Lubrication has always been a problem area in high-performance two-stroke racing engines for a number of different reasons. In this post, we’ll discuss the finer points of 2 stroke racing engine lubrication. Also, we’ll provide an answer to the question of what is the best 2 stroke racing oil.
A recuperative oil circulation system with the oil supply in the crankcase, as in four-stroke engines, is impractical since, for example, the crankcase in 2 stroke go-kart racing engines serves as the pumping chamber.
During the deliver phase, the mixture would pull so much oil into the cylinder that the supply would be depleted in a matter of seconds, not to mention the difficulties that would be experienced during ignition and combustion.
Instead, modest amounts of oil should always be fed into the cylinder, and after lubricating the moving components, such as bearings and pistons, the oil must partake in combustion before being expelled from the cylinder via the exhaust.
If you’re looking for simplicity and certainty, there isn’t a more reasonable solution than the one that is employed in practically all two-stroke high-performance engines today: mixing the oil and gasoline together while you’re filling up the tank.
It should be emphasized at this point that while discussing the operational phases of an engine, the phrase “mixture” refers to the mixture of air and fuel vapor that acts as the active component in which combustion takes place.
The phrase “mixture,” which occurs throughout this article, refers to the fluid condition of oil and gasoline and has nothing to do with the definition of the term “mixture” in its previous context.
“AMSOIL DOMINATOR® Synthetic 2-Stroke Racing Oil extends the life of pistons and bearings by using race-proven anti-friction additive chemistry. High-rpm engines benefit from the film’s superior strength. It helps to reduce ring sticking and plug fouling by burning cleanly. Exhaust power valves may be used with this synthetic racing oil.”
As a result, the oil is put into the engine at the same time as the new charge and in the same ratio every time, so that the stronger the charge, the greater the volume of oil introduced into the engine.
The problem is that only a portion of the oil reaches the working surfaces where it is required, with the remainder flowing into the cylinder while still in suspension, where it creates a variety of problems.
There is a greater risk of detonation because it lessens the detonation resistance and when it burns it removes valuable air from ignition without providing the cycle with a correspondingly meaningful gain in energy.
Finally, the oil generates carbon deposits that impact the spark plug, may produce pre-ignition when in the head, and, over time, can clog the ports, specifically the exhaust port.
The undesirable exhaust smoke produced by a two-stroke racing engine is caused by byproducts from the combustion of the oil in the mixture, which ultimately form oily buildup in the exhaust pipe and silencer, resulting in an ineffective exhaust system.
Certain design characteristics aim to reduce the amount of oil required to lubricate the moving components, hence alleviating some of these problems.
Modern low-power touring engines often use a mixture of approximately 2 percent oil, which results in reduced operating expenses, of course, since the cost of the mixture is proportionate to the quantity of oil used.
Despite the fact that an overabundance of oil in the mixture might hamper thermodynamic efficiency, high performance or racing engines demand significantly more liberal lubrication because of the harsher circumstances and larger mechanical and thermal stresses they are subjected to.
Recent models have made an effort to keep the percentage below 6 or 7 percent, which is sufficient provided the proper types of oil are utilized.
A lack of adequate lubrication will undoubtedly have the greatest impact on the large end bearing, which is under constant and extreme pressure.
Ideally, the incoming new mixture should be guided toward the large end bearing area, therefore depositing part of the oil in suspension on the surface of the bearing. In this manner, the connecting rod is not only lubricated but it is also cooled, which is crucial.
Large openings allow oil to reach the rollers and races of the bearings via the connecting rod’s eye. These openings must not compromise the structural integrity of the connecting rod or restrict the roller track to an unacceptable degree.
The optimal location is found by counter-sinking the two holes in the large end. They should have no influence on the track, which is exposed to the highest amount of pressure, or on the
connecting rod construction, which is most prone to failure.
To eliminate this possibility, the apertures are rounded after heat treatment and before tempering. The small end bearing often only requires one hole at the top, which is produced with the same care as the large end bearing.
