Troubleshooting A Two-Stroke Crankshaft That Won't Turn

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Two-stroke engines are simple and lightweight, but they require careful maintenance. For example, two-stroke oil must be mixed with gasoline to lubricate the crankshaft, connecting rod, and cylinder walls. If this is forgotten, the engine will not last long. Similarly, two-stroke crankshafts require careful modification, or stroking, which can be done by boring the crank halves and inserting an offset bushing. However, this process can be complex and requires careful measurement and specialized tools.

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Two-stroke crankshafts require special two-stroke oil to be mixed with the gasoline

Two-stroke engines are simple and lightweight, but they require careful lubrication to function properly. Unlike four-stroke engines, two-stroke engines use the crankcase as part of the induction tract, so the crankcase is serving as a pressurization chamber to force air and fuel into the cylinder. This means that the crankcase cannot hold a thick oil, and a two-stroke oil must be mixed with the gasoline to lubricate the crankshaft, connecting rod, and cylinder walls.

This two-stroke oil, also known as two-cycle oil, 2-cycle oil, 2T oil, or petroil, is a special type of motor oil intended for use in crankcase compression two-stroke engines, which are typical of small gasoline-powered engines. The oil can be petroleum, castor oil, semi-synthetic, or synthetic oil, and is mixed with petrol/gasoline at a volumetric fuel-to-oil ratio ranging from 16:1 to as low as 100:1.

The difference between regular lubricating oil and two-stroke oil is that the latter must have a much lower ash content to minimize deposits that tend to form when ash is present in the oil during combustion. Additionally, a non-2T-specific oil can turn to gum if mixed with gasoline and not immediately consumed. Two-stroke oil also has different requirements for "stickiness" compared to four-stroke engines.

The use of two-stroke oil in two-stroke engines is essential for proper lubrication and engine longevity. Without it, the engine is likely to fail prematurely.

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In a two-stroke engine, the crankcase serves as a pressurization chamber, so it can't hold thick oil

Two-stroke engines typically use a crankcase-compression design, which means that the fuel-air mixture passes through the crankcase before entering the cylinders. This design does not include an oil sump in the crankcase. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so it can be filled with heavy oil to lubricate the crankshaft bearings. However, in a two-stroke engine, the crankcase serves as a pressurization chamber to force the fuel-air mixture into the cylinder. This means that it cannot hold thick oil. Instead, two-stroke oil is mixed with the fuel and burned in the combustion chamber to lubricate the crankshaft, connecting rod, and cylinder walls.

The two-stroke cycle involves the piston compressing the air-fuel mixture on one side and creating a vacuum on the other side to suck in air and fuel from the carburetor. This vacuum also opens the reed valve, allowing the air-fuel-oil mixture to be drawn in. The piston then moves downward, pushing the compressed fuel-air mixture from the crankcase into the combustion chamber. This design is commonly used in small petrol engines for motorcycles, generators, and garden equipment.

In a four-stroke engine, the engine's lubricating oil is recirculated, whereas in a two-stroke engine, the oil is burned. The oil is stored at the bottom of the crankcase or in a separate reservoir and pressurized by an oil pump before being squirted into the crankshaft and connecting rod bearings. While the crankshaft does not have much contact with the sump oil, some oil may splash onto it due to g-forces or bumpy roads, known as "windage".

The crankcase in a two-stroke engine has a specific function as a pressurization chamber, which is why it cannot hold thick oil like a four-stroke engine's crankcase. This design consideration is an important distinction between two-stroke and four-stroke engines and their lubrication requirements.

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The piston is doing three different things in a two-stroke engine

A two-stroke engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston (one up and one down movement) in one revolution of the crankshaft. In a two-stroke engine, the piston is doing three different things:

  • On one side of the piston is the combustion chamber, where the piston is compressing the air/fuel mixture and capturing the energy released by the ignition of the fuel.
  • On the other side of the piston is the crankcase, where the piston is creating a vacuum to suck in air and fuel from the carburetor through the reed valve. It then pressurizes the crankcase so that air and fuel are forced into the combustion chamber.
  • Meanwhile, the sides of the piston act like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

The piston's multiple functions make two-stroke engines simple and lightweight. They are also advantageous due to their high power-to-weight ratio, ease of maintenance, and ability to be used in any orientation.

