outer rotor cavity has one more cavity than the number of lobes on the star inner rotor. Also the inner rotor runs a little off-centre inside the outer rotor. Both these rotors rotate inside a tightly fitting pump housing. The differing lobe/cavity combination and the off-centre rotation of the inner rotor produces a constantly changing volume between two adjacent lobe/cavity combination. The inlet is placed where this combination has the most space between them while the outlet is at the point where their inter-space is the smallest. This arrangement squeezes the oil out of the pump and can flow against considerable resistance. Hence the pump generates pressure. All pretty involved to read but simple enough to understand if you see the accompanying figure and photo of the pump.
Vane Pumps are named so because their main pumping elements are vanes moving eccentrically inside a circular pumping chamber (see pic). The central rotor has slots for vanes within which they slide in and out responding to the eccentricity in the rotation of the central rotor with respect to the outer cavity. Vane pumps are rarely found on motorcycle engines though they find automotive applications for power steering pumps, as superchargers and as pumps for automatic transmission systems.
The lubrication system pumps live a hard life. For example there’s only a coarse screen filter at the suction end of the oil pump. This is to allow free flow of oil into the pump but comes with the disadvantage that the oil being pumped does not get really clean until after it has passed through the pump and gets past the micronic oil filter installed between the pump and the rest of the engine.
Meet the Lifeblood!
Oil can best be understood by knowing what all it is expected to do for an engine. First and foremost and most obviously it must lubricate the moving parts, reduce friction drastically between them and remove the possibility of metal to metal contact between components that move relative to each other. The next important function of oil is as a heat carrier – it must help cool the engine by absorbing and carrying away excess heat. Dry sump systems have a slight edge in this over wet sump since the oil gets pumped to a remote tank and so has more distance and time in its travel to cool off. Also the tank is usually away from the most intense heat source in the engine – the combustion chamber which is not the case in with wet sump systems. Oil is also responsible for forming a ‘seal’ between the piston rings and the cylinder walls. Oil fills up the microscopic irregularities in both the surfaces and allows the parts to slide over each other without metal to metal contact. The same oil also protects the engine internals from corrosion and also keeps the insides clean of extraneous deposits. All that put together is quite a load of work for a mere liquid but then it is a very special liquid. Let’s find out why and how.
Oil is a byproduct of petroleum refining. In fact Petroleum literally means ‘oil of the rocks’! Petra is ‘rocks’ in latin and oleum is ‘oil’. Rather hard to imagine oneself wringing rocks and getting oil from them. In fact there isn’t any wringing involved in actually extracting crude oil because it has pooled into large subterranean (fancy word for underground) deposits and all one needs to do is drill a hole deep enough to reach them and pump the liquid out. Of course it is not that simple in practice but that’s the underlying principal nevertheless. These deposits are not just confined to land but are also as numerous under sea and that’s where off-shore drilling rigs come into the picture. The basics though remain the same as on land with the only difference that the ‘land’ here is under hundreds or even thousands of feet of water. The name ‘crude’ arises from the fact that the extracted oil can only be put to use by us when it has been ‘refined’ and so quite naturally the opposite of ‘refined’ had to be ‘crude’. Crude oil was created through the heating and compression of organic materials over a long period of time. Most of the oil we extract today comes from the remains of prehistoric algae and zooplankton whose remains settled on the bottom of an ocean, lake or underground. Over time this organic material combined with mud and was then heated to high temperatures from the pressure created by heavy layers of sediment. This process, known as Diagenesis, changes the chemical composition first into a waxy compound called Kerogen and then, with increased heat, into a liquid through a process called Catagenesis. In a sense we are riding our motorcycles fuelled and lubricated by liquid dinosaurs!
Lubricating oil is formed by mixing two main constituents – the Base Stock and the Additives. Base stocks are what comes either from dead Dinosaurs, or pure synthetically synthesized stuff or a mix of the two. That’s where the names Mineral, Fully Synthetic and Semi Synthetic come from. Then there are the additives that are meant to compliment the oil in its functional abilities. It is the additives that make the oil capable of enhancing the existing properties of oil as well as adding new ones like anti-foaming, anti-wear, detergent abilities etc. Engine oil works in a tough environment with usual feed pressures of 50-60 psi increasing at some 10 psi for every 1000 rpm of engine speed. Even then the figures seem nothing compared to the real deal that comes when the oil gets squeezed between the bearing surfaces, gear teeth and clutch plates and the pressure within those small spaces rises to a couple of thousand psi! Adding to the woes is the heat. Oils usually operate at temperatures well above the boiling point of water. For best abilities, oil should be operating around 110 degree C consistently. If the oil heats up north of 140 degree C irreversible degeneration within the oil happens, it gets ‘burnt’ and needs to be changed as it will no longer perform well inside the engine. Oils getting ‘dirty’ comes at times from this overheating but usually results from an accumulation of contaminants (dust, tiny metal shavings, moisture, acids etc.) and chemical changes within the oil due to thermal and mechanical loads. But heat is the best weapon if you want to kill any oil.
