Two-stroke engine lubrication Part 2 OilChat#45

When it comes to two-stroke or two-cycle (2T) engine oil every two-stroke enthusiast is an ‘expert’. Some old-timers still believe castor based 2T oil is a gift from heaven, while lots of fanatics can’t resist the ‘pleasing’ aromatic odour of mineral oil being burned in an engine and these days many 2T buffs swear by synthetics. Amid all these opinions there still is a lot of smoke and mirrors and plenty of spin around the subject. In this issue of OilChat we will endeavour to clear some of the fallacies surrounding two-stroke oil.

 

Compared to four-stroke engines, two-stroke engines have far less moving parts. They are therefore less expensive to build, more compact and significantly lighter.  2T engines generally also have a higher power-to-weight ratio than four-strokes. For these reasons, two-stroke engines are ideal in applications such as lawn mowers, chainsaws, grass and hedge trimmers, outboard motors, microlight airplanes, karts, off-road motorcycles and two-wheel racing applications.

2T engines come in a variety of sizes and designs, ranging from tiny little engines for model airplanes to powerful marine outboard motors. Traditionally two-stroke oil was mixed with the petrol, but many recent designs pump the lubricant from a separate tank into the engine. These designs are often referred to as auto-lube systems.  It is still a total-loss system with the oil being burnt with the fuel in the same way as in pre-mix systems.

Considering that 2T engines are lubricated by a mere whiff of oil introduced into the engine in vapour form, two-stroke engine oil must have excellent lubricity. 2T oils must also be clean burning since the oil is combusted with the fuel/air mixture. In bygone days castor-based oils were the preferred lubricant for two-stroke engines due to its high film strength and “wetting ability” (the capability of the oil to spread out on metal surfaces). In addition the chemical structure of castor oil allows it to polymerise at high temperatures to form a sticky wax type material, often referred to as castor varnish. This wax has lubricating properties. In the event of oil starvation the wax separates the metal surfaces for a short period of time.

The down side of castor oil is it that does not burn completely. This results in fouled spark plugs, combustion deposits, piston rings sticking in their grooves and excessive smoke. Furthermore in modern engines where tolerances between parts are much finer, the build-up of castor varnish can lead to degradation in engine performance.

Modern two-stroke engine oils are generally blended using one of the following three base fluids:

Mineral Oils are primarily used for lower performance 2T engines. These oils are good rust preventatives and have great lubricating properties. Mineral oil does not burn all that well and leaves deposits behind when it does burn. Typically a 2T engine running on mineral oil will have gummy deposits in the ring groves and burnt carbon in the exhaust port and on top of the piston. This is a pretty big deal in modern two-stroke engines as gummy parts, fouled spark plugs and carbon deposits mean less performance and more maintenance.

Synthetic Fluids offer the best of everything, including good lubricity and superb combustion properties. With little of the mess that petroleum based oils normally deposit, synthetics leave a much
cleaner engine and therefore maintain maximum engine power for extended periods of time. Synthetic oils also produce much less smoke, but they come at a price premium.

Mixtures of Mineral and Synthetic (Semi-Synthetic) Oils are a compromise that meets in the middle. Semi-synthetics cost less than full synthetic oils. They offer reasonably good combustion and lubrication properties at a more realistic price.

The prime function of two-stroke oil is to lubricate (i) the main bearings on the ends of the crankshaft, (ii) the big- and small-end bearings of the piston connecting rod and (iii) the cylinder walls. To prevent deposit formation in the combustion chamber and on the piston, there is no traditional ash type anti-wear (ZDDP) additive in two-stroke oil formulations and the lubricity is mainly provided by the oil film of the base oil. Many 2T oils contain a high molecular weight polyisobutylene (PIB) to enhance the lubricity of the oil, but this may increase carbon deposit.

Another important function of two-stroke oil is to keep deposits inside the engine to a minimum. High performance 2T oils are therefore formulated with a low ash detergent additive to remove such deposits. Many two-stroke oils are pre-diluted with a solvent to facilitate mixing with petrol at all temperatures. 2T oils are normally dyed blue/green to identify its presence in petrol/oil mixtures. Other additives that may be included in 2T oil formulations are combustion improvers and octane enhancers.

Even if a two-stroke oil has good lubricity, high detergency, resists exhaust system blocking and meets low smoke requirements, how does one know whether the oil is suitable for a particular 2T engine. If you take a closer look at the container or product data sheet you will most probably come across specifications such as API TC, ISO L-EGC, JASO FD, etc. More about this in the next issue of OilChat….