INTRODUCTION
Engine cycle
This system manages to pack one power stroke into every two strokes of the piston (up-down). This is achieved by exhausting and recharging the cylinder simultaneously.
The steps involved are:
Two Stroke Spark Ignition (SI) engine:
In a two-stroke SI engine a cycle is completed in two strokes of a piston or one complete revolution (360°) of a crankshaft. In this engine the intake and exhaust strokes are eliminated and ports are used instead of valves. In this cycle, the petrol is mixed with lubricant oil, resulting in a simpler, but more environmentally damaging system, as the excess oils do not burn and are left as a residue. As the piston proceeds downward another port is opened, the fuel/air intake port. Air/fuel/oil mixtures come from the carburetor, where it was mixed, to rest in an adjacent fuel chamber. When the piston moves further down and the cylinder doesn’t have anymore gases, fuel mixture starts to flow to the combustion chamber and the second process of fuel compression starts. The design carefully considers the point that the fuel-air mixture should not mix with the exhaust, therefore the processes of fuel injection and exhausting are synchronized to avoid that concern. It should be noted that the piston has three functions in its operation:
The major components of a two-stroke spark ignition engine are the:
Cylinder: A cylindrical vessel in which a piston makes an up and down motion.
Piston: A cylindrical component making an up and down movement in the cylinder.
Combustion chamber: A portion above the cylinder in which the combustion of the fuel-air mixture takes place.
Intake and exhaust ports: An intake port allows the fresh fuel-air mixture to enter the combustion chamber and an exhaust port discharges the products of combustion.
Crankshaft: A shaft which converts the reciprocating motion of the piston into a rotary motion.
Connecting rod: A rod which connects the piston with the crankshaft.
Spark plug: An ignition-source located at the cylinder head that is used to initiate the combustion process.
Operation
When the piston moves from bottom dead center (BDC) to top dead center (TDC) the fresh air and fuel mixture enters the crank chamber through the intake port. The mixture enters due to the pressure difference between the crank chamber and the outer atmosphere while simultaneously the fuel-air mixture above the piston is compressed.
Ignition: With the help of a spark plug, ignition takes place at the top of the stroke. Due to the expansion of the gases the piston moves downwards covering the intake port and causes the fuel-air mixture inside the crank chamber to be compressed. When the piston is at bottom dead center the burnt gases escape from the exhaust port.At the time the transfer port is uncovered the compressed charge from the crank chamber enters into the combustion chamber through the transfer port. The fresh charge is deflected upwards by a hump provided on the top of the piston and removes the exhaust gases from the combustion chamber. Again the piston moves from bottom dead center to top dead center and the fuel-air mixture is compressed when the both the exhaust port and transfer ports are covered. The cycle is repeated.
Sparks Fly
You can understand a two-stroke engine by watching each part of the cycle. Start with the point where the spark plug fires. Fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites. The resulting explosion drives the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder
Fuel Intake
As the piston finally bottoms out, the intake port is uncovered. The piston’s movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel,Note that in many two-stroke engines that use a cross-flow design, the piston is shaped so that the incoming fuel mixture doesn’t simply flow right over the top of the piston and out the exhaust port.
The Compression Stroke
Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor.Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. It’s called a two-stoke engine because there is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust stroke.
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.
If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline. Now that you understand the two-stroke cycle you can see why. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston’s connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can’t hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn’t going to last very long.
Applications
These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination of light weight and twice the power gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs.
Disadvantages of the Two-stroke
A two-stroke engine is an internal combustion engine that completes
the process cycle in one revolution of the crankshaft (an up stroke and a
down stroke of the piston, compared to twice that number for a
four-stroke engine). This is accomplished by using the end of the
combustion stroke and the beginning of the compression stroke to perform
simultaneously the intake and exhaust (or scavenging) functions. In
this way, two-stroke engines often provide high specific power, at least
in a narrow range of rotational speeds. The functions of some or all of
the valves required by a four-stroke engine are usually served in a
two-stroke engine by ports that are opened and closed by the motion of
the piston(s), greatly reducing the number of moving parts. Gasoline
(spark ignition) versions are particularly useful in lightweight
(portable) applications, such as chainsaws, and the concept is also used
in diesel compression ignition engines in large and weight insensitive
applications, such as ships and locomotives.The first commercial
two-stroke engine involving in-cylinder compression is attributed to
Scottish engineer Dugald Clerk.
This system manages to pack one power stroke into every two strokes of the piston (up-down). This is achieved by exhausting and recharging the cylinder simultaneously.
The steps involved are:
- Intake and exhaust occur at bottom dead center. Some form of pressure is needed, either crankcase compression or super-charging.
- Compression stroke: Fuel-air mix is compressed and ignited. In case of diesel: Air is compressed, fuel is injected and self-ignited.
- Power stroke: Piston is pushed downward by the hot exhaust gases.
Two Stroke Spark Ignition (SI) engine:
In a two-stroke SI engine a cycle is completed in two strokes of a piston or one complete revolution (360°) of a crankshaft. In this engine the intake and exhaust strokes are eliminated and ports are used instead of valves. In this cycle, the petrol is mixed with lubricant oil, resulting in a simpler, but more environmentally damaging system, as the excess oils do not burn and are left as a residue. As the piston proceeds downward another port is opened, the fuel/air intake port. Air/fuel/oil mixtures come from the carburetor, where it was mixed, to rest in an adjacent fuel chamber. When the piston moves further down and the cylinder doesn’t have anymore gases, fuel mixture starts to flow to the combustion chamber and the second process of fuel compression starts. The design carefully considers the point that the fuel-air mixture should not mix with the exhaust, therefore the processes of fuel injection and exhausting are synchronized to avoid that concern. It should be noted that the piston has three functions in its operation:
- The piston acts as the combustion chamber with the cylinder and compresses the air/fuel mixture, receives back the liberated energy, and transfers it to the crankshaft.
