Two-stroke engine full explaination.(with advantage disadvantage).

Basic:

 A two-stroke (or two-cycle) engine is a type of internal combustion engine which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaft revolution.  

History: 

The first commercial 2 stroke engine was credited to Dugald Clerk, a Scottish engineer who patented his plan in 1881. Clerk design was totally distinct, having a separate charging cylinder. Englishman Joseph Day was also credited for the crankcase-scavenged engine for using an expanse below the piston as a charging pump. A popular German inventor, Karl Benz produced a two-stroke engine on 31 December 1879, which he gained copyright In 1880. 
The first confirmed two-stroke engine was attributed to a man who started producing twin-cylinder water-cooled motorcycles in 1908. His name is Yorkshire man Alfred Angus Scott. 

Advantages of two-stroke engine :  

• Simpler in design because don't have valve only port. 
• Power developed is twice as compared to same dimensions four-stroke engine with the same operating speed. 
• The work required to overcome the friction of the exhaust and suction strokes is saved. 
• Burnt gases do not remain in the clearance space because of scavenging. 
• Two-stroke engines can work in any position since oil flow is not a concern with any valves to worry about. 
• It occupies lesser space. 
• Lighter in weight because the lighter flywheel is used. 
• Easy to maintain. 
• Less cost. 
• It produces Uniform torque. 
• Power to weight ratio is high. 

Disadvantages of two-stroke engine :  

• A high-speed two-stroke engine is less efficient. 
• When the inlet valve of the engine is opened for the intake of the air-fuel mixture, the exhaust valve is also open so the part of the fuel is wasted this increase fuel consumption and reduce overall efficiency. 
• Effective compression is less in case of a two-stroke engine. 
• These engines are liable to cause heavier combustion of lubricating oil. 
• With heavy loads, it gets heated due to excessive heat produced. 
• Thermal efficiency is also less. 
• It causes pollution and smoky. 

Application: 

• These engines are used in small vehicles like mopeds, scooters etc. 
• Small gasoline engines are used for lawn movers. 
• It is also used for small electric generator set, pumping set, outboard motor boats. 
• Two stroke diesel engines are used for ship propulsion. 

 

 

 

Parts: 

• Piston: It moves from BDC to TDC. One stroke of piston is define as movement of piston form one extreme (TDC or BDC) to other extreme (BDC to TDC). 
  
• Cylinder: It is same as used in four stroke engine except it does not have valve mechanism. It has two ports on side wall of cylinder. 
  
• Crankshaft: It is used to convert reciprocating motion of piston into rotary motion. 
  
• Inlet and Exhaust Port: Two stroke engines contain ports except valves. These ports open and closed due to piston movement. When the piston is moving towards TDC inlet ports opens and when it moves toward BDC exhaust port opens. 
  
• Transfer Port: These engines contain one extra port which is known as transfer port. It is connected from crankcase to combustion chamber. Its main function is to supply the charge from crankcase to combustion chamber when piston is moving from TDC to BDC. 
  
• Inlet and Exhaust Manifold: These are connected to inlet and exhaust port and regulate the flow of charge and exhaust gases. 
  
• Flywheel: It need smaller flywheel compare to four stroke engine because less power fluctuation. 
  
• Crankcase: This is the part where crankshaft situated. The inlet port is also connected with crankcase. First charge enters into crankcase and send to combustion chamber through transfer port. 

 

 

 

 

 

 

Suction Stroke and Compression Stroke: 

Suction means charge drawn into engine cylinder or in case of two stroke engine in crankcase and compression means compressed the previous drawn charge into engine cylinder. In two strokes engines, both these process take place simultaneously. When the piston moves from BDC to TDC, the inlet port opens and partially vacuum created into crankcase which accelerates charge drawn into crankcase. Simultaneously the piston compressed the charge available in engine cylinder or combustion chamber. The exhaust port remain closed during this stroke. 
 

Power and Exhaust Stroke: 

In this stroke, piston moves from TDC to BDC. Inlet port remains open for first half of this stroke and closed in other half. Simultaneously, exhaust port remains closed during first half of this stroke and open into second half. Transfer port opens into second half of this stroke. 

There is a deflector in engine cylinder which regulate the fresh charge does not exhausted with exhaust gases. 
During this piston stroke, in SI engines, spark plug produces sparks. This spark ignited the charge which create a high pressure force. This force moves piston form TDC to BDC. When the piston reaches in middle, the exhaust port and transfer port opens simultaneously. This exhausted burnt gases out from engine cylinder and transfer port supplied fresh fuel-air mixture into engine cylinder from crankcase for further cycle. 
In CI engines, injector injects fuel during this stroke. This fuel burns due to heat produced by compression which create a high pressure force. This force moves piston form TDC to BDC. When the piston reaches in middle, the exhaust port and transfer port opens simultaneously. This exhausted burnt gases out from engine cylinder and transfer port supplied fresh air into engine cylinder from crankcase for further cycle. 

 

A two-stroke engine contains two processes:  

Compression stroke: The inlet port opens, the air-fuel mixture enters the chamber and the piston moves upwards compressing this mixture. A spark plug ignites the compressed fuel and begins the power stroke.  

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

 

 

 

 

 

 

Two-stroke diesel and gasoline engine: 

Small gasoline-powered two-stroke engines recognized as crankcase-compression engines. They are lubricated by a petrol mixture in a total-loss system. The oil mixed at a preceding time with the petrol fuel, in a ratio of about 1:50. The oil forms emissions as oily droplets in the exhaust or by being seared.  This help to produce extra exhaust emissions with extreme hydrocarbons, than four-stroke engines of equal power output. Due to the equal opening time of the intake and exhaust ports in some two-stroke engine design. Some amount of unburden fuel gases are allowed to leave the exhaust stream. However, small air-cooled engines with high combustion temperatures may give high NOISE emissions. 

Two-stroke diesel engine entirely depends on the heat of compression in order to ignite. A uni flow diesel engine receives air to the chamber, and the exhaust gases leave through an overhead poppet valve. 
In the case of  ported and loop-scavenged engines, intake and exhaust take place through piston-controlled ports. All two-stroke diesel are all scavenged by forced induction. Though some designs use a mechanically driven roots blower, whilst marine diesel normally uses exhaust-driven turbochargers, using electrically driven auxiliary blowers for low-speed operation when turbochargers are unable to deliver enough air. 

A propeller is directly fitted to a marine two-stroke diesel engines, making it run in both directions. The valve timing and fuel injection are mechanically readjusted by using a different set of cams on the camshaft. 

Modern two-stroke engines are designed with a power valve system. They are fitted manually in or around the exhaust ports. The working of these valves is as follows: 
The exhaust port is transformed by closing off the top part of the port, which alters the timing. Or by altering the quantity of exhaust, which changes the vibrant frequency of the expansion chamber. 

One of the greatest advantages of two-stroke engines is because of direct injection. It helps in abolishing some pollution and waste produce by the carburetor two-stroke; the amount of fuel/air mixture penetrating the cylinder instantly goes out unburden through the exhaust port. 
The two systems use is a low-pressure air-assisted injection and high-pressure injection.