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working principle of 2 stroke ic engine

working principle of 2 stroke ic engine

A two-stroke engine is one that completes its operating cycle in one revolution of the crankshaft or two strokes of the piston. working principle of 2 stroke ic engine In this engine the operations of the intake and exhaust processes of the four-stroke engine are taken care of by the incoming fresh charge, which is compressed in the crankcase or by a separate blower, near the bottom dead center of the engine piston. will happen. , the engine piston only needs to compress the new charge and expand the combustion products.

Since a two-stroke engine has twice the number of revolutions per minute as a four-stroke engine operating at the same speed and with the same number of cylinders, it theoretically produces twice the power while operating at the same average effective pressure. Like a four-stroke engine, the power output of this engine depends on the number of air per minute available for combustion.

 

working principle of 2 stroke ic engine

Many two-stroke engines have greatly simplified engine construction, using the piston as a slide valve, with intake and exhaust ports cut into the side of the cylinder. If the engine runs on the Otto cycle, the charge consists of a proper mixture of fuel and air, whereas the charge for a diesel or dual combustion cycle consists of pure air. working principle of 2 stroke ic engine There are several difficulties in applying the two-stroke principle to small high-speed CI engines, although these are by no means insurmountable. For SI engines, the two-stroke cycle principle is used for a large number of engines, from small single-cylinder model engines producing a fraction of a kilowatt to the largest aircraft engines capable of 2500 kW or more units.

Types of two-stroke engines

There are basically two types of two-stroke engines depending on the scavenging method used:

The details of the above two types of two-stroke engines are briefly discussed in the following sections.

Crankcase scavenged engine One of the simplest types of two-stroke engines. In this engine, the charge (fuel-air mixture in SI engines and air in CI engines) is compressed in the crankcase below the piston during the expansion stroke. This engine has three ports.

IC machine

(i) Crankcase swept engine

(ii) Separately designed machine

(i) Intake port in crankcase

(ii) Port of transfer

(iii) Port of exit

Crankcase-scavenged two-stroke engine

The compressed charge forces the combustion products into the engine cylinder through the charge transfer port. This process is called scavenging, and engines of this type are called crankcase-scavenged engines.

As the piston moves down, it first opens the exhaust ports, and the cylinder pressure drops to atmospheric levels as the combustion products exit these ports. working principle of 2 stroke ic engine In addition, the downward movement of the piston exposes the transfer ports, which allow the slightly compressed mixture in the crankcase or air (depending on the type of engine) to enter the engine cylinder. The top and ports of the piston are usually designed so that fresh air is directed toward the top of the cylinder before flowing toward the exhaust ports.

It is intended to clear the top of the cylinder of combustion products and reduce the flow of fresh fuel-air mixture directly through the exhaust ports. The projection on the piston is called a deflector. As the piston returns from the bottom center, the transfer ports and exhaust ports are closed and charge compression begins. The movement of the piston during compression reduces the pressure in the crankcase so that fresh mixture or air can be drawn into the crankcase through the inlet reed valve. Ignition and expansion occur normally, and the cycle repeats itself. The engine is undercharged by one cylinder displacement volume due to flow restrictions at the inlet reed valve and transfer port.

Optional machine

Another type of engine that uses an external device such as a blower to clean the combustion products is called an externally or separately scavenged engine. Details of this type of machine . In these engines the air-fuel mixture is supplied to the inlet manifold at a slightly higher pressure. As the piston moves downward along the expansion side, it opens the exhaust ports approximately 60◦ before the bottom center.

After about 10◦, when the pressure in the cylinder is low enough, the inlet ports are opened and the cleaning process takes place. The inlet port is designed so that most of the air flows through the top of the cylinder, moves down the exhaust side, and forms an annulus before reaching. This ensures cleanliness of the upper cylinder block. Piston deflectors are not used because they are heavy and overheat at high outputs. Compared to normal crankcase scavenged engines with deflector pistons, the scavenging process is more efficient in this type of engines.

Opposite of piston or end-to-end scavenged engine:

Another type of two-stroke engine is called the opposed piston type or sometimes called the end-to-end scavenged engine. Details of this type of machine. In this way the discharge ports are opened first. The inlet port is designed so that the air or mixture enters tangentially, giving a reciprocating motion. working principle of 2 stroke ic engine Vortex helps prevent mixing of new charge and combustion products during the cleaning process. At the beginning of the compression stroke, the exhaust ports close first.

As the flow passes through the inlet ports the cylinder pressure begins to increase and the inlet ports close when the pressure is close to the inlet manifold pressure. Note that in this opposed piston engine the two pistons are slightly out of phase. In loop-scavenged engines the port timing is symmetrical so that the exhaust port closes after the inlet port closes. Timing prevents this type of engine from filling its cylinders to full inlet pressure. In end-to-end scavenged engines, in-cylinder counterflow is eliminated, reducing the chance of fresh and spent gases mixing. So cleaning should be efficient.

Two-stroke air capacity

Except when a large valve overlap is used, a four-stroke engine retains almost all of the charge entering the cylinder during the intake process, meaning none is lost through the exhaust valves. So an air meter at the engine inlet will measure ˙ma, the amount of air available for combustion, and engine output will be proportional to ˙ma.

A two-stroke engine’s exhaust ports remain open during most cleaning. This cannot avoid some loss of fresh air or mixture through the open exhaust port at that time. As the charge increases in the cylinder, the increased charge is discharged through the exhaust port. Hence the power output of a two-stroke engine is not proportional to the amount of charge entering the engine, but to the amount of charge placed in the cylinder. The amount of charge that can be retained in the cylinder is called the air capacity of a two-stroke engine.

Advantages and disadvantages of two-stroke engines

Two-stroke engines have certain advantages and disadvantages compared to four-stroke engines. The main advantages and disadvantages are briefly discussed in the following sections.

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Advantages of two-stroke engine

(i) As there is one work stroke for each revolution, power is developed

It is twice as powerful as a four-stroke engine of similar dimensions and running at the same speed.

Work required to overcome discharge and suction friction

The stroke was saved.

(iii) Since there is one working stroke in each revolution, a more uniform turning occurs

The torque is obtained at the crankshaft, therefore, a light flywheel is required.

(iv) – stroke engines are lighter than four-stroke engines

Power output and speed.

(v) For the same output, two-stroke engines occupy less space.

(vi) Construction of two-stroke cycle engine is simple because it

Port instead of valve. This significantly reduces maintenance issues.

(vii) In two-stroke engines, the burnt gases are not discharged due to scavenging.

For four-stroke engines, stay within the clearance range

Disadvantages of two-stroke engine

(i) high velocity -stroke engines are much less efficient

Measuring ability.

(ii) In machines operating in Otto cycle, part of the fresh mixture is lost.

Because it comes out of the exhaust port during cleaning. This increases fuel consumption and reduces thermal efficiency.

(iii) Some part of the piston stroke is lost by providing ports

In the case of a two-stroke engine the effective compression is less.

(iv) Two-stroke engines cause high consumption of lubricating oil.

(v) Two-stroke engines overheat due to overheating due to overload

prepared. Also at light load, engine operation is not very smooth due to increased dilution of charge.

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