- The piston motion creates a vacuum that sucks the fuel/air mixture from the carburetor and pushes it from the crankcase (adjacent chamber) to the combustion chamber.
- The sides of the piston act like the valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.
The major components of a two-stroke spark ignition engine are the:
Cylinder: A cylindrical vessel in which a piston makes an up and down motion.
Piston: A cylindrical component making an up and down movement in the cylinder.
Combustion chamber: A portion above the cylinder in which the combustion of the fuel-air mixture takes place.
Intake and exhaust ports: An intake port allows the fresh fuel-air mixture to enter the combustion chamber and an exhaust port discharges the products of combustion.
Crankshaft: A shaft which converts the reciprocating motion of the piston into a rotary motion.
Connecting rod: A rod which connects the piston with the crankshaft.
Spark plug: An ignition-source located at the cylinder head that is used to initiate the combustion process.
Operation
When the piston moves from bottom dead center (BDC) to top dead center (TDC) the fresh air and fuel mixture enters the crank chamber through the intake port. The mixture enters due to the pressure difference between the crank chamber and the outer atmosphere while simultaneously the fuel-air mixture above the piston is compressed.
Ignition: With the help of a spark plug, ignition takes place at the top of the stroke. Due to the expansion of the gases the piston moves downwards covering the intake port and causes the fuel-air mixture inside the crank chamber to be compressed. When the piston is at bottom dead center the burnt gases escape from the exhaust port.At the time the transfer port is uncovered the compressed charge from the crank chamber enters into the combustion chamber through the transfer port. The fresh charge is deflected upwards by a hump provided on the top of the piston and removes the exhaust gases from the combustion chamber. Again the piston moves from bottom dead center to top dead center and the fuel-air mixture is compressed when the both the exhaust port and transfer ports are covered. The cycle is repeated.
Sparks Fly
You can understand a two-stroke engine by watching each part of the cycle. Start with the point where the spark plug fires. Fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites. The resulting explosion drives the piston downward. Note that as the piston moves downward, it is compressing the air/fuel mixture in the crankcase. As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases out of cylinder
Fuel Intake
As the piston finally bottoms out, the intake port is uncovered. The piston’s movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel,Note that in many two-stroke engines that use a cross-flow design, the piston is shaped so that the incoming fuel mixture doesn’t simply flow right over the top of the piston and out the exhaust port.
The Compression Stroke
Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke. As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor.Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle. It’s called a two-stoke engine because there is a compression stroke and then a combustion stroke. In a four-stroke engine, there are separate intake, compression, combustion and exhaust stroke.
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/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
- Meanwhile, the sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.
If you have ever used a two-stroke engine, you know that you have to mix special two-stroke oil in with the gasoline. Now that you understand the two-stroke cycle you can see why. In a four-stroke engine, the crankcase is completely separate from the combustion chamber, so you can fill the crankcase with heavy oil to lubricate the crankshaft bearings, the bearings on either end of the piston’s connecting rod and the cylinder wall. In a two-stroke engine, on the other hand, the crankcase is serving as a pressurization chamber to force air/fuel into the cylinder, so it can’t hold a thick oil. Instead, you mix oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls. If you forget to mix in the oil, the engine isn’t going to last very long.
Applications
- The two-stroke engine was very popular throughout the 20th century in motorcycles and small-engined devices, such as chainsaws and outboard motors, and was also used in some cars, a few tractors and many ships. Part of their appeal was their simple design (and resulting low cost) and often high power-to-weight ratio. Because of the lower cost to rebuild and maintain, the two stroke engine was incredibly popular in the dirt bike and motocross industry for many years, until recently when the EPA required the industry to switch to four-stroke engines because they emit less pollution than two stroke engines[dubious – discuss][list membership disputed]. Many designs use total-loss lubrication, with the oil being burned in the combustion chamber, causing “blue smoke” and other types of exhaust pollution. This is a major reason for two-stroke engines being replaced by four-stroke engines in many applications.
- Two-stroke engines do not have valves, which simplifies their construction and lowers their weight.
- Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost.
- Two-stroke engines can work in any orientation, which can be important in something like a chainsaw. A standard four-stroke engine may have problems with oil flow unless it is upright, and solving this problem can add complexity to the engine.
These advantages make two-stroke engines lighter, simpler and less expensive to manufacture. Two-stroke engines also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination of light weight and twice the power gives two-stroke engines a great power-to-weight ratio compared to many four-stroke engine designs.
Disadvantages of the Two-stroke
- Two-stroke engines don’t last nearly as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear a lot faster.
- Two-stroke oil is expensive, and you need about 4 ounces of it per gallon of gas. You would burn about a gallon of oil every 1,000 miles if you used a two-stroke engine in a car.
- Two-stroke engines do not use fuel efficiently, so you would get fewer miles per gallon.
- Two-stroke engines produce a lot of pollution — so much, in fact, that it is likely that you won’t see them around too much longer. The pollution comes from two sources. The first is the combustion of the oil. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit huge clouds of oily smoke. Each time a new charge of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port. That’s why you see a sheen of oil around any two-stroke boat motor. The leaking hydrocarbons from the fresh fuel combined with the leaking oil is a real mess for the environment.
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