A four stroke diesel engine is an internal combustion engine that completes one working cycle in four strokes (movements) of the piston ā that is, in two revolutions of the crankshaft. It runs on diesel as fuel and works on the diesel cycle (constant pressure cycle). Unlike a petrol engine, a diesel engine has no spark plug; the fuel is ignited by the heat produced when air is compressed to a very high pressure. The four strokes are the suction stroke, compression stroke, power (expansion) stroke and exhaust stroke.
A four stroke diesel engine completes one cycle in four strokes (two crankshaft revolutions).
The four strokes are: suction, compression, power (expansion) and exhaust.
During suction, only pure air is drawn in (no fuel).
Air is compressed to a high ratio, becoming very hot (about 500ā600Ā°C).
Diesel is injected at the end of compression and ignites by itself (compression ignition).
A diesel engine has no spark plug; it uses a fuel injector.
It works on the diesel (constant-pressure) cycle; only the power stroke produces work.
Suction (Intake) stroke: The piston moves down, the inlet valve opens, and only pure air is drawn into the cylinder. (In a diesel engine, no fuel enters at this stage ā only air.)
Compression stroke: Both valves close and the piston moves up, compressing the air to a very high pressure. This raises the temperature of the air to about 500ā600Ā°C ā high enough to ignite diesel.
Power (Expansion/Working) stroke: At the end of compression, diesel is sprayed into the hot compressed air through a fuel injector. The fuel ignites on its own due to the high temperature (compression ignition), and the burning gases expand and push the piston down forcefully. This is the stroke that produces power.
Exhaust stroke: The exhaust valve opens and the piston moves up, pushing out the burnt gases from the cylinder. The cycle then repeats.
A four stroke diesel engine works on the diesel cycle, also called the constant-pressure cycle.
Key points: ā¢ It draws in only air during suction, not a fuel-air mixture. ā¢ The air is compressed to a high ratio (about 14:1 to 22:1), making it very hot. ā¢ Fuel is injected at the end of compression and ignites by itself due to the heat ā this is called compression ignition. ā¢ There is no spark plug; instead a fuel injector is used.
The engine completes the four strokes in two revolutions of the crankshaft. Only one stroke (the power stroke) produces power; the other three are prepared by the engine's momentum (helped by the flywheel).
Diesel Engine: ā¢ Draws in only air during suction ā¢ Fuel (diesel) is injected at the end of compression ā¢ Uses compression ignition (no spark plug) ā¢ Has a fuel injector ā¢ High compression ratio (about 14:1 to 22:1) ā¢ Works on the diesel (constant-pressure) cycle ā¢ More fuel-efficient; used in trucks, buses, trains
Petrol Engine: ā¢ Draws in a mixture of petrol and air ā¢ Mixture is compressed at a lower ratio ā¢ Uses spark ignition (has a spark plug) ā¢ Has a carburettor or fuel injection for mixing ā¢ Lower compression ratio (about 6:1 to 10:1) ā¢ Works on the Otto (constant-volume) cycle ā¢ Lighter and smoother; used in cars and motorcycles
The four strokes are: (1) Suction stroke ā only pure air is drawn into the cylinder; (2) Compression stroke ā the air is compressed to a high pressure and becomes very hot; (3) Power stroke ā diesel is injected, ignites by itself due to the heat, and pushes the piston down to produce power; and (4) Exhaust stroke ā the burnt gases are pushed out of the cylinder.
A four stroke diesel engine draws in only air, compresses it to a high ratio so it becomes very hot, and then sprays diesel into the hot air. The fuel ignites on its own due to the high temperature (compression ignition), and the expanding gases push the piston to produce power. The burnt gases are then exhausted, and the cycle repeats over four strokes (two crankshaft revolutions).
A diesel engine does not need a spark plug because it uses compression ignition. The air is compressed to a very high ratio, which raises its temperature to about 500ā600Ā°C. When diesel is injected into this very hot air, it ignites by itself without any spark. A fuel injector, not a spark plug, is used.
A diesel engine draws in only air, compresses it to a high ratio, and injects fuel that ignites by compression (no spark plug); it works on the diesel cycle. A petrol engine draws in a petrol-air mixture, compresses it at a lower ratio, and ignites it with a spark plug; it works on the Otto cycle. Diesel engines are more fuel-efficient and are used in trucks and buses, while petrol engines are lighter and used in cars and bikes.
What Happens as Angle of Incidence Increases Gradually
As angle of incidence increases in a denser medium: refracted ray bends away, reaches 90Ā° at critical angle, then total internal reflection occurs beyond it.
What is a Lubricant? Types and Functions in Physics
Learn what a lubricant is in physics. Discover how lubricants reduce friction between moving surfaces, preventing wear and tear in machinery.
What is Ampere's Circuital Law? (Class 12 Physics)
Learn Ampere's Circuital Law in Electromagnetism. Understand the line integral formula, the permeability of free space (Ī¼ā), and its application for solenoids.
What is an Ebonite Rod in Physics Experiments?
Learn what an ebonite rod is and why it is used in physics. Understand how rubbing an ebonite rod with animal fur creates static electricity and negative charges.
What Is a Semiconductor? ā Definition, Types and Examples
A semiconductor has conductivity between a conductor and insulator. Examples: silicon, germanium. Types: intrinsic and extrinsic (n-type, p-type). Used in transistors and ICs.
Turn this guide into revision flashcards, a practice exam, or an AI-generated podcast ā free, no signup required.