Increased lubrication is required due to the usage of smooth bronze bushings, which is provided by drilling adequate lateral or inclined holes on the inner surface of the bushing, as well as oilways on the inner surface of the bushing to transport the oil to the wrist pin.
“AMSOIL DOMINATOR® Synthetic 2-Stroke Racing Oil was designed to survive the extreme temperatures and pressures associated with racing and high-performance 2 stroke go-kart racing engines, thanks to the use of specially designed synthetic base oils. Additional protection is provided by anti-friction additive chemistry, which has been tested and verified in racing.”
In order to prevent the bushing from rotating and clogging the oilways, this operation on the bushing is performed after it has been placed into the eye of the connecting rod.
Oilways are only required if the lateral spacers are located at the big-end bearing. From this perspective, lateral anchoring the small end bearing improves operating conditions for the large end bearing, allowing the mixture in the crankcase to cover the rollers more freely.
The main bearings do not require as much lubrication. Their location, though, partly insulated by the internal flywheels and therefore not impacted by the mixture, may pose problems unless precautions are taken to ensure that at least a nominal volume of oil is used to minimize them.
As long as the stiffness of the crankshaft allows for a safe flexing distance from the cylinder axis, it is common practice to place the main bearing on the transmission side in an outside location. As a result of this arrangement, the bearing may benefit from the transmission’s superior lubrication conditions, while the internal seal prevents it from entering the crankcase.
Another option is to isolate the main bearings between two seals and feed in oil from reservoirs that collect the lubricant ejected from the primary transmission, allowing for an uninterrupted flow of oil. Well-placed drains help to return the oil back where it came from.
Between the main bearings and the seals of the crankshaft, it is common practice to divert the oil into tiny reservoirs where it may be collected after separating from the mixture owing to turbulence or centrifugal force.
A strategy, to increase lubrication of the main bearings, is to provide a large enough chamber between the bearing and seal to produce pulsation of the mixture, which then penetrates the bearing moving parts during pressure changes in the crankcase.
By developing a separate lubrication system of total loss type, and operating in multiple ways, earlier efforts were made to prevent the issues of gas and oil mixture lubrication.
In response to the crankcase depression, oil was taken via oil-ways drilled in the crankshaft, which were blocked during the pre-compression phase. Pressure was transferred from the crankcase to the pressurised oil tank by a pipe with a non-return valve.
Engine load, density and viscosity, and ambient temperature were all taken into account to determine how much oil was fed in. Mechanical oil pumps were sometimes utilized, with a pre-determined flow rate.
“To be the best 2 stroke racing oil, AMSOIL designed it with synthetic base oils. This 2 stroke racing oil will burn cleanly and contains strong high-temperature detergents for outstanding deposit management. This product prevents ring sticking, exhaust port blockage, and pre-ignition by reducing the amount of power-robbing carbon. Improve performance in spite of extreme operating environments found in high performance 2 stroke engines.”
The primary flaw with all of these approaches is the non-automatic nature of the feed, which should change with load and rotational speed, or in other words, with throttle opening.
One of the main drawbacks of all total loss oil systems is the necessity to constantly monitor the oil level in the tank, which is recharged at a slower rate than the fuel tank, and thus prone to being ignored and running out, or a gradual malfunction in the oil feed, which is always hard to adjust.
These are the reasons why two-stroke engine builders that specialize in touring engines have traditionally opted to employ mixture lubrication.
When the ratio of oil used and the manner of preparation is followed, even the most simple of methods may protect the user from the threat of running out of oil.
Having the mixture ready to go before it is poured into the tank is really required since otherwise, it would sink to the bottom and be fed into the carburetor in excess at first, and then insufficiently later on when the fuel level drops.
To lubricate the exhaust piston, which is hotter and less in touch with the fuel-oil mixture, twin cylinder engines need a pump in addition to the fuel-oil mix system.
