Two-Stroke Engine Timing Diagrams Explained

TWO-STROKE ENGINE

TIMING DIAGRAMS

EXPLAINED

HANDBOOK

Second Edition, 2026

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Table of Contents

Introduction............................................................................................................1

Timing Diagram Purpose......................................................................................1

Timing Diagram Reference Points.......................................................................2

Two Stroke Engine Timing Diagram General Overview.....................................2

Main Phases...........................................................................................................3

Marine Slow Speed Two Stroke Engine Cycle.....................................................4

Exhaust Timing.......................................................................................................4

Scavenge Timing....................................................................................................5

Air Supply and Charging........................................................................................6

Port and Valve Overlap..........................................................................................6

Fuel Injection and Combustion Timing...............................................................7

Reading a Two-Stroke Timing Diagram...............................................................7

Introduction

Two-stroke engine timing diagrams show when key events occur in relation to crankshaft

rotation and piston movement. Unlike four-stroke engines, which use mechanically operated

valves, two-stroke engines use different methods to control gas flow. Some rely on ports

uncovered and covered by the piston, others rely on cylinder-head valves, whilst others utilize a

mixture of both ports and valves.

Timing diagrams help visualize how intake, compression, combustion, exhaust, and scavenging

overlap within a single 360 degree crankshaft revolution. Although timing diagrams can be used

for all two-stroke engine designs, this article focuses only on slow speed marine diesel engines

and their associated timing diagrams.

Timing Diagram Purpose

A two-stroke engine completes a full operating cycle in one crankshaft revolution, or 360

degrees of rotation. A timing diagram maps significant events -such as port opening and closing

times- onto this rotation, using the crankshaft angle as reference.

Simplified Two-Stroke Engine Timing Diagram

Because multiple processes occur almost simultaneously in a two-stroke engine, timing

diagrams are essential for understanding how fresh charge enters the cylinder, how exhaust

gasses are discharged, and how these events are coordinated to produce continuous power.

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Two-Stroke Engine Timing Diagrams Explained | pg 1

Timing Diagram Reference Points

Timing diagrams are drawn against the crankshaft angle, with 360 degrees representing one

complete cycle. The two key piston positions are top dead center (TDC), where the piston

reaches its highest point, and bottom dead center (BDC), where the piston reaches its lowest

point.

Top Dead Center (TDC) and Bottom Dead Center (BDC)

The BDC and TDC positions divide the cycle into upward and downward piston movement,

however, most events occur before or after these reference points to take advantage of gas

flow inertia and pressure differences within the system.

Two Stroke Engine Timing Diagram General Overview

The below diagram shows the timing events of a large marine two-stroke diesel engine during

one crankshaft revolution.

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Two-Stroke Engine Timing Diagrams Explained | pg 2

Two-Stroke Engine Timing Diagram

Main Phases:

●Expansion / Power stroke (0-120°):

■Begins just after combustion (near TDC).

■Expanding gasses force the piston downward towards BDC.

●Exhaust period (120-240°):

■As the piston nears BDC, the exhaust valve opens (EVO) about 60° before BDC.

■Burnt gasses begin leaving the cylinder.

■The exhaust valve remains open after BDC and closes at EVC, about 60° after BDC.

●Scavenging period (138-222°):

■The scavenge ports open (SPO) about 42° before BDC.

■Pressurized scavenge air enters the cylinder and pushes exhaust gasses out via the

exhaust gas valve.

■The scavenge ports close at SPC, about 42° after BDC.

●Compression stroke (240-360°):

■After the scavenge ports and exhaust valve close, the piston moves upward towards

TDC and compresses the air trapped in the cylinder.

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Two-Stroke Engine Timing Diagrams Explained | pg 3

●Fuel Injection (345-20°):

■Near the end of compression, fuel is injected just before TDC.

■Fuel Injection Start (FIS) occurs about 15° before TDC.

■Fuel Injection End (FIE) occurs about 20° after TDC.

The exact degree positioning of each event depends upon the engine design. For example, some

engines may inject fuel later than 15 before TDC°.

A more detailed description of the entire two-stroke engine cycle is given in the next section.

Marine Slow Speed Two Stroke Engine Cycle

Exhaust Timing

In a large marine two-stroke diesel engine, the exhaust valve is typically the first component to

open during the downward power stroke. As the piston approaches bottom dead center, the

hydraulically or mechanically actuated exhaust valve opens in the cylinder cover, allowing

high-pressure combustion gasses to discharge into the exhaust receiver; this rapid release of

pressure reduces cylinder pressure before scavenging air enters.