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The piston compresses the air/fuel mixture, capturing energy from fuel ignition

The piston plays a crucial role in capturing energy from fuel ignition. In an internal combustion engine, the piston compresses the air-fuel mixture, which is then ignited to release energy. This process involves four distinct piston strokes: intake, compression, power, and exhaust. During the intake stroke, the piston moves down, creating low pressure in the cylinder, allowing the air-fuel mixture to be drawn in. The compression stroke follows, where the piston compresses the air-fuel mixture, increasing its temperature and pressure. This compression is essential for capturing energy, as it allows more energy to be released during ignition.

The compression ratio, which depends on the piston's position, is crucial for engine performance. A higher compression ratio generally leads to improved fuel efficiency and higher combustion pressure. However, it also increases the operator's effort when starting the engine. After compression, the spark plug initiates ignition, causing the fuel to rapidly combine with oxygen and release energy in the form of heat. This combustion process drives the power stroke, where the expanding gases push the piston, converting the energy into mechanical work.

The role of the piston is central to capturing and utilising the energy from fuel ignition. By compressing the air-fuel mixture, the piston creates the necessary conditions for ignition and combustion. The compression stroke ensures that the mixture is at the optimal temperature and pressure for ignition. This process also increases fuel vaporisation, allowing for a more complete burning of the mixture. The energy released during combustion drives the piston back down the cylinder, converting the energy from fuel ignition into usable mechanical energy.

The piston's movement during the power stroke harnesses the energy from combustion. The expanding gases exert force on the piston, which is transferred through the connecting rod to rotate the crankshaft. This process ultimately drives the vehicle's wheels. The piston's role in capturing and converting energy from fuel ignition highlights the importance of precise piston design and materials to withstand high temperatures and pressures. Overall, the piston's ability to compress, capture, and convert energy from the air-fuel mixture is a fundamental aspect of internal combustion engines.

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The piston also creates a vacuum to suck in air/fuel from the carburetor

The movement of a piston creates a vacuum that draws in air and fuel into the engine. This is known as the manifold vacuum, and it is the difference in air pressure between the engine's intake manifold and the Earth's atmosphere. The piston's movement on the induction stroke and the airflow through a throttle in the intake manifold cause this vacuum.

The vacuum created by the piston is essential for the proper functioning of the carburetor, a device that mixes air and fuel for internal combustion engines. The carburetor utilizes the principle of vacuum to draw fuel from the fuel bowl and create an optimal mixture for combustion. As the piston moves downward during the intake stroke, it creates a low-pressure area within the intake manifold, forming a vacuum. This vacuum effect is further enhanced by the venturi, a tapered section in the carburetor that accelerates airflow, causing a significant drop in pressure.

The downward movement of the piston opens the throttle valve, allowing outside air to flow into the venturi. As the air passes through the venturi, its speed increases, creating a stronger vacuum. This vacuum effect draws fuel from the fuel bowl through the jet, where it is atomized into fine droplets. These fine droplets have a larger surface area, allowing them to mix more thoroughly with the air, resulting in efficient combustion.

The size of the droplets and the amount of fuel delivered into the airstream are critical factors in the air-fuel mixture. The viscosity and specific gravity of the fuel, as well as the temperature, can affect the atomization process. Warmer fuel tends to be less dense and atomizes better than colder fuel. Additionally, the position of the throttle valve and the size of the jet play a role in regulating the amount of fuel drawn into the airstream.

In summary, the piston's movement creates a vacuum that is crucial for the proper functioning of the carburetor. The carburetor utilizes this vacuum to draw in air and fuel, creating an optimal mixture for combustion. The design of the carburetor, along with the piston's movement, ensures efficient engine performance.

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