When performance over a specific level is expected, this becomes essential. The newest variants give oil based on the throttle opening. In certain circumstances, it is off below a specific throttle opening since the engine needs no additional lubrication.
Recent racing and touring engines employ differential feed pumps more and more. That is, their output changes with speed and load, and they lubricate the whole engine.
A low oil indication, generally a warning light, reduces the danger of running out of oil. Obviously, they aren’t utilized in race engines or on bikes. It is customary for the lubricant to be delivered to the crankshaft by oilways to the large end bearing, where it is dispersed onto the cylinder walls by centrifugal force, therefore completing its work.
If it seems that the lubrication conditions of the cylinder wall most strained by the piston are inadequate, it will be essential to provide oil directly to the affected region in the proper amount.
Variable output oil pumps are either reciprocating with variable stroke pistons or gear pumps with continuous output. Because surplus oil is not recovered but is forced out via the exhaust, the setting is critical to the system’s efficiency and oil consumption.
To reduce oil consumption, which affects the performance of two-stroke engines, researchers have experimented with mixing gasoline and oil in separate tanks.
The same variable output pumps are employed here, coupled to the throttle to adjust the oil flow to the engine. The engine may benefit from a higher oil percentage being injected to achieve optimum performance or when the engine is under enormous stress.
Compare this to what occurs with regular lubrication, which is accomplished by a prepared mixture, in which a consistent amount of oil is required to satisfy the engine while maintaining an adequate safety margin, regardless of the operating circumstances.
Oil is pumped into the induction pipe downstream of the carburetor through automated mixing. It is difficult for it to mix uniformly with the air and gasoline. This characteristic, which seems to be a hindrance at first glance, may sometimes be used to one’s benefit. This requires an explanation of the physics of lubrication by mixing in order to be understood accurately.
The oil in the tiny droplets of mixture that reach the crankcase as mist is only slightly impacted by the evaporation that occurs in the carburetor’s diffuser. This is especially true when it comes into contact with heated engine components, which causes the oil to separate from the gasoline.
In the crankcase, the film of oil that covers the crankshaft components grows until it reaches the cylinder walls. Thereafter, the lubrication travels to the crankcase wall reaching the main bearings. The oil included in the portion of the mixture that does not touch the inner walls is transferred into the combustion chamber, where it does more damage than benefit.
This detrimental impact is mitigated slightly, incidentally, by the turbulence created by the connecting rod and internal fly-wheel. The option of using less oil introduces another disadvantage affecting the oil’s lubricating function in the crankcase. It will not completely separate from the gasoline, even at the relatively high temperatures of the crankcase, crankshaft, and related components. Thus, the oil’s lubricating properties are diminished, since viscosity is proportional to density.
Wear rises dramatically when a lubricant’s viscosity, which controls how well it adheres to the wear surface, is lowered. Parts having sliding contact, such as cylinders, pistons, and rings are mostly to blame.
Using oil with a higher viscosity, such as 50 SAE, is required in engines that are subjected to a lot of wear and tear. Dilution seems to have little effect on the viscosity of this kind of oil, but it can lead to more carbon deposits in the combustion chamber.
To avoid deposits in the tank and to ensure lubrication regularity, total separation of the mixture in the crankcase is preferred.
“In order to get the most out of high-heat, high-rpm 2 stroke go-kart racing engines, you need an anti-friction formula with excellent film strength. AMSOIL DOMINATOR® Synthetic 2-Stroke Racing Oil burns clean and helps avoid ring sticking and plug fouling. It protects coated and uncoated racing pistons.”
In this regard, oil injection may be advantageous if done properly. It ensures a better separation of gasoline and oil, which arrives as undiluted droplets. The droplets are big enough to prevent being swept away by the transfer flow before lubricating the crankcase moving components.