Water is a by-product of combustion and there’s usually some little amount of blow by past the piston rings into the oil below. This blowby also adds some miniscule amounts of fuel to the oil. Also condensation inside the oil sump (since it is exposed to the outside air through a vent) adds water too. This is why it is important to run your engine at operating temperature for an appreciable time to help heat the oil inside enough for it to evaporate both these damaging contaminants. So take that longer circuitous route to office if your office is just a couple of kilometres away. 10 minutes of riding is not enough to heat the oil inside for it to clean itself. But however well the evaporation takes place some bit of this dirty stuff gets left behind in the oil over time and turns it darker in colour. This sludge, soot and carbon not just affect the slipperiness of the oil but also have the potential to clog the oil filter. A clogged filter can lead to a disastrous drop in oil pressure! How? Well, all oil filters come with a bypass valve which is there to allow the oil to bypass a clogged filter and so not starve the engine of its oil needs if the filter gets blocked. Better feed dirty than die of hunger. But since a working filter puts some resistance to the oil feed from the pump and the bypass valve offers none, there’s a dramatic drop in oil pressure.
Synth, Semi or Mineral – that’s the real question!
Again there’s that eternal question amongst bikers (and automobilists and truckers and motorboaters etc. for that matter). Whether to go mineral, semi-synthetic or fully synthetic? Well, the only really straight answer to this is to go by the engine manufacturer’s recommendations. But of course you want to think beyond the mere ‘following of advice’ and so here’s an attempt to help you make an informed decision. There’s a load of variables that can affect your choice of oil vis-a-vis where you live (which decides how hot or cold is your riding environment), how much and how fast or slow you usually ride and of course the type of bike you ride. Most riders feel that synthetic oils are the best and universal panacea to all lubrication ills. It is really not that simple.
Castrol, the world leader in the field of automotive lubrication products, has recently launched a new engine oil Castrol POWER1 Cruise, which is especially made keeping in mind the requirements of cruiser motorcycles. It is available in 2.5L pack size and has been launched at an introductory price of Rs.999/-
Let’s take up the ‘where you live’ factor first. Most of our country is mostly warm, moderately hot, hot or even very hot. Fully synthetic oils have an advantage, and a substantial advantage it is, over semi synthetic or mineral oils in cold and extremely cold conditions. These oils retain their stated viscosity even at temperatures as low as minus 45 degree C! Of course even Drass up north does not hit such a low in the peak of winter. The advantage of course can be garnered even at temperatures in single digits because synthetic oils will flow well even at start up when the engine is still stone cold while mineral or even semi-synthetic oils will not flow well initially till they are sufficiently warmed up. So if you live north of Gujarat and do short duration runs in winters to your office, you would do well to fill her up with synthetic oils for the coldest months at least.
Synthetics are also equally great at the other end of the temperature spectrum – that is very hot conditions. Some of these oils can go right up to even 200 degree C without suffering permanent thermal damage when Mineral oils break down chemically and fail at around 140 degree C. Thinking of times when the outside air temperature hits 45 degree C and you insist on three-digit cruising speeds on an air cooled engine bike, then going synthetic is anytime recommended. However staid commuting even during such abominably hot weather can easily be handled by even mineral oils. But before jumping in for fully synthetic oils, do make sure the oil is suitable for wet clutches. There are certain additives to oils that form a slippery coating on the surfaces exposed to it. But if that happens to the plates in your bike’s clutch, you will have a ‘slipping’ clutch in place of a ‘gripping’ clutch. Mineral oils and semi-synthetic oils are free from such additives and so safe from that aspect.
And last but not the least, synthetic oils cost about twice their equivalent mineral oils. Semi-synthetics fall somewhere in the middle of these two, tilting a trifle towards mineral oils. It is true that synthetic oils need to be changed at longer intervals but in the kind of environment and traffic conditions offered by our country (dust, humidity, contaminated or adulterated fuel, stop and go traffic, interrupted cruising, heat etc.) , stretching the oil change interval is not really advisable. So if you are regular at changing your bike’s engine oil at 3000-4000 km intervals, are a sedate rider who is mainly an in-city rider and also live in a place that does not experience extreme weather conditions either way, staying with mineral or at most going for semi-synthetic is good enough for you. But if you usually rev up the engine, are hard pressed to remember when you’d changed oil last, usually have cash to spare and live in places that see extreme weather (Jaipur and everywhere north of that) then go in for fully synthetic oils for the sake of your machine. Again be sure that specific oil has a grade recommended by your engine manufacturer and that it is oil suitable for wet clutches if your bike has one. But whatever oil you use, it is not advisable to stretch it beyond 6000-7000 kms at the most of whatever kind of usage.