What is an Internal Combustion Engine? Name the various types.

An Internal Combustion Engine (ICE) is an engine in which the combustion of fuel takes place inside the engine cylinder or combustion chamber, and the heat energy released by combustion is converted into mechanical work.

This is in contrast to an External Combustion Engine, such as a steam engine, where combustion takes place externally in a boiler to produce steam, which is then used to develop mechanical power.

Classification of Internal Combustion Engines

Internal combustion engines may be classified according to the following:

1. According to the operating cycle

• Two-stroke cycle engine
• Four-stroke cycle engine

In a two-stroke cycle engine, the complete cycle is completed in two piston strokes, or one revolution of the crankshaft.

In a four-stroke cycle engine, the complete cycle is completed in four piston strokes, or two revolutions of the crankshaft.

2. According to the method of ignition

(a) Spark Ignition (SI) Engine

A mixture of fuel and air is admitted to the cylinder and compressed. Ignition is initiated by an electric spark from a spark plug.

(b) Compression Ignition (CI) Engine

Only air is admitted and compressed to a high pressure and temperature. Fuel is then injected into the cylinder in a finely atomised spray at high pressure. Ignition occurs due to the high temperature of the compressed air.

Large marine diesel engines are compression-ignition engines.

3. According to the method of scavenging

This classification applies particularly to two-stroke engines.

(a) Cross-flow scavenging

The scavenge ports and exhaust ports are located on opposite sides of the cylinder liner.

(b) Loop or return-flow scavenging

The scavenge ports and exhaust ports are located on the same side of the cylinder liner, and the incoming scavenge air follows a loop or return path before leaving through the exhaust ports.

(c) Uniflow scavenging

The scavenge ports are located near the lower end of the cylinder liner, while exhaust gases are discharged through an exhaust valve located in the cylinder cover.

The flow of scavenge air and exhaust gas is predominantly in one direction.

Modern low-speed marine two-stroke engines generally employ uniflow scavenging.

4. According to engine speed

A commonly used classification is:

• Slow-speed engine: below 400 rpm
• Medium-speed engine: 400–1400 rpm
• High-speed engine: above 1400 rpm

5. According to cylinder arrangement

(a) In-line engine

The cylinders are arranged vertically in one line.

(b) V-type engine

The cylinders are arranged in two banks inclined to each other, forming a V configuration.

(c) Opposed-piston engine

Two pistons operate in opposite directions within the same cylinder.

6. According to method of air supply

(a) Naturally aspirated engine

Fresh air enters the cylinder solely due to the suction created by the movement of the piston.

(b) Supercharged engine

Air is supplied to the cylinder at a pressure higher than atmospheric pressure by means of a blower or compressor.

(c) Turbocharged engine

Air is supplied under pressure by a turbocharger, which is driven by the energy of the engine exhaust gases.

7. According to fuel-injection system

• Jerk-type fuel injection system
• Common-rail fuel injection system
• Electronically controlled fuel injection system

In a conventional mechanically controlled engine, fuel injection and exhaust-valve operation are controlled by cam-operated mechanisms.

In an electronically controlled engine, fuel injection and exhaust-valve operation are controlled by an electronic control system, which regulates the timing and duration according to engine operating conditions.

8. According to fuel used

• Diesel-fuel engine
• Gas-fuel engine
• Dual-fuel engine

A dual-fuel engine is capable of operating on two fuels, for example liquid fuel and natural gas (LNG).

Modern dual-fuel marine engines can utilise boil-off gas (BOG) from LNG cargo as fuel, depending on the engine design and operating mode.

Types of Two-Stroke Marine Engines According to Scavenging

For oral examination, remember:

Cross-flow → Loop/Return-flow → Uniflow

Modern large marine two-stroke engines use uniflow scavenging because it provides an effective and relatively direct flow of scavenge air through the cylinder.

Important Marine Terminology

Use these terms in the oral:

Incorrect/less precise	Preferred terminology
Air outlet	Exhaust valve
Air inlet holes	Scavenge ports
Exhaust holes	Exhaust ports
Fuel spray	Finely atomised fuel spray
Air blower	Scavenge-air blower
Turbo	Exhaust-gas turbocharger
Burning	Combustion
Fuel burning by itself	Self-ignition
Power stroke	Expansion/power stroke
Top of cylinder	Cylinder cover
Cylinder top	Upper end of cylinder liner
Bottom holes	Scavenge ports
Fuel pump	Fuel injection pump / fuel pump

Best MEO Oral Answer

“Sir, an internal combustion engine is an engine in which combustion of fuel takes place inside the engine cylinder or combustion chamber, and the heat energy released is converted into mechanical work.

The main classifications are: according to cycle—two-stroke and four-stroke; according to ignition—spark ignition and compression ignition; according to scavenging—cross-flow, loop or return-flow, and uniflow; according to speed—slow-speed, medium-speed and high-speed; according to cylinder arrangement—in-line and V-type; according to air supply—naturally aspirated, supercharged and turbocharged; and according to fuel-injection system—jerk-type, common-rail and electronically controlled systems.”