The temperature of the oil in the tank and the delay in the supply frequently affect the equipment’s efficiency. Mixture lubrication avoids these issues since the amount of oil that enters the engine is always proportionate to the amount of fuel, regardless of temperature.
Because solubility is not critical in automated mixing, incompatible oils and fuels may be used. Normal mixing cannot achieve this. Castor oil, for example, is not soluble in gas. But it is in methyl alcohol.
Mineral oils are completely soluble in gas and benzene, making them ideal for two-stroke engines. They aren’t soluble in alcohol, so use vegetable oil. Historically, castor oil was widely favored for all types of racing engines, including four- and two-stroke. It is still used with alcohol fuels today, although it has been mostly phased out in favor of more appropriate grades made accessible by the petrochemical sector.
Castor oil’s viscosity, which remains high even at very high temperatures, is a significant benefit. Because of its high acidity, the engine components can become damaged. The rings and bearing cages can be harmed by the gumminess that forms where the oil collects the most.
Oils labeled as gum and acid-free must still be flushed out of engines after they have run on castor oil in order to maintain their consistency and efficiency. Most motorsport rules call for gas-based mixes. Mineral oils with adequate additives to enhance their qualities are preferred, and modern procedures are continually changing.
Experiments with non-crude oils like chemical or synthetic oils have also proven successful. Two-stroke racing oils are characterized by the fact that they are designed to minimize friction by
inserting a liquid layer between the two surfaces that come into contact.
This reduces the coefficient of friction from anywhere between 0.01 and 0.04. As long as there is a microscopic amount of pressure exerted on the surfaces that need lubrication, an oil film must cling fully to them. The higher the viscosity, the more noticeable this property becomes.
Specifically, metal seizing is caused by very high loads that tear down the oil coating, allowing direct contact between moving components. The drag created by the oil films adhering to both surfaces creates some friction force.
The more slippery the oil, the less of an issue this is likely to be. Because of this, the second attribute is essential. Oil oxidation resistance and chemical stability are significant in four-stroke engines, but not in two-stroke engines, where used oil is regularly replenished.
However, a decreased propensity to create combustion leftovers is a desirable trait. Avoiding lead and ash deposits on the combustion chamber walls and piston crown is critical. With the use of detergent oils, it is possible to prevent piston rings from sticking because of deposits in their grooves, which might affect compression.
However, when used in the combustion chamber, detergent oils perform less well. Modified 2 stroke engines exposed to greater mechanical loads and temperatures than intended often need better lubrication. This is more prevalent in engines with large unit cylinder capacity.
If the engine is built to operate on a low-oil mixture, increasing the proportion of oil in the mixture is appropriate. These engines’ piston rings are purposefully broad to lower particular loads. Increasing the lubricating openings in the small and large end bearing regions may enhance their operating conditions.
Cutouts in the piston skirt enable this. In cases of overheating large end bearings, moving the lateral spacers to the small end may help. This makes it simpler for the oil mist to penetrate between the rollers, into the cages, and on the roller tracks.
Oilways directing oil from the crankcase walls may help lubricate the main bearings. The piston is best lubricated by direct introduction into the cylinder. The skirt may be pierced with holes to allow the fuel-oil mixture to reach the cylinder walls, which is a partial cure. The holes’ edges must be smooth to prevent friction on the oil. They must never be put near the exhaust ports.
Because chromed barrels have a lower coefficient of friction, pistons and rings operate better. This equals higher performance even with a modest amount of oil. The same great results are achieved with chromed rings in cast iron barrels, currently standard in racing engines.
Increasing the amount of oil in the mixture during engine running-in guarantees precise matching of cylinder piston and rings. But not too much, since too much lubrication might generate deposits. This clogs the piston and sticks the rings. With cool running at low load, this results in unsatisfactory running in.
Hopefully, you’ve gotten some helpful information on the basics concerning 2 stroke racing engine lubrication. Also, our answer on what is the best 2 stroke racing oil will help you make your 2 stroke racing more competitive and successful!