Understanding the Fuel/Oil Ratio in Two Stroke Engines

cub-cadet-logoWhy do two stroke engines need oil mixed with the fuel? How do fuel/oil ratios affect performance? Let’s take a look at how these motors work, and what you need to do to keep the two stroke motor in your Cub Cadet running right.

Why Do Two Strokes Need Oil in their Fuel?

Four stroke engines, like the ones found in cars and larger lawn care equipment, have valves at the top of the cylinder head that stay closed during the compression and ignition strokes to contain the explosion and make power. This is followed by two more strokes, the exhaust and intake strokes, which use the pumping motion of the piston to push out exhaust gases and pull in fresh air and fuel. Each time fuel is ignited, the piston has to move up and down twice for a total of four strokes, hence the name.

Two stroke engines move the intake and exhaust to the sides of the cylinder wall. To burn fuel, the piston moves down to let exhaust pass out of the cylinder and fresh air and fuel pass into the cylinder, then moves up to compress and ignite the fuel/air mixture. This means the piston isn’t pumping gases through the cylinder, so the crankshaft is used in its place. The intake passes through the crankcase where a specially-shaped crankshaft pushes fuel and air into the cylinder. These incoming gases push out the exhaust gases.

What does all this have to do with oil? In a four stroke engine, the crankcase is separate from the combustion chamber, so the oil can sit in the bottom where the crankshaft splashes it around the inside of the engine. The crankcase on a two stroke doesn’t leave space for the oil to let the crankshaft pump the fuel/air mixture. Instead, oil is added directly to the fuel where it can pass through the engine to lubricate internal components.

Why Are Two Stroke Engines Used in Hand-held Lawn Care Equipment?

All things being equal, a two stroke engine should make double the power of a four stroke engine because it ignites fuel twice as often. Combine that with a simplified design that doesn’t require a cam, and you get an engine that provides the power needed for the equipment with a lot less weight.

Since the oil is stored in the crankcase, tilting a four stroke engine can cause that oil to leak into places it doesn’t belong, like the cylinder. Since there’s no oil stored inside a two stroke engine, it can operate just fine at any angle, making it a great choice for equipment like chainsaws that have to be used in a number of different positions.

What Makes Two Stroke Oil Different from Four Stroke Oil?

Four stroke oil is designed to last for a long time inside the crankcase, withstanding heat while creating a coating on internal components that prevents metal to metal contact.

Two stroke oil only needs to work briefly, coating internal components as it flows through the intake, while burning off cleanly inside the combustion chamber to keep the engine clean and keep emissions levels under control.

How Does the Fuel/Oil Ratio Affect the Engine?

The engine requires a certain amount of oil to maintain lubrication, while the carburetor is tuned to add a specific amount of fuel to the air to keep the engine running. Using too little oil can result in accelerated engine wear, while using too much causes the engine to run lean, causing high combustion chamber temperatures that will damage the cylinder and head.

If you have different equipment requiring different fuel/oil ratios, you need fuel blended for each ratio. Most recent Cub Cadet two strokes need a 40:1 ratio. This requires 3.2 oz of oil for every U.S. gallon or 25 ml of oil for every liter of gas. Some equipment needs a 50:1 ratio, which uses 2.6 oz. of oil for every gallon or 20 ml for every liter. There’s even some old equipment designed for a 32:1 ratio, which means 5 oz of oil has to be added per gallon or 31.25 ml per liter of gas. Check the owner’s manuals to find out which ratio is correct for your equipment.

Getting Parts for Your Cub Cadet Two Stroke

If you’re having problems with your Cub Cadet’s engine, or you need other parts to maintain it, you can get it from www.cubparts.com. We’re a certified Cub Cadet dealer, carrying parts for everything from classic tractors to new string trimmers, and we can ship your order anywhere in the U.S. and Canada.

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