Mech

What are IC Engines?

To first know IC Engines, we must know what are Engines

What is an engine?

An Engine is the device that transforms one form of the energy into another form

OR

Machine that does job of the energy conversion is called the Engine.

What are IC Engines

Engines are mainly classified into 2 major categories:

1-Internal Combustion Engines (IC Engines)

2-External Combustion Engines (EC Engines)

IC Engines

A flowchart showing classification of Engines

Today we will discuss IC Engines.

There are two types of engines:

1-Spark ignition engines (SI Engines)

2-Compression ignition engines (CI Engines)

Above mentioned engines are further classified on the basis of working principles, as:

Spark-ignition engines are of two types:

1-Four stroke spark-ignition engine

2-Two stroke spark-ignition engine

TYPES OF IC ENGINES Spark ignition engine

Same happens to compression-ignition engines:

1-Four stroke compression-ignition engine

2-Two stroke compression-ignition engine

Now we will discuss them one by one.

Four stroke spark-ignition engine: in this engine, the cycle of operations is completed in four strokes of the piston or two revolutions of the crankshaft. During four strokes, these four events to be completed:

  1. Suction
  2. Compression
  3. Combustion and Expansion
  4. Exhaust

Sometimes these are considered five events when expansion is considered as the separate event.

Two stroke spark-ignition engine: in this type, cycle of operations is completed in two strokes of the piston or one of the crankshaft. During two strokes, these two events happen:

  1. Compression
  2. Expansion

Isn’t amazing is this, you would be wondering where the other two strokes have gone!!

Now we’ll discuss briefly the above mentioned processes.

Suction Stroke: This is also called Intake stroke and it starts when the piston moves from TDC (Top Dead Centre) to BDC (Bottom Dead Centre). This movement of the piston draws the air-fuel mixture into the cylinder. When piston reaches Bottom Dead Centre (BDC), the suction stroke ends.

Compression Stroke: The mixture taken into the cylinder is compressed by the return stroke of the piston from Bottom Dead Centre (BDC) to Top Dead Centre (TDC).

Combustion and Expansion: At the end of the compression stroke, the mixture is ignited with the help of a spark plug located on the cylinder head. The high pressure of the burnt gases forces the piston towards BDC. Of the four strokes only during this stroke Power is produced.

Exhaust Stroke: At the end of the expansion stroke the exhaust valve opens and the burnt gases escape when piston starts moving from BDC to TDC.

Each cylinder of the four stroke engine completes the above mentioned four processes.

Now is the turn for Compression-Ignition Engines.

1-Four stroke compression-ignition engine: It is same to the four stroke spark-ignition engines,:

  1. Suction
  2. Compression
  3. Expansion
  4. Exhaust

4 Stroke Engine

In compression-ignition engines, the compression ratio is at much higher level. Compression ratio of an SI-Engine is between 6 – 10 while for CI-Engine it is from 16 – 20.

In CI-Engine, during suction stroke only air instead of fuel-air mixture is inducted. Due to high compression ratio the temperature in compression stroke is sufficiently high to self ignite the fuel which is injected into the combustion chamber.

Two stroke compression-ignition engine: This is same as the two stroke SI-Engine described above. in this type, cycle of operations is completed in two strokes of the piston or one of the crankshaft. During two strokes, these two events happen:

  1. Compression
  2. Expansion

Now we’ll discuss briefly the above mentioned processes.

Suction Stroke: Air alone is included during this stroke.

Compression Stroke: Air sucked during the suction stroke is compressed into this stroke. Fuel injects nearly at the end of this stroke. Due to the high compression, this fuel burns out and produces energy.

Expansion Stroke: Due to the energy produced by the burnt fuel, piston moves downward to BDC in this stroke.

Exhaust Stroke: In this, piston travels from BDC to TDC pushing out the products of combustion.

Comparison of SI and CI Engines:

In four stroke engines, there is one power stroke for every two revolutions of the crankshaft. In both SI and CI Engines operating on four stroke cycle, power can be obtained only in every two revolution of the crankshaft.

Since both SI and CI Engines have much in common, it is worth to compare them based on important parameters like basic cycle of operation, fuel induction, compression ratio etc.

Enrollment in local colleges, 2005

Description SI Engine CI Engine
Basic cycle Works on otto cycle Works on Diesel cycle
Fuel Gasoline, a highly volatile fuel. Diesel oil, a non volatile fuel.
Introduction of fuel A gaseous mixture of fuel and air is introduced during the suction stroke. A carburettor and an ignition system is necessary. Fuel is injected directly into the combustion chamber at high pressure at the end of compression stroke.
Load control Throttle controls the quantity of fuel-air mixture. The quantity of fuel is regulated.
Ignition Requires an ignition system with spark plug in the combustion chamber. Self-ignition occurs due to high temperature of air because of the high compression. Ignition system and spark plug are not necessary.
Compression ratio 6 – 10. Upper limit is fixed by antiknock quality of fuel. 16 – 20. Upper limit is limited by weight increase of the engine.
Speed Due to light weight and also due to homogeneous combustion, they are high speed engines. Due to heavy weight and also due to heterogeneous combustion, they are low speed engines.

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