I’ve heard it said that there are two types of people in the racing powertrain world: those who simply love two-stroke engines and those who absolutely hate them. When it comes to two-stroke motors there seems to be no halfway point.
Perhaps you agree with me, perhaps not, but assuming you are not someone whose allegiance is to the electric cause, you simply can’t deny the pure thrill of riding or driving a two-stroke powered machine and the challenge it creates. But along with the power surges and inevitably narrow torque band there simply have to be downsides – that of sticking piston rings and/or the most feared of all, especially for bikers, rapid and unexpected piston seizure!
Two-strokes however come in many different shapes and sizes, from marine propulsion units rated at over 80 MW to the humblest of leaf-blower 50 cc designs, and each design type will most likely require a different type of lube oil formulation. For the purpose of this article therefore I will concentrate on the most simple of crossflow designs with either no gearbox (as in the case of many fixed-drive karting applications) or a separate gearbox of the type used in Class IV karting/Superkarting or motocross.
250 cc twin-cylinder Superkart
The lubricants I will discuss are those where the incoming fuel-lube mix charge is drawn through the crankcase and into the cylinder via a number of ports before being compressed and eventually ejected through the exhaust ports.
Having no complex valvetrain mechanism and consequently fewer engine parts – and therefore reduced weight – together with one bang per piston cycle, as opposed to one every two cycles, these engines generally have superior performance over their four-stroke cousins for a given piston displacement. Simpler, lighter and more powerful, what’s not to like about that? Unfortunately according to many racing authorities – a lot!
Indeed, during the years of Motorcycle Grand Prix racing, before the current MotoGP, the dominant 500 cc two-stroke bikes were some of the most powerful and spectacular machines ever. Having up to 200 bhp (as in the case of the 10 world title-winning Honda NSR500) with incredible surges of tyre-shredding power and devoid of the electronic controls of today, they have been described as the most ‘unrideable’ and the ‘biggest, baddest, most evil machines’ ever to grace the race track.
It is little wonder then that the FIM became concerned about where top-level racing motorcycle development was going. That, together with increasing criticism of the environmental footprint, meant that two-stroke engines were progressively banned from top-level motorcycle racing.
The engine oils used in these two-strokes are unlike anything to be found in the more familiar four-stroke competition units. The oils in four-strokes have to do many more things than just lubrication and the separation of the moving parts to prevent engine wear. They also have to cool the engine and minimise the formation of carbonaceous deposits on hot surfaces and prevent any lacquers by using detergent additive technology.
These deposits will be captured by the bulk oil in the sump and dispersed into the oil to stop them coagulating and blocking the oilways. Since our four-stroke also generates acids during the combustion process, these will accumulate in the oil in the sump and have to be neutralised by adding some type of base additive or additives, plus there is the principle of 'overbasing’, a method of packing residual base into the oil without making them highly alkaline.
If that isn’t enough, the lube in our four-stroke unit might need corrosion inhibitors or anti-foam agents.
In many engines, particularly those with small sumps and/or capable of high revs, where the volume of oil is limited, there may be insufficient time for the foam produced in the crankcase to subside naturally and for the air and/or exhaust gases to separate out. Should that happen, the foam will gradually build up and eventually spill out through the breather vents or, even worse, find its way past the oil seals creating a serious fire risk.
I had this happen once on a prototype engine, and it is no fun when heavily aerated oil spews out near a glowing-hot exhaust system in the test cell!
As well as all these requirements, should your four-stroke engine have an integral gearbox, sharing the cylinder lube with that of the gearbox, extreme pressure additives might be needed to protect the flanks of the gear teeth as they slide in and out of contact as the gears rotate. Finally, antioxidants are introduced to reduce the effects of thermo-oxidative stress on the oil and its additives when exposed to unburned fuel and exhaust gases to maintain the performance of the product. Our four-stroke engine oil works very hard.
Crossflow two-stroke arrangement
By comparison, our particular crossflow two-stroke design doesn’t need to be quite so active in some ways. Sure, for a given horsepower the engine might be working at a higher speed, but the role of the lubricant for our simple cylinder has been reduced to one of controlling engine friction and wear, improving the environmental footprint and ensuring that the oil remains chemically stable during its life time.
Since two-stroke oils are consumed during the combustion phase and not stored temporarily in the engine sump or dry-sump oil tank, this last requirement effectively relates only to extending the shelf life of the product outside the engine.
Thus the balance of properties required in two-stroke engine oils are very different from those used in four-strokes, and this is why neither should be used in the other’s application.
Mixed with the fuel, either as part of the in-tank pre-mix fuel treatment or in the form of a cylinder oil injection system, two-stroke oil will inevitably be burned as part of the combustion process. Any unburned residue, if not ejected via the exhaust port, will build up on the parts exposed to the hot gases, the combustion chamber, spark plug, piston, rings and exhaust ports. Where surface temperatures are comparatively low, this might result in black carbonaceous, sooty deposits.
