How Diesel Engines Work

How diesel engines work? Let’s briefly explain.

Chapter 1 Diesel Engine Working

Below is the structural drawing, let’s look at the key spare parts first.

Fuel Injector

Injector nozzle

Cam operated by camshaft

Cylinder head

Intake port

Combustion chamber

Valve spring

Valve stem


Exhaust port

Piston ring


Cylinder block

Cooling jacket

Small end bearing


Connecting rod


Big end bearing

Crankshaft web



Chapter 2 Intake/Suction Stroke

The piston moves down (TDC to BDC) during the intake stroke.

The intake valve opens during the intake stroke to allow fresh air from the atmosphere into the engine cylinder. The motion of the intake valve is controlled by a camshaft.

The exhaust valve remains closed during the intake stroke.

The downward motion of the piston creates partial vacuum inside the cylinder.

The pressure difference generated from the downward motion of the piston pulls the air molecules into the cylinder.

The intake valve closes at the end of the intake stroke.

Chapter 3 Compression Stroke

The piston moves from BDC to TDC during the compression stroke.

The air encompassed in the cylinder during the intake stroke is compressed during this stroke.

During the compression stroke both the valves remain closed. This isolates the combustion chamber from the ambient air.

Compression raises the pressure and temperature of this air. Compression ratios ranging from 12:1 to 22:1 is used.

At the end of the compression stroke, the fuel injector injects a metered quantity of fuel in a finely atomized from into the cylinder.

Chapter 4 Power Stroke

Compression raises the pressure and temperature of air to match the auto or self ignition temperature of Diesel.

These fuel molecules absorb heat from the surrounding hot air and vaporize. This results into combustion and subsequent generation of energy.

Combustion of fuel generates energy. The trust produced due to the combustion of fuel pushes the piston down from TDC to BDC.

Both the valves remain closed during the power stroke.

This downward motion of the piston provides positive driving energy to rotate the wheels.

Driving energy is channeled to the wheels through an array of complex mechanical parts.

Chapter 5 Exhaust Stroke

Combusiton of fuel produces residue gases having high temperature and pressure.

These gasses are composed of unburnt fuel particles and a number of harmful pollutants like CO,CO2,NOX, HC and SOX.

The intake valve remains closed during the exhaust stroke.

The exhaust valve opens during the exhaust stroke. This allows smooth disposal of the gasses into the exhaust port and further down the exhaust system.

The upward motion of the piston and the pressure difference between the cylinder and the atmosphere drives the exhaust gases out during this stroke.

Exhaust gasses are channeled through the exhaust system to be purified and muffled before being released in to the atmosphere.

Chapter 6 Four Strokes in Diesel Engine





Chapter 7 Stages of Combustion

The blue curve in the graph represents the pressure buildup during the compression stroke.

1, Delay period

2, Uncontrolled combustion

3, Controlled combustion

4, After burn

Chapter 8 Valve Timing Diagram for Four Stroke Diesel Engine

The intake valve opens early with respect to the TDC position of the piston. This compensates for the slow operation of the valves.

Similarly the late closing (after the BDC position of the piston) is meant to accommodate maximum air molecules inside the cylinder.

Fuel is not injected instantaneously, but injection is spread over a very short interval of time.

This spread assists in smooth and controlled combustion.

To achieve maximum evacuation of exhaust gases, the exhaust valve is timed to open early.

After the piston reaches the TDC (during the exhaust stroke), a part of the gases is still left in the clearance volume. The late closing of exhaust valve assists in the removal of these remaining gases.