Part 1

Two-stroke engines are internal combustion engines that complete a power cycle with two piston strokes during just one crankshaft turn. The reason it can do so is that the engine is designed so that it can take in gas for combustion and exhaust waste at the same time. They were first designed in around 1881, and over the years their designs have changed and improved. The current engines that we see today are mostly based on the prototype designed more than 100 years ago.

 A two-stroke engine contains two processes:

·       Compression stroke: Compression is caused by an opening of the inlet port and the entry of air and fuel into the chamber, followed by the rising piston, which compresses the air/fuel mixture. A spark plug then ignites this compressed mixture, causing the power stroke to begin.

·       Power stroke: The heated gas exerts high pressure on the piston, the piston moves downward (expansion), and waste heat is exhausted.

Two-stroke engines are found in a variety of small propulsion applications, such as motorbikes, Raksha, scooters, go-karts, snowmobiles and outboard motors (An outboard motor is a propulsion system for boats).

And due to their high power-to-weight ratio and ability to be used in any orientation, two-stroke engines are common in handheld outdoor power tools including leaf blowers, chainsaws, and string trimmers.

Two-stroke gasoline engines are preferred when mechanical simplicity, lightweight, and high power-to-weight ratio are design priorities. As the oil reservoir does not depend on gravity, they can operate in any direction.

ADVANTAGES OF TWO-STROKE ENGINES

Two-stroke engines have two main advantages over four-stroke engines: They are simpler and lighter, and they produce about twice as much power relative to their size.

Some applications such as lawn mowers and leaf blowers simply cannot have a four-stroke engine as it would not be practical and far too expensive. And since a two-stroke engine can operate at any angle it makes it easy to use for handheld machinery which requires mobility.

Furthermore, the production costs and retail prices of machinery using two-stroke engines are significantly lower, which makes it more accessible to a larger percentage of the population.

These two main advantages have made a two-stroke engine a very popular choice both for manufacturers and consumers.

However today we are seeing a decline in the use of two-stroke engines for some machinery as we become more aware of the long-term effects of two-stroke engines.

DISADVANTAGES OF TWO-STROKE ENGINES

Whilst a Two-stroke engine is cheap and easy to use, they don't last nearly as four-stroke engines. Purchasing a vehicle is a long-term commitment and requires a large capital, so consumers would expect a long life span for their machinery. The lack of a dedicated lubrication system in a two-stroke engine means that the parts would wear out a lot faster making long-distance travel difficult and eventually leading to constant repairs whereas a normal vehicle would only typically require service at each 3000-5000 KM range.

For smaller applications a Two-stroke engine is cheaper to run, it requires a small amount of gas and lubricant which is mixed to operate the machine. However, when required to run for an extended period, a two-stroke engine becomes very expensive. The need to constantly mix oil and gas would mean that two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon. For example, a two-stroke engine would use almost 4 Liters of lubricating fluid every 1000 KM whereas a four-stroke engine would require a change of 4 Liters every 3000KM

Thirdly, two-stroke engines produce a lot of pollution - so much that it is likely that you won't see them around much longer due to the newer environmental regulations. The pollution comes from two sources:

•         The first is the smoke from the combustion of the oil. Combustion of oil produces smoke no matter how well the engine is designed, and a badly worn two-stroke engine can emit huge clouds of hazardous oily smoke.

•         Secondly, each time a new charge of air/fuel is loaded into the combustion chamber, part of the oil and fuel solution leaks out through the exhaust port. That's why you see a layer of oil around any two-stroke boat motor.

By understanding how the operation of an engine can affect the oil used and how oil can affect the engine, we can better appreciate the difference between water-cooled two-stroke oil and one formulated for an air-cooled two-stroke engine. The components used in these two oils, and the reasons for their uses are shown below: 

Water-cooled two-cycle engine oils require higher levels of heavy oil to prevent piston and cylinder wall scuffing. Because of the high piston temperature, light oils evaporate too fast from the piston cylinder contact area. On the other hand, oil with a heavy base vaporizes at very high temperatures, resisting evaporation and remaining in place to provide lubrication to the piston and cylinder. Air-cooled oil formulations must have much lower levels of heavy base oil than water-cooled engine oils. These oils require only a small amount of heavy oil to protect against piston scuffing and seizure at peak temperatures. High levels of heavy base oils in oil formulated for air-cooled engines can cause engine deposits. These deposits form as a result of the incomplete burning of the heavy oil. The deposits can cause piston ring sticking and can eventually plug or disrupt the flow of the exhaust system, resulting in power loss and possible engine damage.

 Detergent Additives

It should not be used in water-cooled two-cycle oil formulations. When burned with the fuel, detergents produce an ash deposit in the cylinders. This ash deposit can foul spark plugs, form exhaust port deposits that cause loss of power, and possibly create cylinder hot spots that can cause destructive pre-ignition. On the other hand, the only way to protect air-cooled two-cycle engines against piston rings sticking at their high peak temperatures is to include some detergent additives in the oil formulation. Detergents provide high-temperature deposit control not available from other additives used in the oil. However, in the air-cooled engine, any ash deposits that could form from the detergents are dislodged by engine vibration and exhausted from the engine.

Ash

Ash is the non-combustible residue of lubrication oil or fuel. Detergent additives contain metallic derivatives, such as calcium, barium, and magnesium sulfonate which are common sources of ash. Ash deposits can impair engine efficiency and power. But, detergents are an important component of engine oil that help control varnish deposits, piston ring deposits, and rust (yes, rust) by keeping insoluble combustion particles from adhering to metal surfaces. In some cases, detergents neutralize acids formed from the combustion of the fuel mixture. Ash deposits may have a grayish color, where carbon residue is usually black and sooty.

 Carbon residue

On the other hand, carbon residue is different from ash. It is formed from unburned and partially burned fuel, and from burning of the crankcase lubricant. Water from the condensation of combustion products along with carbon residue from fuel contributes to engine piston deposits. Carbon deposits are normally black and have a sooty appearance. Oils formulated for outboard engines require a large amount of antioxidant and dispersant additives to control deposit formation since these oils do not contain detergents. Outboard oils also contain a large number of rust inhibitors because an outboard engine's continuous contact with water makes rust prevention an important requirement. Air-cooled oils contain lower amounts of antioxidants and dispersants since the detergent additives do most of the work of preventing deposit accumulation. These oils also contain rust inhibitor additives to protect against rust that can form from water that enters the engine due to condensation.

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