Two Stroke Fuel/Oil Ratios
Lubrication of the moving parts in a two stroke engine (without oil injection) is dependent on the oil that is pre- mixed with the fuel. The quantity of oil needed for lubrication is relatively small but must be constantly re-supplied.
As the rising piston draws the atomized mix into the crankcase through the one-way reed valve, the oil tends to separate out into tiny droplets lubricating the bearings, cylinder wall and piston. As the piston descends, the gasoline (along with a quantity of oil) is forced through the transfer ports into the combustion chamber side of the piston where it is compressed and burned. Generally, the oil stays in the crankcase longer while the gasoline passes quickly through. Here is where the fuel to oil mix ratio comes into play. Too little oil in the mix and you risk piston scoring and possible seizure, too much oil and excess carbon deposits form in the combustion chamber and exhaust ports, and even richer mixes will send unburned oil out the exhaust where it will pool in the muffler and drip from the exhaust pipe. Also, when too much oil is present, spark plug fouling becomes a problem. An ideal ratio allows enough oil for lubrication while minimizing oil buildup in the crankcase. Some important factors help determine this ratio starting with the maximum RPM of the engine. The higher the rpm the more oil is needed for adequate lubrication. Because the fuel/oil mix moves through the engine faster as the rpm increases, a high revving (10,000 rpm) two stroke may require a 16:1 ratio to maintain a sufficient lubrication presence inside the engine. (Oil injection by a metering pump has been used as a way of dealing with these varying lubrication needs. These pumps use a control cable which moves in conjunction with the throttle cable delivering more oil as the rpm increases and very little at idle.) Another factor in determining the amount of oil needed is the type of bearings used in the engine. As a rule, plain bearings require more oil whereas needle bearings and ball bearings require less oil. Most brands of two stroke oil made from natural petroleum are specified at ratios ranging from 32:1 to 50:1. Synthetic two stroke oil however may be recommended by the manufacturer at ratios as high as 100:1. These high ratios are necessary because some synthetics do not “flash off” hot engine parts as easily as natural oil and if used at the same ratios will result in excessive crankcase oil build up leading to a similar condition stated earlier where unburned oil is blown out the exhaust ports into the muffler. Modern two stroke oil, both natural and synthetic, are formulated to lubricate and burn cleanly at the oil manufacturers recommended mix ratios² and are the preferred choice over regular four stroke motor oil.
¹ When using premix, regardless of oil brand or type, a certain amount of oil will accumulate in the crankcase at idle, or in engines with a low maximum rpm such as a Lawn Boy (3200 rpm). This excess oil can only leave the engine by passing through the combustion chamber, and as it does, varying degrees of smoking will occur.
² If you change a mix ratio for a particular engine from say 32:1 to 40:1 making a slightly richer in fuel mix, or go the other way from 40:1 to 32:1 making the mix slightly leaner in fuel, a carburetor re-adjustment may be necessary.
I hope this helps with understanding two strokes
