Here in this article we will discuss about two stroke engine. The topics that we will cover are definition, construction, working, advantages and disadvantages and application of 2 stroke engine.
Two Stroke Engine
What is stroke?
- In two stroke engine the suction and exhaust strokes are eliminated.
- There are only two remaining strokes – these are the compression stroke and Expansion or power stroke and usually called as upward stroke and downward stroke.
- Instead of valves, the two stroke engine consists of inlet and exhaust ports.
- The fresh charge enters into the cylinder through inlet port and burnt gases escapes out through exhaust port.
- The burnt exhaust gases are forced out through the exhaust port by fresh charge which enters the cylinder nearly at the end of the working
stroke through the inlet port.
1. Upward stroke:
- During upward stroke, the piston moves from BDC to TDC and compresses the charge (air-fuel mixture) in the combustion chamber of the cylinder.
- Because of the upward movement of the piston a partial vacuum is created in the crankcase and this allows the entry of the fresh charge into the crankcase through uncovered inlet port.
- The exhaust port and the inlet port remains covered when the piston at the TDC.
- The ignition of the fresh charge is takes place by the spark
- As soon as the combustion of the fresh charge takes place, a large amount of the hot gases is produced which exerts a very high pressure force on the top of the piston. Due to this high pressure force, the piston moves downward and rotates the crankshaft and does useful work.
- During this stroke the inlet port is covered by the piston and the new charge is compressed in the crankcase.
- Further downward movement of the piston uncovers first the exhaust port and the transfer port and the exhaust starts through the exhaust port.
- As soon as the transfer port opens, the charge through it is forced into the cylinder.
- The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out most of the exhaust gases.
- The piston is now at BDC position. The cylinder is completely filled with the fresh charge but it is somewhat diluted with the exhaust gases.
- Finally the cycle event is then repeated. We get two strokes for the single revolution of the crankshaft.
Port Timing diagram for a two stroke cycle engine
- Two stroke engine gives a working stroke for each revolution of the crankshaft. The four stroke engine gives a working stroke for each two revolution of the crankshaft.
- Power developed by the two stroke engine is twice that developed by the four stroke engine for the same engine speed and volume.
- Lighter flywheel is required in two stroke engine because of the more turning moment on the crankshaft.
- For the same power, a two stroke engine is more compact, light and requires less space than a four stroke engine, therefore is used in motorcycles and scooters.
- It is simpler in construction and mechanism.
- There is no valve and valve mechanism in it. The ports can be easily designed and covered and uncovered by the movement of the piston itself.
- It has high mechanical efficiency.
- It requires fewer spare parts due to its simple design.
- It has high fuel consumption.
- Thermal efficiency is less than four stroke engine.
- The charge is diluted by the burnt gases due to incomplete scavenging.
- It produces greater noise.
- It does more consumption of the lubricating oil.
- There is a greater wear and tear of moving parts.
- It is used in light weight vehicles like scooters, motorcycles, mopeds etc which uses gasoline as fuel.
- It is also used in many diesel engines, mainly industrial and marine engines, as well as in some heavy trucks and machinery