Good to know - You can access the Diesel Engine Fundamentals 1 and Diesel Engine Fundamentals 2

courses along with over 100 hours of additional engineering video lessons and tutorials.

Marine Engine Exhaust Gas Valve

The exhaust valve remains open through the lower part of the stroke and closes after the piston

has started to rise on the compression stroke, according to the engine’s valve timing arrangement.

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Two-Stroke Engine Timing Diagrams Explained | pg 4

Scavenge Timing

After the exhaust valve opens, the scavenge ports in the cylinder liner are uncovered by the

descending piston. Pressurized scavenge air, supplied by turbochargers or auxiliary blowers,

then enters the cylinder through these ports.

Scavenge timing is carefully arranged so that exhaust gasses have already begun leaving the

cylinder before fresh air enters; this promotes efficient cylinder clearing and charging. The

incoming air sweeps the remaining exhaust gasses upward toward the open exhaust valve,

preparing the cylinder for the next cycle.

Intake Ports (bottom) and Exhaust Valve (top)

The scavenge ports close as the piston rises from bottom dead center, whilst the exhaust valve

closes slightly thereafter; this sequence improves cylinder charging efficiency and reduces the

loss of fresh air into the exhaust system.

Good to know - in large marine engines, the scavenging process is usually a uniflow scavenging process

because air enters through ports at the lower end of the cylinder and exits through the exhaust valve at

the top. There are three main types of scavenging modes used by piston engines: cross-flow, uniflow,

and loop-flow.

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Two-Stroke Engine Timing Diagrams Explained | pg 5

Engine Scavenging Modes

Air Supply and Charging

Fresh air is compressed externally by the turbocharger compressor. At low engine loads,

electrically driven or mechanically driven auxiliary blowers also supply air; this compressed

air is delivered to the scavenge air receiver, then distributed to each cylinder through the

scavenge ports when uncovered by the piston.

As the piston moves towards top dead center, it passes and closes the scavenge ports, causing the

trapped air charge to be compressed within the cylinder. Cylinder pressure and temperature

then increase rapidly until conditions are suitable for fuel injection and auto-ignition.

Good to know - unlike small crankcase-scavenged two-stroke engines, a large marine diesel engine does

not use the crankcase to compress intake air. Large marine engines only draw fresh air in via the

scavenge ports, they do not draw an air/fuel mixture, like is common for small two-stroke engines.

Port and Valve Overlap

Timing diagrams for large marine two-stroke engines show that the exhaust valve and scavenge

ports are open simultaneously for a short period of time around bottom dead center; this overlap

is essential for effective scavenging.

During this period, incoming scavenge air pushes the remaining exhaust gasses upward and out

through the exhaust valve. Correct overlap timing improves gas exchange, reduces residual

exhaust contamination, and increases combustion efficiency.

Poor engine timing may lead to excessive fresh air passing directly into the exhaust system,

consequently reducing engine efficiency. Alternatively, if poor timing causes too little air to be

supplied, exhaust gas may remain within the cylinder and a resultant reduction in engine power

will occur.

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Two-Stroke Engine Timing Diagrams Explained | pg 6

Fuel Injection and Combustion Timing

Large marine two-stroke engines are compression-ignition engines, so there is no spark

ignition system (no spark plug). Instead, fuel is injected directly into the cylinder near the end of

the compression stroke. The fuel atomizes, mixes with the hot compressed air, and auto-ignites.

Timing diagrams are used to show the start of injection and end of injection. Fuel injection

begins shortly before top dead center so that peak cylinder pressure occurs just after top

dead center during the power stroke; this allows the expanding combustion gasses to act on the

piston when the crankshaft geometry is most favorable for producing torque (a form of power).

If peak pressure occurs too early before top dead center, the piston will be forced upward against

the rising pressure; this creates mechanical stress, rough running, and reduced overall engine

efficiency.

Good to know - excessively early combustion can produce sharp pressure rises and a metallic knocking

sound, often referred to as 'knock' or 'diesel knock'.

Marine Slow Speed Two-Stroke Engine

Reading a Two-Stroke Timing Diagram

When reading a two-stroke timing diagram, focus on the sequence and duration of port and valve

openings rather than the separate strokes. Small changes in port height or piston geometry can

significantly affect timing, impacting power, efficiency, and emissions. Short durations with early

exhaust port closing favor low to mid RPM torque, while longer durations with late exhaust port

closing produce high RPM power. Comparing diagrams makes it possible to understand the

engine’s intended RPM range and application.

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Two-Stroke Engine Timing Diagrams Explained | pg 7