The Four Stages of a Diesel Engine

The four stages of a typical four-stroke diesel engine are: 1) intake, 2) compression, 3) power (combustion/expansion), and 4) exhaust. These strokes describe the mechanical sequence that turns the linear motion of a piston into rotary power, with fuel auto-igniting due to high compression rather than a spark.

The Four Strokes Explained

Below is a concise, step-by-step look at what happens in each stroke, including valve positions and when fuel is introduced in a modern diesel engine.

1. Intake stroke: The piston moves down from top dead center (TDC) to bottom dead center (BDC). The intake valve opens, the exhaust valve is closed, and only air is drawn into the cylinder (no fuel yet).
2. Compression stroke: The piston rises from BDC to TDC with both valves closed, compressing the air to a high pressure and temperature. Near the end of this stroke—just before TDC—high-pressure fuel is injected directly into the hot, compressed air.
3. Power (combustion/expansion) stroke: The injected fuel auto-ignites and burns as the piston passes TDC, forcing the piston down to BDC. Both valves remain closed, and the expanding gases deliver work to the crankshaft. In the ideal Diesel cycle, most heat addition occurs at (approximately) constant pressure during the early part of this stroke.
4. Exhaust stroke: The exhaust valve opens (often slightly before BDC of the power stroke to start blowdown). The piston moves from BDC back to TDC, pushing spent gases out. A short overlap period may follow, where the intake valve begins to open as the exhaust valve is still closing, improving cylinder scavenging.

Taken together, these four strokes span 720 degrees of crankshaft rotation and repeat continuously under load, converting fuel energy into usable mechanical power with high efficiency.

How Diesel Four-Stroke Operation Differs from Gasoline Engines

While the mechanical four-stroke sequence is the same, diesel engines compress only air and rely on compression-induced autoignition—no spark plugs. Fuel is injected directly into the combustion chamber at the end of compression, and combustion is largely mixing-controlled. Diesels typically run higher compression ratios (about 14:1 to 22:1), enabling better thermal efficiency and torque at low engine speeds.

Two-Stroke Diesels Exist—But They’re Different

Some large industrial and marine diesels use a two-stroke cycle with ports and/or valves plus scavenging blowers or turbochargers to manage gas exchange. Though they still rely on compression ignition, their gas-exchange process and timing differ significantly from the four-stroke sequence summarized above.

Real-World Timing and Control

Modern diesel engines use precise electronic control for injection timing, pressure, and multiple injection events (pilot, main, and sometimes post-injection) to reduce noise, control NOx and particulate emissions, and improve efficiency. Valve timing typically includes advance/retard and overlap to optimize breathing, often alongside turbocharging and exhaust gas recirculation (EGR) systems.

Summary

The four stages of a four-stroke diesel engine are intake, compression, power (combustion/expansion), and exhaust. Air is drawn in, compressed to raise temperature, fuel is injected to auto-ignite and produce work, and exhaust gases are expelled—completing a 720-degree cycle that delivers efficient, high-torque performance characteristic of diesel powertrains.

What is the 4 stage engine cycle?

In an engine, a stroke refers to the movement of the piston in the cylinder as part of the combustion process. There are typically four strokes involved in the operation of a four-stroke engine: intake, compression, power and exhaust.

What are the 4 stages of the diesel engine cycle?

The four separate strokes are termed:

• Intake: Also known as induction or suction.
• Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C.
• Combustion: Also known as power or ignition.
• Exhaust: Also known as outlet.

What is a stage 5 diesel engine?

EU Stage V is the latest stage of engine emission legislation designed to reduce pollution from off-highway machines, affecting European territories only. In order to meet these guidelines, Autostop and a Diesel Particulate Filter (DPF) has been introduced across all applicable machines.

What are the 4 phases of diesel injection?

1.1 Combustion in diesel engine
The four stages are ignition delay, premixed burning, mixing controlled combustion, and late burning/combustion.