Cylinder barrel showing intake ports and above them the transfer ports
Where surface temperatures are higher, other deposits can be baked onto the surfaces in the form of hard lacquers. Once formed, the deposits are often difficult to remove, so in an effort to minimise their build-up, detergents and dispersants are added to the oil.
In four-stroke engines these detergents/dispersants might take the form of calcium/magnesium organic compounds consisting of a metal (the calcium or magnesium) ‘head’ of the molecule in the compound and an oil soluble organic ‘tail’. While the actual chemical reaction is quite complex, the organic tail of the compound is dissipated, while the metal ‘head’ can form a nucleus around which other exhaust products can accumulate in the form of airborne particulates commonly referred to as ‘ash’ or more accurately, ‘sulphated ash’.
In two-stroke applications, oils use only low-ash or completely ashless detergent/dispersant additives. Where ashless detergents are deemed necessary (in many low-performance applications, pleasure craft or garden tools) these will be synthesised from totally organic compounds.
One such ‘additive’ that has been suggested to me is hydrazine (H2N2), an unstable compound more commonly known as ‘rocket fuel’. However, it would need to be introduced in very small amounts to avoid other issues, but other combinations of hydrogen and nitrogen, for example those based on ammonia (NH3) could be used.
Comparison of old and new pistons. Note the light brown coloured lacquer around the piston pin boss
When it comes to minimising friction and wear, the situation again differs from normal automotive understanding. Having no highly loaded cam lobes introducing boundary-layer lubrication into the mix, the situation regarding our two-stroke is somewhat eased.
At times during the motion of a four-stroke’s cam, the velocity of the cam surface relative to the tappet surface falls to zero. At this point the oil film could very easily break down, and with metal-to-metal contact, friction and wear could take place. Ordinarily zinc metal based additives are used to protect the surfaces, but with no cams in our two-stroke engine there is simply no need for these zinc-based ZDDPs.
It follows that two-stroke units do not use these anti-wear additives, preferring to rely almost totally on the viscosity of the oil to maintain the oil film thickness between surfaces. And as the oil film thickness tends to follow the viscosity of the oil, two-strokes tend to use thicker, more viscous oils, usually of the SAE 30 or SAE 40 grade.
It is not just the viscosity of the engine oil that matters here though, the type of oil and its composition are equally important.
Classification of engine oils
Paraffinic oils, as used in engine lubricants, are categorised into various groups according to their level of refinement, and are denoted by the amount of ‘saturates’ they contain, the amount of sulphur within them and their ‘Viscosity Index’, a reference to their rate of ‘thinning’ with increasing temperature. These are laid out in the table above/below.
In the petrochemicals industry, the amount of saturates in an oil is an indication of the refinement of a product and its stability in use. Unfortunately, the more saturated the product is, the harder it is to accept additive chemistry. Thus a Group I mineral oil will readily accept additives whereas a more refined Group III offering is less so.
When we move to the more exotic Group IV oils the situation gets even worse, which is one reason why Group IV (PAO) oils are often blended with Group V esters. When it comes to two-stroke oils therefore we often find that the slightly more expensive Group II or III oils will be blended with the cheaper Group I products to generate the precise characteristics required.
For those with deeper pockets who can afford the best in protection, ester-based blends might be preferable. One such oil, classed as a tri-ester, comes from the castor bean, which also happens to be classed as a vegetable oil and therefore considered sustainable in the modern world. Produced from a combination of pressing the bean and further refining, this oil has some impressive properties in that it is attracted to heat.
As such, it makes an excellent lubricant for two-stroke engines, preventing them from seizure, albeit at the expense of what may now be deemed excessive smoke and the tendency to encourage ring-sticking. From my own experience the only downside to using castor bean products is its unsuitability for use with things like power valves and that of the ease of mixing in the fuel.
The latter is not so much an issue with premixed fuel so long as it is freshly mixed, but difficulties can be experienced with oil injection systems. More important perhaps, users should be aware that castor-based oils are generally incompatible with mineral-derived products, and the two should not be mixed.
Now to the vexed questions of what fuel-to-oil ratio to use or how lean to run the engine to give maximum performance but avoid piston seizure. A familiar topic of conversation in any karting paddock, the general opinion unsurprisingly is that racers paying for their own engine rebuilds generally always seem to err on the safe side.
I’d therefore like to finish with a comment from an ex-motorcycle racer from the period when two-strokes were king, and who is still running a team, albeit using four-stroke engines. He said, “I remember one sales guy telling me I could run my [two-stroke] bike on his oil at 100:1 ratio. I told him I had paperwork for him to sign first, obliging him to pay for my ruined bike and hospital bills when this 100:1-ratio oil failed. He left and I never saw him again.”
Seems to say it all. Stay safe.